Boundary-bound reactions: Pattern formation with and without hydrodynamics.

We study chemical pattern formation in a fluid between two flat plates and the effect of such patterns on the formation of convective cells. This patterning is made possible by assuming the plates are chemically reactive or release reagents into the fluid, both of which we model as chemical fluxes. We consider this as a specific example of boundary-bound reactions.

The -Link Swimmer in Three Dimensions: Controllability and Optimality Results.

The controllability of a fully three-dimensional -link swimmer is studied. After deriving the equations of motion in a low Reynolds number fluid by means of Resistive Force Theory, the controllability of the minimal 2-link swimmer is tackled using techniques from Geometric Control Theory. The shape of the 2-link swimmer is described by two angle parameters.

Patterns of bacterial motility in microfluidics-confining environments.

Understanding the motility behavior of bacteria in confining microenvironments, in which they search for available physical space and move in response to stimuli, is important for environmental, food industry, and biomedical applications. We studied the motility of five bacterial species with various sizes and flagellar architectures (, , , , and ) in microfluidic environments presenting various levels of confinement and geometrical complexity, in the absence of external flow and concentration gradients. When the confinement is moderate, such as in quasi-open spaces with only one limiting wall, and in wide channels, the motility behavior of bacteria with complex flagellar architectures approximately follows the hydrodynamics-based predictions developed for simple monotrichous bacteria.

Fight the flow: the role of shear in artificial rheotaxis for individual and collective motion.

To navigate in complex fluid environments, swimming organisms like fish or bacteria often reorient their bodies antiparallel or against the flow, more commonly known as rheotaxis. This reorientation motion enables the organisms to migrate against the fluid flow, as observed in salmon swimming upstream to spawn. Rheotaxis can also be realized in artificial microswimmers - self-propelled particles that mimic swimming microorganisms.

Microswimmer Propulsion by Two Steadily Rotating Helical Flagella.

Many theoretical studies of bacterial locomotion adopt a simple model for the organism consisting of a spheroidal cell body and a single corkscrew-shaped flagellum that rotates to propel the body forward. Motivated by experimental observations of a group of magnetotactic bacterial strains, we extended the model by considering two flagella attached to the cell body and rotating about their respective axes. Using numerical simulations, we analyzed the motion of such a microswimmer in bulk fluid and close to a solid surface.

Using Chemical Pumps and Motors To Design Flows for Directed Particle Assembly.

Mechanical and electrical pumps are conventionally used to drive fluid flow in microfluidic devices; these pumps require external power supplies, thus limiting the portability of the devices. Harnessing catalytic reactions in solution allows pumping to be shifted into the chemical realm and alleviates the need for extraneous equipment. Chemical "pumps" involve surface-bound catalytic patches that decompose dissolved reagents into the products of the reaction.

Flow-Driven Assembly of Microcapsules into Three-Dimensional Towers.

By harnessing biochemical signaling and chemotaxis, unicellular slime molds can aggregate on a surface to form a long, vertical stalk. Few synthetic systems can self-organize into analogous structures that emerge out of the plane. Through computational modeling, we devise a mechanism for assembling tower-like structures using microcapsules in solution as building blocks.

Convective Self-Sustained Motion in Mixtures of Chemically Active and Passive Particles.

We develop a model to describe the behavior of a system of active and passive particles in solution that can undergo spontaneous self-organization and self-sustained motion. The active particles are uniformly coated with a catalyst that decomposes the reagent in the surrounding fluid. The resulting variations in the fluid density give rise to a convective flow around the active particles.

Synthetic quorum sensing in model microcapsule colonies.

Biological quorum sensing refers to the ability of cells to gauge their population density and collectively initiate a new behavior once a critical density is reached. Designing synthetic materials systems that exhibit quorum sensing-like behavior could enable the fabrication of devices with both self-recognition and self-regulating functionality. Herein, we develop models for a colony of synthetic microcapsules that communicate by producing and releasing signaling molecules.

Solutal and thermal buoyancy effects in self-powered phosphatase micropumps.

Immobilized enzymes generate net fluid flow when exposed to specific reagents in solution. Thus, they function as self-powered platforms that combine sensing and on-demand fluid pumping. To uncover the mechanism of pumping, we examine the effects of solutal and thermal buoyancy on the behavior of phosphatase-based micropumps, using a series of reactants with known thermodynamic and kinetic parameters.

Harnessing catalytic pumps for directional delivery of microparticles in microchambers.

The directed transport of microparticles in microfluidic devices is vital for efficient bioassays and fabrication of complex microstructures. There remains, however, a need for methods to propel and steer microscopic cargo that do not require modifying these particles. Using theory and experiments, we show that catalytic surface reactions can be used to deliver microparticle cargo to specified regions in microchambers.

Harnessing surface-bound enzymatic reactions to organize microcapsules in solution.

By developing new computational models, we examine how enzymatic reactions on an underlying surface can be harnessed to direct the motion and organization of reagent-laden microcapsules in a fluid-filled microchannel. In the presence of appropriate reagents, surface-bound enzymes can act as pumps, which drive large-scale fluid flows. When the reagents diffuse through the capsules' porous shells, they can react with enzymatic sites on the bottom surface.

Convective flow reversal in self-powered enzyme micropumps.

Surface-bound enzymes can act as pumps that drive large-scale fluid flows in the presence of their substrates or promoters. Thus, enzymatic catalysis can be harnessed for “on demand” pumping in nano- and microfluidic devices powered by an intrinsic energy source. The mechanisms controlling the pumping have not, however, been completely elucidated.

Hydrodynamic analysis of flagellated bacteria swimming in corners of rectangular channels.

The influence of nearby solid surfaces on the motility of bacteria is of fundamental importance as these interactions govern the ability of the microorganisms to explore their environment and form sessile colonies. Reducing biofouling in medical implants and controlling the transport of bacterial cells in a microfluidic device are two applications that could benefit from a detailed understanding of swimming in microchannels. In this study, we investigate the self-propelled motion of a model bacterium, driven by rotating a single helical flagellum, in such an environment.

Self-Propelled Nanomotors Autonomously Seek and Repair Cracks.

Biological self-healing involves the autonomous localization of healing agents at the site of damage. Herein, we design and characterize a synthetic repair system where self-propelled nanomotors autonomously seek and localize at microscopic cracks and thus mimic salient features of biological wound healing. We demonstrate that these chemically powered catalytic nanomotors, composed of conductive Au/Pt spherical Janus particles, can autonomously detect and repair microscopic mechanical defects to restore the electrical conductivity of broken electronic pathways.

Designing Synthetic Microcapsules That Undergo Biomimetic Communication and Autonomous Motion.

Inspired by the collective behavior of slime molds and amoebas, we designed synthetic cell-like objects that move and self-organize in response to self-generated chemical gradients, thereby exhibiting autochemotaxis. Using computational modeling, we specifically focused on microcapsules that encompass a permeable shell and are localized on an adhesive surface in solution. Lacking any internal machinery, these spherical, fluid-filled shells might resemble the earliest protocells.

Hydrodynamic analysis of flagellated bacteria swimming near one and between two no-slip plane boundaries.

The motility of swimming bacteria near solid surfaces has implications in a wide range of scenarios, including water treatment facilities, microfluidics, and biomedical implants. Using the boundary element method to numerically solve the equations of low Reynolds number fluid flow, we investigate the dynamics of a model swimmer propelled by rotating a single helical flagellum. Building on previous simulation results for swimmers near a single plane boundary, we introduce a second, parallel boundary and show that the bacterial trajectories change as the two plates are brought closer together.

Self-assembly of microcapsules regulated via the repressilator signaling network.

One of the intriguing challenges in designing active matter is devising systems that not only self-organize, but also exhibit self-regulation. Inspired by biological regulatory networks, we design a collection of self-organizing, self-regulating microcapsules that move in response to self-generated chemical signals. Three microcapsules act as localized sources of distinct chemicals that diffuse through surrounding fluid.

Designing Bioinspired Artificial Cilia to Regulate Particle-Surface Interactions.

Biological cilia play a critical role in a stunning array of vital functions, from enabling marine organisms to trap food and expel fouling agents to facilitating the effective transport of egg cells in mammals. Inspired by the performance of these microscopic, hair-like filaments, researchers are synthesizing artificial cilia for use in lab-on-a-chip devices. There have, however, been few attempts to harness the artificial cilia to regulate the movement of particulates in these devices.

Fluid-driven motion of passive cilia enables the layer to expel sticky particles.

Inspired by marine organisms that utilize active cilia to prevent the biofouling of their surfaces, we use computational and theoretical modeling to determine if passive cilia, which are driven to undulate by an oscillatory shear flow, can be harnessed for antifouling applications. By modeling the oscillating shear flow near a ciliated wall within a channel, we show that the fluid-driven motion of cilia enables the layer to repel adhesive particles away from the surface. Compared to the behavior of the system in a non-oscillating shear, the oscillations also help transport the particles more rapidly along the flow direction.

Active ciliated surfaces expel model swimmers.

Continually moving cilia on the surface of marine organisms provide a natural defense against biofouling. To probe the physical mechanisms underlying this antifouling behavior, we integrate the lattice Boltzmann and immersed boundary methods and undertake the first computational studies of the interactions between actuated, biomimetic cilia and a model swimmer. We find that swimmers are effectively "knocked away" from the ciliated surface through a combination of steric repulsion and locally fluctuating flows.