Contact Line Stability
Part of our research program involves looking at the stability of contact lines in mensici during evaporation and condensation. At low heat inputs or low condensation rates, a meniscus will generally have a stable contact line with a particular curvature profile and contact angle. As the heat input increases, we have noticed that the contact line remains stable for a while although the contact angle begins to increase. At a particular value of the heat input, the contact line loses its stability and begins to oscillate. This oscillation increases in frequency and amplitude as the heat input increases.
Both the curvature, contact angle, and adsorbed film thickness ahead of the contact line oscillate. The following figures show the oscillations. The fluid seems to behave similarly to waves on a beach.
Curvature oscillations as a function of time. Notice that the curvature gradient increases during recession and decreases during advancement.
Oscillations in contact angle and adsorbed film thickness as a function of time. Notice that the contact angle and adsorbed film thickness appear to be out of phase with one another.
Finally, you can see the complete oscillatory behavior by clicking on the movie below. It is a Quicktime of an oscillating meniscus of a fluorocarbon liquid, HFE-7000. The small droplets seen in the figure represent a second, higher boiling impurity present in the fluid. Evaporative distillation isolates this fluid quite efficiently since the fluorocarbon is 99.8% pure to start with.
More Information
A Study of the Oscillating Corner Meniscus in a Vertical Constrained Vapor Bubble System