A colloidal particle undergoing diffusiophoretic migration towards higher salt concentration (indicated by the intensity of the blue colour). b, Migration of a negative colloidal particle in a solution of NaCl. Diffusion of a binary electrolyte such as NaCl induces an electric field (orange arrow), which prevents any steady separation of charge. (In the absence of the electric field, the smaller and more mobile chloride ions would diffuse more quickly than the larger sodium ions; the lengths of the black arrows denote the diffusive contribution to ion flux.) The electric field generated acts on negatively charged colloidal particles to cause them to migrate to the right. c, Migration of a negative colloidal particle in a solution of KClO3. In the absence of an electric field, the more mobile potassium ions diffuse more quickly than the negative chlorate ions (the length of the black arrows denote the diffusive contribution to ion flux). The electric field (orange arrow) that arises to prevent charge separation points in the opposite direction to that in NaCl because the positive ion now has the higher diffusion coefficient. The electric field acts on negatively charged colloidal particles to cause them to migrate to the left.