Ablation near the electrode in a flashtube. The high-energy electrical arc slowly erodes the glass, leaving a frosted appearance.
Ablation is removal or destruction of material from an object by vaporization, chipping, or other erosive processes. Examples of ablative materials are described below, and include spacecraft material for ascent and atmospheric reentry, ice and snow in glaciology, biological tissues in medicine and passive fire protection materials.
Biology
Biological ablation is the removal of a biological structure or functionality.
Genetic ablation is another term for gene silencing, in which gene expression is abolished through the alteration or deletion of genetic sequence
information. In cell ablation, individual cells in a population or
culture are destroyed or removed. Both can be used as experimental
tools, as in loss-of-function experiments.
Glaciology
In glaciology and meteorology, ablation—the opposite of accumulation—refers to all processes that remove snow, ice, or water from a glacier or snowfield. Ablation refers to the melting of snow or ice that runs off the glacier, evaporation, sublimation, calving,
or erosive removal of snow by wind. Air temperature is typically the
dominant control of ablation, with precipitation exercising secondary
control. In a temperate climate during ablation season, ablation rates
typically average around 2 mm/h.
Where solar radiation is the dominant cause of snow ablation (e.g., if
air temperatures are low under clear skies), characteristic ablation
textures such as suncups and penitentes may develop on the snow surface.
Ablation can refer either to the processes removing ice and snow or to the quantity of ice and snow removed.
Debris-covered glaciers have also been shown to greatly impact
the ablation process. There is a thin debris layer that can be located
on the top of glaciers that intensifies the ablation process below the
ice. The debris-covered parts of a glacier that is experiencing ablation
are sectioned into three categories which include ice cliffs, ponds,
and debris. These three sections allow scientists to measure the heat
digested by the debris-covered area and is calculated. The calculations
are dependent on the area and net absorbed heat amounts in regards to
the entire debris-covered zones. These types of calculations are done to
various glaciers to understand and analyze future patterns of melting.
Moraine
(glacial debris) is moved by natural processes that allow for
down-slope movement of materials on the glacier body. It is noted that
if the slope of a glacier is too high then the debris will continue to
move along the glacier to a further location. The sizes and locations of
glaciers vary around the world, so depending on the climate and
physical geography the varieties of debris can differ. The size and
magnitude of the debris is dependent on the area of glacier and can vary
from dust-size fragments to blocks as large as a house.
There has been many experiments done to demonstrate the effect of
debris on the surface of glaciers. Yoshiyuki Fujii, a professor at the National Institute of Polar Research
designed an experiment that showed ablation rate was accelerated under a
thin debris layer and was retarded under a thick one as compared with
that of a natural snow surface. This science is significant due to the importance of long-term availability of water resources and assess glacier response to climate change.
Natural resource availability is a major drive behind research
conducted in regards to the ablation process and overall study of
glaciers.
Laser ablation
Laser ablation
is greatly affected by the nature of the material and its ability to
absorb energy, therefore the wavelength of the ablation laser should
have a minimum absorption depth. While these lasers can average a low
power, they can offer peak intensity and fluence given by:
![{\displaystyle {\begin{aligned}{\text{Intensity }}(\mathrm {W} /\mathrm {cm} ^{2})&={\frac {{\text{average power }}(\mathrm {W} )}{{\text{focal spot area }}(\mathrm {cm} ^{2})}}\\[5pt]{\text{Peak intensity }}(\mathrm {W} /\mathrm {cm} ^{2})&={\frac {{\text{peak power }}(\mathrm {W} )}{{\text{focal spot area }}(\mathrm {cm} ^{2})}}\\[5pt]{\text{Fluence }}(\mathrm {J} /\mathrm {cm} ^{2})&={\frac {{\text{laser pulse energy }}(\mathrm {J} )}{{\text{focal spot area }}(\mathrm {cm} ^{2})}}\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3b7e4c62ec211d7bd57810456b26a49d5f470cb4)
while the peak power is
Surface ablation of the cornea for several types of eye refractive surgery is now common, using an excimer laser system (LASIK and LASEK). Since the cornea does not grow back, laser is used to remodel the cornea refractive properties to correct refraction errors, such as astigmatism, myopia, and hyperopia. Laser ablation is also used to remove part of the uterine wall in women with menstruation and adenomyosis problems in a process called endometrial ablation.
Recently, researchers have demonstrated a successful technique
for ablating subsurface tumors with minimal thermal damage to
surrounding healthy tissue, by using a focused laser beam from an
ultra-short pulse diode laser source.
Marine surface coatings
Antifouling paints and other related coatings are routinely used to prevent the buildup of microorganisms and other animals, such as barnacles
for the bottom hull surfaces of recreational, commercial and military
sea vessels. Ablative paints are often utilized for this purpose to
prevent the dilution or deactivation of the antifouling agent. Over
time, the paint will slowly decompose in the water, exposing fresh
antifouling compounds on the surface. Engineering the antifouling agents
and the ablation rate can produce long-lived protection from the
deleterious effects of biofouling.
Medicine
In medicine, ablation is the same as removal of a part of biological tissue, usually by surgery. Surface ablation of the skin (dermabrasion, also called resurfacing because it induces regeneration) can be carried out by chemicals (chemoablation), by lasers (laser ablation), by freezing (cryoablation), or by electricity (fulguration). Its purpose is to remove skin spots, aged skin, wrinkles, thus rejuvenating it. Surface ablation is also employed in otolaryngology for several kinds of surgery, such as for snoring. Ablation therapy using radio frequency waves on the heart is used to cure a variety of cardiac arrhythmiae such as supraventricular tachycardia, Wolff–Parkinson–White syndrome (WPW), ventricular tachycardia, and more recently as management of atrial fibrillation. The term is often used in the context of laser ablation, a process in which a laser dissolves a material's molecular bonds. For a laser to ablate tissues, the power density or fluence must be high, otherwise thermocoagulation occurs, which is simply thermal vaporization of the tissues.
Rotoablation is a type of arterial cleansing that consists of
inserting a tiny, diamond-tipped, drill-like device into the affected
artery to remove fatty deposits or plaque. The procedure is used in the
treatment of coronary heart disease to restore blood flow.
Radiofrequency ablation (RFA) is a method of removing aberrant tissue from within the body via minimally invasive procedures.
Microwave ablation (MWA) is similar to RFA but at higher frequencies of electromagnetic radiation.
Bone marrow ablation is a process whereby the human bone marrow cells are eliminated in preparation for a bone marrow transplant. This is performed using high-intensity chemotherapy and total body irradiation. As such, it has nothing to do with the vaporization techniques described in the rest of this article.
Ablation of brain tissue is used for treating certain neurological disorders, particularly Parkinson's disease, and sometimes for psychiatric disorders as well.
Recently, some researchers reported successful results with
genetic ablation. In particular, genetic ablation is potentially a much
more efficient method of removing unwanted cells, such as tumor
cells, because large numbers of animals lacking specific cells could be
generated. Genetically ablated lines can be maintained for a prolonged
period of time and shared within the research community. Researchers at
Columbia University report of reconstituted caspases combined from C. elegans
and humans, which maintain a high degree of target specificity. The
genetic ablation techniques described could prove useful in battling
cancer.
Passive fire protection
Firestopping and fireproofing products can be ablative in nature. This can mean endothermic materials, or merely materials that are sacrificial and become "spent" over time while exposed to fire, such as silicone
firestop products.
Given sufficient time under fire or heat conditions, these products char
away, crumble, and disappear. The idea is to put enough of this
material in the way of the fire that a level of fire-resistance rating can be maintained, as demonstrated in a fire test. Ablative materials usually have a large concentration of organic matter that is reduced by fire to ashes. In the case of silicone, organic rubber surrounds very finely divided silica dust (up to 380 m² of combined surface area of all the dust particles per gram of this dust). When the organic rubber is exposed to fire, it burns to ash and leaves behind the silica dust with which the product started.
Spaceflight
In spacecraft
design, ablation is used to both cool and protect mechanical parts
and/or payloads that would otherwise be damaged by extremely high
temperatures. Two principal applications are heat shields for spacecraft entering a planetary atmosphere from space and cooling of rocket engine nozzles. Examples include the Apollo Command Module that protected astronauts from the heat of atmospheric reentry and the Kestrel second stage rocket engine designed for exclusive use in an environment of space vacuum since no heat convection is possible.
In a basic sense, ablative material is designed so that instead
of heat being transmitted into the structure of the spacecraft, only the
outer surface of the material bears the majority of the heating effect.
The outer surface chars and burns away -- but quite slowly, only
gradually exposing new fresh protective material beneath. The heat is
carried away from the spacecraft by the gases generated by the ablative
process, and never penetrates the surface material, so the metallic and
other sensitive structures they protect, remain at a safe temperature.
As the surface burns and disperses into space, while the remaining solid
material continues to insulate the craft from ongoing heat and
superheated gases. The thickness of the ablative layer is calculated to
be sufficient to survive the heat it will encounter on its mission.
There is an entire branch of spaceflight research involving the search for new fireproofing
materials to achieve the best ablative performance; this function is
critical to protect the spacecraft occupants and payload from otherwise
excessive heat loading. The same technology is used in some passive fire protection applications, in some cases by the same vendors, who offer different versions of these fireproofing products, some for aerospace and some for structural fire protection.
