Topological chaos in active nematics, Amanda J. Tan, Eric Roberts, Spencer Smith, Ulyses Alvarado, Jorge Arteaga, Sam Fortini, Kevin Mitchell, and Linda S. Hirst, Aug 5th, NATURE PHYSICS (2019) Link
Abstract: Active nematics are out-of-equilibrium fluids composed of rod-like subunits, which can generate large-scale, self-driven flows. We examine a microtubule-kinesin-based active nematic confined to two dimensions, exhibiting chaotic flows with moving topological defects. Applying tools from chaos theory, we investigate self-driven advection and mixing on different length scales. Local fluid stretching is quantified by the Lyapunov exponent. Global mixing is quantified by the topological entropy, calculated from both defect braiding and curve extension rates. We find excellent agreement between these independent mea-sures of chaos, demonstrating that the extensile stretching between microtubules directly translates into macroscopic braiding of positive defects. Remarkably, increasing extensile activity (through ATP concentration) does not increase the dimensionless topological entropy. This study represents an application of chaotic advection to the emerging field of active nematics and quantification of the collective motion of an ensemble of defects (through topological entropy) in a liquid crystal.
Rapid bulk assembly of nanoparticles into microstructures is
challenging, but highly desirable for applications in controlled
release, catalysis, and sensing. We report a method to form hollow
microstructures via a two-stage nematic nucleation process, generating
size-tunable closed-cell foams, spherical shells, and tubular networks
composed of closely packed nanoparticles. Mesogen-modified nanoparticles
are dispersed in liquid crystal above the nematic-isotropic transition
temperature (TNI). On cooling through TNI,
nanoparticles first segregate into shrinking isotropic domains where
they locally depress the transition temperature. On further cooling,
nematic domains nucleate inside the nanoparticle-rich isotropic domains,
driving formation of hollow nanoparticle assemblies. Structural
differentiation is controlled by nanoparticle density and cooling rate.
Cahn-Hilliard simulations of phase separation in liquid crystal
demonstrate qualitatively that partitioning of nanoparticles into
isolated domains is strongly affected by cooling rate, supporting
experimental observations that cooling rate controls aggregate size.
Microscopy suggests the number and size of internal voids is controlled
by second-stage nucleation.
The Hirst Lab has been awarded a new three-year NSF grant along with collaborator UC Merced’s Kevin Mitchell.
The grant, combining experiment and theory is titled “Self-mixing Active Fluids”
Active matter is one of the most exciting frontiers in soft matter science. Unlike typical fluids, active fluids are not in equilibrium. Instead, they consume energy locally, translating this energy into internal flows and spontaneous mixing. In this project, mass transport and chaotic mixing in active fluids will be investigated using a fluid consisting of microtubules and kinesin, biological molecules found in the cell. Densely packed microtubules slide antiparallel to each other at a controlled rate due to coupled kinesin molecular motors. An exciting outcome of this work could be the development of a new class of self-mixing active solvents. Such a solvent could revolutionize our understanding of the kinetics of mass transport and chemical reactions. The present proposal concerns much larger length scales than that of standard solvents and will thus serve as an experimental model for these new ideas, helping to establish fundamental laws that govern the behavior of active matter. Since the contents of biological cells are highly complex active materials, far from equilibrium, this work is expected to yield new insights into the role of active materials in biology. A proposed Telluride workshop on transport in active fluids will help to bring together the relatively disparate fields of liquid crystals, biological fluids and nonlinear dynamics. This work will have a significant educational impact at UC Merced, a new university in one of California’s most socio-economically disadvantaged areas. This research will provide the basis for several undergraduate theses and the PIs will use insights from this work to introduce cutting edge materials to their graduate and undergraduate teaching.
This project focuses on transport and mixing in a biologically inspired extensile active nematic. Densely packed microtubules slide antiparallel to each other at a controlled rate due to kinesin molecular motors and the resulting chaotic advection will be measured on different length scales, using the experimental tools of particle tracking, particle image velocimetry, and fluorescence imaging of labeled tracers. Experimental data will be theoretically interpreted using the tools of nonlinear and topological dynamics, thereby merging the fields of chaotic advection and liquid crystals in a unique collaborative effort. Topological entropy will play a central, unifying role in this study. Topological entropy is well known in studies of chaotic advection, but has been thus far overlooked in studies of active nematics. Specific aims are to: 1) use bead tracking and velocity reconstruction, together with tools from nonlinear dynamics, to measure the topological entropy of active nematic mixing; 2) measure the effective diffusivity, enhanced by chaotic advection, of the active nematic on the macroscale; and 3) investigate correlations between molecular-scale dynamics, mesoscale mixing, and macroscale diffusion by varying system parameters.
Nathan Melton, our most recent graduate walked at UC Merced commencement this month. Nathan’s recent work, focused on topological defects in liquid crystals with a new paper just out last month in the journal Nanomaterials.
“Phase-transition-driven nanoparticle assembly in liquid crystal droplets” Charles N. Melton, Sheida T. Riahinasab, Amir Keshavarz, Benjamin J. Stokes and Linda S. Hirst, Nanomaterials, 8, 146 (2018). Link
Nathan has already started work as a postdoctoral scientist at Lawrence Berkeley National Lab at the Advanced Light Source!
Recently Prof Hirst authored a News and Views in Nature focussed on two exciting articles about the cell epithelium as active matter.
“Evidence has been found that a biological tissue might behave like a liquid crystal. Even more remarkably, topological defects in this liquid-crystal system seem to influence cell behaviour. A materials physicist and a biologist discuss what the findings mean for researchers in their fields”.