Phoretic Flows Research Articles

Long-range hydrodynamic communication among synthetic self-propelled micromotors

Long-range communication is ubiquitous among living organisms, but it remains undiscovered among synthetic micro/nanomotors. Here, we report the long-range hydrodynamic communication among synthetic micromotors. With the high activities when powered by a mixed fuel of N2H4 and H2O2, catalytic micromotors move in a stable oblique head-down orientation and thus generate strong, steady converging phoretic flows near a substrate. Consequently, they can emit robust hydrodynamic signals to trigger neighboring motors within a long range and also initiate an “approach-hit-and-run” response when sensing the signals emitted from others. This hydrodynamic communication can be reciprocal or one-way, intraspecific or interspecific, spontaneous or controllable, and it widely exists in various catalytic micromotor systems. Utilizing the hydrodynamic signal, the micromotor can also act as a leader to gather small particles (followers) and exhibit a biomimetic competitive behavior against others for the followers. This work may facilitate the fabrication of intelligent micro/nanorobots for micromanipulations and biomimetic micro/nanorobotic systems.

Light-Programmable Assemblies of Isotropic Micromotors.

“Life-like” nonequilibrium assemblies are of increasing significance, but suffering from limited steerability as they are generally based on micro/nanomotors with inherent asymmetry in chemical composition or geometry, of which the vigorous random Brownian rotations disturb the local interactions. Here, we demonstrate that isotropic photocatalytic micromotors, due to the persistent phoretic flow from the illuminated to shadowed side irrespective of their Brownian rotations, experience light-programmable local interactions (reversibly from attraction to repulsion and/or alignment) depending on the direction of the incident lights. Thus, they can be organized into a variety of tunable nonequilibrium assemblies, such as apolar solids (i.e., immobile colloidal crystal), polar liquids (i.e., phototactic colloidal stream), and polar solids (i.e., phototactic colloidal crystal), which can further be “cut” into a predesigned pattern by utilizing the switching motor-motor interactions at superimposed-light edges. This work facilitates the development of active matters and motile functional microdevices.

Phoretic self-propulsion of helical active particles

Chemically active colloids self-propel by catalysing the decomposition of molecular ‘fuel’ available in the surrounding solution. If the various molecular species involved in the reaction have distinct interactions with the colloid surface, and if the colloid has some intrinsic asymmetry in its surface chemistry or geometry, there will be phoretic flows in an interfacial layer surrounding the particle, leading to directed motion. Most studies of chemically active colloids have focused on spherical, axisymmetric ‘Janus’ particles, which (in the bulk, and in absence of fluctuations) simply move in a straight line. For particles with a complex (non-spherical and non-axisymmetric) geometry, the dynamics can be much richer. Here, we consider chemically active helices. Via numerical calculations and slender body theory, we study how the translational and rotational velocities of the particle depend on geometry and the distribution of catalytic activity over the particle surface. We confirm the recent finding of Katsamba et al. (J. Fluid Mech., vol. 898, 2020, p. A24) that both tangential and circumferential concentration gradients contribute to the particle velocity. The relative importance of these contributions has a strong impact on the motion of the particle. We show that, by a judicious choice of the particle design parameters, one can suppress components of angular velocity that are perpendicular to the screw axis, or even select for purely ‘sideways’ translation of the helix.

Active and Passive Motion in Complex pH‐Fields

AbstractWe create pH landscapes of increasing spatial complexity by placing ion exchange resin beads of different types and sizes in a closed cell featuring a negatively charged bottom substrate. We thus extend previous measurements in a chemically homogeneous background to investigations in chemically structured backgrounds, which evolve in time. We determine local pH gradients by photometry and study the resulting solvent flows along the substrate by tracking passive tracer particles. Analytical modelling of the dispersion of chemicals is possible for situations with dominantly diffusive transport. We then release phoretic swimmers in differently shaped pH fields and study their motion for selected examples. Catalytic Janus swimmers assembled in the wake of a mobile pH source form a modular swimmer showing a noise‐dominated trajectory. For standard modular swimmers with passive cargo, we identify well‐defined and reproducible swimming trajectory types at and around single pH sources. These include swimmer deflection and swing‐by maneuvers, swimmer trapping and escape, as well as circular orbits. More complicated paths result from combining the pH fields of multiple pH sources. Finally, we address swimmer‐swimmer interactions resulting from the superposition of their own pH fields. Our findings clearly demonstrate the high potential of chemically structured environments for swimmer steering. They can further be rationalized in a simple heuristic model considering the interplay of phoretic flows on different length and time scales and its influence on swimmer speed, orientation and emerging propulsion direction. In view of the vast range of possible combinations, our study has to remain preliminary. We anticipate, however, that it will aid the general understanding of transport experiments in pH‐driven systems and other types of phoresis and thus also help design novel and useful strategies for directed transport on the micro‐scale.

Spontaneous polarization and locomotion of an active particle with surface-mobile enzymes

We examine a mechanism of locomotion of active particles whose surface is uniformly coated with mobile enzymes. The enzymes catalyze a reaction that drives phoretic flows but their homogeneous distribution forbids locomotion by symmetry. We find that the ability of the enzymes to migrate over the surface combined with self-phoresis can lead to a spontaneous symmetry breaking instability whereby the homogeneous distribution of enzymes polarizes and the particle propels. The instability is driven by the advection of enzymes by the phoretic flows and occurs above a critical P\'eclet number. The transition to polarized motile states occurs via a supercritical or subcritical pitchfork bifurcations, the latter of which enables hysteresis and coexistence of uniform and polarized states.

Universal optimal geometry of minimal phoretic pumps

Unlike pressure-driven flows, surface-mediated phoretic flows provide efficient means to drive fluid motion on very small scales. Colloidal particles covered with chemically-active patches with nonzero phoretic mobility (e.g. Janus particles) swim using self-generated gradients, and similar physics can be exploited to create phoretic pumps. Here we analyse in detail the design principles of phoretic pumps and show that for a minimal phoretic pump, consisting of 3 distinct chemical patches, the optimal arrangement of the patches maximizing the flow rate is universal and independent of chemistry.

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.

Assembly and Transport of Microscopic Cargos via Reconfigurable Photoactivated Magnetic Microdockers.

The realization of micromotors able to dock and transport microscopic objects in a fluid medium has direct applications toward the delivery of drugs and chemicals in small channels and pores, and the realization of functional wireless microrobots in lab-on-a-chip technology. A simple and general method to tow microscopic particles in water by using remotely controllable light-activated hematite microdockers is demonstrated. These anisotropic ferromagnetic particles can be synthesized in bulk and present the remarkable ability to be activated by light while independently manipulated via external fields. The photoactivation process induces a phoretic flow capable to attract cargos toward the surface of the propellers, while a rotating magnetic field is used to transport the composite particles to any location of the experimental platform. The method allows the assembling of small colloidal clusters of various sizes, composed by a skeleton of mobile magnetic dockers, which cooperatively keep, transport, and release the microscopic cargos. The possibility to easily reconfigure in situ the location of the docker above the cargo is demonstrated, which enables optimize transport and cargo release operations.

Can phoretic particles swim in two dimensions?

Artificial phoretic particles swim using self-generated gradients in chemical species (self-diffusiophoresis) or charges and currents (self-electrophoresis). These particles can be used to study the physics of collective motion in active matter and might have promising applications in bioengineering. In the case of self-diffusiophoresis, the classical physical model relies on a steady solution of the diffusion equation, from which chemical gradients, phoretic flows, and ultimately the swimming velocity may be derived. Motivated by disk-shaped particles in thin films and under confinement, we examine the extension to two dimensions. Because the two-dimensional diffusion equation lacks a steady state with the correct boundary conditions, Laplace transforms must be used to study the long-time behavior of the problem and determine the swimming velocity. For fixed chemical fluxes on the particle surface, we find that the swimming velocity ultimately always decays logarithmically in time. In the case of finite Péclet numbers, we solve the full advection-diffusion equation numerically and show that this decay can be avoided by the particle moving to regions of unconsumed reactant. Finite advection thus regularizes the two-dimensional phoretic problem.

Phoretic flow induced by asymmetric confinement

Internal phoretic flows due to the interactions of solid boundaries with local chemical gradients may be created using chemical patterning. Alternatively, we demonstrate here that internal flows might also be induced by geometric asymmetries of chemically homogeneous surfaces. We characterise the circulatory flow created in a cavity enclosed between two eccentric cylindrical walls of uniform chemical activity. Local gradients of the diffusing solute induce a slip flow along the surface of the cylinders, leading to a circulatory bulk flow pattern which can be solved analytically in the diffusive limit. The flow strength can be controlled by adjusting the relative positions of the cylinders, and an optimal configuration is identified. These results provide a model system for tunable phoretic pumps.

Geometric pumping in autophoretic channels

Many microfluidic devices use macroscopic pressure differentials to overcome viscous friction and generate flows in microchannels. In this work, we investigate how the chemical and geometric properties of the channel walls can drive a net flow by exploiting the autophoretic slip flows induced along active walls by local concentration gradients of a solute species. We show that chemical patterning of the wall is not required to generate and control a net flux within the channel, rather channel geometry alone is sufficient. Using numerical simulations, we determine how geometric characteristics of the wall influence channel flow rate, and confirm our results analytically in the asymptotic limit of lubrication theory.