Transport Mechanisms in
Capillary Condensation of
Water at a Single-Asperity
Nanoscopic Contact
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Abstract
Transport mechanisms involved in capillary condensation
of water
menisci in nanoscopic gaps between hydrophilic surfaces are investigated
theoretically and experimentally by atomic force microscopy (AFM)
measurements of capillary force. The measurements showed an instantaneous
formation of a water meniscus by coalescence of the water layers adsorbed
on the AFM tip and sample surfaces, followed by a time evolution of
meniscus toward a stationary state corresponding to thermodynamic
equilibrium. This dynamics of the water meniscus is indicated by time
evolution of the meniscus force, which increases with the contact
time toward its equilibrium value. Two water transport mechanisms
competing
in this meniscus dynamics are considered: (1) Knudsen diffusion and
condensation of water molecules in the nanoscopic gap and (2) adsorption
of water molecules on the surface region around the contact and flow
of the surface water toward the meniscus. For the case of very hydrophilic
surfaces, the dominant role of surface water transportation on the
meniscus dynamics is supported by the results of the AFM measurements
of capillary force of water menisci formed at sliding tip–sample
contacts. These measurements revealed that fast movement of the contact
impedes on the formation of menisci at thermodynamic equilibrium because
the flow of the surface water is too slow to reach the moving meniscus