Effect of electrostatic interactions on the velocity fluctuations of settling microspheres

Abstract

In general, the sedimentation velocities of microparticles in liquids are not uniform due to long-ranged hydrodynamic interactions. Although the velocity fluctuations have been extensively studied experimentally, numerically, and theoretically, the effect of charges of microparticles on the velocity fluctuations is not yet reported. This is probably due to some difficulties in the detection of optical signals from the micron-sized particles dispersed in water where the refractive index is more or less different, leading to serious scattering and attenuation of visible light. So far, since the dynamic ultrasound scattering technique was a good candidate to observe micron- and nano-sized particles in optically opaque systems, we investigated the velocity fluctuations ΔV of microparticles in water. In this study, we show that the velocity fluctuations of charged silica particles exhibit anomalous rise in the concentration dependence of ΔV and show that such a behavior can be suppressed by adding a small amount of salt.