On the use of peristaltic waves for the transport of soft particles: A numerical study

Peristaltic transport of inelastic circular droplets immersed in an immiscible viscous fluid is numerically studied in a planar two-dimensional channel using the finite-volume method. Numerical results could be obtained for a wide range of droplet's material properties at large deformations. Based on the results obtained in this work, for a particle that is initially placed at the centerline, an increase in the droplet's viscosity is predicted to increase its transport velocity, but the effect can saturate at viscosity ratios as small as two. The transport velocity is shown to linearly increase with the droplet's density, but the effect turns out to be quite weak. An increase in the interfacial tension is found to lower the transport velocity although the effect appears to approach an asymptote. Depending on their size and the Weber number, droplets are predicted to move faster or slower than rigid particles. The transport velocity of droplets is found to increase with an increase in the wave speed or, equivalently, the Reynolds number. Off-center droplets are predicted to migrate toward the wall or toward the centerline. Droplets that migrate toward the centerline remain a short distance away from it under steady conditions. Distribution of surface forces is used to explain some of these results with viscous normal stress predicted to play a key role in controlling the dynamics of droplets in peristaltic flow.