(300e) Colloidal Sorting by Effective Density

Separating one type of molecule from another ? whether those molecules are petroleum-based or pharmaceutically-based for instance ? is a classic chemical engineering problem. We might design distillation columns or membranes as the separating technology. But suppose now that we have a mixtures of colloidal particles in suspension, say particles ranging from 0.1 microns to 10 microns in diameter. How do we sort those particles from one another by size, shape, or density? Sedimentation separation seems like an obvious choice, since larger particles should settle more quickly than smaller particles. Shouldn't they? In this talk we examine how simple intuition has led our lab group astray several times, as well as how a Rayleigh-Benard instability limits the usual sedimentations ? even density gradient sedimentations. We will also examine how we can change the effective density of particles by altering the thickness of the electrical double layer, leading to a simple but powerful colloidal particle sorting technique that takes advantage of the collective nature of the complex fluid.