Catching particles on the moving fluid-fluid interface*

We investigate the drainage of a suspension of non-colloidal particles from an immiscible fluid inside a Hele-Shaw cell. Only for relatively large particles, we observe that particles move slower than the draining mixture and eventually accumulate on the receding interface. The particle accumulation can cause an interfacial instability and lead to particle deposition, if the interface velocity is sufficiently large. This is distinct from the Landau-Levich type coating flows with suspensions, in which only smaller particles are shown to get trapped into the meniscus and are deposited onto the solid substrates. We present experimental results and discuss the physical mechanism of particle accumulation and the resultant instability at the fluid-fluid interface.