Experimental study on the impact behaviors of a water drop on immiscible oil surfaces: bouncing, compound central jet formation and crater evolution

The impact of the drop on an immiscible liquid surface is common in many applications, where differences in physical properties like viscosity and thickness of the target liquid layer are crucial to the impact outcomes. We experimentally investigate the effect of dimensionless thickness beta (ratio of film thickness to drop diameter) and dimensionless viscosity (ratio of film viscosity to drop viscosity) on the water drop impact upon silicone oil surfaces. For the ultra-thin film ( beta = 0.002), drop bouncing resembles when impinging on superhydrophobic surfaces, except that the rupture of oil film results in a residual drop liquid. The dimensionless viscosity determines the receding contact angle, maximum retraction velocity and drop contact time. For the thin film ( beta = 0.1), we observe capillary climbing of oil film due to Neumann's condition and the subsequent compound central jet. In addition, film viscosity plays a significant role in the number of fingers and crown emergence. For the thick film ( beta = 1) and pool ( beta = 5), we derive a theoretical model to predict the maximum penetration depth of the crater in the pool and the maximum expansion width of the crater in the thick film, which is validated by experimental results.