Far-From-Equilibrium Soft Materials Research at Brandeis University
Hydrodynamics of an Active Liquid Crystal
PIs: Seth Fraden, Mike Hagan, and Aparna Baskaran
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.
Oscillator Networks from Reaction-Diffusion Media
PI: Seth Fraden
Inspired by the autonomous nervous system, we seek to develop synthetic systems that create spatio-temporal patterns from oscillatory sub-units. In Engineering reaction–diffusion networks with properties of neural tissue we demonstrated that the oscillation patterns of the Belousov-Zhabotinsky reaction could sculpted into a functional network using microfluidics. Such a material could lay the foundation for the control layer in soft-robotics.
I am currently studying the relationship between network topology and the multiplicity of dynamic steady states that oscillator networks can exhibit.
(Right) Dual column central pattern generator modeled after the lamprey eel spine. The network features columns that propagate signals through excitatory interactions. The columns themselves are linked through inhibitory coupling, this drives each of the columns out of phase. The designed stable attractor of the system is therefore left-right-left-right firing pattern.
(First panel) Finite element simulation of the BZ reaction. The BZ media is confined to the square wells and channels, the surrounding media is selectively permeable to the inhibitor, Bromine, whose concentration field is shown. Snapshots are spaced one oscillation period apart to show the beginning of the shift towards anti-phase synchrony.
(Second Panel) Microscopy image of the completed microfluidic network, the chemical state of the well is indicated by a color change (appears bright in the image).