When a droplet is sliding on surfaces, adaptation of the surface leads to changes of the dynamic contact angles . Hereby two adaptation processes play a role: (i) the adaptation of the surface upon bringing in contact to the droplet (wetting) and (ii) the adaptation of the surface after the droplet passed (dewetting). In order to study both processes, we investigated samples made from polystyrene (PS) polyacrylic acid (PAA) random copolymers by using a tilted-plate method and by sum-frequency generation spectroscopy (SFG). For the wetting process, the advancing and receding contact angles of water droplets decrease when PS/PAA surface adapts to water. We measured a relaxation time of ~1 ms for 40 - 100 nm thick PS/PAA films adapting to water by a tilted-plate method . Here, both water diffusion and polymer reorientation play a role in the adaptation process. For the dewetting process, the sliding droplet velocity decreased for subsequent droplets with different droplet intervals. From the drop interval and the drop velocity, we calculated the time that is required for the surface to (re-)adapt to air. While for the wetting process water diffusion and copolymer reorientation played a role, it is not clear to which extend both effects are present during dewetting. Therefore, we performed SFG experiments on PS/PAA surfaces to determine the contribution of both effects.
 H. J. Butt et al, Langmuir 34 (2018), 11292−11304.
 X Li et al, Langmuir 37(2021), 1571−1577.