arXiv subjects

Thomas R. Powers

Publications and source records attributed to Thomas R. Powers.

At least 19 recordsLinked to original sources

arXiv2025

Mechanical instability generates monodisperse colloidosomes

Formation and rupture of vesicles is a fundamental process underlying diverse phenomena in biology, materials science, and biomedical applications. Vesicles form when the area of a growing disk-like membrane exceeds a critical value at which the edge and bending energies balance each other. Observing such topological transitions in lipid bilayers is a challenge because of their nanoscale dimensions and rapid dynamics. We study a scaled-up model of colloidal membranes assembled from rod-shaped colloidal particles. The unique features of colloidal membranes enable the real-time visualization of spontaneous closure driven by instability relevant to all membrane-based materials. First-principles theory quantitatively predicts the instability point for vesicle formation and intermediate membrane conformations during the disk-to-vesicle transition. The instability generates monodisperse, selectively permeable colloidosomes with size controlled by gravity and membrane thickness, providing a scalable and programmable platform for diverse applications.

cond-mat.soft↗

arXiv2025

Confinement geometry governs the impact of external shear stress on active stress-driven flows in microtubule-kinesin active fluids

Active fluids generate internal active stress and exhibit unique responses to external forces such as superfluidity and self-yielding transitions. However, how confinement geometry influences these responses remains poorly understood. Here, we investigate microtubule-kinesin active fluids under external shear stresses in two geometries. In slab-like confinement (a narrow-gap cavity), external stresses propagated throughout the system, leading to stress competition and a kinematic transition that shifted dynamics from active stress-dominated to shear stress-dominated flow. At the transition, we estimate the active stress to be ~1.5 mPa. Simulation supported that this transition arises from stress competition. In contrast, in ring-like confinement (a toroidal system), external forces acted locally, inducing a mini cavity flow that triggered self-organized reconfiguration rather than direct entrainment. These findings show that the response of active fluids to external forcing depends not only on the magnitude of the applied stress but also on how confinement geometry directs and redistributes that stress, revealing a new approach to controlling active fluid behavior by combining static geometrical design with dynamic external stimuli for real-time modulation of flow patterns. Such control strategies may be applied to microfluidic systems, where external inputs such as micromechanical actuators can dynamically tune active fluid behavior within fixed device geometries, enabling transitions between chaotic and coherent flows for tasks such as mixing, sorting, or directed transport.

cond-mat.soft↗

arXiv2025

Interfacial instability of confined 3D active droplets

Instabilities of fluid-fluid interfaces are ubiquitous in passive soft matter. Adding activity to the interface or either fluid can dramatically change the stability of the interface. Using experiment and theory, we investigate the interfacial instability of a deformable 3D active nematic liquid crystal droplet in the isotropic phase surrounded by a passive fluid and confined between two parallel plates. Spontaneous active flows drive the growth of undulations along the active/passive interface, with the mode number of the fastest-growing mode increasing with droplet radius and decreasing with gap height. We apply the lubrication approximation to a minimal nematohydrodynamic model to determine the growth rates of all interfacial modes. The magnitude of the growth rate is determined by the active timescale and the relaxation timescales associated with liquid crystalline order, as well as capillary and viscous stresses. We find multiple points of agreement between experiment and theory, including the shape evolution of individual droplets, the growth rates of unstable modes averaged across many droplets, and the extensional shear flows observed within droplets.

cond-mat.soft↗

arXiv2025

Brownian Dynamics Simulations of Inclusions in an Active Fluid Bath

We carry out two-dimensional Brownian dynamics simulations of the behavior of rigid inclusion particles immersed in an active fluid bath. The active fluid is modeled as a collection of self-propelled circular disks interacting via a soft repulsive potential and a nematic alignment interaction. The fluid is characterized by its nematic order, polar order and orientational correlation length. The active fluid bath transitions from the isotropic to the nematic phase with increasing number density, increasing nematic interaction strength or increasing Péclet number. The inclusion particles are modeled as rigid assemblies of passive circular disks. Four types of inclusions are considered: a rod-like $I$ shape, a boomerang-like $L$ shape, and stair-like shapes $Z$ and $Z^*$, with opposite handedness. When inclusions are introduced into the active fluid bath, their diffusion is significantly enhanced by the force and torque exerted by the active fluid particles and the chiral inclusion particles exhibit constant rotational drift. These diffusion and rotation enhancements increase as the swimming speed of the active fluid particles increases. The translational motion of the inclusion particles also couples with their orientational motion, and the correlation is modulated by the active fluid particles' swimming speed. This work paves the way for future simulations of inclusions in active fluid baths and suggests potential avenues for controlling transport properties in active materials.

cond-mat.soft↗

arXiv2025

Topology and Kinetic Pathways of Colloidosome Assembly and Disassembly

Liquid shells, such as lipid vesicles and soap bubbles, are ubiquitous throughout biology, engineered matter, and everyday life. Their creation and disintegration are defined by a singularity that separates a topologically distinct extended liquid film from a boundary-free closed shell. Such topology-changing processes are essential for cellular transport and drug delivery. However, their studies are challenging because of the rapid dynamics and small length scale of conventional lipid vesicles. We develop fluid colloidosomes, micron-sized analogs of lipid vesicles. We study their stability close to their disk-to-sphere topological transition. Intrinsic colloidal length and time scales slow down the dynamics to reveal vesicle conformations in real time during their assembly and disassembly. Remarkably, the lowest-energy pathway by which a closed vesicle transforms into a disk involves a topologically distinct cylinder-like intermediate. These results reveal universal aspects of topological changes in all liquid shells and a robust platform for the encapsulation, transport, and delivery of nanosized cargoes.

cond-mat.soft↗

arXiv2024

Low Reynolds number pumping near an elastic half space

Previous studies on peristalsis, the pumping of fluid along a channel by wave-like displacements of the channel walls, have shown that the elastic properties of the channel and the peristaltic wave shape can influence the flow rate. Motivated by the oscillatory flow of cerebrospinal fluid along compliant perivascular spaces, we consider a prescribed wave motion of a single boundary which pumps fluid at small Reynolds number near an elastic half space. We investigate the relationship between flow rate and elastic deformation as a function of the fluid and solid properties. We consider transverse and longitudinal motion of the driving peristaltic wave. We find that a transverse peristaltic wave produces net forward flow and induces elastic motion in which all material points oscillate uniformly. Conversely, a longitudinal peristaltic wave produces a net backward flow and drives elastic motion which is nonuniform in the elastic solid. We use dimensional values relevant to the flow of cerebrospinal fluid and find agreement with in vivo velocity data.

physics.flu-dyn↗

arXiv2023

Chiral fluid membranes with orientational order and multiple edges

We carry out Monte Carlo simulations on fluid membranes with orientational order and multiple edges in the presence and absence of external forces. The membrane resists bending and has an edge tension, the orientational order couples with the membrane surface normal through a cost for tilting, and there is a chiral liquid crystalline interaction. In the absence of external forces, a membrane initialized as a vesicle will form a disk at low chirality, with the directors forming a smectic-A phase with alignment perpendicular to the membrane surface except near the edge. At large chirality a catenoid-like shape or a trinoid-like shape is formed, depending on the number of edges in the initial vesicle. This shape change is accompanied by cholesteric ordering of the directors and multiple $π$ walls connecting the membrane edges and wrapping around the membrane neck. If the membrane is initialized instead in a cylindrical shape and stretched by an external force, it maintains a nearly cylindrical shape but additional liquid crystalline phases appear. For large tilt coupling and low chirality, a smectic-A phase forms. For lower values of the tilt coupling, a nematic phase appears at zero chirality with the average director oriented perpendicular to the long axis of the membrane, while for nonzero chirality a cholesteric phase appears. The $π$ walls are tilt walls at low chirality and transition to twist walls as chirality is increased. We construct a continuum model of the director field to explain this behavior.

cond-mat.soft↗

arXiv2023

Flow states of two dimensional active gels driven by external shear

Using a minimal hydrodynamic model, we theoretically and computationally study active gels in straight and annular two-dimensional channels subject to an externally imposed shear. The gels are isotropic in the absence of externally- or activity-driven shear, but have nematic order that increases with shear rate. Using the finite element method, we determine the possible flow states for a range of activities and shear rates. Linear stability analysis of an unconfined gel in a straight channel shows that an externally imposed shear flow can stabilize an extensile fluid that would be unstable to spontaneous flow in the absence of the shear flow, and destabilize a contractile fluid that would be stable against spontaneous flow in the absence of shear flow. These results are in rough agreement with the stability boundaries between the base shear flow state and the nonlinear flow states that we find numerically for a confined active gel. For extensile fluids, we find three kinds of nonlinear flow states in the range of parameters we study: unidirectional flows, oscillatory flows, and dancing flows. To highlight the activity-driven spontaneous component of the nonlinear flows, we characterize these states by the average volumetric flow rate and the wall stress. For contractile fluids, we only find the linear shear flow and a nonlinear unidirectional flow in the range of parameters that we studied. For large magnitudes of the activity, the unidirectional contractile flow develops a boundary layer. Our analysis of annular channels shows how curvature of the streamlines in the base flow affects the transitions among flow states.

cond-mat.soft↗

arXiv2022

Controlling the shape and topology of two-component colloidal membranes

Changes in the geometry and topology of self-assembled membranes underlie diverse processes across cellular biology and engineering. Similar to lipid bilayers, monolayer colloidal membranes have in-plane fluid-like dynamics and out-of-plane bending elasticity. Their open edges and micron length scale provide a tractable system to study the equilibrium energetics and dynamic pathways of membrane assembly and reconfiguration. Here, we find that doping colloidal membranes with short miscible rods transforms disk-shaped membranes into saddle-shaped surfaces with complex edge structures. The saddle-shaped membranes are well-approximated by Enneper's minimal surfaces. Theoretical modeling demonstrates that their formation is driven by increasing positive Gaussian modulus, which in turn is controlled by the fraction of short rods. Further coalescence of saddle-shaped surfaces leads to diverse topologically distinct structures, including catenoids, tri-noids, four-noids, and higher order structures. At long time scales, we observe the formation of a system-spanning, sponge-like phase. The unique features of colloidal membranes reveal the topological transformations that accompany coalescence pathways in real time. We enhance the functionality of these membranes by making their shape responsive to external stimuli. Our results demonstrate a novel pathway towards control of thin elastic sheets' shape and topology -- a pathway driven by the emergent elasticity induced by compositional heterogeneity.

cond-mat.soft↗

arXiv2021

Taylor dispersion of elongated rods

Particles transported in fluid flows, such as cells, polymers, or nanorods, are rarely spherical. In this study, we numerically and theoretically investigate the dispersion of an initially localized patch of passive elongated Brownian particles constrained to one degree of rotational freedom in a two-dimensional Poiseuille flow, demonstrating that elongated particles exhibit an enhanced longitudinal dispersion. In a shear flow, the rods translate due to advection and diffusion and rotate due to rotational diffusion and their classical Jeffery's orbit. The magnitude of the enhanced dispersion depends on the particle's aspect ratio and the relative importance of its shear-induced rotational advection and rotational diffusivity. When rotational diffusion dominates, we recover the classical Taylor dispersion result for the longitudinal spreading rate using an orientationally averaged translational diffusivity for the rods. However, in the high-shear limit, the rods tend to align with the flow and ultimately disperse more due to their anisotropic diffusivities. Results from our Monte Carlo simulations of the particle dispersion are captured remarkably well by a simple theory inspired by Taylor's original work. For long times and large Peclet numbers, an effective one-dimensional transport equation is derived with integral expressions for the particles' longitudinal transport speed and dispersion coefficient. The enhanced dispersion coefficient can be collapsed along a single curve for particles of high aspect ratio, representing a simple correction factor that extends Taylor's original prediction to elongated particles.

physics.flu-dyn↗

arXiv2021

Axisymmetric membranes with edges under external force: buckling, minimal surfaces, and tethers

We use theory and numerical computation to determine the shape of an axisymmetric fluid membrane with a resistance to bending and constant area. The membrane connects two rings in the classic geometry that produces a catenoidal shape in a soap film. In our problem, we find infinitely many branches of solutions for the shape and external force as functions of the separation of the rings, analogous to the infinite family of eigenmodes for the Euler buckling of a slender rod. Special attention is paid to the catenoid, which emerges as the shape of maximal allowable separation when the area is less than a critical area equal to the planar area enclosed by the two rings. A perturbation theory argument directly relates the tension of catenoidal membranes to the stability of catenoidal soap films in this regime. When the membrane area is larger than the critical area, we find additional cylindrical tether solutions to the shape equations at large ring separation, and that arbitrarily large ring separations are possible. These results apply for the case of vanishing Gaussian curvature modulus; when the Gaussian curvature modulus is nonzero and the area is below the critical area, the force and the membrane tension diverge as the ring separation approaches its maximum value. We also examine the stability of our shapes and analytically show that catenoidal membranes have markedly different stability properties than their soap film counterparts.

cond-mat.soft↗

arXiv2021

Deformation and orientational order of chiral membranes with free edges

Motivated by experiments on colloidal membranes composed of chiral rod-like viruses, we use Monte Carlo methods to determine the phase diagram for the liquid crystalline order of the rods and the membrane shape. We generalize the Lebwohl-Lasher model for a nematic with a chiral coupling to a curved surface with edge tension and a resistance to bending, and include an energy cost for tilting of the rods relative to the local membrane normal. The membrane is represented by a triangular mesh of hard beads joined by bonds, where each bead is decorated by a director. The beads can move, the bonds can reconnect and the directors can rotate at each Monte Carlo step. When the cost of tilt is small, the membrane tends to be flat, with the rods only twisting near the edge for low chiral coupling, and remaining parallel to the normal in the interior of the membrane. At high chiral coupling, the rods twist everywhere, forming a cholesteric state. When the cost of tilt is large, the emergence of the cholesteric state at high values of the chiral coupling is accompanied by the bending of the membrane into a saddle shape. Increasing the edge tension tends to flatten the membrane. These results illustrate the geometric frustration arising from the inability of a surface normal to have twist.

cond-mat.soft↗

arXiv2020

Swimming of microorganisms in quasi-2D membranes

Biological swimmers frequently navigate in geometrically restricted media. We study the prescribed-stroke problem of swimmers confined to a planar viscous membrane embedded in a bulk fluid of different viscosity. In their motion, microscopic swimmers disturb the fluid in both the membrane and the bulk. The flows that emerge have a combination of two-dimensional (2D) and three-dimensional (3D) hydrodynamic features, and such flows are referred to as quasi-2D. The cross-over from 2D to 3D hydrodynamics in a quasi-2D fluid is controlled by the Saffman length, a length scale given by the ratio of the 2D membrane viscosity to the 3D viscosity of the embedding bulk fluid. We have developed a computational and theoretical approach based on the boundary element method and the Lorentz reciprocal theorem to study the swimming of microorganisms for a range of values of the Saffman length. We found that a flagellum propagating transverse sinusoidal waves in a quasi-2D membrane can develop a swimming speed exceeding that in pure 2D or 3D fluids, while the propulsion of a two-dimensional squirmer is slowed down by the presence of the bulk fluid.

cond-mat.soft↗

arXiv2019

Shapes of fluid membranes with chiral edges

We carry out Monte Carlo simulations of a colloidal fluid membrane composed of chiral rod-like viruses. The membrane is modeled by a triangular mesh of beads connected by bonds in which the bonds and beads are free to move at each Monte Carlo step. Since the constituent viruses are experimentally observed to twist only near the membrane edge, we use an effective energy that favors a particular sign of the geodesic torsion of the edge. The effective energy also includes membrane bending stiffness, edge bending stiffness, and edge tension. We find three classes of membrane shapes resulting from the competition of the various terms in the free energy: branched shapes, chiral disks, and vesicles. Increasing the edge bending stiffness smooths the membrane edge, leading to correlations among the membrane normal at different points along the edge. We also consider membrane shapes under an external force by fixing the distance between two ends of the membrane, and find the shape for increasing values of the distance between the two ends. As the distance increases, the membrane twists into a ribbon, with the force eventually reaching a plateau.

cond-mat.soft↗

arXiv2019

Topological structure and dynamics of three dimensional active nematics

Point-like motile topological defects control the universal dynamics of diverse two-dimensional active nematics ranging from shaken granular rods to cellular monolayers. A comparable understanding in higher dimensions has yet to emerge. We report the creation of three-dimensional active nematics by dispersing extensile microtubule bundles in a passive colloidal liquid crystal. Light-sheet microscopy reveals the millimeter-scale structure of active nematics with a single bundle resolution and the temporal evolution of the associated nematic director field. The dominant excitations of three-dimensional active nematics are extended charge-neutral disclination loops that undergo complex dynamics and recombination events. These studies introduce a new class of non-equilibrium systems whose turbulent-like dynamics arises from the interplay between internally generated active stresses, the chaotic flows and the topological structure of the constituent defects.

cond-mat.soft↗

arXiv2019

Force induced formation of twisted chiral ribbons

We study the emergence of helical structures subjected to a stretching force, demonstrating that the force transforms disk-shaped colloidal membranes into twisted chiral ribbons of predetermined handedness. Using an experimental technique that enforces torque-free boundary conditions we simultaneously measure the force-extension curve and quantify the shape of emergent ribbons. An effective theory that accounts for the membrane bending energy and uses geometric properties of the edge to model the internal liquid crystalline degrees of freedom explains both the measured force-extension curve and shape of the twisted ribbons in response to an applied force.

cond-mat.soft↗

arXiv2018

Enhancement of microorganism swimming speed in active matter

We study a swimming undulating sheet in the isotropic phase of an active nematic liquid crystal. Activity changes the effective shear viscosity, reducing it to zero at a critical value of activity. Expanding in the sheet amplitude, we find that the correction to the swimming speed due to activity is inversely proportional to the effective shear viscosity. Our perturbative calculation becomes invalid near the critical value of activity; using numerical methods to probe this regime, we find that activity enhances the swimming speed by an order of magnitude compared to the passive case.

cond-mat.soft↗

arXiv2016

Chiral edge fluctuations of colloidal membranes

We study edge fluctuations of a flat colloidal membrane comprised of a monolayer of aligned filamentous viruses. Experiments reveal that a peak in the spectrum of the in-plane edge fluctuations arises for sufficiently strong virus chirality. Accounting for internal liquid crystalline degrees of freedom by the length, curvature, and geodesic torsion of the edge, we calculate the spectrum of the edge fluctuations. The theory quantitatively describes the experimental data, demonstrating that chirality couples in-plane and out-of-plane edge fluctuations to produce the peak.

cond-mat.soft↗