Adaptation of a Styrene–Acrylic Acid Copolymer Surface to Water
Xiaomei Li, Simon Silge, Alexander Saal, Gunnar Kircher, Kaloian Koynov, Rüdiger Berger, Hans‐Jürgen Butt
Abstract
High Resolution Image Download MS PowerPoint Slide Solid surfaces, in particular polymer surfaces, are able to adapt upon contact with a liquid. Adaptation results in an increase in contact angle hysteresis and influences the mobility of sliding drops on surfaces. To study adaptation and its kinetics, we synthesized a random copolymer composed of styrene and 11–25 mol% acrylic acid (PS/PAA). We measured the dynamic advancing (θ A ) and receding (θ R ) contact angles of water drops sliding down a tilted plate coated with this polymer. We measured θ A ≈ 87° for velocities of the contact line <20 μm/s. At higher velocities, θ A gradually increased to ∼98°. This value is similar to θ A of a pure polystyrene (PS) film, which we studied for comparison. We associate the gradual increase in θ A to the adaptation process to water: The presence of water leads to swelling and/or an enrichment of acid groups at the water/polymer interface. By applying the latest adaptation theory (Butt et al. Langmuir 2018, 34, 11292), we estimated the time constant of this adaptation process to be ≪1 s. For sliding water drops, θ R is ∼10° lower compared to the reference PS surface for all tested velocities. Thus, at the receding side of a sliding drop, the surface is already enriched by acid groups. For a water drop with a width of 5 mm, the increase in contact angle hysteresis corresponds to an increase in capillary force in the range of 45–60 μN, depending on sliding velocity.