Probing the nanoscale: the first contact of an impacting drop

Abstract

When a drop impacts a solid surface, the lubrication pressure in the air deforms its bottom into a dimple. This makes the initial contact with the substrate occur not at a point but along a ring, thereby entrapping a central disc of air. We use ultra-highspeed imaging, with 200 ns time resolution, to observe the structure of this first contact between the liquid and a smooth solid surface. For a water drop impacting onto regular glass we observe a ring of micro-bubbles, owing to multiple initial contacts just before the formation of the fully wetted outer section. These contacts are spaced by a few microns and quickly grow in size until they meet, thereby leaving behind a ring of microbubbles marking the original air-disc diameter. On the other hand, no micro-bubbles are left behind when the drop impacts onto molecularly smooth mica sheets. We thereby conclude that the localized contacts are due to nanometric roughness of the glass surface and the presence of the micro-bubbles can therefore distinguish between glass with 10 nm roughness from perfectly smooth glass. We contrast this entrapment topology with the initial contact of a drop impacting onto a film of extremely viscous immiscible liquid, where the initial contact appears continuous along the ring. Here an azimuthal instability occurs during the rapid contraction at the triple line, also leaving behind micro-bubbles. For low impact velocities the nature of the initial contact changes to one initiated by ruptures of a thin lubricating air film.

Two mechanisms of droplet splashing on a solid substrate

“…These bubbles are formed by an instability of the moving contact line but note that due to the cylindrical symmetry of our calculations these bubbles are in fact tori that would be unstable in 3D. These bubbles are similar to those observed in numerical simulations of drop impacts on a thin liquid film ) and reminiscent of experimental observations by Li et al (2015b). How these bubbles would fare under refinement of the grid has not been investigated but their size would likely vary.…”

“…Measuring these quantities experimentally remains a difficult challenge because of the small values of the Stokes numbers but this has been done eventually using complex optical techniques (Driscoll & Nagel 2011;Kolinski et al 2012;Bouwhuis et al 2012;Klaseboer et al 2014;Li et al 2015a). The results are somehow puzzling since the theoretical predictions have not always been observed or only in a narrow range of Stokes numbers.…”

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

“…This was observed in early imaging (24) and speculated to be due to micro-bubbles. With the high-speed interferometry we have indeed verified this hypothesis and shown that the roughness of the solid plays a key role in this entrapment of the ring of micro-bubbles (26) . Figure 3 (26) .…”

“…Finally, in this experiment we showed that for very low impact velocity the water drop can glide on the air-layer without initial contact between the two liquids. The breakup of this thin air-layer was observed with a spatial resolution of 0.6 µm/px and a time resolution of 200 ns, showing the details of this breakup process in unprecedented detail (26) . This breakup leaves behind a wide axisymmetric band of micro-bubbles (see their figure 5).…”

“…The early interferometric video imaging of the air-layers under impact drops (22)(23)(24)(25)(26) were not time-resolved and could therefore not track the compression of the air inside the central dimple. Our time-resolved imaging has revealed new aspects of the air-layer dynamics (12) , as the bottom of the drop is deformed into a dimple, which makes first contact with the solid not at a point, but rather along a ring which entraps the air-film.…”