High Quality Content by WIKIPEDIA articles! In heat transfer analysis, thermal diffusivity is the ratio of thermal conductivity to volumetric heat capacity. Thermophoresis, also called thermomigration, thermodiffusion, or Sorét effect, or Ludwig-Soret effect, is a phenomenon observed when a mixture of two or more types of motile particles (particles able to move) are subjected to the force of a temperature gradient and the different types of particles respond to it differently. The phenomenon is observed at the scale of one millimeter or less.