Ablation

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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.

Artificial intelligence[edit]

In artificial intelligence (AI), especially machine learning, ablation is the removal of a component of an AI system.[1] The term is by analogy with biology: removal of components of an organism.

Biology[edit]

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.[2]

Electro-Ablation[edit]

Electro-ablation, is an electrochemical process similar to electro-polishing, that removes material from a metallic workpiece to reduce surface roughness.

It is similar to electro-polishing in that it uses a current flowing through an electrolyte from the workpiece (Anode) to remove metal from the surface of the workpiece, producing a smoother surface. It differs from electro-polishing in its current flow, frequency and high rate of metal removal (ablation).

Electro-polishing uses low currents, usually much less than 1 Amp/cm2, that are usually constant or changing at very low frequency. This limits the speed of electro-polishing. Components often requiring several hours of immersed electro-polishing for significant reduction in surface roughness. This reduces the attractiveness of electro-polishing for post-processing of metallic components, such as those produced by Metal Additive Manufacturing (3D Printing of Metals).

Electro-ablation uses a combination of very high current flow (usually in excess of 10 Amps/cm2) combined with very high frequency (up to 200kHz) changes in current flow. This allows electro-ablation to break through highly resistive oxide surfaces, such as those found on Titanium and other exotic metals and alloys. Through software control, electro-ablation processes are able to quickly remove - via ablation - tough protective oxide layers and then immediately reduce the current flow to avoid melting the underlying non-oxidised metal or alloy. This allows very quick surface finishing.

The process is capable of providing surface finishing for a wide range of exotic and widely used metals and alloys, including: Titanium, Stainless Steel, Niobium, Chromium-Cobalt, Inconel, Aluminium and a range of widely available steels and alloys.

Due to its use of very high frequencies, electro-ablation is very effective at achieving high levels of surface finishing in holes, valleys and hidden or internal surfaces on metallic workpieces (parts).

The process is particularly applicable to components produced by Additive Manufacturing process, such as 3D-printed metals. These components tend to be produced with roughness levels well above 5-20 micron. Electro-ablation can be used to quickly reduce the surface roughness to less than 0.8 micron, allowing the post-process to be used for volume production surface finishing.

The electro-ablation process can be applied in two ways, “Brush” or “Immersion” in a bath.

The brush method occurs in air, above a drain tank, with the electrolyte and cathode contained in a brush, that is either handheld or positioned by robotic control. This method is preferred for objects that only require surface finishing on limited areas or on objects that are too large to be used in an Immersion tank.

The Immersion method requires the object to be immersed in a tank containing an electrolyte, usually phosphoric acid, and cathodic probes. This method is preferred for objects produced in a high-volume production environment, where low-skilled labour is available for loading and unloading components, or where longer surface finishing times are required.

Glaciology[edit]

In glaciology and meteorology, ablation—the opposite of accumulation—refers to all processes that remove snow, ice, or water from a glacier or snowfield.[3][page needed] 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.[4] 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.[5]

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.[6]

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.[7]

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.[8] This science is significant due to the importance of long-term availability of water resources and assess glacier response to climate change.[9] Natural resource availability is a major drive behind research conducted in regards to the ablation process and overall study of glaciers.

Laser ablation[edit]

An Nd:YAG laser drills a hole through a block of nitrile. The intense burst of infrared radiation ablates the highly absorbing rubber, releasing an eruption of plasma.

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:

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.[10]

Marine surface coatings[edit]

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[edit]

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.

High-intensity focused ultrasound (HIFU) ablation removes tissue from within the body noninvasively.

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.[11]

Passive fire protection[edit]

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[citation needed] 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[citation needed]). When the organic rubber is exposed to fire, it burns to ash and leaves behind the silica dust with which the product started.

Protoplanetary disk ablation[edit]

Protoplanetary disks are rotating circumstellar disks of dense gas and dust surrounding young, newly formed stars. Shortly after star formation, stars often have leftover surrounding material that is still gravitationally bound to them, forming primitive disks that orbit around the star's equator – not too dissimilarly from the rings of Saturn. This occurs because the decrease in the protostellar material’s radius during formation increases angular momentum, which means that this remaining material gets whipped into a flattened circumstellar disk around the star. This circumstellar disk may eventually mature into what is referred to as a protoplanetary disk: a disk of gas, dust, ice and other materials from which planetary systems may form. In these disks, orbiting matter starts to accrete in the colder mid-plane of the disk from dust grains and ices sticking together. These small accretions grow from pebbles to rocks to early baby planets, called planetesimals, then protoplanets, and eventually, full planets.[12]

As it is believed that massive stars may play a role in actively triggering star formation (by introducing gravitational instabilities amongst other factors),[13] it is plausible that young, smaller stars with disks may be living relatively nearby to older, more massive stars. This has already been confirmed through observations to be the case in certain clusters , e.g. in the Trapezium cluster.[14] Since massive stars tend to collapse through supernovae at the end of their lives, research is now investigating what role the shockwave of such an explosion, and the resulting supernova remnant (SNR), would play if it occured in the line of fire of a protoplanetary disk. According to computationally modelled simulations, a SNR striking a protoplanetary disk would result in significant ablation of the disk, and this ablation would strip a significant amount of protoplanetary material from the disk – but not necessarily destroy the disk entirely.[15] This is an important point because a disk that survives such an interaction with sufficient material leftover to form a planetary system may inherit an altered disk chemistry from the SNR, which could have effects on the planetary systems that later form.

Spaceflight[edit]

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.[16] 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.

See also[edit]

References[edit]

  1. ^ Newell, Allen (1975). D. Raj Reddy (ed.). A Tutorial on Speech Understanding Systems. In Speech Recognition: Invited Papers Presented at the 1974 IEEE Symposium. New York: Academic. p. 43.
  2. ^ Cell Ablation definition, Change Bioscience.
  3. ^ Paterson, W. S. B. 1999. The Physics of Glaciers. Tarrytown, N.Y., Pergamon.
  4. ^ "Glossary of Meteorology". Archived from the original on 2011-09-17. Retrieved 2010-07-05.
  5. ^ Betterton, M. D. (2001-04-26). "Theory of structure formation in snowfields motivated by penitentes, suncups, and dirt cones". Physical Review E. American Physical Society (APS). 63 (5): 056129. arXiv:physics/0007099. doi:10.1103/physreve.63.056129. ISSN 1063-651X. PMID 11414983.
  6. ^ Sakai, Akiko, et al. "Role of supraglacial ponds in the ablation process of a debris-covered glacier in the Nepal Himalayas." IAHS PUBLICATION (2000): 119-132.
  7. ^ Paul, Frank; Huggel, Christian; Kääb, Andreas (2004). "Combining satellite multispectral image data and a digital elevation model for mapping debris-covered glaciers". Remote Sensing of Environment. Elsevier BV. 89 (4): 510–518. doi:10.1016/j.rse.2003.11.007. ISSN 0034-4257.
  8. ^ Fujii, Yoshiyuki (1977). "Field Experiment on Glacier Ablation under a Layer of Debris Cover". Journal of the Japanese Society of Snow and Ice. Japanese Society of Snow and Ice. 39 (Special): 20–21. doi:10.5331/seppyo.39.special_20. ISSN 0373-1006.
  9. ^ Kayastha, Rijan Bhakta, et al. "Practical prediction of ice melting beneath various thickness of debris cover on Khumbu Glacier, Nepal, using a positive degree-day factor." IAHS PUBLICATION 7182 (2000).
  10. ^ Yousef Sajjadi, Amir; Mitra, Kunal; Grace, Michael (2011). "Ablation of subsurface tumors using an ultra-short pulse laser". Optics and Lasers in Engineering. Elsevier BV. 49 (3): 451–456. doi:10.1016/j.optlaseng.2010.11.020. ISSN 0143-8166.
  11. ^ Chelur, Dattananda S.; Chalfie, Martin (February 2007). "Targeted cell killing by reconstituted caspases". Proceedings of the National Academy of Sciences. 104 (7): 2283–8. Bibcode:2007PNAS..104.2283C. doi:10.1073/pnas.0610877104. PMC 1892955. PMID 17283333. Retrieved 2007-03-08.
  12. ^ Sheehan, Patrick (October 2020). "Early onset of planet formation observed in a nascent star system". Nature. 586 (7828): 205–206. doi:10.1038/d41586-020-02748-w.
  13. ^ Lee, Hsu-Tai; Chen, W. P. (10 March 2007). "Triggered Star Formation by Massive Stars". The Astrophysical Journal. 657 (2): 884. doi:10.1086/510893. ISSN 0004-637X.
  14. ^ McCaughrean, Mark J.; O'dell, C. Robert (May 1996). "Direct Imaging of Circumstellar Disks in the Orion Nebula". The Astronomical Journal. 111: 1977. doi:10.1086/117934.
  15. ^ Close, J. L.; Pittard, J. M. (July 2017). "Hydrodynamic ablation of protoplanetary discs via supernovae". Monthly Notices of the Royal Astronomical Society. 469 (1): 1117–1130. doi:10.1093/mnras/stx897. ISSN 0035-8711.
  16. ^ Parker, John and C. Michael Hogan, "Techniques for Wind Tunnel assessment of Ablative Materials", NASA Ames Research Center, Technical Publication, August 1965.

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