Microscopic Mechanisms and Surface Adaptation Effects in Slide-Electrification

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Slide electrification describes a spontaneous charge separation between a moving liquid droplet and a hydrophobic surface. Depending on substrate material and liquid, the drop accumulates either positive or negative charge with the substrate charging oppositely. Here, the drop charge can be influenced by the contact time between surface and water drop and the time between subsequent drops. Models, that describe the charge generation, therefore include a surface adaptation term, very much akin to wetting adaptation phenomena. The microscopic origin of slide electrification and the surface adaptation effects is still unclear. The mechanism has been rationalized phenomenologically, assuming charge pinning at the receding contact line. Here, electron transfer, adsorption/desorption of ions, or an incomplete reorganization of the electric double layer could all lead to the observed effects. Thus, the first goal of our project is a molecular understanding of slide electrification. We will combine experiments with computer simulations and systematically change the solid-liquid interface by varying the three components that are involved: 1) the substrate, 2) the hydrophobic layer 3) and the liquid (type of liquid, salt concentration, pH). For the substrate, the dielectric permittivity and surface chemistry are the important factors contributing to slide electrification. To investigate the contribution of ionic and electronic charge transfer to the slide electrification process, we will also include carbon-based model surfaces with defined electronic conductivity. To control the grafting density and the charge diffusion at the surface, we will change the molecular weight of the hydrophobic layer and the surface roughness. In addition, we will vary the surface chemistry, e.g. with hydroxide and amine functionalized surfaces. To study the interplay between wetting adaptation and charge adaptation, we will study adaptive polymeric and polyelectrolyte surfaces in cooperation with other groups within this SPP. Thus, we want to clarify the mechanisms behind the voltage that forms at the three-phase contact line and the surface adaptation. The application of slide electrification is electricity generation, a potential source of renewable energy. Currently, the efficiency of energy conversion from kinetic droplet energy to electric power is very low: much less than 0,1 %. Therefore, the second goal of the project is to understand the fundamental limits of the slide electrification efficiency. We want to identify substrate – surface layer – liquid combinations that yield improved conversion efficiencies. This will be achieved by computational pre-screening of surface combinations, aiming at those that maximize the surface voltage and the drop charge. The top performing combinations will be then characterized experimentally.