Bridging of liquid drops at chemically structured walls

Area of Science:

• Physical Chemistry
• Surface Science
• Statistical Mechanics

Background:

• Understanding fluid adsorption on patterned surfaces is crucial for materials science and nanotechnology.
• Finite-size effects and long-range forces significantly influence interfacial phenomena and phase transitions.
• Previous studies often simplified surface patterns, limiting insights into complex wetting behaviors.

Purpose of the Study:

• To investigate fluid adsorption and bridging transitions on a wall with three parallel wetting stripes.
• To analyze the impact of stripe width (L) and separation (D1, D2) on surface free energy and droplet configurations.
• To determine the surface phase diagram and identify conditions for different droplet coexistence.

Main Methods:

• Utilized mesoscopic interfacial models and microscopic density functional theory (DFT).
• Analyzed the scaling of nonextensive free-energy contributions with stripe width (lnL).
• Mapped the surface phase diagram, identifying triple points and equilibrium droplet configurations.

Main Results:

• Identified bridging transitions where liquid drops merge on adjacent wetting stripes at molecularly small separations.
• Demonstrated that finite-size effects, scaling with lnL, drive these transitions by modifying surface free energy.
• Revealed a surface phase diagram with two triple points, allowing coexistence of isolated, double, and triple drops.

Conclusions:

• Equilibrium droplet configurations on patterned surfaces reflect substrate symmetry, transitioning from isolated to merged drops with decreasing stripe separation.
• Symmetry-broken configurations exist metastably near the bridging phase boundary.
• Findings have implications for phase transitions on diverse patterned surfaces.