Colloidal deposition under conditions of controlled potential

The rate of colloidal deposition of carbon black onto a microscope slide coated with tin-doped indium oxide has been measured using light scattered from the evanescent wave of a light beam passing through the slide. The rate can be measured in real time and the ITO coat allows the electrode potential of the surface to be changed. The mass transport of the particles is controlled by using the hydrodynamics of the impinging jet; depending on the size of the jet the hydrodynamics can either be wall-pipe or wall-jet. When the potential of the surface is positive there is no barrier to the deposition of the negatively charged particles; under both the wall-pipe and wall-jet regimes the particles deposit as the mass transport-controlled rate. Good agreement is found between the values of the diffusion coefficient of the particles measured in the two regimes. Depending on the age of the slide, when the potential is negative the particles either do not deposit or they deposit into the secondary minimum. The radial variation under the wall-jet regime shows that if they do deposit they do so again at the mass transport-controlled rate. The position of the secondary minimum can be calculated and it is shown that there is good agreement between the calculated position and the penetration of the evanescent wave. For ionic strengths > 10 mmol dm–3 there is no barrier and the particles deposit at all potentials. From this result we estimate a value of the Hamaker constant of 23.8 kBT. Using this value one can then calculate for lower ionic strengths the potential at which the barrier collapses and the particles transfer from the secondary into the primary minimum. Reasonable agreement is found between the theoretical and experimental results.