ABSTRACT
Colloidal particles, when placed in a solution that is not uniform in the concentration of some molecular solute, will spontaneously migrate toward regions of higher or lower concentration of the solute as a result of physical interaction between the solute molecules and each particle. This migration is termed diffusiophoresis [1-9] and has been demonstrated experimentally for both charged [10] and uncharged [11] solutes. Diffusiophoresis is of practical importance in some applications to particle analysis or separation and in certain latex-particle coating processes [1,12]. In a solution of nonionic solute, the solute molecules interact with the particle through the van der Waals/dipole forces. For charged particles in an electrolyte solution, the particle-solute interaction is electrostatic in nature, and its range is the Debye screening length κ−1 (de‰ned right after Equation 7.3). Particles with zeta potentials of order kT/e (~25 mV; e is the elementary electric charge, k is the Boltzmann constant, and T is the absolute temperature) in electrolyte gradients of order 10 M/cm (such gradients are easily attainable in concentration boundary layers) will move by diffusiophoresis at speeds of 10-50 μm/s.
