Chapter 6 - Liquid–Solid Systems

Abstract

This chapter concerns the case where the continuous phase is a liquid. The most obvious difference is that in liquid–solid systems the difference in densities between the phases is much lower than in a gas–solid system, so that the particle settling velocity is also much reduced. In practice, suspensions of solid particles in liquids are divided into “settling” and “nonsettling.” Roughly speaking, particles above about 100μm in size in water show noticeable settling behavior, while those below this size will remain in suspension quite easily. An example of a “nonsettling” system is a pharmaceutical oral suspension of a nonsoluble drug, where the particles are typically in the size range 5–50μm. From the settling velocity Eqs (4.10) and (4.11)Eq (4.10)Eq (4.11), it is apparent that the tendency toward settling is reduced by (1) decreasing the particle size and (2) increasing the liquid viscosity. Even so, bottles of such products often carry the instruction “shake before use!” Other products which are sold as suspensions of particles in a continuous liquid phase include paint, ink, abrasive cleaning products, and foods. If the particle size is below about 1μm, the dispersion is known as colloidal.