At the impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surface ("contact boiling"), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back ("gentle film boiling"), or both forms the Leidenfrost layer and ejects tiny droplets upward ("spraying film boiling"). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading γ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (γ~We(2/5)), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (γ~We(1/4)). We also interferometrically measure the vapor thickness under the droplet.

At impact of a liquid drop on a solid surface an air bubble can be entrapped. Here we show that two competing effects minimize the (relative) size of this entrained air bubble: For large drop impact velocity and large droplets the inertia of the liquid flattens the entrained bubble, whereas for small impact velocity and small droplets capillary forces minimize the entrained bubble. However, we demonstrate experimentally, theoretically, and numerically that in between there is an optimum, leading to maximal air bubble entrapment. Our results have a strong bearing on various applications in printing technology, microelectronics, immersion lithography, diagnostics, or agriculture.The impact of liquid droplets on surfaces is omnipresent in nature and technology, ranging from falling raindrops to applications in agriculture and inkjet printing. The crucial question often is: How well does the liquid wet a surface? The traditional view is that it is the surface tension which gives a quantitative answer. However, it has been shown recently that an air bubble can be entrapped under a liquid drop as it impacts on the surface [1-6]. Also Xu et al. [7,8] revealed the important role of the surrounding air on the impact dynamics, including a possible splash formation. The mechanism works as follows [3][4][5][6]: The air between the falling drop and the surface is strongly squeezed, leading to a pressure buildup in the air under the drop. The enhanced pressure results in a dimple formation in the droplet and eventually to the entrapment of an air bubble (figure 1a). The very simple question we ask and answer in this paper is: For which impact velocity is the entrapped bubble maximal?Our experimental setup is shown in figure 1b and is similar to that of ref.[9] where it is described in detail. An ethanol drop impacts on a smooth glass surface after detaching from a needle, or for velocities smaller than 0.32 m/s, after moving the needle downwards using a linear translation stage. A high-speed side view recording is used to measure the drop diameter and velocity. A synchronized bottom view recording by a high-speed color camera is used to measure the deformed shape of the liquid drop. Colored interference patterns are created by high-intensity coaxial white light, which reflects from both the glass surface and the bottom of the droplet. Using a color-matching approach in combination with known reference surfaces, the complete air thickness profile can be extracted (shown in figure 1c). For experiments done at larger impact velocities (U > 0.76 m/s), we use a pulse of diffused laser light triggered by an optical switch. The thickness of the air film at the rim is assumed to be zero, and the complete air thickness profile can then be obtained from the monochromatic fringe pattern. From these measurements we can determine the dimple height, H d , and the volume of the entrained bubble, V b , at the very moment of impact. This moment is defined by the first wetting of the surface, i.e., the moment when the concentric symmetry of th...

A wearable and flexible pressure sensor is essential to the realization of personalized medicine through continuously monitoring an individual's state of health and also the development of a highly intelligent robot. A flexible, wearable pressure sensor is fabricated based on novel single-wall carbon nanotube /tissue paper through a low-cost and scalable approach. The flexible, wearable sensor showed superior performance with concurrence of several merits, including high sensitivity for a broad pressure range and an ultralow energy consumption level of 10 W. Benefited from the excellent performance and the ultraconformal contact of the sensor with an uneven surface, vital human physiological signals (such as radial arterial pulse and muscle activity at various positions) can be monitored in real time and in situ. In addition, the pressure sensors could also be integrated onto robots as the artificial skin that could sense the force/pressure and also the distribution of force/pressure on the artificial skin.

When a droplet impacts upon a surface heated above the liquid's boiling point, the droplet either comes into contact with the surface and boils immediately (contact boiling), or is supported by a developing vapor layer and bounces back (film boiling, or Leidenfrost state). We study the transition between these characteristic behaviors and how it is affected by parameters such as impact velocity, surface temperature, and controlled roughness (i.e., micro-structures fabricated on silicon surfaces). In the film boiling regime, we show that the residence time of droplets impacting upon the surface strongly depends on the drop size. We also show that the maximum spreading factor G of droplets in this regime displays a universal scaling behavior G $ We 3/10 , which can be explained by taking into account the drag force of the vapor flow under the drop. This argument also leads to predictions for the scaling of film thickness and velocity of the vapor shooting out of the gap between the drop and the surface.In the contact boiling regime, we show that the structured surfaces induce the formation of vertical liquid jets during the spreading stage of impacting droplets.

We study drop impact on a deep pool of the same fluid, with an emphasis on the air layer trapped under the droplets from its formation to its rupture. The penetration velocity of the air layer at a very short time scale prior to its rupture is shown, using an energy argument and experimental verification, to be one-half of the impact velocity. We then deduce the dependence of the rupture position on the liquid viscosity and the impact velocity. We show that the volume of the resulting air bubbles can be related to both those resulting from droplets impacting on solid surfaces and those resulting from rigid spheres impacting on liquid surfaces.