Wettability-independent bouncing on flat surfaces mediated by thin air films

Abstract

The impingement of drops onto solid surfaces 1,2 plays a crucial role in a variety of processes, including inkjet printing, fog harvesting, anti-icing, dropwise condensation and spray coating 3-6. Recent e orts in understanding and controlling drop impact behaviour focused on superhydrophobic surfaces with specific surface structures enabling drop bouncing with reduced contact time 7,8. Here, we report a di erent universal bouncing mechanism that occurs on both wetting and non-wetting flat surfaces for both high and low surface tension liquids. Using high-speed multiple-wavelength interferometry 9 , we show that this bouncing mechanism is based on the continuous presence of an air film for moderate drop impact velocities. This submicrometre 'air cushion' slows down the incoming drop and reverses its momentum. Viscous forces in the air film play a key role in this process: they provide transient stability of the air cushion against squeeze-out, mediate momentum transfer, and contribute a substantial part of the energy dissipation during bouncing. The role of ambient air in drop impact and other dynamic wetting phenomena has long been neglected. Only recently, observations such as the suppression of splashing in drop impact at reduced ambient pressure 10 , the generation of splashes by superhydrophobic spheres falling into a liquid bath 11 , and the entrainment of air by fast-moving contact lines 12 highlighted the relevance of this rather viscous ambient medium. For drop impact, theoretical studies 13-16 suggested that splashing might be related to the presence of a thin lubricating air layer between the impacting drop and the substrate. Subsequent experiments confirmed the transient formation of an air layer with (sub)micrometre thickness 9,17-20. However, it turned out that the air film collapses on a microsecond timescale for typical impact speeds of splashing experiments (of the order of m s −1). However, as we report in this Letter, the air film remains intact if the initial impact speed is reduced to less than ν ∼ 0.5 m s −1. In this case, the drop rebounds without ever directly touching the surface. We release liquid drops of water, glycerol, silicone oil and various other organic liquids (Supplementary Table 1) of millimetric size (R = 0.52. .. 1.03 mm) from a height of several millimetres to a few centimetres to fall onto carefully cleaned and dust-free surfaces of variable wettability (Methods). On their first impact the drops have initial Weber numbers We = ρRν 2 /σ = 0.64. .. 4.3 (ρ: liquid density; R: drop radius; σ : surface tension). For all liquids studied, side-view images taken with a high-speed video camera (Fig. 1a) show that the drops bounce provided that the impact speed is not too high. For water, the maximum impact speed is 0.48 m s −1 , corresponding to We max ≈ 4. Throughout the entire bouncing process, the apparent contact angle observed in side-view images remains close to 180 • , both for clean glass substrates with an equilibrium contact

Contactless prompt tumbling rebound of drops from a sublimating slope

“…These include splash [6][7][8][9][10], phase-change-induced surface levitation [11][12][13][14][15], skating on a film of trapped air [16][17][18], and rebounding [19][20][21][22]. Recently [11], it was demonstrated that drops can rebound after impact on an extremely cold solid carbon dioxide surface (at -79°C, well below the limit of even homogeneous nucleation of water), because of the formation of a sublimated vapor layer acting both as impact cushion and thermal insulator, enabling drops to hover and rebound without freezing.…”

Kelvin–Helmholtz instability in an ultrathin air film causes drop splashing on smooth surfaces

“…However, this interaction is highly complex: Below the drop air is trapped at both the impact center and the expanding front (21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34), and above it the atmosphere constantly interacts with its top surface. As a result, even the very basic question of which part of air plays the essential role is completely unknown.…”

Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides

“…[1][2][3][4][5] There has been considerable focus on understanding their wettability 6,7 and developing them for a wide range of applications including droplet impact resistance, [8][9][10][11][12][13] anti-icing, [14][15][16][17][18][19] dropwise condensation, [20][21][22][23][24] electro-wetting, 25,26 drag reduction, [27][28][29][30][31][32] evaporation, 33,34 and anti-corrosion. [35][36][37][38] An important challenge for broad applicability of these hydrophobic materials is their limited robustness.…”