Antibubbles and fine cylindrical sheets of air

Abstract

Drops impacting at low velocities onto a pool surface can stretch out thin hemispherical sheets of air between the drop and the pool. These air sheets can remain intact until they reach submicron thicknesses, at which point they rupture to form a myriad of microbubbles. By impacting a higher-viscosity drop onto a lower-viscosity pool, we have explored new geometries of such air films. In this way we are able to maintain stable air layers which can wrap around the entire drop to form repeatable antibubbles, i.e. spherical air layers bounded by inner and outer liquid masses. Furthermore, for the most viscous drops they enter the pool trailing a viscous thread reaching all the way to the pinch-off nozzle. The air sheet can also wrap around this thread and remain stable over an extended period of time to form a cylindrical air sheet. We study the parameter regime where these structures appear and their subsequent breakup. The stability of these thin cylindrical air sheets is inconsistent with inviscid stability theory, suggesting stabilization by lubrication forces within the submicron air layer. We use interferometry to measure the air-layer thickness versus depth along the cylindrical air sheet and around the drop. The air film is thickest above the equator of the drop, but thinner below the drop and up along the air cylinder. Based on microbubble volumes, the thickness of the cylindrical air layer becomes less than 100 nm before it ruptures.

Vortex-induced buckling of a viscous drop impacting a pool

“…This was verified by comparing with impacts on fresh pools, as was also done in Ref. [40]. Figure 3 shows the changes in the evolution of the drop shape, as the impact velocity is increased, while keeping the same drop and pool viscosities of 500 and 0.65 cSt respectively.…”

“…It is clear that there is not a trivial relationship between the impact velocity U and the penetration speed of the drop into the pool, as the penetration is affected by impact momentum, gravity through the air-crater formation, as well as the weight of the drop, which is of slightly larger density than the pool. For very low impact velocities, a stable air film can act as a trampoline, causing the drop to rebound from the surface, as was previously studied for these same liquids [40,44]. For the lowest velocities herein, the drop penetrates with a small crater [ Fig.…”

“…Figure 2 shows a schematic of the setup, which mirrors that used by Beilharz et al [40]. The pool liquid is contained in a 50-mm-deep glass container, with a 40×50 mm cross section.…”

Probing the nanoscale: the first contact of an impacting drop

“…The total number of visible micro-bubbles in this entire band around the full periphery is here estimated to be ∼ 12 800, ranging in sizes from 1-5 µm. The bubble sizes are smallest near the first rupture, where the air layer should be thinnest (Beilharz et al (2015)) and grow larger towards the edges of the band (Fig. 5b).…”

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

“…This configuration is challenging for interferometry due to the large curvature of the free surface (20) . However, for one variant of this experiment, where one produces fine cylindrical sheets of air, which form around highly viscous drop impacting on a lower viscosity pool (21) . One such realization is shown in Figure 2, where the drop viscosity is 10,000 cSt silicon oil and the pool is only 1 cSt.…”