Drops can bounce from perfectly hydrophilic surfaces

Drops are well known to rebound from superhydrophobic surfaces and from liquid surfaces. Here, we show that drops can also rebound from a superhydrophilic solid surface such as an atomically smooth mica sheet. However, the coefficient of restitution CR associated with this process is significantly lower than that associated with rebound from superhydrophobic surfaces. A direct imaging method allows us to characterize the dynamics of the deformation of the drop in entering the vicinity of the surface. We find that drop bouncing occurs without the drop ever touching the solid and there is a nanometer-scale film of air that separates the liquid and solid, suggesting that shear in the air film is the dominant source of dissipation during rebound. Furthermore, we see that any discrete nanometer-height defects on an otherwise hydrophilic surface, such as treated glass, completely inhibits the bouncing of the drop, causing the liquid to wet the surface. Our study adds a new facet to the dynamics of droplet impact by emphasizing that the thin film of air can play a role not just in the context of splashing but also bouncing, while highlighting the role of rare surface defects in inhibiting this response. editor’s choice Copyright c © EPLA, 2014 The impact of liquid droplets on solid surfaces is ubiquitous in many natural and industrial settings. Depending on the impact parameters, a liquid drop may spread, break up, splash or bounce [1]. While spreading and splashing occur under a wide range of conditions [1–5], it is traditionally thought that drops will only bounce from solid surfaces that are special: heated beyond the boiling temperature of the impacting liquid [6] or superhydrophobic; textured and functionalized to comply with a Cassie-Baxter state, wherein the liquid is supported by micron-sized asperities and the interstitial air [7–10]; additionally, drops are well known to rebound from liquid surfaces [11]. Here we show that this is not the case: drops can bounce from any smooth enough hydrophilic surface at room temperature as well. Indeed, drops were observed to rebound from glass, silicon wafers and from mica; thus, drop rebound appears to be a general phenomenon for impact upon smooth, defect free surfaces. Our experiments were carried out by impacting drops upon a freshly cleaved mica surface, which is a hydrophilic surface with a vanishing contact angle, as shown in fig. 1(b). We find that if the impact velocity V is below a critical value, Vc = 0.75m/s, then the drop will rebound, while above Vc, the drop will wet the surface. Similar rebound events are observed for impacting drops of silicon oil as well as for impact on clean glass slides. This suggests that when the drop bounces it never contacts the substrate, and instead is cushioned by a thin film of air [12]. Indeed, for V < 0.2m/s, the drop does not visibly detach from the surface and bounce; instead, the drop oscillates above a steady thin film of air without fully retracting away from the solid surface. Similarly, for impact upon a superhydrophobic substrate at low V , the drop oscillates while in partial contact with the substrate [13]. The role of air in droplet impact has recently been the focus of much study and debate [2,5,12,14–23]. To understand the role played by air during the early dynamics of droplet impact, we use Total Internal Reflection (TIR) microscopy to directly observe the impact surface [12], as shown schematically in fig. 1(a). When the droplet enters the evanescent field above the impact surface, partially transmitted light results in a gray-scale image, as shown in fig. 1(c), which can be converted to a height profile h, as shown in fig. 1(d). While TIR can resolve nanometer scales, its range is limited and cannot resolve the dynamics occurring more than