Motion of Particles in Film Flow

The effect of particle size (d p) to film thickness (h o) ratio on the motion of spherical particles in a stable liquid film flowing down an inclined flat surface is studied experimentally. Previously reported models that are based on force and torque balance are modified to predict the motion of particles that are smaller than film thickness. At low d p/h o values, particle velocity is observed to increase nearly linearly with particle size, reflecting the increasing influence of hydrodynamic drag as larger particles expose their surface to regions of higher fluid velocity. Good agreement between model predictions and experimental results is observed for small d p/h o ratios. When d p/h o is in the range of ∼0.7−1, particle velocities are observed to decrease rapidly with an increase in size. This may be attributed to the effect of the proximity of the free interface to the particle surface and also the deformation of the free surface induced by the moving particle. When d p/h o is in the approximate range of 1−1.75, particles ceased to move due to the surface tension acting on the particle along the circumference of the contact radius of the three-phase interface. For particles significantly larger than film thickness (d p/h o greater than about 1.7), particle velocity is observed to increase with its size as the particle motion is aided by the increased contribution from the gravitational force. For the range of film thicknesses and particle sizes studied, there appears to be a d p/h o range in which gravity force begins to dominate over surface tension force.