Aims and objectives of the Priority Programme 2171

Wetting and dewetting, i.e., the respective advancing and receding motion of liquids on surfaces is one of the fascinating everyday phenomena of the natural and technical world that surrounds us. Liquid-repellent surfaces are essential for many plants and animals, as water drops very quickly roll off, maintaining the functionality of the surface. With the increasing miniaturisation of technical fluidic systems, the control of wettability is becoming increasingly important, as at small length scales interfacial effects dominate.

The Priority Programme (SPP) 2171 shall develop the basic physical understanding of dynamic wetting and dewetting processes on flexible, adaptive and switchable surfaces. Flexible surfaces are deformed by capillary interactions and thus provide a feedback mechanism on the static and dynamic behaviour of the liquid. Adaptive surfaces change their physico-chemical properties in the presence of a liquid, e.g., under a drop, or they adapt their wetting behaviour in response to environmental conditions. Switchable surfaces can repeatedly and almost instantaneously change their surface energy or topography in response to external influences, thereby, e.g., enabling periodic wetting dynamics. Switching processes can, e.g., be triggered by electric fields or light irradiation.

In all three cases -- flexible, adaptive and switchable surfaces -- the substrate dynamics couples to the hydrodynamics in the wetting liquid and provides additional time and length scales. The SPP will mainly focus on the dynamics of (de)wetting of simple, low-molecular-weight liquids. Beside this it will consider mixtures of simple liquids, diluted suspensions and surfactant solutions in situations where their (de)wetting dynamics is altered by substrate- or interface-induced internal structuring. This in turn, may influence the substrate dynamics.

Such a coupling of different non-equilibrium processes has a significant influence on the mechanisms of energy dissipation which ultimately determine the overall dynamics. The resulting additional degrees of freedom offer new possibilities for a future targeted control of dynamic wetting processes, but also pose new challenges for experimental investigation and theoretical description.

The highly cooperative and interdisciplinary SPP builds on the rapid advance in length scale-bridging experimental and theoretical methods. It shall establish the prerequisites for a future control of dynamic wetting processes.