Demixing in Binary Mixtures with Differential Diffusivity at High Density.

Spontaneous phase separation, or demixing, is important in biological phenomena such as cell sorting. In particle-based models, an open question is whether differences in diffusivity can drive such demixing. While differential-diffusivity-induced phase separation occurs in mixtures with a packing fraction up to 0.7 [S. N. Weber et al. Binary mixtures of particles with different diffusivities demix, Phys. Rev. Lett. 116, 058301 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.058301], here we investigate whether demixing persists at even higher densities relevant for cells. For particle packing fractions between 0.7 and 1.0 the system demixes, but at packing fractions above unity the system remains mixed, exposing re-entrant behavior in the phase diagram that occurs when phase separation can no longer drive a change in entropy production at high densities. We also find that a confluent Voronoi model for tissues does not phase separate, consistent with particle-based simulations.

Chemically Driven Multimodal Locomotion of Active, Flexible Sheets.

The inhibitor-promoter feedback loop is a vital component in regulatory pathways that controls functionality in living systems. In this loop, the production of chemical A at one site promotes the production of chemical B at another site, but B inhibits the production of A. In solution, differences in the volumes of the reactants and products of this reaction can generate buoyancy-driven fluid flows, which will deform neighboring soft material. To probe the intrinsic interrelationship among chemistry, hydrodynamics, and fluid-structure interactions, we model a bio-inspired system where a flexible sheet immersed in solution encompasses two spatially separated catalytic patches, which drive the A-B inhibitor-promotor reaction. The convective rolls of fluid generated above the patches can circulate inward or outward depending on the chemical environment. Within the regime displaying chemical oscillations, the dynamic fluid-structure interactions morph the shape of the sheet to periodically "fly", "crawl", or "swim" along the bottom of the confining chamber, revealing an intimate coupling between form and function in this system. The oscillations in the sheet's motion in turn affect the chemical oscillations in the solution. In the regime with non-oscillatory chemistry, the induced flow still morphs the shape of the sheet, but now, the fluid simply translates the sheet along the length of the chamber. The findings reveal the potential for enzymatic reactions in the body to generate hydrodynamic behavior that modifies the shape of neighboring soft tissue, which in turn modifies both the fluid dynamics and the enzymatic reaction. The findings indicate that this non-linear dynamic behavior can be playing a critical role in the functioning of regulatory pathways in living systems.

Self-Generated Convective Flows Enhance the Rates of Chemical Reactions.

In chemical solutions, the products of catalytic reactions can occupy different volumes compared to the reactants and thus give rise to local density variations in the fluid. These density variations generate solutal buoyancy forces, which are exerted on the fluid and thus "pump" the fluid to flow. Herein, we examine if the reaction-induced pumping accelerates the chemical reaction by transporting the reactants to the catalyst at a rate faster than passive diffusion. Using both simulations and experiments, we show a significant increase in reaction rate when reaction-generated convective flow is present. In effect, through a feedback loop, catalysts speed up reactions not only by lowering the energy barrier but also by increasing the collision frequency between the reactants and the catalyst.

Solutal-buoyancy-driven intertwining and rotation of patterned elastic sheets.

The intertwining of strands into 3D spirals is ubiquitous in biology, enabling functions from information storage to maintenance of cell structure and directed locomotion. In synthetic systems, entwined fibers can provide superior mechanical properties and act as artificial muscle or structural reinforcements. Unlike structures in nature, the entwinement of synthetic materials typically requires application of an external stimulus, such as mechanical actuation, light, or a magnetic field. Herein, we use computational modeling to design microscale sheets that mimic biology by transducing chemical energy into mechanical action, and thereby self-organize and interlink into 3D spirals, which spontaneously rotate. These flexible sheets are immersed in a fluid-filled microchamber that encompasses an immobilized patch of catalysts on the bottom wall. The sheets themselves can be passive or active (coated with catalyst). Catalytic reactions in the solution generate products that occupy different volumes than the reactants. The resulting density variations exert a force on the fluid (solutal buoyancy force) that causes motion, which in turn drives the interlinking and collective swirling of the sheets. The individual sheets do not rotate; rotation only occurs when the sheets are interlinked. This level of autonomous, coordinated 3D structural organization, intertwining, and rotation is unexpected in synthetic materials systems operating without external controls. Using physical arguments, we identify dimensionless ratios that are useful in scaling these ideas to other systems. These findings are valuable for creating materials that act as "machines", and directing soft matter to undergo self-sustained, multistep assembly that is governed by intrinsic chemical reactions.

Chemical pumps and flexible sheets spontaneously form self-regulating oscillators in solution.

The synchronization of self-oscillating systems is vital to various biological functions, from the coordinated contraction of heart muscle to the self-organization of slime molds. Through modeling, we design bioinspired materials systems that spontaneously form shape-changing self-oscillators, which communicate to synchronize both their temporal and spatial behavior. Here, catalytic reactions at the bottom of a fluid-filled chamber and on mobile, flexible sheets generate the energy to "pump" the surrounding fluid, which also transports the immersed sheets. The sheets exert a force on the fluid that modifies the flow, which in turn affects the shape and movement of the flexible sheets. This feedback enables a single coated (active) and even an uncoated (passive) sheet to undergo self-oscillation, displaying different oscillatory modes with increases in the catalytic reaction rate. Two sheets (active or passive) introduce excluded volume, steric interactions. This distinctive combination of the hydrodynamic, fluid-structure, and steric interactions causes the sheets to form coupled oscillators, whose motion is synchronized in time and space. We develop a heuristic model that rationalizes this behavior. These coupled self-oscillators exhibit rich and tunable phase dynamics, which depends on the sheets' initial placement, coverage by catalyst and relative size. Moreover, through variations in the reactant concentration, the system can switch between the different oscillatory modes. This breadth of dynamic behavior expands the functionality of the coupled oscillators, enabling soft robots to display a variety of self-sustained, self-regulating moves.

Achieving Independent Control over Surface and Bulk Fluid Flows in Microchambers.

To fully realize the potential of microfluidic platforms as useful diagnostic tools, the devices must be sufficiently portable that they function at the point-of-care, as well as remote and resource-poor locations. Using both modeling and experiments, here we develop a standalone fluidic device that is driven by light and operates without the need for external electrical or mechanical pumps. The light initiates a photochemical reaction in the solution; the release of chemical energy from the reaction is transduced into the spontaneous motion of the surrounding fluid. The generated flow is driven by two simultaneously occurring mechanisms: solutal buoyancy that controls the motion of the bulk fluid and diffusioosmosis that regulates motion near the bottom of the chamber. Consequently, the bulk and surface fluid flows can be directed independently of one another. We demonstrate that this exceptional degree of spatiotemporal control provides a new method for autonomously transporting different-sized particles in opposite directions within the chamber. Thus, one device can be used to both separate the particles and drive them to different locations for further processing or analysis. This property is particularly useful for analyzing fluids that contain multiple contaminants or disease agents. Because this system relies on intrinsic hydrodynamic interactions initiated by a portable, small-scale source of light, the device provides the desired level of mobility vital for the next generation of functional fluidic platforms.

Light-Induced Convective Segregation of Different Sized Microparticles.

Using computational modeling, we design a facile method for sorting particles of different sizes in a fluid-filled microchamber. The microchamber is inclined at an angle with respect to the horizontal direction and contains suspended gold nanoparticles as well as the microparticles. With the application of ultraviolet light, the heat generated by illuminating the gold nanoparticles gives rise to thermal buoyancy effects, which drive the flow of the fluid in the chamber. This thermally driven, convective flow can be tailored by varying the intensity of the imposed light and the concentration of the gold nanoparticles in the solution. The competition between the drag force imposed by the fluid flows and the gravitational forces acting on the different sized particles produces the separation of the particles along the chamber's bottom, inclined wall. The separation distance between the particles can be increased by increasing the angle of inclination and the relative difference in the particle sizes. This system provides a label-free, membrane-less, and low-cost approach for sorting particles vital to a wide range of applications.

Emergent topological phenomena in active polymeric fluids.

Polymeric fluids show a wealth of topological phenomena, from entanglement and reptation at microscales to orientational ordering and defect production at macroscales, which can be explained by statistical-mechanical theories. In the presence of activity, the latter must be augmented by forces that cause spontaneous chain motion and fluid flow. Here, using such augmented Langevin equations, we study active polymeric solutions and melts composed of chains of hydrodynamically interacting stresslets. In a spherical volume, contractile chains are unstable and self-knot into entangled melts at both low and high densities. Extensile chains in the same geometry form an unentangled reptating state at low densities and an entangled, coherently moving, non-reptating state at high densities. On a spherical surface, contractile chains show transitions, with increasing areal density, between isotropic, orientationally ordered and micro-phase separated states. Extensile chains in the same geometry show a transition between isotropic and nematic states. In both cases, defects in orientationally ordered states are produced athermally and without conserving topological charge. Our work reproduces the phenomenology of several recent experiments, highlights the importance of hydrodynamic interactions in active polymer fluids, and suggests non-equilibrium kinetic routes to topological structures that are otherwise difficult to obtain in equilibrium.

Linking Catalyst-Coated Isotropic Colloids into "Active" Flexible Chains Enhances Their Diffusivity.

Active colloids are not constrained by equilibrium: ballistic propulsion, superdiffusive behavior, or enhanced diffusivities have been reported for active Janus particles. At high concentrations, interactions between active colloids give rise to complex emergent behavior. Their collective dynamics result in the formation of several hundred particle-strong flocks or swarms. Here, we demonstrate significant diffusivity enhancement for colloidal objects that neither have a Janus architecture nor are at high concentrations. We employ uniformly catalyst-coated, viz. chemo-mechanically, isotropic colloids and link them into a chain to enforce proximity. Activity arises from hydrodynamic interactions between enchained colloidal beads due to reaction-induced phoretic flows catalyzed by platinum nanoparticles on the colloid surface. This results in diffusivity enhancements of up to 60% for individual chains in dilute solution. Chains with increasing flexibility exhibit higher diffusivities. Simulations accounting for hydrodynamic interactions between enchained colloids due to active phoretic flows accurately capture the experimental diffusivity. These simulations reveal that the enhancement in diffusivity can be attributed to the interplay between chain conformational fluctuations and activity. Our results show that activity can be used to systematically modulate the mobility of soft slender bodies.

Colloidal transport by active filaments.

Enhanced colloidal transport beyond the limit imposed by diffusion is usually achieved through external fields. Here, we demonstrate the ballistic transport of a colloidal sphere using internal sources of energy provided by an attached active filament. The latter is modeled as a chain of chemo-mechanically active beads connected by potentials that enforce semi-flexibility and self-avoidance. The fluid flow produced by the active beads and the forces they mediate are explicitly taken into account in the overdamped equations of motion describing the colloid-filament assembly. The speed and efficiency of transport depend on the dynamical conformational states of the filament. We characterize these states using filament writhe as an order parameter and identify ones yielding maxima in speed and efficiency of transport. The transport mechanism reported here has a remarkable resemblance to the flagellar propulsion of microorganisms which suggests its utility in biomimetic systems.

Dynamics of a polyelectrolyte in simple shear flow.

The configurational dynamics of a polyelectrolyte (PE), subjected to a simple shear flow, is studied using Brownian dynamics (BD) and Dissipative Particle Dynamics (DPD) simulations of a bead-spring model with explicit counterions. We explore the effect of counterion condensation on the tumbling and extension of PEs by varying the shear rates for a range of values of the electrostatic coupling parameter A (which is defined as the ratio of the Bjerrum length to the size of the monomer). In all cases, the power spectrum of Rs(t) (which characterizes the projected length of the PE in the flow direction as a function of time) exhibits a power law decay at high frequencies, similar to that for a dumbbell in shear flow. For lower values of A (A ~ 2), the tumbling of the PE is periodic and is always associated with folding and stretching, which is in contrast to the oscillatory transition between the extended and globular states seen at higher values of A (A ~ 15). We observe that for A ~ 2 the tumbling frequency decreases and the average tumbling time increases with hydrodynamic interaction (HI). For A > 15, we observe a critical shear rate γ[combining dot]c below which there is considerable counterion condensation and the PE remains in the globular state with a structure akin to that of a neutral polymer in poor solvent. The γ[combining dot]c and the behavior of the PE above the critical shear rate are dependent on the HI. For a given shear rate, when there is considerable condensed counterion fluctuation, the PE extends as a whole and then collapses by the formation of folds with no observable periodicity in tumbling. When the condensed counterion fluctuations are suppressed, the polymer exhibits periodic tumbling. Simulation artifacts resulting from the implicit nature of the solvent and that due to boundary conditions are discussed by comparing the BD results with that obtained from the DPD simulations incorporating Ewald summation for electrostatics.