Abstract
The Cassie-Wenzel transition of a symmetric binary liquid mixture in contact with a nano-corrugated wall is studied. The corrugation consists of a periodic array of nanopits with square cross sections. The substrate potential is the sum over Lennard-Jones interactions, describing the pairwise interaction between the wall particles C and the fluid particles. The liquid is composed of two species of particles, A and B, which have the same size and equal A-A and B-B interactions. The liquid particles interact between each other also via A-B Lennard-Jones potentials. We have employed classical density functional theory to determine the equilibrium structure of binary liquid mixtures in contact with the nano-corrugated surface. Liquid intrusion into the pits is studied as a function of various system parameters such as the composition of the liquid, the strengths of various interparticle interactions, and the geometric parameters of the pits. The binary liquid mixture is taken to be at its mixed-liquid-vapor coexistence. For various sets of parameters the results obtained for the Cassie-Wenzel transition, as well as for the metastability of the two corresponding thermodynamic states, are compared with macroscopic predictions in order to check the range of validity of the macroscopic theories for systems exposed to nanoscopic confinements. Distinct from the macroscopic theory, it is found that the Cassie-Wenzel transition cannot be predicted based on the knowledge of a single parameter, such as the contact angle within the macroscopic theory.
Highlights
Wetting of solid surfaces by liquids is ubiquitous in nature [1,2,3] as well as in various technological applications [4,5,6,7,8,9]
We have studied the Cassie-Wenzel transition of a symmetric binary liquid mixture at a nano-corrugated surface which exhibits a periodic array of nanopits with a square cross section
The structural properties of the binary liquid mixtures in thermal equilibrium and, in case it applies, in metastable equilibrium have been determined by using density functional theory, which captures the microscopic details of the system
Summary
Wetting of solid surfaces by liquids is ubiquitous in nature [1,2,3] as well as in various technological applications [4,5,6,7,8,9]. With the advent of advanced fabrication techniques, it is possible to tailor the topography of a surface down to the nanoscale [12,13]. One observes that changing the topography, even only on the nanoscale, results in large changes of macroscopic observables, such as the contact angle of a sessile drop. This has encouraged the fabrication of patterned surfaces in order to control wetting. Typical configurations, which liquids exhibit on textured surfaces, are either the Cassie-Baxter (CB) [14] or the Wenzel (W) state [15]. In the CB state (for short named the Cassie state) liquid remains suspended above the substrate surface, whereas in the W state the liquid intrudes the surface cavities, i.e., pits
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