Pair Interaction of Catalytically Active Colloids: From Assembly to Escape

The dynamics and pair trajectory of two self-propelled colloids are reported. The autonomous motions of the colloids are due to a catalytic chemical reaction taking place asymmetrically on their surfaces that generates a concentration gradient of interactive solutes around the particles and actuate particle propulsion. We consider two spherical particles with symmetric catalytic caps extending over the local polar angles $\theta^1_{cap}$ and $\theta^2_{cap}$ from the centers of active sectors in an otherwise quiescent fluid. A combined analytical-numerical technique was developed to solve the coupled mass transfer equation and the hydrodynamics in the Stokes flow regime. The ensuing pair trajectory of the colloids is controlled by the reacting coverages $\theta^j_{cap}$ and their initial relative orientation with respect to each other. Our analysis indicates two possible scenarios for pair trajectories of catalytic self-propelled particles: either the particles approach, come into contact and assemble or they interact and move away from each other (escape). For arbitrary motions of the colloids, it is found that the direction of particle rotations is the key factor in determining the escape or assembly scenario. Based on the analysis, a phase diagram is sketched for the pair trajectory of the catalytically active particles as a function of active coverages and their initial relative orientations. We believe this study has important implications in elucidation of collective behaviors of auotophoretically self-propelled colloids

Curvature Capillary Migration of Microspheres

We address the question: How does capillarity propel microspheres along curvature gradients? For a particle on a fluid interface, there are two conditions that can apply at the three phase contact line: Either the contact line adopts an equilibrium contact angle, or it can be pinned by kinetic trapping, e.g. at chemical heterogeneities, asperities or other pinning sites on the particle surface. We formulate the curvature capillary energy for both scenarios for particles smaller than the capillary length and far from any pinning boundaries. The scale and range of the distortion made by the particle are set by the particle radius; we use singular perturbation methods to find the distortions and to rigorously evaluate the associated capillary energies. For particles with equilibrium contact angles, contrary to the literature, we find that the capillary energy is negligible, with the first contribution bounded to fourth order in the product of the particle radius and the deviatoric curvature. For pinned contact lines, we find curvature capillary energies that are finite, with a functional form investigated previously by us for disks and microcylinders on curved interfaces. In experiments, we show microsphere migrate along deterministic trajectories toward regions of maximum deviatoric curvature with curvature capillary energies ranging from $6 \times10^3 - 5 \times 10^4~k_BT$. These data agree with the curvature capillary energy for the case of pinned contact lines. The underlying physics of this migration is a coupling of the interface deviatoric curvature with the quadrupolar mode of nanometric disturbances in the interface owing to the particle's contact line undulations. This work is an example of the major implications of nanometric roughness and contact line pinning for colloidal dynamics.Comment: 9 pages, 8 figure

Capillary Assembly of Colloids: Interactions on Planar and Curved Interfaces

In directed assembly, small building clocks are assembled into an organized structures under the influence of guiding fields. Capillary interactions provide a versatile route for structure formation. Colloids adsorbed on fluid interfaces distort the interface, which creates an associated energy field. When neighboring distortions overlap, colloids interact to minimize interfacial area. Contact line pinning, particle shape and surface chemistry play important roles in structure formation. Interface curvature acts like an external field; particles migrate and assemble in patterns dictated by curvature gradients. We review basic analysis and recent findings in this rapidly evolving literature. Understanding the roles of assembly is essential for tuning the mechanical, physical, and optical properties of the structure.Comment: 14 pages, 9 figure

Reply to the Comments on "Curvature capillary migration of microspheres" by P. Galatola and A. Wurger

We have studied microparticle migration on curved fluid interfaces in experiment and derived an expression for the associated capillary energy $E$ for two cases, i.e., pinned contact lines and equilibrium contact lines, which differ from expressions derived by others in the literature. In this problem, a particle of radius $a$ makes a disturbance in a large domain characterized by principal radii of curvature $R_1$ and $R_2$. Since $a$ is smaller than all associated geometric and physico-chemical length scales, analysis calls for a singular perturbation approach. We recapitulate these concepts, identify conceptual errors in the Comments about our work, and provide evidence from experiment and simulation that supports our view.Comment: 6 pages, 2 figure

Self-Diffusiophoretic Colloidal Propulsion Near a Solid Boundary

We study the diffusiophoretic self-propulsion of a colloidal catalytic particle due to a surface chemical reaction in a vicinity of a solid wall. Diffusiophoresis is a chemico-mechanical transduction mechanism in which a concentration gradient of an interacting solute produces an unbalanced force on a colloidal particle and drives it along the gradient. We consider a spherical particle with an axisymmetric reacting cap covering the polar angle range $0\le \theta\le \theta_{cap}$ in the presence of a repulsive solute, near an infinite planar wall, and solve the coupled solute concentration and Stokes equations, using a mixture of numerical and analytic arguments. The resulting particle trajectory is determined by $\theta_{cap}$ and the initial orientation of the symmetry axis with respect to the plane. At normal incidence the particle initially moves away from or towards the wall, depending on whether the cap faces towards or away, respectively, but even in the latter case the particle never reaches the wall due to hydrodynamic lubrication resistance. For other initial orientations, when $\theta_{cap}\le 115^{\circ}$ the particle either moves away immediately or else rotates along its trajectory so as to cause the active side to face the wall and the particle to rebound. For higher coverage we find trajectories where the particle skims along the wall at constant separation or else comes to rest. We provide a phase diagram giving the nature of the trajectory (repulsion, rebound, skimming or stationary) as a function of $\theta_{cap}$ and the initial orientation

Capillary Interactions on Fluid Interfaces: Opportunities for Directed Assembly

A particle placed in soft matter distorts its host and creates an energy landscape. This can occur, for example, for particles in liquid crystals, for particles on lipid bilayers or for particles trapped at fluid interfaces. Such energies can be used to direct particles to assemble with remarkable degrees of control over orientation and structure. These notes explore that concept for capillary interactions, beginning with particle trapping at fluid interfaces, addressing pair interactions on planar interfaces and culminating with curvature capillary migration. Particular care is given to the solution of the associated boundary value problems to determine the energies of interaction. Experimental exploration of these interactions on planar and curved interfaces is described. Theory and experiment are compared. These interactions provide a rich toolkit for directed assembly of materials, and, owing to their close analogy to related systems, pave the way to new explorations in materials science.Comment: 37 pages, 28 figure

Capillary migration of microdisks on curved interfaces

The capillary energy landscape for particles on curved fluid interfaces is strongly influenced by the particle wetting conditions. Contact line pinning has now been widely reported for colloidal particles, but its implications in capillary interactions have not been addressed. Here, we present experiment and analysis for disks with pinned contact lines on curved fluid interfaces. In experiment, we study microdisk migration on a host interface with zero mean curvature; the microdisks have contact lines pinned at their sharp edges and are sufficiently small that gravitational effects are negligible. The disks migrate away from planar regions toward regions of steep curvature with capillary energies inferred from the dissipation along particle trajectories which are linear in the deviatoric curvature. We derive the curvature capillary energy for an interface with arbitrary curvature, and discuss each contribution to the expression. By adsorbing to a curved interface, a particle eliminates a patch of fluid interface and perturbs the surrounding interface shape. Analysis predicts that perfectly smooth, circular disks do not migrate, and that nanometric deviations from a planar circular, contact line, like those around a weakly roughened planar disk, will drive migration with linear dependence on deviatoric curvature, in agreement with experiment

Films of bacteria at interfaces: three stages of behaviour

Bacterial attachment to a fluid interface can lead to the formation of a film with physicochemical properties that evolve with time. We study the time evolution of interface (micro)mechanics for interfaces between oil and bacterial suspensions by following the motion of colloidal probes trapped by capillarity to determine the interface microrheology. Initially, active bacteria at and near the interface drive superdiffusive motion of the colloidal probes. Over timescales of minutes, the bacteria form a viscoelastic film which we discuss as a quasi-two-dimensional, active, glassy system. To study late stage mechanics of the film, we use pendant drop elastometry. The films, grown over tens of hours on oil drops, are expanded and compressed by changing the drop volume. For small strains, by modeling the films as 2D Hookean solids, we estimate the film elastic moduli, finding values similar to those reported in the literature for the bacteria themselves. For large strains, the films are highly hysteretic. Finally, from wrinkles formed on highly compressed drops, we estimate film bending energies. The dramatic restructuring of the interface by such robust films has broad implications, e.g. in the study of active colloids, in understanding the community dynamics of bacteria, and in applied settings including bioremediation

Curvature capillary repulsion

Directed assembly of colloids is an exciting field in materials science to form structures with new symmetries and responses. Fluid interfaces have been widely exploited to make densely packed ordered structures. We have been studying how interface curvature can be used in new ways to guide structure formation. On a fluid interface, the area of the deformation field around adsorbed microparticles depends on interface curvature; particles move to minimize the excess area of the distortions that they make in the interface. For particles that are sufficiently small, this area decreases as particles move along principle axes to sites of high deviatoric curvature. We have studied this migration for microparticles on a curved host interface with zero mean curvature created by pinning an oil-water interface around a micropost. Here, on a similar interface, we demonstrate capillary curvature repulsion, that is, we identify conditions in which microparticles migrate away from high curvature sites. Using theory and experiment, we discuss the origin of these interactions and their relationship to the particle's undulated contact line. We discuss the implications of this new type of interaction in various contexts from materials science to microrobotics.Comment: 9 pages, 7 figure

Curvature-driven migration of colloids on lipid bilayers

Colloids and proteins alike can bind to lipid bilayers and move laterally in these two-dimensional fluids. Inspired by proteins that generate membrane curvature, sense the underlying membrane geometry, and migrate to high curvature sites, we explore the question: Can colloids, adhered to lipid bilayers, also sense and respond to membrane geometry? We report the curvature migration of Janus microparticles adhered to giant unilamellar vesicles elongated to present well defined curvature fields. However, unlike proteins, which migrate to minimize membrane bending energy, colloids migrate by an entirely different mechanism. By determining the energy dissipated along a trajectory, the energy field mediating these interactions is inferred to be linear in the local deviatoric curvature, as reported previously for colloids trapped at curved interfaces between immiscible fluids. In this latter system, however, the energy gradients are far larger, so particles move deterministically, whereas the colloids on vesicles move with significant fluctuations in regions of weak curvature gradient. By addressing the role of Brownian motion, we show that the observed curvature migration of colloids on bilayers is indeed a case of curvature capillary migration, with membrane tension playing the role of interfacial tension. Furthermore, since this motion is mediated by membrane tension and shape, it can be modulated, or even turned "on" and "off", by simply varying these parameters. While particle-particle interactions on lipid membranes have been considered in many contributions, we report here an exciting and previously unexplored modality to actively direct the migration of colloids to desired locations on lipid bilayers.Comment: 24 pages, 12 figure