Possibility of non-Fickian mixing at concentration interface between stratified suspensions

Hypothesis: Relative motion of micro-sized particles suspended in liquid is governed by hydrodynamic effect, in contrast to nano-sized particle suspension in which thermal effect is significant. As a result, the mixing behavior of stratified suspensions with micro-sized particles is totally different from those obeying Fick's diffusion law for nano-sized particles. Such a "non-Fickian" mixing of micro-sized particles is determined not only by the concentration difference but also the physical properties of suspensions. Experiments: We conducted an experimental study of gravitational settling of stratified suspensions of micro-sized particles with concentration gradients opposed to gravity. We also performed point-force-type numerical simulations under the same conditions as those in the experiment. Particularly, we focused on the relative motion of particles near the concentration interface, which is an apparent interface between the upper and the lower suspensions having different concentrations. Findings: The experimental and numerical results indicate that, if the number density of particles in suspension is sufficient, the concentration interface seemingly behaves immiscibly and the interface prevents particle mixing. However, a small number of particles cannot maintain the seal of the concentration interface then demonstrates miscible behavior. The mixing mechanism of the suspended particles at the concentration interface is strongly related to the miscible and immiscible characteristics of the interface