autophoresis

Autophoresis is a phenomenon observed in certain colloidal solutions where particles spontaneously move in a directed manner without any external force field. This movement is not due to Brownian motion, which is random. Instead, autophoresis arises from local gradients in the solution created by the particles themselves.

Typically, autophoresis occurs when particles are chemically active or have a surface that interacts with the surrounding medium in a way that generates a gradient. For instance, a catalytic particle might consume or produce a substance at its surface. This local change in concentration can lead to a difference in pressure or diffusion forces acting on the particle, causing it to move.

There are two main types of autophoresis: diffusiophoresis and thermophoresis. Diffusiophoresis is driven by concentration gradients, where particles move towards regions of higher or lower solute concentration. Thermophoresis, on the other hand, is driven by temperature gradients, with particles migrating towards colder or hotter regions.

The direction of movement in autophoresis depends on the nature of the particle-surface interaction and the properties of the surrounding fluid. For example, a particle that produces a solute at its surface might move away from that surface if the solute concentration builds up, or it might move towards regions where the solute is diffused away more rapidly.

Autophoresis is a self-propulsion mechanism that can lead to interesting collective behaviors in suspensions, such as self-assembly or directed motion of the entire suspension. Research in this area is important for understanding micro- and nanomachines and designing smart materials.