Gravity induced bulk flow

Natural convection is the flow induced by the unequal pull of gravity on fluid elements of different densities. For example, if we inject a globule (or layer) of dense aqueous solution marked with a dye into a beaker of water, the dense globule will be observed to sink under the influence of gravity, as illustrated in Figure 4.8. That sinking motion is actually a form of bulk displacement or flow, specifically natural convective flow. 

The stresses acting on the material acting to induce flow, e.g., gravity pulling downwards on a potential arch that may form. The stresses acting on the bulk solid can be determined using mathematical models (1). 

Such a relative motion can be induced by external electric fields or by pressure gradients or bulk forces (e.g. gravity). It is possible that particles move in a quiescent solvent or that the solvent flows through a fixed bed of particles. A detailed description on electrokinetic phenomena is e.g. given by Hunter (1988). Zeta-potential measurements on colloidal suspensions are fiequently conducted via electrophoresis or by means of electroacoustics. Besides this, there are recent techniques based on non-linear optics that are sensitive to interfacial changes. 

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