35 research outputs found
Passive Janus Particles Are Self-propelled in Active Nematics
While active systems possess notable potential to form the foundation of new
classes of autonomous materials, designing systems that can extract functional
work from active surroundings has proven challenging. In this work, we extend
these efforts to the realm of designed active liquid crystal/colloidal
composites. We propose suspending colloidal particles with Janus anchoring
conditions in an active nematic medium. These passive Janus particles become
effectively self-propelled once immersed into an active nematic bath. The
self-propulsion of passive Janus particles arises from the effective
topological charge their surface enforces on the surrounding active fluid. We
analytically study their dynamics and the orientational dependence on the
position of a companion defect. We predict that at sufficiently small
activity, the colloid and companion defect remain bound to each other, with the
defect strongly orienting the colloid to propel either parallel or
perpendicular to the nematic. At sufficiently high activity, we predict an
unbinding of the colloid/defect pair. This work demonstrates how suspending
engineered colloids in active liquid crystals may present a path to extracting
activity to drive functionality.Comment: 14 pages, 9 figure
Multi-particle collision dynamics algorithm for nematic fluids
Research on transport, self-assembly and defect dynamics within confined,
flowing liquid crystals requires versatile and computationally efficient
mesoscopic algorithms to account for fluctuating nematohydrodynamic
interactions. We present a multi-particle collision dynamics (MPCD) based
algorithm to simulate liquid-crystal hydrodynamic and director fields in two
and three dimensions. The nematic-MPCD method is shown to successfully
reproduce the features of a nematic liquid crystal, including a
nematic-isotropic phase transition with hysteresis in 3D, defect dynamics,
isotropic Frank elastic coefficients, tumbling and shear alignment regimes and
boundary condition dependent order parameter fields
Anisotropic run-and-tumble-turn dynamics
Run-and-tumble processes successfully model several living systems. While studies have typically focused on particles with isotropic tumbles, recent examples exhibit “tumble-turns", in which particles undergo 90° tumbles and so possess explicitly anisotropic dynamics. We study the consequences ofsuch tumble-turn anisotropicity at both short and long-time scales. We model run-and-tumble-turn particles as self-propelled particles subjected to an angular potential that favors directions of movement parallel to Cartesian axes. Using agent-based simulations, we study the effects of the interplay between rotational diffusion and an aligning potential on the particles’ trajectories, whichleads to the right-angled turns. We demonstrate that the long-time effect is to alter the tumble-turn time, which governs the long-time dynamics. In particular, when normalized by this timescale, trajectories become independent of the underlying details of the potential. As such, we develop a simplified continuum theory, which quantitatively agrees with agent-based simulations. We find that the purely diffusive hydrodynamic limit still exhibits anisotropic features at intermediate times and conclude that the transition to diffusive dynamics precedes the transition to isotropic dynamics.By considering short-range repulsive and alignment particle-particle interactions, we show how theanisotropic features of a single particle are inherited by global order of the system. We hope thiswork will shed light on how active systems can extend local anisotropic properties to macroscopicscales, which might be important in biological processes occurring in anisotropic environment
Rapid dynamics of cell-shape recovery in response to local deformations
It is vital that cells respond rapidly to mechanical cues within their microenvironment through changes
in cell shape and volume, which rely upon the mechanical properties of cells’ highly interconnected
cytoskeletal networks and intracellular fluid redistributions. While previous research has largely
investigated deformation mechanics, we now focus on the immediate cell-shape recovery response
following mechanical perturbation by inducing large, local, and reproducible cellular deformations using
AFM. By continuous imaging within the plane of deformation, we characterize the membrane and
cortical response of HeLa cells to unloading, and model the recovery via overdamped viscoelastic
dynamics. Importantly, the majority (90%) of HeLa cells recover their cell shape in o1 s. Despite actin
remodelling on this time scale, we show that cell-shape recovery time is not affected by load duration,
nor magnitude for untreated cells. To further explore this rapid recovery response, we expose cells to
cytoskeletal destabilizers and osmotic shock conditions, which uncovers the interplay between actin and
osmotic pressure. We show that the rapid dynamics of recovery depend crucially on intracellular
pressure, and provide strong evidence that cortical actin is the key regulator in the cell-shape recovery
processes, in both cancerous and non-cancerous epithelial cell
Hydrodynamics of Micro-swimmers in Films
One of the principal mechanisms by which surfaces and interfaces affect
microbial life is by perturbing the hydrodynamic flows generated by swimming.
By summing a recursive series of image systems we derive a numerically
tractable approximation to the three-dimensional flow fields of a Stokeslet
(point force) within a viscous film between a parallel no-slip surface and
no-shear interface and, from this Green's function, we compute the flows
produced by a force- and torque-free micro-swimmer. We also extend the exact
solution of Liron & Mochon (1976) to the film geometry, which demonstrates that
the image series gives a satisfactory approximation to the swimmer flow fields
if the film is sufficiently thick compared to the swimmer size, and we derive
the swimmer flows in the thin-film limit. Concentrating on the thick film case,
we find that the dipole moment induces a bias towards swimmer accumulation at
the no-slip wall rather than the water-air interface, but that higher-order
multipole moments can oppose this. Based on the analytic predictions we propose
an experimental method to find the multipole coefficient that induces circular
swimming trajectories, allowing one to analytically determine the swimmer's
three-dimensional position under a microscope.Comment: 35 pages, 11 figures, 5 table
Morphology of depletant-induced erythrocyte aggregates
Red blood cells suspended in quiescent plasma tend to aggregate into multicellular assemblages, including linearly stacked columnar rouleaux, which can reversibly form more complex clusters or branching networks. While these aggregates play an essential role in establishing hemorheological and pathological properties, the biophysics behind their self-assembly into dynamic mesoscopic structures remains under-explored. We employ coarse-grained molecular simulations to model low-hematocrit erythrocytes subject to short-range implicit depletion forces, and demonstrate not only that depletion interactions are sufficient to account for a sudden dispersion-aggregate transition, but also that the volume fraction of depletant macromolecules controls small aggregate morphology. We observe a sudden transition from a dispersion to a linear column rouleau, followed by a slow emergence of disorderly amorphous clusters of many short rouleaux at larger volume fractions. This work demonstrates how discocyte topology and short-range, non-specific, physical interactions are sufficient to self-assemble erythrocytes into various aggregate structures, with markedly different morphologies and biomedical consequences
Twist-induced crossover from 2D to 3D turbulence in active nematics
While studies of active nematics in two dimensions have shed light on various
aspects of the flow regimes and topology of active matter, three-dimensional
properties of topological defects and chaotic flows remain unexplored. By
confining a film of active nematics between two parallel plates, we use
continuum simulations and analytical arguments to demonstrate that the
crossover from quasi-2D to 3D chaotic flows is controlled by the morphology of
the disclination lines. For small plate separations, the active nematic behaves
as a quasi-2D material, with straight topological disclination lines spanning
the height of the channel and exhibiting effectively 2D active turbulence. Upon
increasing channel height, we find a crossover to 3D chaotic flows due to the
contortion of disclinations above a critical activity. We further show that
these contortions are engendered by twist perturbations producing a sharp
change in the curvature of disclinations.Comment: Accepted for PRE Rapid Communication