Self-Motility of an Active Particle Induced by Correlations in the Surrounding Solution

Ionic Diffusiophoresis of Active Colloids via Galvanic Exchange Reactions

In order to move toward realistic applications by extending active matter propulsion reactions beyond the classical catalytic hydrogen peroxide decomposition, we investigate the self-propulsion mechanism of Janus particles. To address the influences of ionic species, we investigate Janus particles driven by a galvanic exchange reaction that consumes and produces ions on one hemisphere. Our galvanophoretic experiments in the regime of thin Debye layers confirm that even the simplest models in active matter are still full of important surprises. We find a logarithmic speed dependence on the fuel concentration, which cannot be explained using the classic ionic self-diffusiophoretic framework. Instead, an approach based on the Poisson-Nernst-Planck equations yields a better agreement with the experiments. We attribute the discrepancy between the two models to the breakdown of two key hypotheses of the ionic self-diffusiophoretic approach.

Enhanced phoretic self-propulsion of active colloids through surface charge asymmetry

Charged colloidal particles propel themselves through asymmetric fluxes of chemically generated ions on their surface. We show that asymmetry in the surface charge distribution provides an additional mode of self-propulsion at the nanoscale for chemically active particles that produce ionic species. Particles of sizes smaller than or comparable to the Debye length achieve directed self-propulsion through surface charge asymmetry even when ionic flux is uniform over its surface. Janus nanoparticles endowed with both surface charge and ionic flux asymmetries result in enhanced propulsion speeds of the order of μm/s or higher. Our work suggests an alternative avenue for specifying surface properties that optimize self-propulsion in ionic media.

Isotropic active colloids: explicit vs. implicit descriptions of propulsion mechanisms

Modeling the couplings between active particles often neglects the possible many-body effects that control the propulsion mechanism. Accounting for such effects requires the explicit modeling of the molecular details at the origin of activity. Here, we take advantage of a recent two-dimensional model of isotropic active particles whose propulsion originates from the interactions between solute particles in the bath. The colloid catalyzes a chemical reaction in its vicinity, which results in a local phase separation of solute particles, and the density fluctuations of solute particles cause the enhanced diffusion of the colloid. In this paper, we investigate an assembly of such active particles, using (i) an explicit model, where the microscopic dynamics of the solute particles is accounted for; and (ii) an implicit model, whose parameters are inferred from the explicit model at infinite dilution. In the explicit solute model, the long-time diffusion coefficient of the active colloids strongly decreases with density, an effect which is not captured by the derived implicit model. This suggests that classical models, which usually decouple pair interactions from activity, fail to describe collective dynamics in active colloidal systems driven by solute-solute interactions.

Conditions for the propulsion of a colloid surrounded by a mesoscale phase separation

We study a two-dimensional model of an active isotropic colloid whose propulsion is linked to the interactions between solute particles of the bath. The colloid catalyzes a chemical reaction in its vicinity, that yields a local phase separation of solute particles. The density fluctuations of solute particles result in the enhanced diffusion of the colloid. Using numerical simulations, we thoroughly investigate the conditions under which activity occurs, and we establish a state diagram for the activity of the colloid as a function of the parameters of the model. We use the generated data to unravel a key observable that controls the existence and the intensity of activity: The filling fraction of the reaction area. Remarkably, we finally show that propulsion also occurs in three-dimensional geometries, which confirms the interest of this mechanism for experimental applications.

Spontaneous propulsion of an isotropic colloid in a phase-separating environment

The motion of active colloids is generally achieved through their anisotropy, as exemplified by Janus colloids. Recently, there was a growing interest in the propulsion of isotropic colloids, which requires some local symmetry breaking. Although several mechanisms for such propulsion were proposed, little is known about the role played by the interactions within the environment of the colloid, which can have a dramatic effect on its propulsion. Here, we propose a minimal model of an isotropic colloid in a bath of solute particles that interact with each other. These interactions lead to a spontaneous phase transition close to the colloid, to directed motion of the colloid over very long timescales and to significantly enhanced diffusion, in spite of the crowding induced by solute particles. We determine the range of parameters where this effect is observable in the model, and we propose an effective Langevin equation that accounts for it and allows one to determine the different contributions at stake in self-propulsion and enhanced diffusion.

Active spheres induce Marangoni flows that drive collective dynamics

For monolayers of chemically active particles at a fluid interface, collective dynamics is predicted to arise owing to activity-induced Marangoni flow even if the particles are not self-propelled. Here, we test this prediction by employing a monolayer of spherically symmetric active [Formula: see text] particles located at an oil-water interface with or without addition of a nonionic surfactant. Due to the spherical symmetry, an individual particle does not self-propel. However, the gradients produced by the photochemical fuel degradation give rise to long-ranged Marangoni flows. For the case in which surfactant is added to the system, we indeed observe the emergence of collective motion, with dynamics dependent on the particle coverage of the monolayer. The experimental observations are discussed within the framework of a simple theoretical mean-field model.