Drop Splashing on a Dry Smooth Surface

Abstract

The corona splash due to the impact of a liquid drop on a smooth dry substrate is investigated with high speed photography. A striking phenomenon is observed: splashing can be completely suppressed by decreasing the pressure of the surrounding gas. The threshold pressure where a splash first occurs is measured as a function of the impact velocity and found to scale with the molecular weight of the gas and the viscosity of the liquid. Both experimental scaling relations support a model in which compressible effects in the gas are responsible for splashing in liquid solid impacts. What is the mechanism for the violent shattering that takes place as a liquid drop hits a smooth dry surface and splashes? How does the energy, originally distributed uniformly as kinetic energy throughout the drop, become partitioned into small regions as the liquid disintegrates into thousands of disconnected pieces? It is not surprising that the velocity of impact, the drop size and shape, or the liquid surface tension has an important effect on the mass and energy distribution of the ejected droplets [1,2]. However, it is perhaps more difficult to imagine that the surrounding air has a significant role to play in this all-too-common occurrence. More to the point, one would hardly expect the splash to disappear if the surrounding atmosphere were removed. Nevertheless this is the case.The elegant shapes formed during a splash have captured the attention of many photographers since the remarkable early images of Worthington showing the shapes that occur as milk or mercury hits a smooth substrate [3]. Many studies have focused on the fingering dynamics [4][5][6][7] and the effect of surface roughness [1,2,8]. In the present study, we focus only on a drop hitting a smooth substrate. The top row of Figure 1 shows four frames from a movie of an alcohol drop hitting a dry glass slide in a background of air at atmospheric pressure. The drop, after impact, spreads and creates a corona with a thickened rim which first develops undulations along the rim and then breaks up due to surface tension. During this process, the thin sheet comprising the corona surface retracts and rips into pieces. These images are reminiscent of the corona caused by a drop hitting a thin layer of fluid photographed by Edgerton and his colleagues [9]. However, in our case we have made sure that the slide is completely dry prior to impact. Our images illustrate an important puzzle: why do we see a corona form at all? At the substrate surface the liquid times. The first frame shows the drop just as it is about to hit the substrate. The next three frames in each row show the evolution of the drop at 0.276 ms, at 0.552 ms and at 2.484 ms after impact. In the top row, with the air at 100 kPa (atmospheric pressure), the drop splashes. In the second row, with the air just slightly above the threshold pressure, P T = 38.4 kPa, the drop emits only a few droplets. In the third row, at a pressure of 30.0 kPa, no droplets are emitted and no splashing occurs. However, there is an un...

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