C5MPT Summit Speaker Series Event
School of Physics and Astronomy
University of Leeds, Leeds LS2 9JT, U.K.
Lecture #1: May 3, 2016 2:00 pm
Donadeo ICE 8-207 Patrick D Daniel/Enbridge Conference Room
Cavitation in Confinement
Bubble formation in water occurs during boiling, when the vapour pressure of the water reaches atmospheric pressure, and during cavitation, when the external pressure decreases below the vapour pressure. In both cases, however, visible bubble formation is almost invariably due to the growth and release of pre-existing, microscopic bubbles in surface cracks. This is why bubbles in a glass of champagne or a pan of boiling water always emerge from the same points on the surface of the vessel. True nucleation of a bubble in water is an extremely rare event, particularly in bulk, but even at a hydrophobic surface. Although simple surface energy considerations show that a thin film of water between two hydrophobic surfaces is metastable towards vapour or dissolved gas, the kinetic barrier to nucleation is expected to be prohibitively high at finite surface separations. In the 1980s experiments with the surface force apparatus and similar instruments showed that bubbles will under some conditions nucleate between two hydrophobic surfaces in water (figure below).
Photographic images of interference fringes and schematics of two curved (R ≈ 2 cm) hydrophobic surfaces in water. The fringes result from passing white light through the back-silvered surfaces, which allows transmission of discrete wavelengths only. If surfaces in contact (left, vertical fringes of constant wavelength for the flattened contact zone) are separated, a bubble may nucleate between the surfaces (right) as these regain their curved shape. The liquid-bubble interface shows up as discontinuities in interference fringes. The bubble remains stable at a finite surface separations, but disappears if the surfaces are brought back into contact.
Although these observations have been confirmed in more recent experiments, it appears as if little systematic work has been carried out, and there is much to learn about the precise conditions necessary for cavitation and the relationship to macroscopic quantities like surface energies and contact angles. I will discuss stability criteria for bridging bubbles and the relationship, or lack thereof, to surface nanobubbles and the force between macroscopic hydrophobic surfaces. Speculation about possible future research directions will follow.