Symmetry-breaking morphological transitions at chemically nanopatterned walls

TL;DR

This paper investigates how fluids interact with chemically patterned walls, revealing symmetry-breaking morphological transitions and bridging phenomena that depend on system size, wall composition, and fluid density, using density functional theory.

Contribution

It introduces a detailed analysis of symmetry-breaking transitions and bridging phenomena at chemically nanopatterned walls using nonlocal DFT, highlighting finite-size effects and the transition to continuous wetting.

Findings

Transitions are weakly first-order but become rounded at small system sizes.

In large systems, an infinite sequence of bridging transitions occurs, becoming indistinguishable from continuous wetting.

The phase diagram and contact angle behavior are characterized for various conditions.

Abstract

We study the structure and morphological changes of fluids that are in contact with solid composites formed by alternating and microscopically wide stripes of two different materials. One type of the stripes interacts with the fluid via long-ranged Lennard-Jones-like potential and tends to be completely wet, while the other type is purely repulsive and thus tends to be completely dry. We consider closed systems with a fixed number of particles that allows for stabilization of fluid configurations breaking the lateral symmetry of the wall potential. These include liquid morphologies corresponding to a sessile drop that is formed by a sequence of bridging transitions that connect neighboring wet regions adsorbed at the attractive stripes. We study the character of the transitions depending on the wall composition, stripes width, and system size. Using a (classical) nonlocal density functional theory (DFT), we show that the transitions between different liquid morphologies are typically weakly first-order but become rounded if the wavelength of the system is lower than a certain critical value $L_c$. We also argue that in the thermodynamic limit, i.e., for macroscopically large systems, the wall becomes wet via an infinite sequence of first-order bridging transitions that are, however, getting rapidly weaker and weaker and eventually become indistinguishable from a continuous process as the size of the bridging drop increases. Finally, we construct the global phase diagram and study the density dependence of the contact angle of the bridging drops using DFT density profiles and a simple macroscopic theory.