Dynamic electrophoretic mobility of concentrated suspensions

In this paper, we first report dynamic mobility (ud*) data obtained with an electroacoustic technique (electrokinetic sonic amplitude (ESA)) on suspensions of alumina with varying volume fractions and ionic strengths. The frequency interval was 1-18 MHz for these measurements. We found the expected reduction of the modulus of ud* with volume fraction associated to the hindering of the particle motion by the interaction with its neighbors. More illustrative is the behavior of the phase angle, normally negative (the mobility lags behind the applied field) but sometimes positive in the frequency region of the Maxwell-Wagner relaxation. This behavior is related to the phase angle of the induced dipole moment. From phase data, we also concluded that the effect of the dipole mentioned is weakened by partial neutralization of the dipoles induced on neighbor particles. Our results indicate that dynamic mobility data are little affected by the possible presence of stagnant-layer conductivity, which adds to the advantage of being suitable for concentrated systems. Furthermore, a good sensitivity to changes in particle size by, for instance, aggregation was also detected. The main features of the experimental mobility spectra were compared with the predictions of two models, one based on direct estimation of particle-particle interactions and the other based on the cell concept. We found that the predictions of the cell model agree best with data obtained at the lowest frequencies (close to the Maxwell-Wagner relaxation), whereas for higher frequencies it appears as if the interaction model would better capture the effects of inertia.