Henry Shum

Fluid transport by individual microswimmers

We discuss the path of a tracer particle as a microswimmer moves past on an infinite straight trajectory. If the tracer is sufficiently far from the path of the swimmer it moves in a closed loop. As the initial distance between the tracer and the path of the swimmer

Convective flow reversal in self-powered enzyme micropumps

\rho

Harnessing catalytic pumps for directional delivery of microparticles in microchambers

decreases, the tracer is displaced a small distance backwards (relative to the direction of the swimmer velocity). For much smaller tracer-swimmer separations, however, the tracer displacement becomes positive and diverges as

Designing Bioinspired Artificial Cilia to Regulate Particle-Surface Interactions.

\rho \to 0

Harnessing surface-bound enzymatic reactions to organize microcapsules in solution

. To quantify this behaviour we calculate the Darwin drift, the total volume swept out by a material sheet of tracers, initially perpendicular to the swimmer path, during the swimmer motion. We find that the drift can be written as the sum of a {\em universal} term which depends on the quadrupolar flow field of the swimmer, together with a non-universal contribution given by the sum of the volumes of the swimmer and its wake. The formula is compared to exact results for the squirmer model and to numerical calculations for a more realistic model swimmer.

Entrainment and scattering in microswimmer-colloid interactions

Significance Surface-bound enzymes act as pumps in the presence of their specific substrates or promoters, thereby combining sensing and fluidic pumping into a single self-powered microdevice. Using a combination of theory and experiments, we have elucidated the mechanism of the prototypical urease-based pump. We find that even simple enzymatic reactions can drive complex, time-dependent flows whose direction and speed depend critically on the relative diffusivities and expansion coefficients of the reactants and products. Our approach allows us to accurately predict the behavior of new pump designs under different conditions. 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. Herein, we combine theory and experiments to demonstrate a previously unreported spatiotemporal variation in pumping behavior in urease-based pumps and uncover the mechanisms behind these dynamics. We developed a theoretical model for the transduction of chemical energy into mechanical fluid flow in these systems, capturing buoyancy effects due to the solution containing nonuniform concentrations of substrate and product. We find that the qualitative features of the flow depend on the ratios of diffusivities δ=DP/DS and expansion coefficients β=βP/βS of the reaction substrate (S) and product (P). If δ>1 and δ>β (or if δ<1 and δ<β), an unexpected phenomenon arises: the flow direction reverses with time and distance from the pump. Our experimental results are in qualitative agreement with the model and show that both the speed and direction of fluid pumping (i) depend on the enzyme activity and coverage, (ii) vary with the distance from the pump, and (iii) evolve with time. These findings permit the rational design of enzymatic pumps that accurately control the direction and speed of fluid flow without external power sources, enabling effective, self-powered fluidic devices.

Synthetic quorum sensing in model microcapsule colonies

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. Here reagents diffuse from a gel reservoir and react with the catalyst-coated surface. Fluid density gradients due to the spatially varying reagent concentration induce a convective flow, which carries the suspended particles until the reagents are consumed. Consequently, the cargo is deposited around a specific position on the surface. The velocity and final peak location of the cargo can be tuned independently. By increasing the local particle concentration, highly sensitive assays can be performed efficiently and rapidly. Moreover, the process can be repeated by introducing fresh reagent into the microchamber.

Self-Propelled Nanomotors Autonomously Seek and Repair Cracks

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. Here, we review recent computational studies on the interactions between actuated artificial cilia and microscopic particles, showing that these cilia are effective at transporting both rigid and deformable particles in microchannels. The findings also reveal that these beating filaments can be used to separate microparticles based on their size and stiffness. Importantly, these studies indicate that artificial cilia can be used to prevent fouling by a wide variety of agents because they can expel both passive particulates and active swimmers from the underlying surface. These results can help guide experimental efforts to fully exploit artificial cilia in controlling particle motion within fluid environments.

An introduction to the hydrodynamics of swimming microorganisms

A model developed for convective aggregation of microcapsules predicts a mechanism for the spontaneous assembly of protocells. 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. The ensuing reaction generates fluid density variations, which result in fluid flows. These flows carry the suspended microcapsules and drive them to aggregate into “colonies” on and near the enzyme-covered sites. This aggregation continues until the reagent has been depleted and the convection stops. We show that the shape of the assembled colonies can be tailored by patterning the distribution of enzymes on the surface. This fundamental physicochemical mechanism could have played a role in the self-organization of early biological cells (protocells) and can be used to regulate the autonomous motion and targeted delivery of microcarriers in microfluidic devices.

Self-assembly of microcapsules regulated via the repressilator signaling network

We use boundary element simulations to study the interaction of model microswimmers with a neutrally buoyant spherical particle. The ratio of the size of the particle to that of the swimmer is varied from