Raphaël Saiseau
Department of Physics, University of Konstanz, 78457 Konstanz, Germany
Wetting beyond the contact line: Texture, evaporation, and composition gradients
Droplet shapes and dynamics are generally described through surface chemistry, force balances, and transport processes near the contact line. While this picture captures many wetting phenomena, it also raises paradoxes associated with the singular nature of the contact line. Additional effects challenge its assumptions: evaporation can decouple contact-line motion from liquid transport, surface texture introduces confinement, and composition gradients generate Marangoni flows. These common effects can combine to produce distinct wetting states.
In this seminar, I will present experiments in which isopropyl alcohol (IPA) vapor condenses into water droplets deposited on hydrophilic micropillar arrays, bringing these three effects together. Preferential condensation near the droplet edge lowers the local surface tension, generating Marangoni stresses that oppose capillary flow into the texture. Varying vapor concentration and pillar spacing produces wetting states ranging from immediate wicking to droplet–film coexistence and temporary pinning at the pillars as the droplet composition evolves.
To describe these nonequilibrium morphologies, we couple thin-film hydrodynamics to composition transport and diffusion-limited evaporation and condensation. The texture is represented through an effective capillary pressure and hydrodynamic mobilities. Numerical and semi-analytical solutions reproduce droplet–film coexistence and relate the balance between Marangoni contraction and capillary wicking to the different wetting regimes.
Finally, spatial and temporal modulation of the vapor field enables droplet transport and extraction of previously imbibed liquid. Vapor composition thus provides reversible wetting control on a fixed substrate, reducing reliance on specific materials or geometrical designs and offering applications in coating, cleaning, and drying of functional surfaces.
