Phoretic

Phoretic describes the motion of dispersed particles driven by gradients in fields or properties of the surrounding medium, rather than by bulk fluid flow. The term covers several phenomena observed in colloidal science and soft matter, most notably electrophoresis, diffusiophoresis, thermophoresis, and magnetophoresis. In each case, a particle moves relative to the surrounding fluid when a persistent gradient is present, even when the bulk fluid is at rest.

Electrophoresis refers to motion under the influence of an electric field acting on charged or polarizable particles. Diffusiophoresis is propulsion in a solute concentration gradient, which can arise from chemical reactions or incomplete mixing. Thermophoresis, or thermodiffusion, describes movement driven by a temperature gradient. Magnetophoresis is motion in a magnetic field gradient, relevant for magnetic colloids. The underlying mechanism typically involves interactions in the fluid near the particle surface that produce a slip velocity, leading to a net hydrodynamic propulsion.

Self-phoretic or autophoretic particles generate their own gradients through surface processes such as catalytic reactions or asymmetric heating, enabling propulsion without external fields. These systems are studied as models of active matter and have potential applications in microfluidics, targeted transport, and the autonomous assembly of micro- or nano-scale components.

Theoretical descriptions usually treat low Reynolds number flow and relate particle velocity to the gradient through a phoretic mobility coefficient. Practical challenges include Brownian motion at small scales and the influence of boundaries or complex fluids on gradient formation and particle trajectories.