Focusing Effect for Sliding Drops Induced by Adaptive Surfaces

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Surface adaptation has been put forward to be one cause for the velocity dependence of advancing and receding contact angles, even at low slide velocity. By considering a dry-to-wet and a wet-to-dry relaxation process, we had proposed a model, which is able to explain these dynamic contact angles (Langmuir 2018, 34, 11292). The aim of our project is to experimentally verify the model and explore, how applicable it is. In the first funding period, we built an inclined plane setup to measure advancing and receding contact angles versus velocity for water drops up to 1 m/s. We synthesized a random poly(styrene/acrylic acid) copolymer PS/AA and demonstrated that for water drops on PS/AA films, advancing-contact-angle-versus-velocity-curves can indeed be fitted with the model. The fit revealed relaxation times < 2 ms of the polymer at the surface. However, we are still missing independent measurements of the dry-to-wet relaxation times, which would be required for full validation of the model. The aim for the second funding period is to investigate polymer surfaces, where we can independently measure the relaxation time of the dry-to-wet and the wet-to-dry transitions. Then, recorded advancing-contact-angle-versus-velocity-dependencies can be validated independently using our model. Therefore, we plan to synthesize three types of polymer brushes: (1) PS and poly(2-vinylpyridine) (PVP) binary brush films, (2) pH-responsive poly(N,N-dimethyl aminoethyl meth¬acrylate) (PDMAEMA) brushes and (3) different block copolymer layers. For series of drops sliding on an adaptive surface we predicted a focusing effect. It is induced by partial adaptation caused by the first drop sliding along the surface. This drop leaves behind a lane of increased surface energy and we expect that subsequent drops will then slide along the same lane. Our aim is to verify this hypothesis.