Session R24: Active Colloids II

Live

Chemically-active droplets slowly solubilizing in a surfactant-laden solution can spontaneously self-propel as a result of the non-linear transport of surfactant molecules and micellar compounds by the interfacial flows induced by these slowly-diffusing solutes. Many recent experiments have reported the intriguing individual behaviour of these droplets, in particular the emergence of straight, curly and chaotic trajectories. Furthermore, such droplets are fundamentally anti-chemotactic : they avoid their own chemical trail as well as the proximity of other droplets. In experiments on multiple-droplet interactions, this results in complex avoiding trajectories. Motivated by these observations, we use theoretical modeling and numerical simulation to rationalise these individual and collective dynamics. In this presentation, I will present some of our recent work on the emergence of complex individual behaviour and on the nonlinear interplay of hydrodynamic and chemical interactions in droplet collisions. In particular, based on a first-of-a-kind analysis of the complete hydro-chemical interactions in a two-droplet collision, I will show how such pairwise collisions may lead to the emergence of a variety of collective dynamical regimes, as well as alignment and scattering of the droplets.   

Live

The development of engineered active colloids that harness the chemical energy of the environment to move has shown promising applications such as bioremediation, micro-mixing, micro-machinery and drug delivery. To mimic the intrinsic asymmetry of flagellates and other motile microorganisms, active colloids are designed with fore-aft asymmetric chemical activity, which define their polar axis and their preferential direction of motion. Recent experiments showed that Janus active particles can spontaneously break their polar symmetry and transition from a persistent Brownian motion to chiral trajectories. Using numerical simulations, we show that such spontaneous chiralization is a generic feature of polar active colloids. As observed in the experiments, the transition from polar to chiral symmetry results in the onset of active particle rotation and the emergence of circular trajectories. We find that the symmetry breaking instability is driven by the advection of a solute that interacts differently with the two portions of the particle surface and it occurs as through supercritical pitchfork bifurcation.   

Live

By introducing the notion of a dynamic overlap concentration scale, we identify universal and previously unreported features of the mechanical properties of active colloids. These features are codified by recognizing that the characteristic length scale of an active particle's trajectory, the run-length, introduces a new concentration scale Φ^*. Large-scale simulations of repulsive active Brownian particles (ABPs) confirm that this new run-length dependent concentration, which is the trajectory-space analog of the overlap concentration in polymer solutions, delineates distinct concentration regimes in which interparticle collisions alter particle trajectories. Using Φ^* and concentration scales associated with colloidal jamming, the mechanical equation-of-state for ABPs can be collapsed onto a set of principal curves that contain a number of previously overlooked features. The inclusion of these features qualitatively alters previous predictions of the behavior for active colloids as we demonstrate by computing the spinodal for a suspension of purely-repulsive ABPs. Our findings suggest that dynamic overlap concentration scales should be of great utility in unraveling the behavior of active and driven systems.   

Live

Reactive suspensions consist of many self-propelled particles in a fluid that catalyze chemical reactions on their surface. This activity generates chemical gradients in the system which, in turn, create a phoretic slip velocity along the particle surface and drives the particles' motion. Finding the particles' velocities requires solving first the Laplace problem for the solute concentration with prescribed chemical fluxes, and then the Stokes problem for the fluid motion forced by the phoretic slip velocities in response to the concentration distribution.
To model these coupled diffusio-phoretic effects and hydrodynamics interactions, we propose a generalization of the classical Force Coupling Method (FCM) to treat both problems in an integrated framework. The new generalised FCM framework will be introduced, focusing specifically on the novel treatment of the diffusion problem and its coupling to the hydrodynamics. Its validation and comparison with other analytical and numerical methods will be presented on several canonical yet generic two-particle interaction problems.   

Live

In active matter, janus microswimmers are often regarded as model systems. Composed of a chemically inert and a catalytic component, microswimmers consume the surrounding fuel and convert free energy into work by using the interfacial phenomenon of diffusiophoresis, or the migration in a gradient of chemicals. They interact in their surroundings via the fluid they move (hydrodynamics) and are mediated by the chemical gradient they generate (phoretic interactions). Common colloids are sensitive to gradients (phoresis) and are advected by flows (hydrodynamics) making it difficult to disentangle the two physical effects. We propose and synthesize a novel type of colloidal tracers with tunable phoretic mobility in order to quantify and disentangle the two effects. The detailed understanding of the interactions lay the groundwork for the design and assembly of complex machines made of machines or assembled from a multitude of active particles.   

Live

Particle transport in microfluidic flows has a wide range of applications in fields of biomedicine, diagnostics, chemical processing, additive manufacturing, and fluid mechanics, etc. In microfluidic flows, where the particle size is comparable to that of the characteristic length of the system, the particles tend to be influenced greatly by the particle-particle interaction forces, and particle-wall interaction forces. In addition to these interaction forces, the particle-fluid interaction is also affected by the particle shape and size. In this study, we firstly analyze the effect of particle shape and size on the particle transport in a microchannel flow by studying the particle transport of spherical and ellipsoidal particles under similar flow conditions. A fully coupled interaction between the fluid flow and the particle transport shows that the ellipsoidal particles undergo simultaneous rotation and transverse motion while being advected along the length of the channel. Such a particle shape-based transport behavior of ellipsoidal particle can be associated with the prevention of the phenomenon of coffee-stain in additive manufacturing. We finally discuss the effect of particle shape and its role in the prevention of coffee-stain.   

Live

The axisymmetric flow patterns due to the rotation and collapse of an actively driven annular chiral fluid monolayer composed of millions of micron-sized colloidal magnets sitting on a passive subphase are studied. Exact representations for the flow field are given in terms of solutions to a singular integral equation. The effects of odd viscosity (a.k.a. Hall viscosity), an allowed but poorly-understood component of the viscous stress tensor in two-dimensional time reversal symmetry-breaking fluids, on the closure of the hole are investigated. Our work suggests that experimentally measuring the closure time could be a novel method of estimating this elusive parameter.   

Live

Active colloidal systems exhibit intriguing active motion and collective behaviors such as living crystals, and giant fluctuations. The active particles used in experiments so far are of D₂ symmetry, such as spherical or rod-like Janus particles. In contrast, active particles used in theoretical and computational studies often have low symmetries and more diverse shapes. In the talk, we will present our recent research results on designing and fabricating colloidal particles of low symmetries, and our experimental studies of their active motions driven by electrohydrodynamic flows.   

Live

Artificial microswimmers have attracted great interest recently, especially for applications in sensing and biology. In particular, micro-propulsion by producing bubbles using decomposition of a chemical fuel is an attractive method, but it requires a fuel source. In this talk, we demonstrate a new optically-powered approach to catalyst-free bubble self-propulsion that works in a broad range of fluids, including deionized water. These devices, approximately 100 microns in size, consist of encapsulated silicon photodiodes connected in series with two external electrodes. Under standard illumination conditions in a microscope, the photovoltaics drive water splitting at the electrodes and eject the resulting bubbles preferentially in one direction, propelling the device through fluid. These devices are fabricated and released in parallel using standard semiconductor technologies. We discuss the fabrication and characterization of these devices, demonstrate working bubble rockets, and discuss their efficiency and potential applications.   

Live

The near-wall pressure of active systems could play a major role in their morphological behavior and assembly states under confinement. Previous studies showed that, except for very specific systems (e.g., without any torque interactions between particles and the wall such as spherical particles), the strength of particle-wall interaction affects the mechanical pressure of active fluids. However, to what extent this dependence is applicable has not been investigated yet. We use Langevin dynamics simulations to study swim pressure of 2D dry active systems of overdamped self-propelled rods with negligible noise. To elucidate effects of wall stiffness on pressure, we introduce a mobile wall to the middle of the simulation box, which exerts asymmetric harmonic repulsions to rods on different sides. By tracking the wall movement and calculating the near-wall pressure for various wall stiffness, we find that the pressure-stiffness dependence is destroyed as wall stiffness increases, suggesting that pressure can reinstate an equation of state at this limit. These findings provide new insight into active pressure and could improve our prediction for the dynamical behavior of confined active systems.   

Live

Motility-induced phase separation (MIPS), the phenomena in which purely repulsive active particles undergo a liquid-gas phase separation, is among the simplest and widely studied examples of a nonequilibrium phase transition. In this talk, we reveal that nearly the entirety of the MIPS phase boundary of three dimensional active Brownian particles is in fact metastable with respect order-disorder coexistence -- the active crystallization transition. At an activity just above the MIPS critical point, the solid-fluid coexistence curve intersects the liquid-gas binodal, forming a triple point where solid, liquid and gas may coexist. The nearly close-packed density of the active solid phase results in a remarkably narrow transition pathway for nucleating an active crystal from a disordered fluid. However, this structural barrier is surmountable at high activity and fluid densities near maximal packing, where the liquid remains decidedly mobile (non-glassy) and local density fluctuations resembling active crystals become increasing probable. This nucleation landscape results in a diminishing lifetime of liquid-gas coexistence with increasing activity as nucleation of the active within the liquid phase becomes remarkably facile.   

Live

Magnetically driven colloids, an example of active matter, are able to demonstrate complex collective behavior and self-organize into coherent dynamic patterns via short- and long-range interactions. Here we present a method for guided self-assembly of ferromagnetic rolling particles energized by a uniform AC magnetic field into a stationary vortex via magnetic interaction with an additional strongly localized magnetic field. By tuning the parameters of the additional field we effectively control vortex dimensions, internal order, and a number of entrapped rollers. We find that vortex self-organization is assisted by field-induced magnetic steering and controlled by a phase shift between alternating magnetic fields. The presented method for assisted self-organization of rolling colloids into a vortex with on-demand characteristics suggests a new paradigm for active matter control and may lead to the development of new approaches for microscopic transport in active particles systems.   

Live

Spontaneous behaviour can arise when a collection of active matter entities encounters, such in bird swarming [1]. Recent studies mimic these events in artificial systems to study and achieve a better understanding of new phenomena,[2] posting as one of the main challenges the guidance of large amount of particles without applying an external stimuli (i.e. electromagnetic fields) [3].

In this work we follow an alternative approach to guide these artificial entities, based on the topology. We demonstrate that depending on the internal structure of the device, our artificial entities, the Janus colloids, are topologically driven, leading to an increase in the particle flow. Topologically controlled transport also shows accumulation of active particles on the corners of the device, due to the non-Hermitian skin effect. This work sets a principle to design patterned surfaces that display spontaneous, robust, and predictable guided particle motion, guaranteed by Hermitian and non-Hermitian topology.

[1] Andrea Cavagna et Al., Annual Review of Condensed Matter Physics 5.1 (2014)
[2] Ana C. Hortelao et Al, Submitted and bioRxiv (2020)
[3] Islam S. M. Khalil et Al., International Journal of Advanced Robotic Systems (2015)