Non-equilibrium sedimentation of colloids: confocal microscopy and Brownian dynamics simulations

Experimental and computational details are presented for an investigation of the transient time evolution of colloidal dispersions confined in a horizontal slit pore and under the influence of gravity (Royall et al 2007 Phys. Rev. Lett. 98 188304). We demonstrate that the interparticle interactions can be well described by those of effective hard spheres by comparing experimental results for the pair distribution function obtained in the homogeneous part of the settling system to the theoretical result for hard spheres in equilibrium. Using an effective hard sphere diameter that is 10% larger than that obtained by static light scattering takes account of the (screened) electrostatic repulsion between particles. As a simple computational model, we use Brownian dynamics computer simulations with hard sphere pair interactions and investigate the time evolution of the one-body density profile during sedimentation. We show that an 'intrinsic clock', that ticks only when trial moves are accepted, facilitates high accuracy of the time evolution of the density profile, even when using relatively large integration time steps for the Langevin equations of motion.