Dynamic Wetting of Adaptive and Responsive Polyelectrolyte Substrates: A multiscale approach

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Wetting of polymer substrates by aqueous solutions is of specific interest in the context of the adhesion of biologically relevant systems like cells or proteins, but also for the technical control of wetting. Polymer substrates which contain charges or polar groups have a strong tendency to swell in water. Therefore, not only elastic but also adaptive properties of the substrate affect the wetting properties. The global aim of the present project is the understanding of the relation between the adaptation/response of polyelectrolyte substrates and their dynamic wetting properties. In the present project adaptation refers to swelling of the polyelectrolyte substrate caused by the (partial) uptake of the wetting liquid which is water or an aqueous salt solution. One class of studied adaptive substrate is responsive to temperature, which raises the question how a change in temperature affects wetting properties.In this context the project aims to contribute to a better understanding of wetting phenomena on nanoscopic/mesoscopic length scale (10 nm - 1 m) lateral and vertical to the surface close to the three-phase contact line (TPCL). Therefore, AFM techniques will be used. To make a link to wetting phenomena on a macroscopic length scale, typical static and dynamic wetting experiments (i.e. sessile drop, drop on a tilted plate, advancing and receding contact angle measurements) will be also carried out. Surface effects like surface elasticity, surface charge and interfacial molecular rearrangement are assumed to govern hysteresis effects on a nano/mesoscale which affects macroscopic wetting phenomena. Three types of surface deformation might occur on the nanoscale and might contribute to the overall profile of the TPCL: 1) A precursor film in front of the droplet, which is dominated by the disjoining pressure, 2) a rim of swollen substrate which is due to transport of liquid from the droplet into the substrate lateral (and vertical) to the substrate surface and 3) a wetting ridge induced by vertical components of the surface stress.The project aims to disentangle the different surface effects on wetting phenomena.As polyelectrolyte substrates polyelectrolyte multilayers (PEMs) and layers/multilayers of adsorbed PNIAPM microgel are used. Both can be considered as gels. Both types of substrates are complementary with respect to their advantages and allows control of surface charge (sign, density), range of achievable thickness, mechanical and rheological properties, hydrophilic/hydrophilic balance and their response to temperature. Cooperations are planned within the SPP with experimentalists and theoreticians in order to get deeper insight into the link between wetting phenomena on different length scales.