Dynamic charging at moving contact lines

Water drops sliding over hydrophobic surfaces can lead to surface charging. In contrast to charging caused by friction between two solid phases, drop slide electrification is largely unexplored. Slide electrification has been consistently reported, but results are difficult to reproduce. No theory or quantitative explanation currently exists.

One reason for the lack of quantitative understanding is that the deposition of charge is a non-equilibrium effect and depends essentially on microscopic processes at the contact line. Slide electrification is relevant for the friction of drops and possible corrosion due to ions deposited on surfaces.

It has potential as a means of power generation.Based on a recently developed lateral adhesion force apparatus (DAFI) and a new theoretical approach to describe slide electrification, we aim for a fundamental understanding of charge separation at sliding drops.

Thus we plan toidentify important parameters for slide electrification (surface chemistry, substrate material, thickness, slide distance, velocity, drop rate, pH value, salt, atmosphere), and construct a fast, inverted Reflectance Interference Microscope (RIM) to image the movement of the sliding contact line with unprecedented temporal and spatial resolution.

RIM will be combined with DAFI and electronics to detect charge transfer. Experiments using macroscopic drops will be complemented by moving micron-sized drops (<1 pL) over surfaces using a liquid probe microscope and simultaneously measuring the charge transfer.

Based on the microscopic processes identified above we develop a theory to predict charge transfer.Using this fundamental understanding, we will explore the potential of slide electrification for electric energy generation.

Our objectives are to design a nano- and microstructured surface, which provides maximal power output, and build small scale devices to generate electric energy.