Self-organized dynamics and the transition to turbulence of confined active nematics

We study how confinement transforms the chaotic dynamics of bulk microtubule-based active nematics into regular spatiotemporal patterns. For weak confinements, multiple continuously nucleating and annihilating topological defects self-organize into persistent circular flows of either handedness. Increasing confinement strength leads to the emergence of distinct dynamics, in which the slow periodic nucleation of topological defects at the boundary is superimposed onto a fast procession of a pair of defects. A defect pair migrates towards the confinement core over multiple rotation cycles, while the associated nematic director field evolves from a distinct double spiral towards a nearly circularly symmetric configuration. The collapse of the defect orbits is punctuated by another boundary-localized nucleation event, that sets up long-term doubly-periodic dynamics. Comparing experimental data to a theoretical model of an active nematic, reveals that theory captures the fast procession of a pair of $+\frac{1}{2}$ defects, but not the slow spiral transformation nor the periodic nucleation of defect pairs. Theory also fails to predict the emergence of circular flows in the weak confinement regime. The developed confinement methods are generalized to more complex geometries, providing a robust microfluidic platform for rationally engineering two-dimensional autonomous flows

Driven Topological Transitions in Active Nematic Films

The topological properties of many materials are central to their behavior, with the dynamics of topological defects being particularly important to intrinsically out-of-equilibrium, active materials. In this paper, local manipulation of the ordering, dynamics, and topological properties of microtubule-based extensile active nematic films is demonstrated in a joint experimental and simulation study. Hydrodynamic stresses created by magnetically actuated rotation of disk-shaped colloids in proximity to the films compete with internal stresses in the active nematic, enabling local control of the motion of the +1/2 charge topological defects that are intrinsic to spontaneously turbulent active films. Sufficiently large applied stresses drive the formation of +1 charge topological vortices in the director field through the merger of two +1/2 defects. The directed motion of the defects is accompanied by ordering of the vorticity and velocity of the active flows within the film that is qualitatively unlike the response of passive viscous films. Many features of the film's response to the disk are captured by Lattice Boltzmann simulations, leading to insight into the anomalous viscoelastic nature of the active nematic. The topological vortex formation is accompanied by a rheological instability in the film that leads to significant increase in the flow velocities. Comparison of the velocity profile in vicinity of the vortex with fluid-dynamics calculations provides an estimate of film viscosity

Rapid prototyping of cyclic olefin copolymer (COC) microfluidic devices

We introduce a low-cost, high yield rapid fabrication method for casting COC microfluidic chips that is appropriate for academic labs and small companies. Devices are comprised of two molded pieces joined together to create a sealed device. The first piece contains the microfluidic features and the second contains the inlet and outlet manifold, a frame for rigidity and a viewing window. The microfluidic features are patterned using a PDMS mold that itself was replica-molded from a photoresist master. Dimensional stability of the microfluidics portion of the COC device is achieved by confining the PDMS mold in an aluminium frame. The mold for the lid is CNC milled from aluminium. Sealing the COC device is accomplished by timed immersion of the lid in a mixture of volatile and non-volatile solvents followed by application of heat and pressure. Surface treatment to render the device fluorophilic is performed using dopamine in assembled devices. (C) 2017 Elsevier B.V. All rights reserved