Active Colloids Research Articles

Motility-induced clustering and meso-scale turbulence in active polar fluids

Meso-scale turbulence was originally observed experimentally in various suspensions of swimming bacteria, as well as in the collective motion of active colloids. The corresponding large scale dynamical patterns were reproduced in a simple model of a polar fluid, assuming a constant density of active particles. Recent, more detailed studies in a variety of experimental realizations of active polar fluids revealed additional interesting aspects, such as anomalous velocity statistics and clustering phenomena. Those phenomena cannot be explained by currently available models for active polar fluids. Herein, we extend the continuum model suggested by Dunkel et al to include density variations and a local feedback between the local density and self-propulsion speed of the active polar particles. If the velocity decreases strong enough with the density, a linear stability analysis of the resulting model shows that, in addition to the short-wavelength instability of the original model, a long-wavelength instability occurs. This is typically observed for high densities of polar active particles and is analogous to the well-known phenomenon of motility-induced phase separation (MIPS) in scalar active matter. We determine a simple phase diagram indicating the linear instabilities and perform systematic numerical simulations for the various regions in the corresponding parameter space. The interplay between the well understood short-range instability (leading to meso-scale turbulence) and the long-range instability (associated with MIPS) leads to interesting dynamics and novel phenomena concerning nucleation and coarsening processes. Our simulation results display a rich variety of novel patterns, including phase separation into domains with dynamically changing irregularly shaped boundaries. Anomalous velocity statistics are observed in all phases where the system segregates into regions of high and low densities. This offers a simple explanation for their occurrence in recent experiments with bacterial suspensions.

Chemokinesis-driven accumulation of active colloids in low-mobility regions of fuel gradients

Many motile cells exhibit migratory behaviors, such as chemotaxis (motion up or down a chemical gradient) or chemokinesis (dependence of speed on chemical concentration), which enable them to carry out vital functions including immune response, egg fertilization, and predator evasion. These have inspired researchers to develop self-propelled colloidal analogues to biological microswimmers, known as active colloids, that perform similar feats. Here, we study the behavior of half-platinum half-gold (Pt/Au) self-propelled rods in antiparallel gradients of hydrogen peroxide fuel and salt, which tend to increase and decrease the rods’ speed, respectively. Brownian Dynamics simulations, a Fokker–Planck theoretical model, and experiments demonstrate that, at steady state, the rods accumulate in low-speed (salt-rich, peroxide-poor) regions not because of chemotaxis, but because of chemokinesis. Chemokinesis is distinct from chemotaxis in that no directional sensing or reorientation capabilities are required. The agreement between simulations, model, and experiments bolsters the role of chemokinesis in this system. This work suggests a novel strategy of exploiting chemokinesis to effect accumulation of motile colloids in desired areas.

Autonomous materials systems from active liquid crystals

Liquid crystals (LCs) are ubiquitous in display technologies. The orientational ordering in the nematic phase of LCs gives rise to structural anisotropy, the ability to form topological defects and extraordinary sensitivity to interfacial events — three features that are not found in traditional, isotropic liquids. Active LC systems represent a particular class of active materials, in which some form of energy is transformed and used to generate motion, providing opportunities for new technologies and offering a platform to investigate matter far from equilibrium. In this Review, we discuss recent advances in the field of active LC systems, including natural systems, such as cell colonies, biopolymers and bacteria, and synthetic systems, which mimic the adaptive and autonomous behaviours found in nature. We investigate the properties of defects and flows, examine LC interfaces and emulsions, and discuss active LC colloids and composites. Finally, we highlight future possibilities and applications of active LC systems. Active liquid crystals are a particular type of active materials in which an inherent structural anisotropy leads to long-range order and topological defects. This Review discusses natural and synthetic active liquid crystals, and explores their autonomous behaviour as well as potential applications.

Computer‐Free Autonomous Navigation and Power Generation Using Electro‐Chemotaxis

Computer‐free autonomous decision making based on environmental cues provides exciting alternatives to classic control systems for robots and smart materials. Although this functionality has been studied in microswimmers and active colloids where energy in the surrounding liquid is prevalent, there are no devices that can provide sufficient power from environmental chemicals to move and steer larger scale robots and vehicles in dry environments. This work overcomes this limitation with an environmentally controlled voltage source (ECVS) that, when directly attached to electric motors on a vehicle, can increase the energy available to the vehicle and provide computer‐free autonomous navigation toward chemical fuels in the environment and away from hazards. The ECVS uses electrochemistry to extract power from the chemical fuels, and the vehicle avoids hazards that reduce the output voltage or electrochemical kinetics. Two ECVSs can also be arranged in series or parallel to perform logical functions based on the chemicals in contact with the ECVSs. This work presents a new method to simultaneously steer and power vehicles and robots without computers by directly responding to a wide variety of chemical fields in their environment using electrochemistry.

Self‐assembled Micro‐nanorobots: From Assembly Mechanisms to Applications

AbstractDue to the extremely tiny size, micro‐nanorobots hold promise in performing tasks at the micro‐nanoscale, which is impossible for macroscale robots. However, the massive fabrication of micro‐nanorobots is of crucial challenge and has been attracting extensive attention among academics. Active colloids can be self‐assembled into ordered structures, and offer a promising solution to this largescale manufacture challenge. Here, we summarize the development and mechanisms of different assembly technologies in the past two decades, and introduce the applications of micro‐nanorobots using self‐assembly strategies in biomedical and physical fields. For clarification, the self‐assembly mechanisms are presented in three categories: chemical, physical and biological. The manufacture of micro‐nanorobots through self‐assembly not only solves the problem of large‐scale manufacturing, but also increases the reconfigurability of structure and improves their adaptability to the environment, which promises further development as microsurgeons and the application of micro‐nanorobots in other fields, such as environmental restoration and micro‐manipulation.

Scalable Chemical Synthesis Route to Manufacture pH-Responsive Janus CaCO3 Micromotors.

A cost-effective scalable chemical route to produce pH-responsive active colloids (ACs) is developed here. For the first time, calcium carbonate particles are half-coated with a silica layer via Pickering emulsion methodology. This methodology allows to create anisotropy on the particles' surfaces and benefit from the decomposition of the calcium carbonate in acidic media to generate self-propulsion. The coupling between the self-diffusiophoretic motion of these ACs and acid concentrations is experimentally investigated in Newtonian media via optical microscopy. With increasing hydrogen-ion concentrations, the pH-responsive colloids experience higher mean-square displacements because of self-propulsion velocities and enhanced long-time diffusivities. Because they are biocompatible and environmentally friendly, these ACs constitute a platform for advanced diagnostics, targeted drug delivery, and water/soil remediation.

Molecular theory of Langevin dynamics for active self-diffusiophoretic colloids.

Active colloidal particles that are propelled by a self-diffusiophoretic mechanism are often described by Langevin equations that are either postulated on physical grounds or derived using the methods of fluctuating hydrodynamics. While these descriptions are appropriate for colloids of micrometric and larger size, they will break down for very small active particles. A fully microscopic derivation of Langevin equations for self-diffusiophoretic particles powered by chemical reactions catalyzed asymmetrically by the colloid is given in this paper. The derivation provides microscopic expressions for the translational and rotational friction tensors, as well as reaction rate coefficients appearing in the Langevin equations. The diffusiophoretic force and torque are expressed in terms of nonequilibrium averages of fluid fields that satisfy generalized transport equations. The results provide a description of active motion on small scales where descriptions in terms of coarse grained continuum fluid equations combined with boundary conditions that account for the presence of the colloid may not be appropriate.

Competing active and passive interactions drive amoebalike crystallites and ordered bands in active colloids

Swimmers and self-propelled particles are physical models for the collective behavior and motility of a wide variety of living systems, such as bacteria colonies, bird flocks, and fish schools. Such artificial active materials are amenable to physical models which reveal the microscopic mechanisms underlying the collective behavior. Here we study colloids in a dc electric field. Our quasi-two-dimensional system of electrically driven particles exhibits a rich and exotic phase behavior exhibiting passive crystallites, motile crystallites, an active gas, and banding. Amongst these are two mesophases, reminiscent of systems with competing interactions. At low field strengths activity suppresses demixing, leading to motile crystallites. Meanwhile, at high field strengths, activity drives partial demixing to traveling bands. We parametrize a particulate simulation model which reproduces the experimentally observed phases.

Colloidal monolayers with cell-like tessellations via interface assisted evaporative assembly

Evaporating sessile drops containing surface active colloids is a promising route to self-assemble two-dimensional nanostructures. The standard protocol is to first self-assemble surface active nanoscale particles at the water-vapour interface and subsequently transfer it on to a solid surface. Colloidal monolayers with very few morphologies have been fabricated, exploiting this bottom-up self-assembly technique. However, the evaporation kinetics under controlled humidity conditions may dramatically alter the microstructure of self-assembled colloidal monolayers at the liquid-vapor interface and that on the solid surfaces, an aspect that has not been fully addressed in the prior studies. To this end, we present an experimental study of evaporation driven self-assembly of soft poly(N-isopropylacrylamide) (pNIPAM) microgel particles loaded in a sessile drop. The surface-active microgel particles spontaneously populate the water-vapour interface facilitating the suppression of the coffee-ring effect and the formation of monolayer stains. The role of evaporation kinetics under controlled humidity conditions on the colloid's microstructure adsorbed to the solvent-air interface and on the morphology of the colloidal monolayer transferred onto the solid surface are studied in detail. The formation of particle-free and particle-rich regions at the water-vapor interface is observed for sessile drops evaporated under saturated humidity conditions. We show that the evaporation induced shrinkage of the interface area and the enhancement of the areal density of microgel particles adsorbed onto the interface leads to a restructuring of the particle-laden interface. The rearrangement of microgel particles along the water-vapor interface resembling the de-wetting assisted patterns is transferred to the solid substrate upon complete evaporation of the solvent. The microgel particles in the deposit assemble into domains with enhanced crystalline order. The evolution of Voronoi entropy across the monolayer deposit patterns obtained by the standard and slow evaporation routes are presented.

Self-Propelled Janus Colloids in Shear Flow.

To fully harness the potential of artificial active colloids, investigation of their response to various external stimuli including external flow is of great interest. Therefore, in this study, we perform experiments on SiO2-Pt Janus particles suspended in an aqueous medium in a capillary subjected to different shear flow rates. Particles were propelled using varied H2O2 (fuel) concentrations. For a particular propulsion speed, with increasing shear flow, a continuous transition in the motion of active Janus particles (JPs) from the usual random active motion to preferential movement along the vorticity direction and then finally to migration along the flow was observed. This transition was accompanied by a significant decline in in-plane fluctuations of the particle trajectories. Another key observation is that the activity of JPs produces a delay in shear-induced rolling, which at moderate flow, allows the JPs to adopt a specific orientation, facilitating their migration along the vorticity direction. At higher flow rates, once shear flow overcomes the activity-induced resistance and initiates rolling, the probability of JPs adopting such preferred orientations reduces. Our analysis further revealed that these transitions are governed by a nondimensional quantity λ, which compares the relative strength of the shear-induced particle flow to the propulsion speed.

Janus Particles: Inverse Solidification Induced by Active Janus Particles (Adv. Funct. Mater. 39/2020)

In article number 2003851, Denys Makarov, Larysa Baraban, and co-workers discuss inverse solidification driven by active colloids, which provides novel insight into the collective effects in mixed colloidal systems comprising of active and passive species. It offers versatile possibilities to study the processes of solidification in various systems out of equilibrium, including the formation of biomolecular condensates or biomineralization, amorphous to polycrystalline transitions, or synthesis of materials under extreme conditions.

Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water–air interface

We investigate the radial thermocapillary flow driven by a laser-heated microbead in partial wetting at the water–air interface. Particular attention is paid to the evolution of the convective flow patterns surrounding the hot sphere as the latter is increasingly heated. The flow morphology is nearly axisymmetric at low laser power (P). Increasing P leads to symmetry breaking with the onset of counter-rotating vortex pairs. The boundary condition at the interface, close to no-slip in the low-P regime, turns about stress-free between the vortex pairs in the high-P regime. These observations strongly support the view that surface-active impurities are inevitably adsorbed on the water surface where they form an elastic layer. The onset of vortex pairs is the signature of a hydrodynamic instability in the layer response to the centrifugal forced flow. Interestingly, our study paves the way for the design of active colloids capable of achieving high-speed self-propulsion via vortex pair generation at a liquid interface.

Universal reshaping of arrested colloidal gels via active doping.

Colloids that interact via a short-range attraction serve as the primary building blocks for a broad range of self-assembled materials. However, one of the well-known drawbacks to this strategy is that these building blocks rapidly and readily condense into a metastable colloidal gel. Using computer simulations, we illustrate how the addition of a small fraction of purely repulsive self-propelled colloids, a technique referred to as active doping, can prevent the formation of this metastable gel state and drive the system toward its thermodynamically favored crystalline target structure. The simplicity and robust nature of this strategy offers a systematic and generic pathway to improving the self-assembly of a large number of complex colloidal structures. We discuss in detail the process by which this feat is accomplished and provide quantitative metrics for exploiting it to modulate the self-assembly. We provide evidence for the generic nature of this approach by demonstrating that it remains robust under a number of different anisotropic short-ranged pair interactions in both two and three dimensions. In addition, we report on a novel microphase in mixtures of passive and active colloids. For a broad range of self-propelling velocities, it is possible to stabilize a suspension of fairly monodisperse finite-size crystallites. Surprisingly, this microphase is also insensitive to the underlying pair interaction between building blocks. The active stabilization of these moderately sized monodisperse clusters is quite remarkable and should be of great utility in the design of hierarchical self-assembly strategies. This work further bolsters the notion that active forces can play a pivotal role in directing colloidal self-assembly.

Feedback-controlled active brownian colloids with space-dependent rotational dynamics

The non-thermal nature of self-propelling colloids offers new insights into non-equilibrium physics. The central mathematical model to describe their trajectories is active Brownian motion, where a particle moves with a constant speed, while randomly changing direction due to rotational diffusion. While several feedback strategies exist to achieve position-dependent velocity, the possibility of spatial and temporal control over rotational diffusion, which is inherently dictated by thermal fluctuations, remains untapped. Here, we decouple rotational diffusion from thermal fluctuations. Using external magnetic fields and discrete-time feedback loops, we tune the rotational diffusivity of active colloids above and below its thermal value at will and explore a rich range of phenomena including anomalous diffusion, directed transport, and localization. These findings add a new dimension to the control of active matter, with implications for a broad range of disciplines, from optimal transport to smart materials.

Formation of self-propelling clusters starting from randomly dispersed Brownian particles

We present a simple chemical strategy for the formation of a self-propelling cluster via the process of capture and assembly of passive colloids on the surface of a chemically active colloid. The two species of colloids that are isotropic and Brownian otherwise interact to form propelling clusters. With the help of coarse-grained numerical simulations, we show that a chemically active colloid can induce diffusiophoretic motility to nearby chemically inert colloids towards itself. This propulsion and then self-assembly can then lead to the formation of active clusters. We observe the formation of propelling dimers, trimers, tetramers, etc. depending on the chemical activity and size of the colloids.

Self-propelled torus colloids.

Suspensions of chemically powered self-propelled colloidal particles are examples of active matter systems with interesting properties. While simple spherical Janus particles are often studied, it is known that geometry is important and recent experiments have shown that chemically active torus-shaped colloids behave differently from spherical colloids. In this paper, coarse-grained microscopic simulations of the dynamics of self-diffusiophoretic torus colloids are carried out in bulk solution in order to study how torus geometric factors influence their active motion. The concentration and velocity fields are key ingredients in self-diffusiophoretic propulsion, and the forms that these fields take in the colloid vicinity are shown to be strong functions of torus geometric parameters such as the torus hole size and thickness of the torus tube. This work utilizes a method where self-diffusiophoretic torus colloids with various geometric and dynamical characteristics can be built and studied in fluid media that include chemical reactions and fluid flows. The model can be used to investigate the collective properties of these colloids and their dynamics in confined systems, topics that are of general importance for applications that use colloidal motors with complex geometries.

Dynamic Assembly of Active Colloids: Theory and Simulation

AbstractBecause they consume energy to drive their motion, systems of active colloids are intrinsically out of equilibrium. In past decades, a variety of intriguing dynamic patterns have been observed in systems of active colloids, which offer a new platform for studying non‐equilibrium physics, in which computer simulation and analytical theory have played an important role. Recent progress in understanding the dynamic assembly of active colloids by using numerical and analytical tools is reviewed, including progress in understanding the motility induced phase separation in the past decade, followed by discussion on the effect of shape anisotropy and hydrodynamics on the dynamic assembly of active colloids.

Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification.

A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of a singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO2-Pt Janus micromotors, when subject to both UV light and AC electric fields, move up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO2-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such "tilting-induced retardation" could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.

Light-switchable propulsion of active particles with reversible interactions

Active systems such as microorganisms and self-propelled particles show a plethora of collective phenomena, including swarming, clustering, and phase separation. Control over the propulsion direction and switchability of the interactions between the individual self-propelled units may open new avenues in designing of materials from within. Here, we present a self-propelled particle system, consisting of half-gold-coated titania (TiO2) particles, in which we can quickly and on-demand reverse the propulsion direction, by exploiting the different photocatalytic activities on both sides. We demonstrate that the reversal in propulsion direction changes the nature of the hydrodynamic interaction from attractive to repulsive and can drive the particle assemblies to undergo both fusion and fission transitions. Moreover, we show these active colloids can act as nucleation sites, and switch rapidly the interactions between active and passive particles, leading to reconfigurable assembly and disassembly. Our experiments are qualitatively described by a minimal hydrodynamic model.

Formation of stable and responsive collective states in suspensions of active colloids

Many animal species organise into disordered swarms, polarised flocks or swirls to protect from predators or optimise foraging. Previous studies suggest that such collective states are related to a critical point, which could explain their balance between robustness to noise and high responsiveness regarding external perturbations. Here we provide experimental evidence for this idea by investigating the stability of swirls formed by light-responsive active colloids which adjust their individual motion to positions and orientations of neighbours. Because their behaviour can be precisely tuned, controlled changes between different collective states can be achieved. During the transition between stable swirls and swarms we observe a maximum of the group’s susceptibility indicating the vicinity of a critical point. Our results support the idea of system-independent organisation principles of collective states and provide useful strategies for the realisation of responsive yet stable ensembles in microrobotic systems.