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Active fluid
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<h2 id="terminology">Terminology</h2> <p>The terms “active fluids”, “active <a href="/facts/Nematics/ehCCST89">nematics</a>” and “active <a href="/facts/Liquid_crystal/ehCCST89">liquid crystals</a>” have been used almost synonymously to denote hydrodynamic descriptions of dense active matter.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> While in many respects they describe the same phenomenon, there are subtle differences between them. “Active nematics” and “active liquid crystals” refers to systems where the constituent elements have <a href="/facts/Nematic/ehCCST89">nematic order</a> whereas “active fluids” is the more generic term combining systems with both nematic and polar interactions. </p> <h2 id="examples-and-observations">Examples and observations</h2> <p>There are wide range of cellular and intracellular elements which form active fluids. This include systems of <a href="/facts/Microtubule/ENL1TxSe">microtubule</a>, <a href="/facts/Bacteria/wC8xSCsU">bacteria</a>, <a href="/facts/Spermatozoon/sTG4GWc0">sperm cells</a> as well as inanimate <a href="/facts/Microswimmer/pzqfhI5b">microswimmers</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> It is known that these systems form a variety of structures such as regular and irregular <a href="/facts/Lattice_(order)/5ak6ufky">lattices</a> as well as seemingly random states in two dimensions. </p> <h3>Pattern formation</h3> <p>Active fluids have been shown to organize into regular and irregular lattices in a variety of settings. These include irregular <a href="/facts/Hexagonal_crystal_family/sSSok8wo">hexagonal lattices</a> by microtubules<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> and regular vortex lattice by sperm cells.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> From topological considerations, it can be seen that the constituent element in quasi stationary states of active fluids should necessarily be vortices. But very less is known, for instance, about the length scale selection in such systems. </p> <h3>Active turbulence</h3> <p>Chaotic states exhibited by active fluids are termed as active turbulence.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> Such states are qualitatively similar to <a href="/facts/Turbulence/NpgFUhG5">hydrodynamic turbulence</a>, by virtue of which they are termed active turbulence. But recent research has indicated that the statistical properties associated with such flows are quite different from that of hydrodynamic turbulence.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p> <h2 id="mechanism-and-modelling-approaches">Mechanism and modelling approaches</h2> <p>The mechanism behind the formation of various structures in active fluids is an area of active research. It is well understood that the structure formation in active fluids is intimately related to <a href="/facts/Crystallographic_defect/LHJymDro">defects</a> or <a href="/facts/Disclination/FqTITKum">disclinations</a> in the <a href="/facts/Order_parameter/eLP2BY8R">order parameter</a> field<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> (the orientational order of the constituent agents). An important part of research on active fluids involve modelling of dynamics of these defects to study its role in pattern formation and turbulent dynamics in active fluids. Modified versions of <a href="/facts/Vicsek_model/nrbhmQMA">Vicsek model</a> are among earliest and continually used approach to model active fluids.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Such models have been shown to capture the various dynamical states exhibited by active fluids.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> More refined approaches include derivation of <a href="/facts/Continuum_limit/bSDQhaU9">continuum limit</a> hydrodynamic equations for active fluids<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> and adaptation of liquid crystal theory by including the activity terms.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> </p> <h2 id="potential-applications">Potential applications</h2> <p>A few technological applications for active fluids have been proposed such as powering of molecular motors through active turbulence and patterned state.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> Furthermore, given the innumerable applications liquid crystals find in various technologies, there have been proposals to augment them by using active liquid crystals.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Active_matter/jH9Ph4Yq">Active matter</a></li> <li><a href="/facts/Vicsek_model/nrbhmQMA">Vicsek model</a></li> <li><a href="/facts/Navier%E2%80%93Stokes_equations/48pQugEa">Navier–Stokes equations</a></li> <li><a href="/facts/Soft_matter/62NTslbq">Soft matter</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Saintillan, David (2018). 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