From Bouncing to Floating: Noncoalescence of Drops on a Fluid Bath

When a drop of a viscous fluid is deposited on a bath of the same fluid, it is shown that its coalescence with this substrate is inhibited if the system oscillates vertically. Small drops lift off when the peak acceleration of the surface is larger than $g$. This leads to a steady regime where a drop can be kept bouncing for any length of time. It is possible to inject more fluid into the drop to increase its diameter up to several centimeters. Such a drop remains at the surface, forming a large sunk hemisphere. When the oscillation is stopped, the two fluids remain separated by a very thin air film, which drains very slowly ($\sim 30\min$). An analysis using lubrication theory accounts for most of the observations.

Figure 1

Two photographs showing a drop of radius 2 mm as it bounces on the liquid surface. The arrows show the direction of the bath motion.

Figure 2

(a) The observed acceleration thresholds for the bouncing of drops of various sizes as a function of the forcing frequency. (b) The rescaling of the data by Eq. (1): plot of $(\gamma _m{}^C-g)/l$ as a function of the frequency. The symbols corresponding to drops of various sizes are the same as in (a). (c) Inset: Sketch of the geometry of the drop and of the separating film.

Figure 3

(a) A large floating drop, as seen from the side. (b) The interference fringes of the air film as observed when the drop is lit from the top with white light. The black dot at the center is the reflection of the camera’s lens.

Figure 4

(a) The local thickness of an air film (as measured from the interference fringes) as a function of the nondimensional distance $r/R_H$ to the drop axis. The evolution at four different times shows the early stages of the film thinning. (b) The thinning air films as a function of time for three different drops with $R_H=1.2\mathrm{cm}$ and for which the initial film thickness was $1.5\mu \mathrm{m}$. The dashed line is the thinning estimated by Eq. (3). (c) Inset: Sketch of the geometry of the floating drop.