Failure of thermocapillary-driven permanent nonwetting droplets

Abstract

A droplet may be prevented from molecular contact with a solid surface by providing a thin, lubricating film of surrounding fluid between the solid and liquid surfaces. In this study, we exploit thermocapillary convection, caused by a temperature difference maintained between the droplet and the unwetted surface, to provide this lubricating film. This state may be sustained indefinitely ͑permanent nonwetting͒ if the load applied to the droplet does not exceed a threshold. Failure of such systems may be categorized as either film or pinning failures, depending on whether the lubrication film is breached, resulting in a molecular contact between the droplet and the solid surface, or the droplet is forced from its support by losing its pinning contact line. In this work, loads that trigger film and pinning failures are quantified, and their mechanisms explained. Results show that larger loads can be sustained for systems with an elevated temperature difference and for droplets of higher viscosity.

Buckling instability of squeezed droplets

“…In this paper we show that the shape instability observed by Nagy and Neitzel [23] arises with the same geometrical criterion as that of the Plateau liquid bridge, namely when the droplet surface becomes tangent to the pinned surface at the point of contact (apparent contact angle of π). We demonstrate that this instability criterion does not depend on the droplet contact angle on the upper surface.…”

“…When the droplet is compressed, by displacing the top surface, below a threshold the droplet maintains axisymmetry, but upon reaching a critical conformation, the droplet bulges to one side indicating a shape instability. Subsequent Surface Evolver [26] simulations of the same setup verified the geometric asymmetry [23].…”

“…In order to corroborate our asymptotic predictions we appeal to numerical simulations using Surface Evolver (SE) [26]. Nagy and Neitzel also used the same program to confirm the existence of their observed instability in an idealized setting [23]. In Surface Evolver the droplet shape is discretized into triangular facets whose positions are defined by a position vector X hence the energy is E(X) [30].…”

“…non-wetting scenario [23][24][25]. The peak load occurs at the critical shape, beyond which the droplet undergoes a limit-point buckling instability, and collapses.…”

“…In recent experiments by Nagy and Neitzel, droplets pinned to a bottom surface and compressed by a perfectly non-wetting surface from above have been shown to develop a geometric asymmetry at a critical deformation [23]. Experimentally, the perfectly non-wetting condition on the top surface is obtained using a thin layer of air maintained by thermocapillary convection between the cold surface (of arbitrary contact angle) and the hot droplet, leading to an effective contact angle of 180 • [24,25].…”