Problem addressed
It has been observed that water drops sliding over hydrophobic surfaces leave negative electric charges on the surface. The drops themselves acquire a positive charge. In contrast to charging caused by friction between two solids, drop slide electrification is largely unexplored. No theory or quantitative explanation existed, when we started the project.
We studied spontaneous charging of sliding drops and presented a model to describe charging kinetics. We found that spontaneous charging increases drop friction by three effects: Long-rang Coulomb forces, electrowetting and an increases of the solid surface energy. Another effect of drop charging is that charged solutes are deposited behind moving drops, depending on their charge. Surfactants reduce drop charging.
Relevance for society
In general, an understanding of dynamic wetting is relevant for many daily phenomena and industrial applications. To improve processes such as printing, make heat exchangers more efficient, allow for a fast drainage of water on the windows of cars, and increase mobility of droplets in microfluidics a good understanding of dynamics wetting is essential. The results are further relevant for coating technology because the deposition of solutes depends substantially on their charge.
In the process of analyzing slide electrification, new methods to characterize surfaces in general are developed. During the first two years of the project, it turned out that slide electrification is related to the electric surface potential of the solid surface in the liquid. For many materials, this surface potential is important, for example for the stabilization of dispersions. Surface potentials are, however, notoriously difficult to measure. Thus, we intend to explore the potential to apply slide electrification as a standard method for measuring surface potentials.
Objectives
The first objective was to establish a protocol on how to measure charge separation reproducibly. When multiple drops move over a surface, the charge deposited by the first drop influences charge deposition by the following drops. We established that charging can be quantified by analyzing series of few hundred drops run down inclined planes. We developed a phenomenological theory to describe the process and we identified the important parameters.
The second objective was to explore to which degree electrostatic forces influence drop motion. To answer this question, we developed a new method to measure these forces. It turned out that electrostatic charging of water drops moving over insulating surfaces influences drop motion substantially.
Remaining objectives are:
• Understand how charges are deposited at the free solid surface although it is energetically unfavorable. We intend to come up with a microscopic theory for charge separation and verify it by experiments.
• Analyze what happens with the deposited charges. How are the surface charges eventually neutralized? How do they react? Do these reactions lead to surface corrosion? Here, the main problem is to find suitable analytical techniques to identify the originating chemical species.
• Design a device, which optimizes the generation of electric energy. What is the most efficient design for an electric generator?
