The sediment of mixtures of charged colloids: Segregation and inhomogeneous electric fieldsJ. Zwanikken and R. van Roij
Institute for Theoretical Physics, Utrecht University Leuvenlaan 4, 3584 CE Utrecht, The Netherlands
received 19 April 2005; accepted 6 June 2005
published online 13 July 2005
We theoretically study sedimentation-diffusion equilibrium of dilute binary, ternary, and polydisperse mixtures of colloidal particles with different buoyant masses and/or charges. We focus on the low-salt regime, where the entropy of the screening ions drives spontaneous charge separation and the formation of an inhomogeneous macroscopic electric field. The resulting electric force lifts the colloids against gravity, yielding highly nonbarometric and even nonmonotonic colloidal density profiles. The most profound effect is the phenomenon of segregation into layers of colloids with equal mass-per-charge, including the possibility that heavy colloidal species float onto lighter ones.
82.70.Dd - Colloids.
05.20.Jj - Statistical mechanics of classical fluids.
© EDP Sciences 2005