Evaporation and instabilities of microscopic capillary bridges

Abstract

The formation and disappearance of liquid bridges between two surfaces can occur either through equilibrium or nonequilibrium processes. In the first instance, the bridge molecules are in thermodynamic equilibrium with the surrounding vapor medium. In the second, chemical potential gradients result in material transfer; mechanical instabilities, because of van der Waals force jumps on approach or a Rayleigh instability on rapid separation, may trigger irreversible film coalescence or bridge snapping. We have studied the growth and disappearance mechanisms of laterally microscopic liquid bridges of three hydrocarbon liquids in slit-like pores. At rapid slit-opening rates, the bridges rupture by means of a mechanical instability described by the Young-Laplace equation. Noncontinuum but apparently reversible behavior is observed when a bridge is held at nanoscopic surface separations H close to the thermodynamic equilibrium Kelvin length, 2r Kcos , where rK is the Kelvin radius and is the contact angle. During the course of slow evaporation (at H > 2r Kcos ) and subsequent regrowth by capillary condensation (at H < 2r Kcos ), the refractive index of the bridge may vary continuously and reversibly between that of the bulk liquid and vapor. The evaporation process becomes irreversible only at the very final stage of evaporation, when the refractive index of the fluid attains virtually that of the vapor. Measured refractive index profiles and the time-dependence of evaporating neck diameters also seem to differ from predictions based on a continuum picture of bridge evaporation far from the critical point. We discuss these findings in terms of the probable density profiles in evolving liquid bridges.T he equilibrium and dynamic aspects of phase changes are often influenced by the presence of surfaces and interfaces. For example, in the case of homogeneous nucleation of a liquid phase, a significant supersaturation must be reached before vapor starts to condense to form droplets because of the activation barrier in the free energy that hinders the nucleation of clusters smaller than a certain size. Interfaces can lower this activation barrier and speed up the nucleation process. From a practical point of view, the nucleation, condensation, and evaporation of fluids in confined geometries play a major role in adhesion, the behavior of moist granular materials, and the transport of fluids through porous media.Much less attention has been paid to the process of evaporation or disappearance of capillary-held liquids than to their formation (condensation or nucleation). The surface force apparatus (SFA; refs. 1-3) provides a powerful means with which to study the behavior of highly confined fluids and has been used to study various aspects of capillary condensed liquids. Recently, we reported some preliminary results on the evaporation of microscopic cyclohexane bridges (4). The major result from this study was the observation of lateral refractive index gradients across the evaporating bridges once they shrink below a cert...

Evaporation-induced evolution of the capillary force between two grains

“…To our knowledge, no comprehensive studies on capillary bridge evaporation have been undertaken. There are a few notable exceptions of a specifically focused work on nano-scale separations (<100 nm) by Gao [11] and Maeda et al [12], or spheres at direct contact, Cutts and Burns [13], or on drying of colloidal crystals by Zhou et al [14]. Most of the previous studies on the capillary forces in two-grain liquid bridges deal with extension of the bridges at a constant liquid volume or a constant suction [12,[15][16][17][18].…”

Subharmonic excitation in amplitude modulation atomic force microscopy in the presence of adsorbed water layers

“…The prediction of subharmonics due to water impacts could thus be used to investigate processes such as nucleation time of water columns in the nanoscale for which a physical understanding is still emerging. 24,27,29,47,48 …”

Direct observation of self-assembled chain-like water structures in a nanoscopic water meniscus

“…A downward portion between two subsequent bundles of water chains gradually changed as the distance decreased, suggesting that the observed oscillatory forces are different from the mechanical instabilities of the Page 9 of 25 meniscus, such as pinning/unpinning of the contact line (of which the transition would be abrupt), 64 or a coupling between evaporation/condensation of vapor (which are highly unstable phenomena). 65 We also excluded the instability of the feedback loop as the origin of the observed oscillatory forces because the feedback signal is stable even with such abrupt changes in the square wave in our routine feedback optimization procedure (see Experimental Section). The oscillatory forces had amplitudes of 3-12 nN in normal force channel, and periodicities of 1.04±0.25 nm and 0.94±0.16 nm for valley-valley and for peak-peak distances, respectively.…”