EXAMINING VISCOSITY AND SURFACE WETTABILITY ON LAMELLA LIFT DYNAMICS AND DROPLET SPLASHING

摘要

The mechanisms that initiate splashing on smooth, dry surfaces are complex and differ from those on rough or prewetted surfaces. This form of splashing is greatly influenced by the surrounding gas pressure. In this work we examine the effects of droplet viscosity, surface wettability, and gas pressure on the splashing dynamics of single droplets. In previous studies droplet viscosity has been shown to both promote and inhibit splashing. In the current study this contradictory result is tested across a wide range of fluid viscosities. The impact energy required for splashing is minimized within a range of Reynolds number of ~100-500. Eventually, splashing appears to become impossible with sufficiently high viscosity due to the slowing of splashing dynamics beyond a certain time window of opportunity. Hydrophobic and hydrophilic coatings were also applied to a smooth surface in order to change the wetting characteristics of the water droplets. It was found that the hydrophilic surface required higher gas pressure (density) for splashing to occur and vice versa for the hydrophobic surface. Focusing on the spreading lamella, a momentum balance was derived with consideration of the chemical affinity or adhesive force of the liquid to the impact surface. The lamella lift from the surface was assumed to be induced by the displaced surrounding gas during spreading. This provides an explanation for the vertical velocity component of corona splashing seen on dry, smooth surfaces. In light of the lamella lift, instability within the spreading droplet is predicted to arise through Rayleigh-Taylor theory, and subsequent timescales of secondary drop formation are examined. By comparing splash thresholds on hydrophobic and hydrophilic surfaces, the effects of the adhesive force are demonstrated and quantified. The adhesive force between the lamella and impact surface plausibly explains the seemingly paradoxical effect of droplet viscosity to promote splashing for low-viscosity fluids.