Dynamic colloidal interactions between protein-stabilised particles - experiment and simulation

Experimental data are compared with computer simulation of interactions between two colloidal particles in a laminar shear field, when one particle is fixed to a wall and a second particle is freely mobile in the shear field in the narrow gap formed by a second parallel, moving wall. Both colloidal and hydrodynamic interactions are taken into account in simulating the scattering of the mobile particle by the fixed particle. A detailed explanation is given of the experimental procedure required to perform and observe such particle collisions successfully, in order to extract the information required to compare with the simulated results. Polystyrene latex particles and oil droplets of diameter ca. 5 mu m, stabilised by pure beta-casein, alpha(s1)-casein, sodium caseinate or gelatine, were studied in water over a range of pH values and ionic strengths. Within experimental error, the scattering in these systems could not be detected as varying markedly, due mainly to random noise introduced by Brownian motion, in agreement with the simulation results for a wide range of DLVO plus simple steric-type colloidal interaction potentials. It is concluded that the dynamic steric interactions for all the different protein layers, as measured by this technique, may be quite similar, or that a more complex type of dynamic interaction may be involved.