Phoresis kernel theory for passive and active spheres with nonuniform phoretic mobility.

By introducing geometry-based phoresis kernels, we establish a direct connection between the translational and rotational velocities of a phoretic sphere and the distributions of the driving fields or fluxes. The kernels quantify the local contribution of the field or flux to the particle dynamics. The field kernels for both passive and active particles share the same functional form, depending on the position-dependent surface phoretic mobility.

Nonstoichiometric Salt Intercalation as a Means to Stabilize Alkali Doping of 2D Materials.

Although doping with alkali atoms is a powerful technique for introducing charge carriers into physical systems, the resulting charge-transfer systems are generally not air stable. Here we describe computationally a strategy towards increasing the stability of alkali-doped materials that employs stoichiometrically unbalanced salt crystals with excess cations (which could be deposited during, e.g.

Modulating drift dynamics of circle swimmers by periodic potentials.

We propose a method to modulate the drifting motion of overdamped circle swimmers in steady fluid flows by means of static sinusoidal potentials. Using Langevin formalism, we study drift velocity as a function of potential strength and wavelength with and without diffusional motion. Drift velocity is essentially quantized without diffusion, but in the presence of noise, the displacement per cycle has a continuous range.

Mutual information and breakdown of the Perron-Frobenius scenario in zero-temperature triangular Ising antiferromagnets on cylinders.

A nominally two-dimensional spin model wrapped onto a cylinder can profitably be viewed, especially for long cylinders, as a one-dimensional chain. Each site of such a chain is a ring of spins with a complex state space. Traditional correlation functions are inadequate for the study of correlations in such a system and need to be replaced with something like mutual information.

Memory effects in spiral diffusion of rotary self-propellers.

The coupling of deterministic rotary motion and stochastic orientational diffusion of a self-propeller leads to a spiral trajectory of the expected displacement. We extend our former analysis of spiral diffusion [Phys. Rev.

Spontaneous directional flow of active magnetic particles.

We predict the emergence of large-scale polar order and spontaneous directional flows in a class of self-propelled autonomous particles that interact via passive repulsion between off-center sites. The coupling of active motion with the passive torque acting about the particle centers results in hybrid active-passive interactions responsible for a macroscopic phase transition from an isotropic state to a polar-aligned state in systems of particles with front interaction sites. We employ a continuum kinetic theory to explain that the emergence of long-ranged orientational order, which occurs in unbounded domains at finite densities, can be externally activated independently of the self-propulsion mechanism and drives a macroscopic particle flow in a direction selected by symmetry breaking.

Swimmers Heal on the Move Following Catastrophic Damage.

Herein, we describe the development of 2D self-healing small-scale swimmers capable of autonomous propulsion and "on-the-fly" structural recovery in large containers. Incorporation of magnetic NdFeB microparticles in specialized printed strips results in rapid reorientation and reattachment of the moving tail to its complementary broken static piece to restore the original swimmer structure and propulsion behavior. The swimmers display functional recovery independent of user input.

Smart Materials for Microrobots.

Over the past 15 years, the field of microrobotics has exploded with many research groups from around the globe contributing to numerous innovations that have led to exciting new capabilities and important applications, ranging from drug delivery, to intracellular biosensing, environmental remediation, and nanoscale fabrication. Smart responsive materials have had a profound impact on the field of microrobotics and have imparted small-scale robots with new functionalities and distinct capabilities. We have identified four large categories where the majority of future efforts must be allocated to push the frontiers of microrobots and where smart materials can have a major impact on such future advances.

Multigear Bubble Propulsion of Transient Micromotors.

Transient, chemically powered micromotors are promising biocompatible engines for microrobots. We propose a framework to investigate in detail the dynamics and the underlying mechanisms of bubble propulsion for transient chemically powered micromotors. Our observations on the variations of the micromotor active material and geometry over its lifetime, from initial activation to the final inactive state, indicate different bubble growth and ejection mechanisms that occur stochastically, resulting in time-varying micromotor velocity.

Built-In Active Microneedle Patch with Enhanced Autonomous Drug Delivery.

The use of microneedles has facilitated the painless localized delivery of drugs across the skin. However, their efficacy has been limited by slow diffusion of molecules and often requires external triggers. Herein, an autonomous and degradable, active microneedle delivery platform is introduced, employing magnesium microparticles loaded within the microneedle patch, as the built-in engine for deeper and faster intradermal payload delivery.

A Macrophage-Magnesium Hybrid Biomotor: Fabrication and Characterization.

Magnesium (Mg)-based micromotors are combined with live macrophage (MΦ) cells to create a unique MΦ-Mg biohybrid motor system. The resulting biomotors possess rapid propulsion ability stemming from the Mg micromotors and the biological functions provided by the live MΦ cell. To prepare the biohybrid motors, Mg microparticles coated with titanium dioxide and poly(l-lysine) (PLL) layers are incubated with live MΦs at low temperature.

Engineering Contactless Particle-Particle Interactions in Active Microswimmers.

Artificial self-propelled colloidal particles have recently served as effective building blocks for investigating many dynamic behaviors exhibited by nonequilibrium systems. However, most studies have relied upon excluded volume interactions between the active particles. Experimental systems in which the mobile entities interact over long distances in a well-defined and controllable manner are valuable so that new modes of multiparticle dynamics can be studied systematically in the laboratory.

Dynamic stabilization of Janus sphere trans-dimers.

We experimentally investigated the self-assembly of chemically active colloidal Janus spheres into dimers. The trans-dimer conformation, in which the two active sites are oriented roughly in opposite directions and the particles are osculated at their equators, becomes dominant as the hydrogen peroxide fuel concentration increases. Our observations suggest high spinning frequency combined with little translational motion is at least partially responsible for the stabilization of the trans-dimer as activity increases.

Acoustic actuation of bioinspired microswimmers.

Acoustic actuation of bioinspired microswimmers is experimentally demonstrated. Microswimmers are fabricated in situ in a microchannel. Upon acoustic excitation, the flagellum of the microswimmer oscillates, which in turn generates linear or rotary movement depending on the swimmer design.

Spiral diffusion of rotating self-propellers with stochastic perturbation.

Translationally diffusive behavior arising from the combination of orientational diffusion and powered motion at microscopic scales is a known phenomenon, but the peculiarities of the evolution of expected position conditioned on initial position and orientation have been neglected. A theory is given of the spiral motion of the mean trajectory depending upon propulsion speed, angular velocity, orientational diffusion, and rate of random chirality reversal. We demonstrate the experimental accessibility of this effect using both tadpole-like and Janus sphere dimer rotating motors.

Acoustofluidic actuation of in situ fabricated microrotors.

We have demonstrated in situ fabricated and acoustically actuated microrotors. A polymeric microrotor with predefined oscillating sharp-edge structures is fabricated in situ by applying a patterned UV light to polymerize a photocrosslinkable polyethylene glycol solution inside a microchannel around a polydimethylsiloxane axle. To actuate the microrotors by oscillating the sharp-edge structures, we employed piezoelectric transducers which generate tunable acoustic waves.

Geometrical Performance of Self-Phoretic Colloids and Microswimmers.

Within a unified formulation-encompassing self-electrophoresis, self-diffusiophoresis, and self-thermophoresis-we provide a simple integral kernel transforming the relevant surface flux to particle velocity for any spheroid with axisymmetric surface activity and uniform phoretic mobility. Appropriate scaling of the speed allows a dimensionless measure of the motion-producing performance of the motor shape and activity distribution across the surface. For bipartite designs with piecewise uniform flux over complementary surface regions, the performance is mapped out over the entire range of geometry (from discotic through spherical to rodlike shapes) and of bipartitioning, and intermediate aspect ratios that maximize performance are identified.

Guiding chiral self-propellers in a periodic potential.

Ingenious suggestions continue to be made for separation of racemic mixtures according to the inert structural chirality of the constituents. Recently discovered self-motile micro- or nanoparticles express dynamical chirality, i.e.

Self-consistent nonlocal feedback theory for electrocatalytic swimmers with heterogeneous surface chemical kinetics.

We present a self-consistent nonlocal feedback theory for the phoretic propulsion mechanisms of electrocatalytic micromotors or nanomotors. These swimmers, such as bimetallic platinum and gold rods catalyzing decomposition of hydrogen peroxide in aqueous solution, have received considerable theoretical attention. In contrast, the heterogeneous electrochemical processes with nonlocal feedback that are the actual "engines" of such motors are relatively neglected.

Selectively manipulable acoustic-powered microswimmers.

Selective actuation of a single microswimmer from within a diverse group would be a first step toward collaborative guided action by a group of swimmers. Here we describe a new class of microswimmer that accomplishes this goal. Our swimmer design overcomes the commonly-held design paradigm that microswimmers must use non-reciprocal motion to achieve propulsion; instead, the swimmer is propelled by oscillatory motion of an air bubble trapped within the swimmer's polymer body.

Gaussian memory in kinematic matrix theory for self-propellers.

We extend the kinematic matrix ("kinematrix") formalism [Phys. Rev. E 89, 062304 (2014)], which via simple matrix algebra accesses ensemble properties of self-propellers influenced by uncorrelated noise, to treat Gaussian correlated noises.

Kinematic matrix theory and universalities in self-propellers and active swimmers.

We describe an efficient and parsimonious matrix-based theory for studying the ensemble behavior of self-propellers and active swimmers, such as nanomotors or motile bacteria, that are typically studied by differential-equation-based Langevin or Fokker-Planck formalisms. The kinematic effects for elementary processes of motion are incorporated into a matrix, called the "kinematrix," from which we immediately obtain correlators and the mean and variance of angular and position variables (and thus effective diffusivity) by simple matrix algebra. The kinematrix formalism enables us recast the behaviors of a diverse range of self-propellers into a unified form, revealing universalities in their ensemble behavior in terms of new emergent time scales.

Nanomotor mechanisms and motive force distributions from nanorotor trajectories.

Nanomotors convert chemical energy into mechanical motion. For a given motor type, the underlying chemical reaction that enables motility is typically well known, but the detailed, quantitative mechanism by which this reaction breaks symmetry and converts chemical energy to mechanical motion is often less clear, since it is difficult experimentally to measure important parameters such as the spatial distribution of chemical species around the nanorotor during operation. Without this information on how motor geometry affects motor function, it is difficult to control and optimize nanomotor behavior.

Glass-like dynamics in the cell and in cellular collectives.

Prominent fluctuations, heterogeneity, and cooperativity dominate the dynamics of the cytoskeleton as well as the dynamics of the cellular collective. Such systems are out of equilibrium, disordered, and remain poorly understood. To explain these findings, we consider a unifying mechanistic rubric that imagines these systems as comprising phases of soft condensed matter in proximity to a glass or jamming transition, with associated transitions between solid-like versus liquid-like phases.

Chiral diffusion of rotary nanomotors.

Neither a purely deterministic rotary nanomotor nor a purely orientational diffuser exhibits long-term translational motion, but coupling rotation to orientational diffusion yields translational diffusion. We demonstrate that this effective translational diffusion can easily dominate the ordinary thermal translational diffusion for experimentally relevant nanomotors, and that this effective diffusion is chiral. Unpowered chiral particles do not exhibit chiral diffusion, but a nanorotor has both handedness and an instantaneous direction of powered motion, thus-unlike an unpowered particle-its diffusional motion can distinguish left from right.