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...

Reply: In the preceding Comment [1], Sefiane raises the point about whether the effect of evaporative cooling would increase the surface tension and thus influence the results and interpretation of our experiments on splashing [2]. We did, in fact, check this effect. However, we note that in our analysis the surface tension does enter explicitly into the expression for the threshold pressure. The issue is, therefore, not whether surface tension is relevant but only whether varies significantly due to evaporative cooling. We believe that the following experiments, which we performed, rule out evaporative cooling as having a strong effect on our data.According to Sefiane's argument, the higher the evaporation rate, the lower will be the temperature so that the surface tension will increase and reduce the splashing. However, we have changed the evaporation rate and see no effect. We first put extra ethanol (which is the liquid used in our drop) into the chamber and then pulled a vacuum to the desired pressure. The vapor from the extra ethanol saturates the chamber and lowers the evaporation rate of the drop to about 1=2 of that in open air. We performed experiments in this saturated environment, and the results are the same as for the unsaturated case within experimental error bars [3]: For a 3.4 mm diameter drop impacting at speed 3:62 0:05 m=s, we find a threshold pressure P T 38 2 kPa, and for an impact speed 4:0 0:1 m=s, we find P T 37 2 kPa. These points are plotted in Fig. 1 along with the original data in the unsaturated atmosphere. This clearly rules out Sefiane's argument that evaporation is important in our experiments.We also measured the surface tension directly versus pressure with the pendant drop method. From the balance of and gravity, we can derive from the shape of a static

A low-viscosity drop breaking apart inside a viscous fluid is encountered when air bubbles, entrained in thick syrup or honey, rise and break apart. Experiments, simulations, and theory show that the breakup under conditions in which the interior viscosity can be neglected produces an exceptional form of singularity. In contrast to previous studies of drop breakup, universality is violated so that the final shape at breakup retains an imprint of the initial and boundary conditions. A finite interior viscosity, no matter how small, cuts off this form of singularity and produces an unexpectedly long and slender thread. If exterior viscosity is large enough, however, the cutoff does not occur because the minimum drop radius reaches subatomic dimensions first.