This paper reports on experimental and theoretical investigations of the impact of a droplet onto a spherical target. Spatial and temporal variation of film thickness on the target surface is measured. Three distinct temporal phases of the film dynamics are clearly visible from the experimental results, namely the initial drop deformation phase, the inertia dominated phase, and the viscosity dominated phase. Experiments are also conducted to study the effect of droplet Reynolds number and target-to-drop size ratio on the dynamics of the film flow on the surface of the target. It is observed that for a given target-to-drop size ratio, the nondimensional temporal variation of film thickness collapses onto a single curve in the first and second phases. The transition to the viscosity dominated regime occurs earlier for the low Reynolds number cases and residual thickness is also larger. A simplified quasi-one-dimensional approach has been used to model the flow on the spherical target. The theory accounts for the inertial and viscous effects. Gravity and the curvature of the target are also taken into account. An analytical expression for the time-dependent film profile on the sphere is obtained for the inviscid, inertia dominated phase of spreading. Then, the evolution equation for the film thickness near the north pole in the viscosity dominated phase is obtained and solved. Good agreement is observed between the theoretical predictions and the measurements when the values of the drop and target diameters are comparable. No adjustable parameters have been used in the model.
Studies on the evaporation of suspended microlitre droplets under atmospheric conditions have observed faster evaporation rates than the theoretical diffusion-driven rate, especially for rapidly evaporating droplets such as ethanol. Convective flow inside rapidly evaporating droplets has also been reported in the literature. The surrounding gas around the evaporating droplet has, however, been considered to be quiescent in many studies, the validity of which can be questioned. In the present work, we try to answer this question by direct experimental observation of the flow. The possible causes of such a flow are also explored.
Drop-surface interaction is predominant in nature as well as in many industrial applications. Superhydrophobic surfaces show potential for various applications as they show complete drop rebound. In a recent work, it has been reported that the drop lift-off time on a superhydrophobic substrate could be further reduced by introducing a macro-ridge. The macro-ridge introduces asymmetry on the morphology of drop spreading and retraction on the surface. This changes the hydrodynamics of drop retraction and reduces the lift-off time. Keeping practical applications in view, we decorate the surface with multiple ridges. The morphology of the hydrodynamic asymmetry is completely different for the drops impacting onto the tip of the ridges from those impacting onto the middle of the valley between the ridges. We show that the morphology forms the key to the lift-off time. We also show that the outward flow from the ridge triggers a Laplace pressure driven de-wetting on the tip of the ridge, thus aiding the lift-off time. At the end of this work, we propose a ridge to ridge separation that effectively reduces the lift-off times for impacts both at the tip of the ridge and offset from it.
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