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.
Highlights
When a drop impacts on a pool surface at low velocity, it can bounce from the surface (Couder et al 2005; Molacek & Bush 2013), entrap a bubble under its centre (Thoroddsen, Etoh & Takehara 2003; Liow & Cole 2007) or stretch out a hemispherical layer of air which breaks up into a multitude of microbubbles, D
We start by showing some of the main previously described phenomena in figure 3. These include rebounding from the pool surface, when all the kinetic energy of the drop is overcome by the surface energy of the deformed crater within the pool
For still larger impact velocity, we show, in figure 3(c), the early breakup of the air layer which entraps a central air disc, a problem extensively studied for higher impact velocities (Thoroddsen et al 2003)
Summary
When a drop impacts on a pool surface at low velocity, it can bounce from the surface (Couder et al 2005; Molacek & Bush 2013), entrap a bubble under its centre (Thoroddsen, Etoh & Takehara 2003; Liow & Cole 2007) or stretch out a hemispherical layer of air which breaks up into a multitude of microbubbles,. Saylor & Testik (2012) showed that the breakup of this air layer is inherently random for water, while Saylor & Bounds (2012) demonstrated that the thin air films are much more stable for some other liquids, such as silicone oils and ethanol (Sundberg-Anderson & Saylor 2014) This repeatability allowed Thoroddsen et al (2012) to study details of the air-film breakup, employing triggered imaging with an ultra-high-speed CCD video camera (Etoh, Poggemann & Kreider 2003). In the present study we observe these air films for silicone oils when we increase the viscosity of the drop liquid while keeping the pool liquid at low viscosity This slows down the shape deformations of the drop and allows the formation of thin air layers with new geometries, which cannot be realized when the drop and pool are of the same viscosity. The image in figure 1(b) shows such a case
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