RESUMEN
A long-standing goal in colloidal active matter is to understand how gradients in fuel concentration influence the motion of phoretic Janus particles. Here, we present a theoretical description of the motion of a spherical phoretic Janus particle in the presence of a radial gradient of the chemical solute driving self-propulsion. Radial gradients are a geometry relevant to many scenarios in active matter systems and naturally arise due to the presence of a point source or sink of fuel. We derive an analytical solution for the Janus particle's velocity and quantify the influence of the radial concentration gradient on the particle's trajectory. Compared to a phoretic Janus particle in a linear gradient in fuel concentration, we uncover a much richer set of dynamic behaviors including circular orbits and trapped stationary states. We identify the ratio of the phoretic mobilities between the two domains of the Janus particle as a central quantity in tuning their dynamics. Our results provide a path for developing optimum protocols for tuning the dynamics of phoretic Janus particles and mixing fluid at the microscale. In addition, this work suggests a method for quantifying the surface properties of phoretic Janus particles, which have proven to be challenging to probe experimentally.
RESUMEN
When immersed in solution, surface-active particles interact with solute molecules and migrate along gradients of solute concentration. Depending on the conditions, this phenomenon could arise from either diffusiophoresis or the Marangoni effect, both of which involve strong interactions between the fluid and the particle surface. We introduce here a numerical approach that can accurately capture these interactions, and thus provide an efficient tool to understand and characterize the phoresis of soft particles. The model is based on a combination of the extended finite element-that enable the consideration of various discontinuities across the particle surface-and the particle-based moving interface method-that is used to measure and update the interface deformation in time. In addition to validating the approach with analytical solutions, the model is used to study the motion of deformable vesicles in solutions with spatial variations in both solute concentration and temperature.
RESUMEN
The realization of micromotors able to dock and transport microscopic objects in a fluid medium has direct applications toward the delivery of drugs and chemicals in small channels and pores, and the realization of functional wireless microrobots in lab-on-a-chip technology. A simple and general method to tow microscopic particles in water by using remotely controllable light-activated hematite microdockers is demonstrated. These anisotropic ferromagnetic particles can be synthesized in bulk and present the remarkable ability to be activated by light while independently manipulated via external fields. The photoactivation process induces a phoretic flow capable to attract cargos toward the surface of the propellers, while a rotating magnetic field is used to transport the composite particles to any location of the experimental platform. The method allows the assembling of small colloidal clusters of various sizes, composed by a skeleton of mobile magnetic dockers, which cooperatively keep, transport, and release the microscopic cargos. The possibility to easily reconfigure in situ the location of the docker above the cargo is demonstrated, which enables optimize transport and cargo release operations.
RESUMEN
Self-motile Janus colloids are important for enabling a wide variety of microtechnology applications as well as for improving our understanding of the mechanisms of motion of artificial micro- and nanoswimmers. We present here micro/nanomotors which possess a reversed Janus structure of an internal catalytic "chemical engine". The catalytic material (here platinum (Pt)) is embedded within the interior of the mesoporous silica (mSiO2)-based hollow particles and triggers the decomposition of H2O2 when suspended in an aqueous peroxide (H2O2) solution. The pores/gaps at the noncatalytic (Pt) hemisphere allow the exchange of chemical species in solution between the exterior and the interior of the particle. By varying the diameter of the particles, we observed size-dependent motile behavior in the form of enhanced diffusion for 500 nm particles, and self-phoretic motion, toward the nonmetallic part, for 1.5 and 3 µm ones. The direction of motion was rationalized by a theoretical model based on self-phoresis. For the 3 µm particles, a change in the morphology of the porous part is observed, which is accompanied by a change in the mechanism of propulsion via bubble nucleation and ejection as well as a change in the direction of motion.