View source for Anisotropy

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[[File:WMAP 2010.png|thumb|300px|[[Wilkinson Microwave Anisotropy Probe|WMAP]] image of the (extremely tiny) anisotropies in the [[cosmic background radiation]]]]
'''Anisotropy''' ({{IPAc-en|ˌ|æ|n|.|@|-|,_|,|ae|n|.|ai|ˈ|s|ɒ|t|r|.|ə|p|.|i}}) is the property of being directionally dependent, which implies different properties in different directions, as opposed to [[isotropy]]. It can be defined as a difference, when measured along different axes, in a material's [[Physical property|physical]] or [[List of materials properties#Mechanical properties|mechanical properties]] ([[absorbance]], [[refractive index]], [[Electrical resistivity and conductivity|conductivity]], [[tensile strength]], etc.)

An example of anisotropy is light coming through a [[polarizer]]. Another is [[wood]], which is easier to split along its [[wood grain|grain]] than across it.

==Fields of interest==

===Computer Graphics===

In the field of [[computer graphics]], an anisotropic surface changes in appearance as it rotates about its [[Normal (geometry)|geometric normal]], as is the case with [[velvet]].

[[Anisotropic filtering]] (AF) is a method of enhancing the image quality of textures on surfaces that are far away and steeply angled with respect to the point of view. Older techniques, such as [[bilinear filtering|bilinear]] and [[trilinear filtering]], do not take into account the angle a surface is viewed from, which can result in [[aliasing]] or blurring of textures. By reducing detail in one direction more than another, these effects can be reduced.

=== Chemistry ===
A chemical anisotropic [[filter (chemistry)|filter]], as used to filter particles, is a filter with increasingly smaller interstitial spaces in the direction of filtration so that the [[Anatomical terms of location#Proximal and distal|proximal]] [[region]]s filter out larger particles and [[Anatomical terms of location#Proximal and distal|distal]] regions increasingly remove smaller particles, resulting in greater flow-through and more efficient filtration.

In [[Nuclear magnetic resonance spectroscopy|NMR spectroscopy]], the orientation of nuclei with respect to the applied magnetic field determines their [[chemical shift]].  In this context, anisotropic systems refer to the electron distribution of molecules with abnormally high electron density, like the pi system of [[benzene]]. This abnormal electron density affects the applied magnetic field and causes the observed chemical shift to change.

In [[fluorescence spectroscopy]], the [[fluorescence anisotropy]], calculated from the [[polarization (waves)|polarization]] properties of fluorescence from samples excited with plane-polarized light, is used, e.g., to determine the shape of a macromolecule.
Anisotropy measurements reveal the average angular displacement of the fluorophore that occurs between absorption and subsequent emission of a photon.

=== Real-world Imagery===
Images of a gravity-bound or man-made environment are particularly anisotropic in the orientation domain, with more image structure located at orientations parallel with or orthogonal to the direction of gravity (vertical and horizontal).

===Physics===<!-- This section is linked from [[Birefringence]] -->
[[File:Plasma-lamp 2.jpg|thumb|300px|right|A [[plasma globe|plasma lamp]] displaying the nature of [[Plasma (physics)|plasmas]], in this case, the phenomenon of "filamentation"]]

[[Physicists]] from [[University of California, Berkeley]] reported about their detection of the cosine anisotropy in [[cosmic microwave background radiation]] in 1977. Their experiment demonstrated the [[Doppler shift]] caused by the movement of the earth with respect to the [[Big Bang|early Universe matter]], the source of the radiation.<ref>{{cite web | url=http://www.muller.lbl.gov/COBE-early_history/anisotropy-PRL.pdf | title=Detection of Anisotropy in the Cosmic Blackbody Radiation | publisher=[[Lawrence Berkeley Laboratory]] and [[Space Sciences Laboratory]], [[University of California, Berkeley]] | date=5 October 1977 | accessdate=15 September 2013 | author=Smoot G. F. |author2=Gorenstein M. V. |author3= [[Richard A. Muller|Muller R. A.]] |last-author-amp=yes }}</ref> Cosmic anisotropy has also been seen in the alignment of galaxies' rotation axes and polarisation angles of quasars.

Physicists use the term anisotropy to describe direction-dependent properties of materials. [[Magnetic anisotropy]], for example, may occur in a [[plasma (physics)|plasma]], so that its magnetic field is oriented in a preferred direction. Plasmas may also show "filamentation" (such as that seen in [[lightning]] or a [[plasma globe]]) that is directional.

An ''anisotropic liquid'' has the fluidity of a normal liquid, but has an average structural order relative to each other along the molecular axis, unlike water or [[chloroform]], which contain no structural ordering of the molecules. [[Liquid crystals]] are examples of anisotropic liquids.

Some materials [[heat conduction|conduct heat]] in a way that is isotropic, that is independent of spatial orientation around the heat source.  Heat conduction is more commonly anisotropic, which implies that detailed geometric modeling of typically diverse materials being thermally managed is required. The materials used to transfer and reject heat from the heat source in [[electronics]] are often anisotropic.<ref name="Nature8April2013">{{cite journal|last1=Tian|first1=Xiaojuan|last2=Itkis|first2=Mikhail E|last3=Bekyarova|first3=Elena B|last4=Haddon|first4=Robert C|title=Anisotropic Thermal and Electrical Properties of Thin Thermal Interface Layers of Graphite Nanoplatelet-Based Composites|url=http://www.nature.com/articles/srep01710|journal=Scientific Reports|volume=3|accessdate=11 August 2016|archiveurl=https://web.archive.org/web/20160811183641/http://www.nature.com/articles/srep01710|archivedate=11 August 2016|date=8 April 2013|doi=10.1038/srep01710|pmc=3632880}}</ref>

Many [[crystal]]s are anisotropic to [[light]] ("optical anisotropy"), and exhibit properties such as [[birefringence]]. [[Crystal optics]] describes light propagation in these media. An "axis of anisotropy" is defined as the axis along which isotropy is broken (or an axis of symmetry, such as normal to crystalline layers). Some materials can have multiple such [[optical axis|optical axes]].

===Geophysics and Geology===
[[Seismic anisotropy]] is the variation of seismic wavespeed with direction. Seismic anisotropy is an indicator of long range order in a material, where features smaller than the seismic [[wavelength]] (e.g., crystals, cracks, pores, layers or inclusions) have a dominant alignment. This alignment leads to a directional variation of [[Elasticity (physics)|elasticity]] wavespeed. Measuring the effects of anisotropy in seismic data can provide important information about processes and mineralogy in the Earth; indeed, significant seismic anisotropy has been detected in the Earth's [[crust (geology)|crust]], [[mantle (geology)|mantle]] and [[Earth's inner core|inner core]].

[[Geological]] formations with distinct layers of [[sedimentary]] material can exhibit electrical anisotropy; [[electrical conductivity]] in one direction (e.g. parallel to a layer), is different from that in another (e.g. perpendicular to a layer). This property is used in the gas and [[oil exploration]] industry to identify [[hydrocarbon]]-bearing sands in sequences of [[sand]] and [[shale]]. Sand-bearing hydrocarbon assets have high [[resistivity]] (low conductivity), whereas shales have lower resistivity. [[Formation evaluation]] instruments measure this conductivity/resistivity and the results are used to help find oil and gas in wells.

The [[hydraulic conductivity]] of [[aquifer]]s is often anisotropic for the same reason. When calculating groundwater flow to [[drainage|drains]]<ref>R.J.Oosterbaan, 1997, The energy balance of groundwater flow applied to subsurface drainage in anisotropic soils by pipes or ditches with entrance resistance. On line: [http://www.waterlog.info/pdf/enerart.pdf] {{Webarchive|url=https://web.archive.org/web/20090219221547/http://waterlog.info/pdf/enerart.pdf |date=19 February 2009 }}. The corresponding free EnDrain program can be downloaded from: [http://www.waterlog.info/endrain.htm].</ref> or to [[Water well|wells]],<ref>R.J.Oosterbaan, 2002, Subsurface drainage by (tube)wells, 9 pp. On line: [http://www.waterlog.info/pdf/wellspac.pdf]. The corresponding free WellDrain program can be downloaded from: [http://www.waterlog.info/weldrain.htm]</ref> the difference between horizontal and vertical permeability must be taken into account, otherwise the results may be subject to error.

Most common rock-forming [[mineral]]s are anisotropic, including [[quartz]] and [[feldspar]]. Anisotropy in minerals is most reliably seen in their [[optical mineralogy|optical properties]]. An example of an isotropic mineral is [[garnet]].

===Medical acoustics===
Anisotropy is also a well-known property in medical ultrasound imaging describing a different resulting [[echogenicity]] of soft tissues, such as tendons, when the angle of the transducer is changed. Tendon fibers appear hyperechoic (bright) when the transducer is perpendicular to the tendon, but can appear hypoechoic (darker) when the transducer is angled obliquely. This can be a source of interpretation error for inexperienced practitioners.

===Materials Science and Engineering===
Anisotropy, in materials science, is a material's directional dependence of a physical property. This is a critical consideration for materials selection in engineering applications.  For monocrystalline material, anisotropy is associated with the crystal symmetry.<ref>{{cite book |last1=Newnham |first1=Robert E. |title=Properties of Materials: Anisotropy, Symmetry, Structure |publisher=Oxford University Press |isbn=978-0198520764 |edition=1st}}</ref> Tensor descriptions of material properties can be used to determine the directional dependence of that property. When a material is polycrystalline, the directional dependence on properties is often related to the processing techniques it has undergone. A material with randomly oriented grains will be isotropic, whereas materials with texture will be often be anisotropic. Textured materials are often the result of processing techniques like hot rolling, wire-drawing, and heat treatments.

Mechanical properties of materials, such as Young's modulus, creep, are often dependent on the direction of measurement.<ref>{{cite book |last1=Courtney |first1=Thomas H. |title=Mechanical Behavior of Materials |publisher=Waveland Pr Inc |isbn=978-1577664253 |edition=2nd}}</ref> Fourth rank [[tensor]] properties, like the elastic constants, are anisotropic, even for materials with cubic symmetry. The Young's modulus relates stress and strain when an isotropic material is elastically deformed; to describe elasticity in an anisotropic material, stiffness (or compliance) tensors are used instead. In metals, anisotropic elasticity behavior is prevalent in all single crystals, with the exception of Tungsten, due to the fact there are only two independent stiffness coefficients in the stiffness tensor (while other cubic crystals have three).  For face centered cubic materials like Copper, the elastic modulus is highest along the <111> direction, normal to the close packed planes.

The anisotropy ratio is a value computed to compare elastic anisotropy in materials. It is given by the following expression:

<math>\text{anisotropy ratio} = \frac{C_{11} - C_{12}}{2\cdot C_{44}}</math>

where the C values are stiffness coefficients in Voight notation. For an isotropic material, the ratio is one.

Fiber-reinforced or layered [[composite material]]s exhibit anisotropic mechanical properties, due to orientation of the reinforcement material. In many fiber-reinforced composites like carbon fiber or glass fiber based composites, the weave of the material (e.g. unidirectional or plain weave) can determine the extent of the anisotropy of the bulk material.<ref>{{cite web |title=Fabric Weave Styles |url=https://compositeenvisions.com/fabric-weave-styles/ |website=Composite Envisions |accessdate=23 May 2019}}</ref> The tunability of orientation of the fibers, allows for application-based designs of composite materials, depending on the direction of stresses applied onto the material.

Amorphous materials such as glass and polymers are typically isotropic. Due to the highly randomized orientation of [[macromolecul]]es in polymeric materials, [[polymer]]s are in general described as isotropic. However, polymers can be engineered to have directionally dependent properties through processing techniques or introduction of anisotropy-inducing elements. Researchers have built composite materials with aligned fibers and voids to generate anisotropic [[hydrogel]]s, in order to mimic hierarchically ordered biological soft matter.<ref>{{cite journal |last1=Sano |first1=Koki |last2=Ishida |first2=Yasuhiro |last3=Aida |first3=Tazuko |title=Synthesis of Anisotropic Hydrogels and Their Applications |journal=Angewandte Chemie International Edition |date=16 October 2017 |volume=57 |issue=10}}</ref> 3D printing, especially Fused Deposition Modeling, can introduce anisotropy into printed parts. This is due to the fact that FDM is designed to extrude and print layers of thermoplastic materials.<ref>{{cite journal |last1=Wang |first1=Xin |last2=Jiang |first2=Man |last3=Gou |first3=Jihua |last4=Hui |first4=David |title=3D printing of polymer matrix composites: A review and prospective |journal=Composites Part B: Engineering |date=1 February 2017 |volume=110 |pages=442–458 |doi=10.1016/j.compositesb.2016.11.034}}</ref> This creates materials that are strong when tensile stress is applied in parallel to the layers and weak when the material is perpendicular to the layers.

===Microfabrication===

Anisotropic etching techniques (such as [[deep reactive ion etching]]) are used in microfabrication processes to create well defined microscopic features with a high [[aspect ratio]].  These features are commonly used in [[microelectromechanical systems|MEMS]] and [[microfluidic]] devices, where the anisotropy of the features is needed to impart desired optical, electrical, or physical properties to the device. Anisotropic etching can also refer to certain chemical etchants used to etch a certain material preferentially over certain crystallographic planes (e.g., KOH etching of [[silicon]] [100] produces pyramid-like structures)

===Neuroscience===

[[Diffusion tensor imaging]] is an [[magnetic resonance imaging|MRI]] technique that involves measuring the fractional anisotropy of the random motion ([[Brownian motion]]) of water molecules in the brain.  Water molecules located in [[White matter|fiber tracts]] are more likely to be anisotropic, since they are restricted in their movement (they move more in the dimension parallel to the fiber tract rather than in the two dimensions orthogonal to it), whereas water molecules dispersed in the rest of the brain have less restricted movement and therefore display more isotropy.  This difference in fractional anisotropy is exploited to create a map of the fiber tracts in the brains of the individual.

===Atmospheric Radiative Transfer===
[[Radiance]] fields (see [[BRDF]]) from a reflective surface are often not isotropic in nature. This makes calculations of the total energy being reflected from any scene a difficult quantity to calculate. In [[remote sensing]] applications, anisotropy functions can be derived for specific scenes, immensely simplifying the calculation of the net reflectance or (thereby) the net [[irradiance]] of a scene.
For example, let the [[BRDF]] be <math>\gamma(\Omega_i, \Omega_v)</math> where 'i' denotes incident direction and 'v' denotes viewing direction (as if from a satellite or other instrument). And let P be the Planar Albedo, which represents the total reflectance from the scene.
:<math>P(\Omega_i) = \int_{\Omega_v}\gamma(\Omega_i, \Omega_v)\hat{n}\cdot d\hat\Omega_v</math>
:<math>A(\Omega_i, \Omega_v) = \frac{\gamma(\Omega_i, \Omega_v)}{P(\Omega_i)}</math>

It is of interest because, with knowledge of the anisotropy function as defined, a measurement of the [[BRDF]] from a single viewing direction (say, <math>\Omega_v</math>) yields a measure of the total scene reflectance (Planar Albedo) for that specific incident geometry (say, <math>\Omega_i</math>).

==See also==
*[[Circular symmetry]]

==References==
{{Reflist}}

== External links ==
{{Wiktionary|anisotropy}}
*[http://www.knitty.com/ISSUEsummer05/FEATsum05TBP.html "Gauge, and knitted fabric generally, is an anisotropic phenomenon"]
*[https://web.archive.org/web/20100303080919/http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=99&pageid=1028022659 "Overview of Anisotropy"]
*[http://www.doitpoms.ac.uk/tlplib/anisotropy/index.php DoITPoMS Teaching and Learning Package: "Introduction to Anisotropy"]

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[[Category:Orientation (geometry)]]
[[Category:Asymmetry]]