A unified description of hydrophilic and superhydrophobic surfaces in terms of the wetting and drying transitions of liquids

Abstract

Clarifying the factors that control the contact angle of a liquid on a solid substrate is a long-standing scientific problem pertinent across physics, chemistry, and materials science. Progress has been hampered by the lack of a comprehensive and unified understanding of the physics of wetting and drying phase transitions. Using various theoretical and simulational techniques applied to realistic fluid models, we elucidate how the character of these transitions depends sensitively on both the range of fluid–fluid and substrate–fluid interactions and the temperature. Our calculations uncover previously unrecognized classes of surface phase diagram which differ from that established for simple lattice models and often assumed to be universal. The differences relate both to the topology of the phase diagram and to the nature of the transitions, with a remarkable feature being a difference between drying and wetting transitions which persists even in the approach to the bulk critical point. Most experimental and simulational studies of liquids at a substrate belong to one of these previously unrecognized classes. We predict that while there appears to be nothing particularly special about water with regard to its wetting and drying behavior, superhydrophobic behavior should be more readily observable in experiments conducted at high temperatures than at room temperature.

Classical density-functional theory applied to the solid state

“…The mean-field term is already given explicitly and we just note that w att (r 1 , r 2 ) = w att (|r 1 − r 2 |), the attractive part of the potential. Finally, φ(r) represents any external one-body field (this plays no role in the present work) and μ = μ − k B T ln ( λ l ) 3 , where μ is the chemical potential and λ is the thermal wavelength. The model functional has been presented for a single-component system but generalization to multiple components is straightforward.…”

“…Conceptually, cDFT calculations involve the minimization of a functional of the local density resulting in both the equilibrium density distribution and the free energy of the system. Notable recent applications include the description of wetting phenomena [3], the calculation of hydration free energies and microscopic structure of molecular solutes [4], and the description of crystallization pathways [5]. An important part of this utility lies in the highly developed description of correlations due to excluded-volume effects that are arise whenever molecules interact via potentials having divergent short-ranged repulsion as is the case, e.g., in simple fluids and is captured in such commonly used models as the Lennard-Jones, Stillinger-Weber, and hard-core Yukawa potentials.…”

Playing the long game wins the cohesion–adhesion rivalry

“…Our understanding of wetting and drying transitions stems from the seminal work of Nakanishi and Fisher (3,4): a single class of surface phase transitions with wetting and drying becoming equivalent at the bulk critical point with temperature T C . In PNAS, Evans et al (5) predict 3 additional classes of surface phase diagrams, with the differences in the 4 total classes arising from the ranges of the ff and fw interactions (5).…”

“…A fundamental observable characterizing wetting/ drying is the contact angle, θ, the angle of intersection between the solid wall (w) and the liquid-vapor (l-v) interface at the 3-phase contact line [see figure 1 of Evans et al (5)]. The contact angle quantifies the balance of surface tensions, γ, where the 3 phases meet through Young's equation, γ lv cosθ = γ wv − γ wl .…”

“…Physically, θ is a manifestation of the competition between fw and ff interactions, that is, adhesion versus cohesion. Evans et al (5) answer the important question, "How do the ranges of these competing interactions affect wetting and drying transitions?" Most earlier work focused on cases where both ff and fw interactions were truncated at some length scale, such that both were short-ranged (SR), often an approximation made to facilitate efficient numerical simulations.…”