Hydrodynamics of Active Fluids
Active fluids transduce energy from the environment into useful work at the molecular scale. In contrast, classical fluids are only driven at the macro-scale by boundaries and pressure gradients. The distributed and embedded nature of the force-injection of these fluids gives rise to striking phenomena such as self-circulation and turbulence even at vanishing Reynolds number. One class of active fluids are those composed of elongated active units, such as ellipsoidal swimmers or microtubules. At high density, these systems spontaneously create a nematic phase with orientational order. Like passive liquid crystals found in display technology, these systems can possess defects and topology-dependent configurations. The hydrodynamic flows created by molecular activity, however, compete with boundary-imposed ordering.
I am currently using continuum-level models to understand the impact of boundary conditions on the flow and defect dynamics. I am also working with experimentalists to develop image processing techniques to quantify observed dynamics.
(Top) Finite element simulation results showing the nematic director field (black lines) circulation of two +1/2 defects (magenta arrows). Boundary conditions are parallel anchoring for the director field and no-slip for the flow field. Points from four initial quadrants are labeled in yellow, red, green and cyan to convey the complex deformations undergone by material points.
(Left) Finite element results showing the director field (black lines), -1/2 defects (blue), +1/2 defects (magenta arrows). Perpendicular anchoring is imposed through an energy-penalty boundary condition; however, the flow "combs" over the director in many locations. The high defect density occurs because the active stress is high. From Insensitivity of active nematic liquid crystal dynamics to topological constraints, PRE.