MPIDS Colloquium: A physicist’s View of hydrophobicity: drying is critical
14:15 - 15:15
Prof. Bob Evans
HH Wills Physics Laboratory, University of Bristol, UK
Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
Prandtl Lecture Hall
All physical scientists would agree that for water (or another liquid) at a flat substrate a contact an-gle θ > 90º defines the substrate as hydrophobic (solvophobic): it prefers gas to liquid. Is there an effective indicator of local ordering of the liquid, manifest at microscopic distances from the sub-strate, that correlates with the macroscopic (thermodynamic) contact angle?
We show such an indicator is the local compressibility χ(z) which measures fluctuations in the local density of the adsorbed liquid. For distances z within one or two molecular diameters of a weakly adsorbing substrate, where θ approaches 180º (drying), classical Density Functional Theory and Grand Canonical Monte Carlo simulations for a LJ liquid and for the realistic SPC/E model of wa-ter, show that χ(z) takes values that are orders of magnitude larger than in the bulk liquid. Such be-haviour is characteristic of a critical drying transition. For substrate-fluid potentials that exhibit dis-persion forces (power-law decay), critical drying occurs in the limit of vanishing attraction. By con-trast, the wetting transition is always first order.
(Work with M.C. Stewart and N. B. Wilding.)