Abstract
In this paper we contrast the early impact stage of a highly viscous drop onto a liquid versus a solid substrate. Water drops impacting at low velocities can rebound from a solid surface without contact. This dynamic is mediated through lubrication of a thin air layer between the liquid and solid. Drops can also rebound from a liquid surface, but only for low Weber numbers. Impacts at higher velocities in both cases lead to circular contacts which entrap an air disc under the centre of the drop. Increasing the drop viscosity produces extended air films for impacts on a smooth solid surface even for much larger velocities. These air films eventually break through random wetting contacts with the solid. Herein we use high-speed interferometry to study the extent and thickness profile of the air film for a large-viscosity drop impacting onto a viscous film of the same liquid. We demonstrate a unified scaling of the centreline height of the air film for impacts on both solid and liquid, when using the effective impact velocity. On the other hand, we show that the large-viscosity liquid film promotes air films of larger extent. Furthermore, the rupture behaviour becomes fundamentally different, with the air film between the two compliant surfaces being more stable, lacking the random wetting patches seen on the solid. We map the parameter range where these air films occur and explore the transition from gliding to ring contact at the edge of the drop dimple. After the air film ruptures, the initial contraction occurs very rapidly and for viscosities greater than 100 cSt the retraction velocity of the air film is ${\sim}0.3~\text{m}~\text{s}^{-1}$, independent of the liquid viscosity and impact velocity, in sharp contrast with theoretical predictions.
Highlights
Impacting drops are present and important in many natural and industrial processes and have been a research topic of continued interest
Using the equipment outlined above, this technique can measure a maximum film thickness of ∼30 μm when the long-distance microscope is narrowly focused on the liquid free surface; only the interference due to the intervening air layer is seen in the images, but not the drop surface further from the centre
The impact velocity and drop size are similar in both cases
Summary
Impacting drops are present and important in many natural and industrial processes and have been a research topic of continued interest There have been many studies investigating the physics of the air-disc entrapment for drops impacting solid surfaces (see Thoroddsen et al 2005; Mandre, Mani & Brenner 2009; Hicks & Purvis 2010; Bouwhuis et al 2012; de Ruiter et al 2012; van der Veen et al 2012; Li & Thoroddsen 2015; Philippi, Lagrée & Antkowiak 2016; Langley, Li & Thoroddsen 2017; Li et al 2017; Langley et al 2018). Langley et al (2017) explored a much larger range of viscosities (10–2 × 107 cSt) and velocities (0.3–5.2 m s−1) impacting on smooth solid surfaces, and found that higher-viscosity drops glide over a thin layer of air even at high impact velocities of ∼5 m s−1. By impacting a liquid surface, we eliminate possible asperities from the surface, allowing further investigation into the transition between gliding and ring contact at the perimeter of the dimple
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