Impact of ultra-viscous drops: air-film gliding and extreme wetting

Abstract

A drop impacting on a solid surface must push away the intervening gas layer before making contact. This entails a large lubricating air pressure which can deform the bottom of the drop, thus entrapping a bubble under its centre. For a millimetric water drop, the viscous-dominated flow in the thin air layer counteracts the inertia of the drop liquid. For highly viscous drops the viscous stresses within the liquid also affect the interplay between the drop and the gas. Here the drop also forms a central dimple, but its outer edge is surrounded by an extended thin air film, without contacting the solid. This is in sharp contrast with impacts of lower-viscosity drops where a kink in the drop surface forms at the edge of the central disc and makes a circular contact with the solid. Larger drop viscosities make the central air dimple thinner. The thin outer air film subsequently ruptures at numerous random locations around the periphery, when it reaches below 150 nm thickness. This thickness we measure using high-speed two-colour interferometry. The wetted circular contacts expand rapidly, at orders of magnitude larger velocities than would be predicted by a capillary–viscous balance. The spreading velocity of the wetting spots is ${\sim}0.4~\text{m}~\text{s}^{-1}$ independent of the liquid viscosity. This may suggest enhanced slip of the contact line, assisted by rarefied-gas effects, or van der Waals forces in what we call extreme wetting. Myriads of micro-bubbles are captured between the local wetting spots.

Probing the nanoscale with high-speed interferometry of an impacting drop

“…(a) Monochromatic light is either shone from the bottom (Light 2) through a half-silvered mirror, for reflective interferometry, or from the top (Light 1) for transmission interferometry (12) . (b) Twocolor interferometry setup, used in Langley et al (15) . (c,d) Interference fringes under the impacting drop, from transmitted (c) and reflective (d) interferometry with monochromatic light (12) .…”

“…The long-distance microscope (Leica Z16 APO) has magnification up to 29.4 and can reach around 1 µm/px resolution even at the highest frame-rate. In limited cases we used a two colors setup (red and blue) to measure absolute thicknesses (15) , but this could only be applied up to 16 kfps and was used to supplement the high-speed monochromatic results, as shown in sec. 3.4.…”

“…In those studies a critical impact Weber numbers has been identified as 4, below which the drop can bounce without contacting the solid (33) . This can even occur for hydrophilic solid (15) .…”

“…Here the kink in the free surface, which makes the first contact with the solid (12,31) , is greatly damped and the drop does not make contact along a ring, but rather it glides on a thin air-layer (15) . This extended air-layer thins until it ruptures at numerous random locations, as is shown in Fig.…”

“…4(a). Skating on a layer of air has been suggested for low-viscosity drops (31)(32)(33) , but only realized for very low impact velocities (15) . In those studies a critical impact Weber numbers has been identified as 4, below which the drop can bounce without contacting the solid (33) .…”