Penetration behaviour of individual hydrophilic particle at a gas–liquid interface

The penetration behaviour of a hydrophilic particle impacting on a gas–liquid interface was studied both experimentally and mathematically. The aim of this study was to determine the critical impact velocity below which a falling hydrophilic particle would remain on a horizontal liquid surface. A model to predict the critical velocity has been developed based on energy balance of both the particle and liquid volume in the vicinity of the impact zone. The model also includes the effect of hydrophobicitiy (contact angle) of the particle as well as the change in potential energy of the impacted liquid. Experiments were performed using spherical glass beads of diameter 0.97–1.66 mm, and using liquids with varying density (1000–1182 kg/m3), viscosity (1.002–4.796 mPa s) and surface tension (50.31–87.42 mN/m). High speed video camera was used to obtain the particle impact velocity, cavity profile and velocity of the three-phase contact line (TPCL) at the critical conditions. The TPCL line velocity and cavity profile were used as inputs for the model. The fitted advancing contact angle was employed in the model. It was found that the model was in good agreement with the experimental observations, and the fitted advancing contact angle agreed with the combined molecular-hydrodynamic model well.