Hydrodynamics of Soft Active Matter
Active materials are a new class of soft materials maintained out of equilibrium by internal energy sources. The key property that distinguishes active matter from more familiar non-equilibrium systems, such as a fluid under shear, is that the energy input that maintains the system out of equilibrium comes from the constituents, rather than the boundaries.
Probably the most promising example of an active material (with a view to applications) is the cell cytoskeleton, responsible for movement (motility) in cells. It provides a fascinating prototype for the design of man-made biomimetic responsive materials. The stages of the cell’s life cycle show the formation of highly nonlinear structures. Many other examples of such materials can be found in biological contexts: bacterial colonies, motor proteins, and the cell cytoskeleton are only some of them. Non-biological systems, for example a layer of vibrated granular rods, also display the same features. The active systems studied theoretically and experimentally so far consist of elongated self-propelled particles of two types: polar ones, with a head and a tail, and apolar (nematic) ones that are head-tail symmetric. A really good starting point to study and explore the dynamics of active system is the following detailed review:
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Systems constituted of a suspension of active units in a fluid, and confined by an imposed geometry, are called active liquid crystals. Their dynamics are mainly governed by the interaction between the average suspension orientation and density, and can be described in terms of pressure, as well as elastic and active stresses exerted on the fluid. This gives rise to phase transitions typical of liquid crystals, along with a range of new phenomena. |
Active polar filmsActive polar fluids exhibit a rich behaviour. Indeed, like active nematics, they exhibit steady spontaneous flow. Moreover, they show concentration banding and spontaneous symmetry breaking in the orientation and concentration.
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Topological defects in Active NematicsApolar particles can form phases with nematic order, characterized by a macroscopic axis of mean orientation identified by a unit vector n and global symmetry for n → −n, as in equilibrium nematic liquid crystals.
Topological defects are spatially inhomogeneous configurations of the director field that cannot be transformed continuously into a uniform state. In active liquid crystals, they can move as a consequence of the presence of self-propelling units. Their dynamics is still subject of study, both analytical and experimental. References:
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