Ordering dynamics of blue phases entails kinetic stabilization of amorphous networks

The cubic blue phases of liquid crystals are fascinating and technologically promising examples of hierarchically structured soft materials, comprising ordered networks of defect lines (disclinations) within a liquid crystalline matrix. We present the first large-scale simulations of their domain growth, starting from a blue phase nucleus within a supercooled isotropic or cholesteric background. The nucleated phase is thermodynamically stable; one expects its slow orderly growth, creating a bulk cubic. Instead, we find that the strong propensity to form disclinations drives the rapid disorderly growth of a metastable amorphous defect network. During this process the original nucleus is destroyed; re-emergence of the stable phase may therefore require a second nucleation step. Our findings suggest that blue phases exhibit hierarchical behavior in their ordering dynamics, to match that in their structure.Comment: 11 pages, 5 figures, 2 supplementary figures, 2 supplementary tables, accepted by PNA

Colloids in active fluids: Anomalous micro-rheology and negative drag

We simulate an experiment in which a colloidal probe is pulled through an active nematic fluid. We find that the drag on the particle is non-Stokesian (not proportional to its radius). Strikingly, a large enough particle in contractile fluid (such as an actomyosin gel) can show negative viscous drag in steady state: the particle moves in the opposite direction to the externally applied force. We explain this, and the qualitative trends seen in our simulations, in terms of the disruption of orientational order around the probe particle and the resulting modifications to the active stress.Comment: 5 pages, 3 figure

Thermodynamics of Blue Phases In Electric Fields

We present extensive numerical studies to determine the phase diagrams of cubic and hexagonal blue phases in an electric field. We confirm the earlier prediction that hexagonal phases, both 2 and 3 dimensional, are stabilized by a field, but we significantly refine the phase boundaries, which were previously estimated by means of a semi-analytical approximation. In particular, our simulations show that the blue phase I -- blue phase II transition at fixed chirality is largely unaffected by electric field, as observed experimentally.Comment: submitted to Physical Review E, 7 pages (excluding figures), 12 figure

Bulk rheology and microrheology of active fluids

We simulate macroscopic shear experiments in active nematics and compare them with microrheology simulations where a spherical probe particle is dragged through an active fluid. In both cases we define an effective viscosity: in the case of bulk shear simulations this is the ratio between shear stress and shear rate, whereas in the microrheology case it involves the ratio between the friction coefficient and the particle size. We show that this effective viscosity, rather than being solely a property of the active fluid, is affected by the way chosen to measure it, and strongly depends on details such as the anchoring conditions at the probe surface and on both the system size and the size of the probe particle.Comment: 12 pages, 10 figure

Colloidal templating at a cholesteric - oil interface: Assembly guided by an array of disclination lines

We simulate colloids (radius $R \sim 1\mu$m) trapped at the interface between a cholesteric liquid crystal and an immiscible oil, at which the helical order (pitch p) in the bulk conflicts with the orientation induced at the interface, stabilizing an ordered array of disclinations. For weak anchoring strength W of the director field at the colloidal surface, this creates a template, favoring particle positions eitheron top of or midway between defect lines, depending on $\alpha = R/p$. For small $\alpha$, optical microscopy experiments confirm this picture, but for larger $\alpha$ no templating is seen. This may stem from the emergence at moderate W of a rugged energy landscape associated with defect reconnections.Comment: 5 pages, 4 figure

Structure of Blue Phase III of Cholesteric Liquid Crystals

We report large scale simulations of the blue phases of cholesteric liquid crystals. Our results suggest a structure for blue phase III, the blue fog, which has been the subject of a long debate in liquid crystal physics. We propose that blue phase III is an amorphous network of disclination lines, which is thermodynamically and kinetically stabilised over crystalline blue phases at intermediate chiralities}. This amorphous network becomes ordered under an applied electric field, as seen in experiments