Hysteresis

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Hysteresis 

Electric displacement field D of a ferroelectric material as the electric field E is first decreased, then increased. The curves form a hysteresis loop.

Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of the moment often form a loop or hysteresis curve, where there are different values of one variable depending on the direction of change of another variable. This history dependence is the basis of memory in a hard disk drive and the remanence that retains a record of the Earth's magnetic field magnitude in the past. Hysteresis occurs in ferromagnetic and ferroelectric materials, as well as in the deformation of rubber bands and shape-memory alloys and many other natural phenomena. In natural systems it is often associated with irreversible thermodynamic change such as phase transitions and with internal friction; and dissipation is a common side effect.

Hysteresis can be found in physics, chemistry, engineering, biology, and economics. It is incorporated in many artificial systems: for example, in thermostats and Schmitt triggers, it prevents unwanted frequent switching.

Hysteresis can be a dynamic lag between an input and an output that disappears if the input is varied more slowly; this is known as rate-dependent hysteresis. However, phenomena such as the magnetic hysteresis loops are mainly rate-independent, which makes a durable memory possible.

Systems with hysteresis are nonlinear, and can be mathematically challenging to model. Some hysteretic models, such as the Preisach model (originally applied to ferromagnetism) and the Bouc–Wen model, attempt to capture general features of hysteresis; and there are also phenomenological models for particular phenomena such as the Jiles–Atherton model for ferromagnetism.

Etymology and history

The term "hysteresis" is derived from ὑστέρησις, an Ancient Greek word meaning "deficiency" or "lagging behind". It was coined in 1881 by Sir James Alfred Ewing to describe the behaviour of magnetic materials.

Some early work on describing hysteresis in mechanical systems was performed by James Clerk Maxwell. Subsequently, hysteretic models have received significant attention in the works of Ferenc Preisach (Preisach model of hysteresis), Louis Néel and Douglas Hugh Everett in connection with magnetism and absorption. A more formal mathematical theory of systems with hysteresis was developed in the 1970s by a group of Russian mathematicians led by Mark Krasnosel'skii.

Types

Rate-dependent

One type of hysteresis is a lag between input and output. An example is a sinusoidal input X(t) that results in a sinusoidal output Y(t), but with a phase lag φ:

${\displaystyle {\begin{aligned}X(t)&=X_{0}\sin \omega t\\Y(t)&=Y_{0}\sin \left(\omega t-\varphi \right).\end{aligned}}}$

Such behavior can occur in linear systems, and a more general form of response is

${\displaystyle Y(t)=\chi _{\text{i}}X(t)+\int _{0}^{\infty }\Phi _{\text{d}}(\tau )X(t-\tau )\,\mathrm {d} \tau ,}$

where ${\displaystyle \chi _{\text{i}}}$ is the instantaneous response and ${\displaystyle \Phi _{d}(\tau )}$ is the impulse response to an impulse that occurred ${\displaystyle \tau }$ time units in the past. In the frequency domain, input and output are related by a complex generalized susceptibility that can be computed from ${\displaystyle \Phi _{d}}$; it is mathematically equivalent to a transfer function in linear filter theory and analogue signal processing.

This kind of hysteresis is often referred to as rate-dependent hysteresis. If the input is reduced to zero, the output continues to respond for a finite time. This constitutes a memory of the past, but a limited one because it disappears as the output decays to zero. The phase lag depends on the frequency of the input, and goes to zero as the frequency decreases.

When rate-dependent hysteresis is due to dissipative effects like friction, it is associated with power loss.

Rate-independent

Systems with rate-independent hysteresis have a persistent memory of the past that remains after the transients have died out. The future development of such a system depends on the history of states visited, but does not fade as the events recede into the past. If an input variable X(t) cycles from X₀ to X₁ and back again, the output Y(t) may be Y₀ initially but a different value Y₂ upon return. The values of Y(t) depend on the path of values that X(t) passes through but not on the speed at which it traverses the path. Many authors restrict the term hysteresis to mean only rate-independent hysteresis. Hysteresis effects can be characterized using the Preisach model and the generalized Prandtl−Ishlinskii model.

In engineering

Control systems

In control systems, hysteresis can be used to filter signals so that the output reacts less rapidly than it otherwise would by taking recent system history into account. For example, a thermostat controlling a heater may switch the heater on when the temperature drops below A, but not turn it off until the temperature rises above B. (For instance, if one wishes to maintain a temperature of 20 °C then one might set the thermostat to turn the heater on when the temperature drops to below 18 °C and off when the temperature exceeds 22 °C).

Similarly, a pressure switch can be designed to exhibit hysteresis, with pressure set-points substituted for temperature thresholds.

Electronic circuits

Sharp hysteresis loop of a Schmitt trigger

Often, some amount of hysteresis is intentionally added to an electronic circuit to prevent unwanted rapid switching. This and similar techniques are used to compensate for contact bounce in switches, or noise in an electrical signal.

A Schmitt trigger is a simple electronic circuit that exhibits this property.

A latching relay uses a solenoid to actuate a ratcheting mechanism that keeps the relay closed even if power to the relay is terminated.

Hysteresis is essential to the workings of some memristors (circuit components which "remember" changes in the current passing through them by changing their resistance).

Hysteresis can be used when connecting arrays of elements such as nanoelectronics, electrochrome cells and memory effect devices using passive matrix addressing. Shortcuts are made between adjacent components (see crosstalk) and the hysteresis helps to keep the components in a particular state while the other components change states. Thus, all rows can be addressed at the same time instead of individually.

In the field of audio electronics, a noise gate often implements hysteresis intentionally to prevent the gate from "chattering" when signals close to its threshold are applied.

User interface design

A hysteresis is sometimes intentionally added to computer algorithms. The field of user interface design has borrowed the term hysteresis to refer to times when the state of the user interface intentionally lags behind the apparent user input. For example, a menu that was drawn in response to a mouse-over event may remain on-screen for a brief moment after the mouse has moved out of the trigger region and the menu region. This allows the user to move the mouse directly to an item on the menu, even if part of that direct mouse path is outside of both the trigger region and the menu region. For instance, right-clicking on the desktop in most Windows interfaces will create a menu that exhibits this behavior.

Aerodynamics

In aerodynamics, hysteresis can be observed when decreasing the angle of attack of a wing after stall, regarding the lift and drag coefficients. The angle of attack at which the flow on top of the wing reattaches is generally lower than the angle of attack at which the flow separates during the increase of the angle of attack.

Backlash

Moving parts within machines, such as the components of a gear train, normally have a small gap between them, to allow movement and lubrication. As a consequence of this gap, any reversal in direction of a drive part will not be passed on immediately to the driven part. This unwanted delay is normally kept as small as practicable, and is usually called backlash. The amount of backlash will increase with time as the moving parts wear away.

In mechanics

Elastic hysteresis

Elastic hysteresis of an idealized rubber band. The area in the centre of the hysteresis loop is the energy dissipated due to internal friction.

In the elastic hysteresis of rubber, the area in the centre of a hysteresis loop is the energy dissipated due to material internal friction.

Elastic hysteresis was one of the first types of hysteresis to be examined.

The effect can be demonstrated using a rubber band with weights attached to it. If the top of a rubber band is hung on a hook and small weights are attached to the bottom of the band one at a time, it will stretch and get longer. As more weights are loaded onto it, the band will continue to stretch because the force the weights are exerting on the band is increasing. When each weight is taken off, or unloaded, the band will contract as the force is reduced. As the weights are taken off, each weight that produced a specific length as it was loaded onto the band now contracts less, resulting a slightly longer length as it is unloaded. This is because the band does not obey Hooke's law perfectly. The hysteresis loop of an idealized rubber band is shown in the figure.

In terms of force, the rubber band was harder to stretch when it was being loaded than when it was being unloaded. In terms of time, when the band is unloaded, the effect (the length) lagged behind the cause (the force of the weights) because the length has not yet reached the value it had for the same weight during the loading part of the cycle. In terms of energy, more energy was required during the loading than the unloading, the excess energy being dissipated as thermal energy.

Elastic hysteresis is more pronounced when the loading and unloading is done quickly than when it is done slowly. Some materials such as hard metals don't show elastic hysteresis under a moderate load, whereas other hard materials like granite and marble do. Materials such as rubber exhibit a high degree of elastic hysteresis.

When the intrinsic hysteresis of rubber is being measured, the material can be considered to behave like a gas. When a rubber band is stretched it heats up, and if it is suddenly released, it cools down perceptibly. These effects correspond to a large hysteresis from the thermal exchange with the environment and a smaller hysteresis due to internal friction within the rubber. This proper, intrinsic hysteresis can be measured only if the rubber band is thermally isolated.

Small vehicle suspensions using rubber (or other elastomers) can achieve the dual function of springing and damping because rubber, unlike metal springs, has pronounced hysteresis and does not return all the absorbed compression energy on the rebound. Mountain bikes have made use of elastomer suspension, as did the original Mini car.

The primary cause of rolling resistance when a body (such as a ball, tire, or wheel) rolls on a surface is hysteresis. This is attributed to the viscoelastic characteristics of the material of the rolling body.

Contact angle hysteresis

The contact angle formed between a liquid and solid phase will exhibit a range of contact angles that are possible. There are two common methods for measuring this range of contact angles. The first method is referred to as the tilting base method. Once a drop is dispensed on the surface with the surface level, the surface is then tilted from 0° to 90°. As the drop is tilted, the downhill side will be in a state of imminent wetting while the uphill side will be in a state of imminent dewetting. As the tilt increases the downhill contact angle will increase and represents the advancing contact angle while the uphill side will decrease; this is the receding contact angle. The values for these angles just prior to the drop releasing will typically represent the advancing and receding contact angles. The difference between these two angles is the contact angle hysteresis.

The second method is often referred to as the add/remove volume method. When the maximum liquid volume is removed from the drop without the interfacial area decreasing the receding contact angle is thus measured. When volume is added to the maximum before the interfacial area increases, this is the advancing contact angle. As with the tilt method, the difference between the advancing and receding contact angles is the contact angle hysteresis. Most researchers prefer the tilt method; the add/remove method requires that a tip or needle stay embedded in the drop which can affect the accuracy of the values, especially the receding contact angle.

Bubble shape hysteresis

The equilibrium shapes of bubbles expanding and contracting on capillaries (blunt needles) can exhibit hysteresis depending on the relative magnitude of the maximum capillary pressure to ambient pressure, and the relative magnitude of the bubble volume at the maximum capillary pressure to the dead volume in the system. The bubble shape hysteresis is a consequence of gas compressibility, which causes the bubbles to behave differently across expansion and contraction. During expansion, bubbles undergo large non equilibrium jumps in volume, while during contraction the bubbles are more stable and undergo a relatively smaller jump in volume resulting in an asymmetry across expansion and contraction. The bubble shape hysteresis is qualitatively similar to the adsorption hysteresis, and as in the contact angle hysteresis, the interfacial properties play an important role in bubble shape hysteresis.

The existence of the bubble shape hysteresis has important consequences in interfacial rheology experiments involving bubbles. As a result of the hysteresis, not all sizes of the bubbles can be formed on a capillary. Further the gas compressibility causing the hysteresis leads to unintended complications in the phase relation between the applied changes in interfacial area to the expected interfacial stresses. These difficulties can be avoided by designing experimental systems to avoid the bubble shape hysteresis.

Adsorption hysteresis

Hysteresis can also occur during physical adsorption processes. In this type of hysteresis, the quantity adsorbed is different when gas is being added than it is when being removed. The specific causes of adsorption hysteresis are still an active area of research, but it is linked to differences in the nucleation and evaporation mechanisms inside mesopores. These mechanisms are further complicated by effects such as cavitation and pore blocking.

In physical adsorption, hysteresis is evidence of mesoporosity-indeed, the definition of mesopores (2–50 nm) is associated with the appearance (50 nm) and disappearance (2 nm) of mesoporosity in nitrogen adsorption isotherms as a function of Kelvin radius. An adsorption isotherm showing hysteresis is said to be of Type IV (for a wetting adsorbate) or Type V (for a non-wetting adsorbate), and hysteresis loops themselves are classified according to how symmetric the loop is. Adsorption hysteresis loops also have the unusual property that it is possible to scan within a hysteresis loop by reversing the direction of adsorption while on a point on the loop. The resulting scans are called "crossing," "converging," or "returning," depending on the shape of the isotherm at this point.

Matric potential hysteresis

The relationship between matric water potential and water content is the basis of the water retention curve. Matric potential measurements (Ψ_m) are converted to volumetric water content (θ) measurements based on a site or soil specific calibration curve. Hysteresis is a source of water content measurement error. Matric potential hysteresis arises from differences in wetting behaviour causing dry medium to re-wet; that is, it depends on the saturation history of the porous medium. Hysteretic behaviour means that, for example, at a matric potential (Ψ_m) of 5 kPa, the volumetric water content (θ) of a fine sandy soil matrix could be anything between 8% to 25%.

Tensiometers are directly influenced by this type of hysteresis. Two other types of sensors used to measure soil water matric potential are also influenced by hysteresis effects within the sensor itself. Resistance blocks, both nylon and gypsum based, measure matric potential as a function of electrical resistance. The relation between the sensor's electrical resistance and sensor matric potential is hysteretic. Thermocouples measure matric potential as a function of heat dissipation. Hysteresis occurs because measured heat dissipation depends on sensor water content, and the sensor water content–matric potential relationship is hysteretic. As of 2002, only desorption curves are usually measured during calibration of soil moisture sensors. Despite the fact that it can be a source of significant error, the sensor specific effect of hysteresis is generally ignored.^[19]

In materials

Magnetic hysteresis

Main article: Magnetic hysteresis

 

Theoretical model of magnetization m against magnetic field h. Starting at the origin, the upward curve is the initial magnetization curve. The downward curve after saturation, along with the lower return curve, form the main loop. The intercepts h_c and m_rs are the coercivity and saturation remanence.

When an external magnetic field is applied to a ferromagnetic material such as iron, the atomic domains align themselves with it. Even when the field is removed, part of the alignment will be retained: the material has become magnetized. Once magnetized, the magnet will stay magnetized indefinitely. To demagnetize it requires heat or a magnetic field in the opposite direction. This is the effect that provides the element of memory in a hard disk drive.

The relationship between field strength H and magnetization M is not linear in such materials. If a magnet is demagnetized (H = M = 0) and the relationship between H and M is plotted for increasing levels of field strength, M follows the initial magnetization curve. This curve increases rapidly at first and then approaches an asymptote called magnetic saturation. If the magnetic field is now reduced monotonically, M follows a different curve. At zero field strength, the magnetization is offset from the origin by an amount called the remanence. If the H-M relationship is plotted for all strengths of applied magnetic field the result is a hysteresis loop called the main loop. The width of the middle section is twice the coercivity of the material.

A closer look at a magnetization curve generally reveals a series of small, random jumps in magnetization called Barkhausen jumps. This effect is due to crystallographic defects such as dislocations.

Magnetic hysteresis loops are not exclusive to materials with ferromagnetic ordering. Other magnetic orderings, such as spin glass ordering, also exhibit this phenomenon.

Physical origin

Main article: Ferromagnetism

The phenomenon of hysteresis in ferromagnetic materials is the result of two effects: rotation of magnetization and changes in size or number of magnetic domains. In general, the magnetization varies (in direction but not magnitude) across a magnet, but in sufficiently small magnets, it does not. In these single-domain magnets, the magnetization responds to a magnetic field by rotating. Single-domain magnets are used wherever a strong, stable magnetization is needed (for example, magnetic recording).

Larger magnets are divided into regions called domains. Across each domain, the magnetization does not vary; but between domains are relatively thin domain walls in which the direction of magnetization rotates from the direction of one domain to another. If the magnetic field changes, the walls move, changing the relative sizes of the domains. Because the domains are not magnetized in the same direction, the magnetic moment per unit volume is smaller than it would be in a single-domain magnet; but domain walls involve rotation of only a small part of the magnetization, so it is much easier to change the magnetic moment. The magnetization can also change by addition or subtraction of domains (called nucleation and denucleation).

Magnetic hysteresis models

The most known empirical models in hysteresis are Preisach and Jiles-Atherton models. These models allow an accurate modeling of the hysteresis loop and are widely used in the industry. However, these models lose the connection with thermodynamics and the energy consistency is not ensured. A more recent model, with a more consistent thermodynamical foundation, is the vectorial incremental nonconservative consistent hysteresis (VINCH) model of Lavet et al. (2011)

Applications

Main article: Magnet § Common uses

There are a great variety of applications of the hysteresis in ferromagnets. Many of these make use of their ability to retain a memory, for example magnetic tape, hard disks, and credit cards. In these applications, hard magnets (high coercivity) like iron are desirable so the memory is not easily erased.

Magnetically soft (low coercivity) iron is used for the cores in electromagnets. The low coercivity reduces that energy loss associated with hysteresis. The low energy loss during a hysteresis loop is also the reason why soft iron is used for transformer cores and electric motors.

Electrical hysteresis

Electrical hysteresis typically occurs in ferroelectric material, where domains of polarization contribute to the total polarization. Polarization is the electrical dipole moment (either C·m⁻² or C·m). The mechanism, an organization of the polarization into domains, is similar to that of magnetic hysteresis.

Liquid–solid-phase transitions

Hysteresis manifests itself in state transitions when melting temperature and freezing temperature do not agree. For example, agar melts at 85 °C (185 °F) and solidifies from 32 to 40 °C (90 to 104 °F). This is to say that once agar is melted at 85 °C, it retains a liquid state until cooled to 40 °C. Therefore, from the temperatures of 40 to 85 °C, agar can be either solid or liquid, depending on which state it was before.

In biology

Cell biology and genetics

Main article: Cell biology

Hysteresis in cell biology often follows bistable systems where the same input state can lead to two different, stable outputs. Where bistability can lead to digital, switch-like outputs from the continuous inputs of chemical concentrations and activities, hysteresis makes these systems more resistant to noise. These systems are often characterized by higher values of the input required to switch into a particular state as compared to the input required to stay in the state, allowing for a transition that is not continuously reversible, and thus less susceptible to noise.

Cells undergoing cell division exhibit hysteresis in that it takes a higher concentration of cyclins to switch them from G2 phase into mitosis than to stay in mitosis once begun.

Biochemical systems can also show hysteresis-like output when slowly varying states that are not directly monitored are involved, as in the case of the cell cycle arrest in yeast exposed to mating pheromone. Here, the duration of cell cycle arrest depends not only on the final level of input Fus3, but also on the previously achieved Fus3 levels. This effect is achieved due to the slower time scales involved in the transcription of intermediate Far1, such that the total Far1 activity reaches its equilibrium value slowly, and for transient changes in Fus3 concentration, the response of the system depends on the Far1 concentration achieved with the transient value. Experiments in this type of hysteresis benefit from the ability to change the concentration of the inputs with time. The mechanisms are often elucidated by allowing independent control of the concentration of the key intermediate, for instance, by using an inducible promoter.

Main article: Chromatin

Darlington in his classic works on genetics discussed hysteresis of the chromosomes, by which he meant "failure of the external form of the chromosomes to respond immediately to the internal stresses due to changes in their molecular spiral", as they lie in a somewhat rigid medium in the limited space of the cell nucleus.

Main article: Morphogen

In developmental biology, cell type diversity is regulated by long range-acting signaling molecules called morphogens that pattern uniform pools of cells in a concentration- and time-dependent manner. The morphogen sonic hedgehog (Shh), for example, acts on limb bud and neural progenitors to induce expression of a set of homeodomain-containing transcription factors to subdivide these tissues into distinct domains. It has been shown that these tissues have a 'memory' of previous exposure to Shh. In neural tissue, this hysteresis is regulated by a homeodomain (HD) feedback circuit that amplifies Shh signaling. In this circuit, expression of Gli transcription factors, the executors of the Shh pathway, is suppressed. Glis are processed to repressor forms (GliR) in the absence of Shh, but in the presence of Shh, a proportion of Glis are maintained as full-length proteins allowed to translocate to the nucleus, where they act as activators (GliA) of transcription. By reducing Gli expression then, the HD transcription factors reduce the total amount of Gli (GliT), so a higher proportion of GliT can be stabilized as GliA for the same concentration of Shh.

Immunology

There is some evidence that T cells exhibit hysteresis in that it takes a lower signal threshold to activate T cells that have been previously activated. Ras GTPase activation is required for downstream effector functions of activated T cells. Triggering of the T cell receptor induces high levels of Ras activation, which results in higher levels of GTP-bound (active) Ras at the cell surface. Since higher levels of active Ras have accumulated at the cell surface in T cells that have been previously stimulated by strong engagement of the T cell receptor, weaker subsequent T cell receptor signals received shortly afterwards will deliver the same level of activation due to the presence of higher levels of already activated Ras as compared to a naïve cell.

Neuroscience

See also: Refractory period (physiology)

The property by which some neurons do not return to their basal conditions from a stimulated condition immediately after removal of the stimulus is an example of hysteresis.

Respiratory physiology

Lung hysteresis is evident when observing the compliance of a lung on inspiration versus expiration. The difference in compliance (Δvolume/Δpressure) is due to the additional energy required to overcome surface tension forces during inspiration to recruit and inflate additional alveoli.

The transpulmonary pressure vs Volume curve of inhalation is different from the Pressure vs Volume curve of exhalation, the difference being described as hysteresis. Lung volume at any given pressure during inhalation is less than the lung volume at any given pressure during exhalation.

Voice and speech physiology

A hysteresis effect may be observed in voicing onset versus offset. The threshold value of the subglottal pressure required to start the vocal fold vibration is lower than the threshold value at which the vibration stops, when other parameters are kept constant. In utterances of vowel-voiceless consonant-vowel sequences during speech, the intraoral pressure is lower at the voice onset of the second vowel compared to the voice offset of the first vowel, the oral airflow is lower, the transglottal pressure is larger and the glottal width is smaller.

Ecology and epidemiology

Hysteresis is a commonly encountered phenomenon in ecology and epidemiology, where the observed equilibrium of a system can not be predicted solely based on environmental variables, but also requires knowledge of the system's past history. Notable examples include the theory of spruce budworm outbreaks and behavioral-effects on disease transmission.

It is commonly examined in relation to critical transitions between ecosystem or community types in which dominant competitors or entire landscapes can change in a largely irreversible fashion.

In ocean and climate science

Complex ocean and climate models rely on the principle.

In economics

Main article: Hysteresis (economics)

Economic systems can exhibit hysteresis. For example, export performance is subject to strong hysteresis effects: because of the fixed transportation costs it may take a big push to start a country's exports, but once the transition is made, not much may be required to keep them going.

When some negative shock reduces employment in a company or industry, fewer employed workers then remain. As usually the employed workers have the power to set wages, their reduced number incentivizes them to bargain for even higher wages when the economy again gets better instead of letting the wage be at the equilibrium wage level, where the supply and demand of workers would match. This causes hysteresis: the unemployment becomes permanently higher after negative shocks.

Permanently higher unemployment

The idea of hysteresis is used extensively in the area of labor economics, specifically with reference to the unemployment rate. According to theories based on hysteresis, severe economic downturns (recession) and/or persistent stagnation (slow demand growth, usually after a recession) cause unemployed individuals to lose their job skills (commonly developed on the job) or to find that their skills have become obsolete, or become demotivated, disillusioned or depressed or lose job-seeking skills. In addition, employers may use time spent in unemployment as a screening tool, i.e., to weed out less desired employees in hiring decisions. Then, in times of an economic upturn, recovery, or "boom", the affected workers will not share in the prosperity, remaining unemployed for long periods (e.g., over 52 weeks). This makes unemployment "structural", i.e., extremely difficult to reduce simply by increasing the aggregate demand for products and labor without causing increased inflation. That is, it is possible that a ratchet effect in unemployment rates exists, so a short-term rise in unemployment rates tends to persist. For example, traditional anti-inflationary policy (the use of recession to fight inflation) leads to a permanently higher "natural" rate of unemployment (more scientifically known as the NAIRU). This occurs first because inflationary expectations are "sticky" downward due to wage and price rigidities (and so adapt slowly over time rather than being approximately correct as in theories of rational expectations) and second because labor markets do not clear instantly in response to unemployment.

The existence of hysteresis has been put forward as a possible explanation for the persistently high unemployment of many economies in the 1990s. Hysteresis has been invoked by Olivier Blanchard among others to explain the differences in long run unemployment rates between Europe and the United States. Labor market reform (usually meaning institutional change promoting more flexible wages, firing, and hiring) or strong demand-side economic growth may not therefore reduce this pool of long-term unemployed. Thus, specific targeted training programs are presented as a possible policy solution. However, the hysteresis hypothesis suggests such training programs are aided by persistently high demand for products (perhaps with incomes policies to avoid increased inflation), which reduces the transition costs out of unemployment and into paid employment easier.

Additional considerations

Models of hysteresis

Each subject that involves hysteresis has models that are specific to the subject. In addition, there are hysteretic models that capture general features of many systems with hysteresis. An example is the Preisach model of hysteresis, which represents a hysteresis nonlinearity as a linear superposition of square loops called non-ideal relays. Many complex models of hysteresis arise from the simple parallel connection, or superposition, of elementary carriers of hysteresis termed hysterons.

A simple and intuitive parametric description of various hysteresis loops may be found in the Lapshin model. Along with the smooth loops, substitution of trapezoidal, triangular or rectangular pulses instead of the harmonic functions allows piecewise-linear hysteresis loops frequently used in discrete automatics to be built in the model. There are implementations of the hysteresis loop model in Mathcad and in R programming language.

The Bouc–Wen model of hysteresis is often used to describe non-linear hysteretic systems. It was introduced by Bouc and extended by Wen, who demonstrated its versatility by producing a variety of hysteretic patterns. This model is able to capture in analytical form, a range of shapes of hysteretic cycles which match the behaviour of a wide class of hysteretical systems; therefore, given its versability and mathematical tractability, the Bouc–Wen model has quickly gained popularity and has been extended and applied to a wide variety of engineering problems, including multi-degree-of-freedom (MDOF) systems, buildings, frames, bidirectional and torsional response of hysteretic systems two- and three-dimensional continua, and soil liquefaction among others. The Bouc–Wen model and its variants/extensions have been used in applications of structural control, in particular in the modeling of the behaviour of magnetorheological dampers, base isolation devices for buildings and other kinds of damping devices; it has also been used in the modelling and analysis of structures built of reinforced concrete, steel, masonry and timber. The most important extension of Bouc-Wen Model was carried out by Baber and Noori and later by Noori and co-workers. That extended model, named, BWBN, can reproduce the complex shear pinching or slip-lock phenomenon that earlier model could not reproduce. BWBN model has been widely used in a wide spectrum of applications and have been incorporated in several software codes such as OpenSees.

Energy

When hysteresis occurs with extensive and intensive variables, the work done on the system is the area under the hysteresis graph.

Hashtag activism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Hashtag_activism 

Hashtag activism refers to the use of Twitter's hashtags for Internet activism. The Hashtag, has become one of the many ways that social media contributes to civic engagement and social movements. The use of the hashtag on social media provides users with an opportunity to share information and opinions about social issues in a way that others (followers) can interact and engage as part of a larger conversation with the potential to create change. The hashtag itself consists of a word or phrase that is connected to a social or political issue, and fosters a place where discourse can occur. Social Media, provides an important platform for historically marginalized populations. Through the use of hashtags these groups are able to communicate, mobilize, and advocate for issues less visible to the mainstream.

Supporters of the power of hashtag activism believe that it allows users to connect with individuals from all over the world and share information quickly. Critics, on the other hand, doubt that hashtag activism leads to real change, as users are simply indicating that they care enough to post a hashtag, rather than taking practical action to make a difference.

History

The Hashtag, as it is currently used across social media sites was first proposed on Twitter by former Google developer Chris Messina, in 2007. The purpose of the invention was to create a meta tag that allowed users to track dynamic content in relation to a particular event or topic. Twitter's initial response to the use of the hashtag for such purposes was negative. The social media site posted that "these things were for nerds" but later that year saw the impact with use of the #sandiegofire which allowed users to track content related to the fire, the safety of loved ones, and general updates. The use of the hashtag has now spread to other social media sites including Facebook, Instagram, and Tik Tok. Since 2007, the hashtag has been used to coordinate conversations online, find supporters, and arrange protests allowing for users to be active participate in social and political movements.

Notable examples

This is a dynamic list and may never be able to satisfy particular standards for completeness. You can help by adding missing items with reliable sources.

The following notable examples are organized by categories: human rights, awareness, political, and trends.

Race

#BlackLivesMatter

Main article: Black Lives Matter

The Black Lives Matter movement calls for an end to police brutality and the killings of African-Americans in the U.S. The #BlackLivesMatter hashtag was first started by Alicia Garza, Patrisse Cullors, and Opal Tometi as a response to the trial and later acquittal of George Zimmerman who shot and killed 17-year-old Trayvon Martin. The hashtag saw a revival in 2014, after the shooting of Michael Brown in Ferguson, Missouri, and after a grand jury did not indict police officer Daniel Pantaleo in the death of Eric Garner.

#IStandWithAhmed

Main article: Ahmed Mohamed clock incident

	President Obama	Twitter
	@POTUS44

Cool clock, Ahmed. Want to bring it to the White House? We should inspire more kids like you to like science. It's what makes America great.

September 16, 2015

IstandwithAhmed: In 2015, a teenage student named Ahmed Mohamed was arrested at his high school in Irving, Texas after his teacher mistook his reassembled clock for a bomb. Ultimately, he was not convicted of any crimes, but he was suspended from school. Shortly after his story hit the news, a tech blogger named Anil Dash tweeted a picture of Ahmed being arrested in his NASA T-shirt along with the "#IstandwithAhmed." His tweet went viral and drew accusations of racism and Islamophobia against the school. It sparked an online movement where many individuals, including scientists and engineers, tweeted their support for Ahmed under the same hashtag.

#TakeAKnee

Main articles: U.S. national anthem protests (2016–present) and Quarterback kneel

#TakeAknee has been a movement since 2016 and was created with the intention of calling attention to the police brutality and racial inequality taking place in America. This movement was enacted primarily by NFL athletes, most notably Colin Kaepernick, through kneeling for the duration of the national anthem; this act has stirred significant controversy because it is interpreted by nationalists as being a disrespectful act that insults the American flag, veterans, and the values the flag represents. This movement ultimately led to #BoycottNFL and controversy that resulted in the NFL ban requiring players to stand for the national anthem, or stay in the locker room. #TakeAKnee is often known as “the U.S. National Anthem Protest”, and is often compared to protests during the civil rights era, lending to a chain of protests led by athletes in different sports. While the police brutality being faced by African Americans was being protested, white American athletes were also seen taking a knee. As a whole, the #TakeAKnee movement created controversy questioning the legal and constitutional rights of individuals and their ability to protest the U.S. National Anthem.

#MyAsianAmericanStory

In August 2015, " a 15-year-old high student named Jason Fong created #MyAsianAmericanStory to highlight immigration stories of Asian Americans after presidential candidate Jeb Bush made a remark about Asian people and their "anchor babies." Fong stated that he started the hashtag to show that "Asian-Americans are part of the American narrative." Users of the tag tweeted about their diverse family immigration histories and encounters with racism. Fong said he was inspired to start #MyAsianAmericanStory in part by hashtags such as #BlackLivesMatter and participated in other hashtag campaigns such as #Asians4BlackLives in order to show his support for dismantling “a broken system that protects police misconduct.”

#thisis2016

In October 2016, following an anti-Asian incident in New York City and the subsequent open letter to the victim from Michael Luo, The New York Times released a video entitled "#thisis2016: Asian-Americans Respond". The video featured Asian Americans who had experienced racism. #thisis2016 subsequently emerged as a hashtag to highlight racism Asian Americans faced. Eventually, #BrownAsiansExist came to prominence following an open letter written to The New York Times expressing their disappointment in the lack of South and Southeast Asian Americans in their "#thisis2016" video. #BrownAsiansExist more broadly highlights the erasure of South Asian and Southeast Asian Americans in the American media's portrayal of Asian Americans.

#OscarsSoWhite

Main article: 87th Academy Awards

#OscarSoWhite is a hashtag campaign started by BroadwayBlack.com managing editor April Reign and was sparked by the Oscars nominees in 2016. All of the 20 actors nominated for lead and supporting actor categories were white, despite multiple films that year starring African American leads that had received critics' prizes and guild awards. The campaign sparked a conversation about diversity, representation, and racism in the film industry. The movement is connected to causing enough external pressure to significantly change the racial composition of Academy membership. Following the peak of the hashtag's popularity, the Academy instated 41% minority voters and 46% female voters. Production companies felt the pressure as well, and subsequently diversified their casting and staffing decisions as well, hiring Ava Duvernay, an African-American female director, to head the production of A Wrinkle in Time and hiring non-white actors in the traditionally white Star Wars series.

#PublishingPaidMe

Main article: PublishingPaidMe

Women's rights

#YesAllWomen

Main article: YesAllWomen

YesAllWomen is a Twitter hashtag and social media campaign in which users share examples or stories of misogyny and violence against women. #YesAllWomen was created in reaction to another hashtag #NotAllMen, to express that all women are affected by sexism and harassment, even though not all men are sexist. The hashtag quickly became used by women throughout social media to share their experiences of misogyny and sexism. The hashtag was popular in May 2014 surrounding discussions of the 2014 Isla Vista killings.

#ShoutYourAbortion

Main article: ShoutYourAbortion

#ShoutYourAbortion is a hashtag and social media campaign used on Twitter that encourages women who have experience with abortion to break the silence surrounding it. The hashtag was created by American writer Lindy West and friends Amelia Bonow and Kimberly Morrison in response to the US House of Representatives efforts to defund Planned Parenthood following the Planned Parenthood 2015 undercover videos controversy.

#ilooklikeanengineer

Main article: I Look Like an Engineer

In August 2015, the #ilooklikeanengineer campaign started. The movement was started by Isis Anchalee to promote discussion of gender issues. Anchalee created the hashtag in response to backlash regarding her face being featured in an ad campaign for the company she works for. One year after the creation of #ilooklikeanengineer 250,000 people had used the hashtag.

#MeToo

Main article: Me Too movement

#MeToo is a Twitter hashtag that raises awareness about sexual assault by encouraging survivors to share their stories. The hashtag was initially first used in 2007 by Tarana Burke but was later popularized and brought to the attention of the media on October 15, 2017, when Alyssa Milano, using Twitter, encouraged individuals to speak up about their experience with assault and say 'Me Too'. Initially meant to simply raise awareness but has developed into a movement and as of October 2018, the hashtag has been used 19 million times. The movement has sparked many other movements like #HowIWillChange and has also led to certain punishments towards the perpetrators. As a reaction, the #HimToo hashtag was created. It that refers to the social media campaign for false rape allegation.

#MosqueMeToo

Main article: Mosque Me Too movement

In February 2018, the Mosque Me Too movement started, following the Me Too movement which gained worldwide prominence in October 2017 and the following months. Muslim women started sharing their experiences of sexual abuse at Muslim holy sites and on pilgrimages such as Hajj, Mecca, Saudi Arabia, using the hashtag #MosqueMeToo.

#WomensMarch

Main articles: 2017 Women's March, 2018 Women's March, and 2019 Women's March

On January 21, 2017, an estimated 2.6 million individuals marched around the world in response to the rhetoric of newly elected President Donald Trump. The march was organized primarily online through Facebook. Now occurring annually, the goal of the Women's March is to raise awareness and advocates for human rights through peaceful protest.

Similar to other hashtag movements, #WomensMarch has an online presence. The movement has a Facebook page that is active, verified under the name Women's March, that was created on November 20, 2016. As of April 2, 2019 the page is liked by over 800,000 individuals and has a following of more than 850,000 users. Outside of the official page, there are multiple pages defined by geographic region including but not limited to Women's March on Connecticut, Women's March on San Diego, and Women's March Milan. In addition to Facebook, the Women's March Movement has an active profile on Instagram and as of April 2019 the page has 1.2 million followers.

#EleNão

Main article: Ele Não movement

 

Protester in Porto Alegre, Brazil, participating in the Ele Não movement.

On September 19, 2018, the Ele Não movement ("Ele Não" is Portuguese for "not him"), also known as the protests against Jair Bolsonaro, were demonstrations led by women which took place in several regions of Brazil and the world. The main goal was to protest against Bolsonaro and his presidential campaign and his sexist declarations.

#WhereisPengShuai

Main article: WhereIsPengShuai

After the November 2021 disappearance of Chinese Tennis athlete, Peng Shuai, the hashtag #WhereisPengShuai appeared on Twitter and was Tweeted by athletes such as Serena Williams and Naomi Osaka.

Other examples

A 2012 Twitter discussion among women working in games, collated under the hashtag #1reasonwhy, indicated that sexist practices such as the oversexualization of female video game characters, workplace harassment and unequal pay for men and women were common in the games industry.

The hashtag #NotYourAsianSidekick was initiated by Suey Park and Juliet Shen in December 2013 on Twitter. Suey Park is a freelance writer who is most known for her Twitter campaign to cancel the Colbert Show, while Juliet Shen ran a blog on Asian American feminism. They started the hashtag town hall as a way to create a platform for structured conversation around misogyny and issues specific to Asian American women. In less than 24 hours, #NotYourAsianSidekick had been used over 45,000 times.

DeafTalent is a hashtag used to highlight through social media the capabilities of the deaf and hard of hearing community. Prior to the hashtag's emergence, in the creative industry, hearing actors had been cast in deaf roles. The SAGE Deaf Studies Encyclopedia wrote, "In response to this, the social media hashtags #DeafTalent and #POCDeafTalent were created. The hashtags, while originally used to point out problematic portrayals of deaf characters and sign language in the media, are now also used to celebrate the wide breadth and multiplicity of deaf actors, artists, and other talent in the world."

The hashtag, #Boymom, has taken to social media platforms in order to display the so-called "chaotic" and "messy" experiences mothers of boys go through. Instagram appears to be the platform with the most #Boymom hashtags, with nearly 12.9 million hashtags. Interestingly, #Girlmom falls significantly behind in numbers with only 4.8 million. Currently, there is much speculation surrounding this hashtag. People argue that it creates an environment for children where they feel unable to fully express their gender. Some people even go as far as to say that it perpetuates the sexist idea that sons are valued more than daughters.

Awareness

#Kony2012

Main article: Kony 2012

 

A Kony 2012 poster

Kony 2012 is a short film produced by Invisible Children, Inc. (authors of Invisible Children). It was released on March 5, 2012.

The film's purpose was to promote the charity's "Stop Kony" movement to make African cult and militia leader, indicted war criminal and the International Criminal Court fugitive Joseph Kony globally known in order to have him arrested by the end of 2012, when the campaign expired. The film spread virally through the #Kony2012 hashtag.

#WhyIStayed

Main article: WhyIStayed/WhyILeft

In 2014, a media release of security camera footage that appeared to show NFL player, Ray Rice, punching his then-fiancée, Janay Rice, sparked public conversation on why victims of abuse stay in abusive relationships. In response to this question, writer and domestic abuse survivor Beverly Gooden started the #WhyIStayed campaign via Twitter in an effort to "change the tone of the conversation". The hashtag began to trend nationally five hours after its creation and was used more than 46,000 times that day. Beverly appeared on NPR's All Things Considered to discuss hashtag activism.

#BringBackOurGirls

First Lady Michelle Obama initiated the #BringBackOurGirls hashtag.

 

Main article: Chibok schoolgirls kidnapping

Boko Haram kidnapped over 200 schoolgirls from Chibok, Nigeria in May 2014, refusing to return the girls. The hashtag #BringBackOurGirls was created and used in hopes of keeping the story in the news and bringing international attention to it. The hashtag was first used by a corporate lawyer named Ibrahim Abdullahi, and has also been used by the likes of First Lady Michelle Obama, who used it to raise awareness for the kidnapped girls. The hashtag in itself has received 2 million retweets.

#AmINext

Main article: AmINext

In the Fall of 2014, a Canadian Inuit woman named Holly Jarrett created the #AmINext hashtag campaign to raise awareness about the Canadian Government's lack of response to the high rate of violence against Indigenous women. The campaign involves people taking photos of themselves with signs holding "#AmINext" and posting it to social media. The campaign was meant to encourage a national conversation about the invisibility and vulnerability of the female Indigenous demographic and call attention to the minimal efforts of the Government in investigating the murders and disappearances. Holly was personally inspired to carry out the campaign as her cousin, Loretta Saunders, an Inuit woman from Labrador, went missing and was ultimately found dead on the side of a Canadian highway. After the campaign, the government filed a national DNA missing person's index and introduced 30 safety initiatives to help indigenous women.

#PrayforParis

Main article: PrayForParis

The epicenter of Paris encountered unexpected terrorist attacks, leading to worldwide efforts to spread awareness about this incident. During this event, terrorists were wearing suicide belts and the subsequent violence was shocking. The terrorists were planning to enter the stadium along with other people. Despite the person being prevented from entering, it demonstrated the severity of how people are risking their own lives, indirectly affecting others. Following the incident, more than 70 million people began to share this news on various social media platforms in order to reach a broader audience. For example, on Facebook, the social media platform enabled users to change their profile picture to a transparent overlay of the French flag. The purpose of changing a user's Facebook profile picture was to indicate support to the victims of this unfortunate event. Twitter was also utilized. However, rather than creating a transparent overlay on a Twitter's user profile, a hashtag was created to emphasize support. This simple hashtag of #PrayforParis allowed users to spread support so that audiences were not only informed about the event, but could also click on a hyperlink to learn more about the cause and other user's perspectives. Although social media platforms were useful in spreading awareness, the effect of younger individuals were just as notable. For example, a young child drew his thoughts on paper, including the message: "Shot after shot, bang after bang, wasting innocent lives!"

#FakeNews

Main article: Fake news

While "fake news" or politically motivated disinformation (PMD) is not a new occurrence, the sentiment and spread of distrust of news coverage has become more notable since the 2016 U.S. elections cycle. The hashtag, #FakeNews, gained major popularity in 2016 when Donald Trump claimed that the negative press coverage he received was due to the spread of false stories. Since the emergence of this hashtag, there has been an increase in policy-related bills and laws regarding the proliferation of inaccurate information globally, which further politicized the issue and raised concerns of impending censorship. The emergence of social media has allowed for "fake news" to spread much quicker than regular news and information, pushing technology companies to take a more active role in detecting and removing "fake news".

"Pope Francis shocks world, endorses Donald Trump for president”

An example of #FakeNews comes from a website named WTOF 5 News. The headline reads: “Pope Francis shocks world, endorses Donald Trump for president” With the help from Facebook, this fake news article received over 960,000 engagements from the popular social media site, making it one of the post popular fake news articles of 2016.

#ProtectOurWinters

Main article: Protect Our Winters

Protect Our Winters is a movement and a nonprofit organization started by snowboarder, Jeremy Jones and other winter sport athletes to raise awareness about global warming and climate change. The movement started in 2016 as a response to it being one of the hottest years. The movement demonstrates the effects of global warming with data about the winter sports industry and rise in carbon dioxide percentage. Protect Our Winters or POW calls for people to not only be aware of the effects global warming but to take action by volunteering, voting for legislature or donating to the cause.

#FreeBritney

Main articles: Free Britney movement and Britney Spears conservatorship dispute

In April 2019, the Britney Spears fan podcast Britney's Gram uncovered the first alleged abuses within her conservatorship arrangement and created the hashtag #FreeBritney. The term itself had been in use by certain fans since 2008. Alongside advocating for the termination and investigation of Spears's conservatorship, the hashtag and accompanying social movement have been used to raise awareness of guardianship and conservatorship abuse across the United States. The hashtag has led to legislative change in California surrounding legal procedures within conservatorship, and has inspired various bipartisan inquiries of guardianship and conservatorship law in Congress.

#flygskam

Flygskam is a Swedish word that literally translates as “flight shame”. It's the name of an anti-flying movement that originated in Sweden last year, which encourages people to stop taking flights to lower carbon emissions. The idea was originally championed by Olympic athlete Bjorn Ferry and gained momentum after teenage activist Greta Thunberg's mother, the opera singer Malena Ernman, publicly announced she would stop flying, with various Swedish celebrities the following suit. Thunberg herself traveled largely by train during her recent two-week tour of Europe. The activism has seen real results in Sweden as the sales of airline ticket sales declined by 4% in January 2020 due to the increasing public awareness to the carbon footprint resulted by commercial flights.

#CoronaVirus

#Coronavirus, #COVID-19, and #Covid_19 represent a few of the most common hashtags referring to Coronavirus 2019 that started in Wuhan, China. The Hashtag has increased rapidly with the World Health Organization's (WHO) declaration of the virus as pandemic on the 11th of March, 2020. Looking at the trajectory of this Hashtag on Twitter from Symplur, it shows a notable decrease in the number of Hashtags from 50763 on 13 April 2020 to 35795 on 18 April 2020.

#PublishingPaidMe

Main article: PublishingPaidMe

Created in 2020 by L.L. McKinney, this hashtag was used by authors to discuss the pay differences received by publishers for black and non-black authors.

Political

#ArabSpring

Main article: Arab Spring

#ArabSpring spread across social media early 2011, spreading awareness of the anti-government protest in the North Africa and the Middle East. The #ArabSpring is Twitter hashtag used in anti-government protests across the Middle East in 2010.

A memorial service held at Harder Stadium after the Isla Vista killings

#NotOneMore

Main article: 2014 Isla Vista killings

The hashtag #NotOneMore developed shortly after the May 23, 2014, shooting in Isla Vista, Santa Barbara, California. During this incident, six students attending the University of California, Santa Barbara, lost their lives. Richard Martinez, the father of one of the victims, quickly spoke out about gun control, calling for stricter gun control during memorial ceremonies and rallies, chanting "Not One More!" The phrase became a hashtag on social media afterwards. Richard also worked with Everytown's digital team to create a tool to allow participants to send postcards to their senators, congressional representatives, and governor containing the phrase "Not One More."

#NODAPL

#NODAPL, also referred to as the Dakota Access Pipeline protests, is a Twitter hashtag and social media campaign for the struggle against the proposed and partially built Dakota Access Pipeline. The role social media played in this movement is so substantial that the movement itself is now often referred to by its hashtag: #NoDAPL. The hashtag reflected a grassroots campaign that began in early 2016 in reaction to the approved construction of the Dakota Access Pipeline in the northern United States. The Standing Rock Sioux and allied organizations took legal action aimed at stopping construction of the project, while youth from the reservation began a social media campaign which gradually evolved into a larger movement with dozens of associated hashtags. The campaign aimed to raise awareness on the threat of the pipeline on the sacred burial grounds as well as the quality of water in the area. In June 2021, a federal judge struck down the Standing Rock Sioux Tribe's lawsuit, but left the option of reopening the case should any prior orders be violated.

 

The #noDAPL hashtag used in real life, outside social media.

#Oromoprotests

In 2014, IOYA (The International Oromo Youth Association) created the #Oromoprotests hashtag to bring awareness to Oromo student protests against the Ethiopian government's plan to expand Addis Ababa and annex areas occupied by Oromo farmers and residents. The hashtag was utilized again starting in late November/December 2015 to bring attention to renewed Oromo protests and the Ethiopian government's violent crackdown on students, journalists and musicians. While the Oromo held protests before, this was the first time the Oromo could be united across the country by using new social media platforms.

#Sosblakaustralia

In March 2015, an activism campaign took hold in Australia. #Sosblakaustralia was a campaign started in a small aboriginal town in Western Australia. This campaign was to combat an initiative that would close down 150 rural aboriginal communities. Though this movement started in a rural community of 200 #Sosblakaustralia with poor Internet connection, it eventually spread to thousands of followers including Australian celebrities such as Hugh Jackman, this caused the movement to expand as far as London. In 18 days this movement had over 50,000 followers and had reached over 1,000,000 people worldwide.

#IdleNoMore

Main article: Idle No More

In the Winter of 2012–2013, in Canada, a campaign was started by Canadian indigenous activists using #IdleNoMore in order to combat future legislation that would threaten indigenous land and water. The movement has continued to grow and unify native communities and the hashtag has made expansion possible. Idle No More started in Canada it has spread to native people around the world including the United States and Australia where indigenous people face similar issues. The use of the hashtag and social media has been instrumental in spreading Idle No More's message to indigenous people around the world giving those who otherwise would be voiceless a means to participate in activism.

#UmbrellaRevolution

Main article: 2014 Hong Kong protests

 

A sign of the Umbrella Revolution protest in Hong Kong

The response of the umbrella became a symbol in Admiralty, Mong Kok and Causeway Bay districts, Hong Kong to protest about the free election systems in China. The protestors had been camped on the streets and the public parks. The umbrella was used to protect the protesters in defence of the democratic political process in 2014 when police used tear gas in attempts to get them to leave. "Umbrella Revolution" and "Umbrella Movement" have been used to identify this event through British media outlet BBC. through social network services such as Twitter and Instagram made the events in Hong Kong reach many other people not directly involved with the protest with the use of #UmbrellaRevolution and created a worldwide social awareness to how Hong Kong was responding to support of the democratic process.

#MarchforOurLives

Main article: March for Our Lives

The March for Our Lives protest began after the Marjory Stoneman Douglas High School shooting in Parkland, Florida on February 14, 2018. In response to a surge of gun violence in schools and the 17 dead after the Parkland shooting, people began to rally around the hashtag #neveragain. The hashtag, indicative of the larger movement against gun violence, spread across social networks and went viral in early 2018.

Additionally, the movement organized walkouts in remembrance of the lives lost due to gun violence. In March 2018, hundreds of marches were organized across the country in support of stricter gun laws, many of which were met with resistance from anti-protesters. Since February 2018 there have been 123 laws passed nationwide at the state-level to address concerns of gun control.

On February 17, 2018, a Facebook page was started by students to encourage their participation in the movement; and as of April 2019 the page has been liked by more than 280,000 individuals and has a following of more than 300,000. The Instagram page @marchforourlives is live and as of April 2019 has over 200 posts and just over 360,000 followers.

#PutItToThePeople

Main article: People's Vote

The United Kingdom-based People's Vote campaign group was launched in April 2018 and calls for a confirmatory public vote on the final Brexit deal between the United Kingdom and the European Union and uses #PutItToThePeople as its activism hashtag.

#EndFathersDay

In 2014, some editors spoofed being black feminists, spreading the meme, #EndFathersDay. Fox News picked the hoax to denounce. After much research, the fake accounts were outed.

#NoBanNoWall

Main article: Protests against Donald Trump

#NoBanNoWall is a hashtag and social media campaign created in response to Donald Trump's purported "Muslim ban" and 2016 presidential campaign promises to build a physical wall on the US-Mexico border. In 2017, President Donald Trump issued multiple executive orders threatening to break up families and turn away refugees. Saki Barzinji and Imraan Siddiqi started #NoBanNoWall in an effort to rally Muslim, Latino, and other communities to stand up against xenophobic immigration policies. On January 25, 2017, protestors gathered at Washington Square Park and chanted, “No ban; no wall,” which inspired the Twitter hashtag #NoBanNoWall to protest Trump's travel ban. The impact of the movement was seen in airports immediately after the hashtag started trending. A judge in New York accepted a request from the ACLU to provide a warrant blocking deportation in airports. The movement became a platform for people to share stories of them or their families immigrating to the US, and worked to combat the growing public fear of certain foreigners.

#FireDrillFridays

Inspired by Greta Thunberg and started by Greenpeace and Jane Fonda, #FireDrillFridays brings awareness to the climate change crisis. Calling for a Green New Deal in the United States government, the movement organized protests on the Capitol every Friday beginning in October 2019. The campaign also advocates for complete stoppage of new fossil fuel projects and to phase out existing fossil fuel projects. #FireDrillFridays gained popularity with celebrity arrests. Due to the COVID-19 pandemic, Fonda moved #FireDrillFridays online to continue rallying for government action.

#WalkAway

The #WalkAway campaign is a social-media campaign that was launched ahead of the United States 2018 mid-term elections with the stated purpose of encouraging liberals to leave the Democratic Party. The campaign has been organizing events in support of Donald Trump. It was noted and criticized for using astroturfing methods of gathering support, by counterfeiting a popular movement of people who have left the party.

Other examples

In September 2014, The Hokkolorob Movement (Let The Voice Raise Movement) started. It is a series of protests initiated by the students of Jadavpur University in Kolkata, India that began on September 3, 2014. The term "hok kolorob" ("make some noise") was first used as a hashtag on Facebook.

Trends

#icebucketchallenge and #ALS

Main article: Ice Bucket Challenge

 

The #icebucketchallenge is an act where a bucket of ice water is dumped over the head of an individual and documented by videos or pictures, and a "challenge" is issued to another person (or persons) to do the same. The "challenged" individual then either has to respond by dumping ice water on their head, or donate money to a Motor neuron disease or amyotrophic lateral sclerosis (ALS, also referred to as Lou Gehrig's disease) charity. However, doing both is also an option. The encouragement of the challenge is to circulate the video or photo on social media websites and applications with their community, friends, and family to show their support in raising awareness of ALS/MND. The involvement of #icebucketchallenge with the global audience of social media generated so much awareness and support that in early August 2014, the national ALS charity foundation president Barbara Newhouse, directly attributed the movement to a fundraising "surge" of $168,000 that accumulated in just a week. That figure contrasted with the $14,000 raised in the same time the year prior prompted the CEO and her 38 years in the industry to view the difference in support as "crazy." A month after the August 2014 fundraising week the number of videos that were directly associated with the #icebucketchallenge was tallied on the Facebook website from June 1 to September 1 at 17 million, according to the Facebook Newsroom. As the videos continued to climb, so did the challenges. Eventually, public figures such as James Franco, Charlie Rose, and even former president George W. Bush took an activist role in raising money for research and awareness of the ALS disease.

#Hallyu

Main article: Korean Wave

The Hallyu Wave, which literally translates to “flow of Korea” (or more commonly known as the Korean Wave) represents the social movement in connection to South Korean culture and entertainment. Economically, Hallyu is tremendously profitable, attracting millions of tourists and fans per year and produces many forms of entertainment, such as Korean pop music and television drama series, ultimately generating billions of US dollars in annual revenue which further strengthens its economic prosperity and stability. Furthermore, being a powerful social movement in its own right, Hallyu holds considerable influence in politics as well. The ninth president of South Korea, Roh Moo-hyun, once stated that Hallyu can be used to improve or repair the tense relations between the Koreas. Still, North Korea does not have its own rendition of Hallyu and even rejects it; for example, when Psy's "Gangnam Style" was released in 2012, North Korea viewed the song with contempt because while South Korea was attracting positive attention, it was also undermining the impoverished conditions of North Korea at the same time.

LGBT rights

In 2014, protests of the then-recently enacted anti-gay laws included targeting the corporate sponsors for the 2014 Winter Olympics in Sochi Russia. Among the sponsors was McDonald's, whose marketing included the hashtag #CheersToSochi, which was hijacked by the queer activist group Queer Nation.

In June 2015, The United States Supreme Court ruled in favor of same-sex marriage nationwide. This led to the creation of the hashtag #lovewins. This hashtag earned over 4.5 billion impressions on Twitter. President Barack Obama joined in and tweeted using the hashtag.

#Girlslikeus

Started by Janet Mock in 2012. This hashtag was used to support Jenna Talackova during the Miss Universe pageant.

#DignidadLiteraria

Main article: DignidadLiteraria

#OwnVoices

This hashtag is used to showcase authors from marginalized or underrepresented groups who write characters from those same groups of people. This hashtag represents stories being told by people who have lived those experiences.

Main article: Diversity in young adult fiction § #ownvoices

Effectiveness

Hashtag activism has been referred to by critics as a form of slacktivism. Slacktivism is using social media as a vehicle to provide minimal support for a political or social issue. Hashtags, likes, shares, and re-posts are all seen by critics as superficial support that provide minimal change and require little effort. The ease of hashtag activism has led to concerns that it might lead to overuse and public fatigue. Critics worry that hashtag activism allows participants to be satisfied with a public symbol of concern, rather than taking action. The passive nature of the practice can lack the passion once displayed in movements from earlier eras.

Other critiques of hashtag activism stem from the belief that online social movements are often started by privileged individuals, rather than by those who are most affected by the cause itself.

Notable critics of hashtag activism include former Alaska governor Sarah Palin, Canadian journalist Malcolm Gladwell, and Nigerian American writer Teju Cole, who all have argued that hashtag activism encourages lazy, ineffective participation that does not elicit any change.

While critics worry that hashtag activism results in a lack of true action offline, supporters believe it to be an easy and effective way for users to share their opinions, find information, and provide moral support to issues. The hashtag allows users to interact with one another to mobilize efforts without supporters needing to be in the same physical location or the same time of day. As posting a hashtag or reading posts linked to the hashtag requires little effort, more people are able to participate. This can build awareness by connecting it to personal stories, experiences, emotions and thoughts. Identifying shared experiences builds rhetorical connections between people who would never otherwise meet, enabling users of hashtags such as #MeToo and #BlackLivesMatter to support and validate each other and helps to expand and enrich awareness by providing access to multiple narratives.

Supporters such as Bev Goodman, who initiated the #WhyIStayed movement about domestic violence, believe that hashtag activism can begin a chain of events that can lead to sustained engagement and political action. For instance, in 2012 when the Susan G. Komen for the Cure foundation decided to stop funding mammograms through Planned Parenthood, social media users tweeted, #standwithpp, and #singon; later that same week, Komen reversed its decision. Social media fosters a fast-paced environment that allows organizations to quickly see how individuals react to policies and decisions, which in turn creates the opportunity for those responses to be taken into account if the organization is open to changing the policy or decision.