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

When a system undergoes a transition from a liquid to a solid phase, it passes through multiple intermediate structures before reaching the final state. However, our knowledge on the exact pathways of this process is limited, mainly due to the difficulty of realizing direct observations. Here, we experimentally study the evolution of symmetry and density for various colloidal systems during liquid-to-solid phase transitions, and visualize kinetic pathways with single-particle resolution. We observe the formation of relatively-ordered precursor structures with different symmetries, which then convert into metastable solids. During this conversion, two major cross-symmetry pathways always occur, regardless of the final state and the interaction potential. In addition, we find a broad decoupling of density variation and symmetry development, and discover that nucleation rarely starts from the densest regions. These findings hold for all our samples, suggesting the possibility of finding a unified picture for the complex crystallization kinetics in colloidal systems.

Droplet impacting on solid or liquid interfaces is a ubiquitous phenomenon in nature. Although complete rebound of droplets is widely observed on superhydrophobic surfaces, the bouncing of droplets on liquid is usually vulnerable due to easy collapse of entrapped air pocket underneath the impinging droplet. Here, we report a superhydrophobic-like bouncing regime on thin liquid film, characterized by the contact time, the spreading dynamics, and the restitution coefficient independent of underlying liquid film. Through experimental exploration and theoretical analysis, we demonstrate that the manifestation of such a superhydrophobic-like bouncing necessitates an intricate interplay between the Weber number, the thickness and viscosity of liquid film. Such insights allow us to tune the droplet behaviours in a well-controlled fashion. We anticipate that the combination of superhydrophobic-like bouncing with inherent advantages of emerging slippery liquid interfaces will find a wide range of applications.

Splashing occurs when a liquid drop hits a dry solid surface at high velocity. This paper reports experimental studies of how the splash depends on the roughness and the texture of the surfaces as well as the viscosity of the liquid. For smooth surfaces, there is a "corona" splash caused by the presence of air surrounding the drop. There are several regimes that occur as the velocity and liquid viscosity are varied. There is also a "prompt" splash that depends on the roughness and texture of the surfaces. A measurement of the size distribution of the ejected droplets is sensitive to the surface roughness. For a textured surface in which pillars are arranged in a square lattice, experiment shows that the splashing has a four-fold symmetry. The splash occurs predominantly along the diagonal directions. In this geometry, two factors affect splashing the most: the pillar height and spacing between pillars.