Spreading and Evaporation of Drops on Patterned Surface*

In processes like inkjet printing, film coating, and the application of agrochemicals, drops usually spread and evaporate on rough surfaces. In this work, we investigated the spreading of partially wetting water drops and fully wetting 5 cSt silicone oil drops on patterned polydimethylsiloxane (PDMS) surfaces with varying roughness. The PDMS substrates, featuring microscale isotropic pillars and anisotropic stripes, were fabricated using soft lithography techniques.

On isotropic pillar surfaces, water drops initially exhibit the Cassie-Baxter state and subsequently transition to the Wenzel state. During this transition, the contact line retracts isotropically. Conversely, on anisotropic striped surfaces, water drops elongate along the stripe lines, forming a "panini" shape. The imbalance of contact forces causes the drops to shrink initially along the stripes, followed by contraction in directions parallel and perpendicular to the stripe. For both cases, the contact radius decreases stepwise, with the contact angle exhibiting periodic oscillations.

In contrast, drops of 5 cSt silicone oil spread rapidly and fully coat the PDMS surface. Due to the compatibility of silicone oil with PDMS, the liquid is absorbed into the substrate, leading to film thinning. A pattern emerges where larger drops remain centred in the pillar gaps resulting from thin film instability. On striped patterns, silicone oil exhibits ultrafast spreading along the stripe direction driven by capillary forces in the gaps.