Attempts to design more efficient energy converters are hindered by a lack of fundamental understanding. 17–20 However, the efficiency of charge separation by flowing liquids is still much lower than that of conventional electric generators. 1–16 To this end, much literature has focused on how charging affects the dynamic wetting of surfaces, the movement of drops, and contact angle hysteresis. With respect to applications, the motivation is to convert the kinetic energy of a flowing liquid directly to electrical energy. This study is motivated by both applied and fundamental research. 1 Introduction Here, we analyze the charging of aqueous drops sliding down hydrophobic surfaces. Given that nearly every surface in our lives comes in contact with water, this water-dependent surface charging may be a ubiquitous process that we can begin to understand through the proposed theory. All of our experimental charge saturation results can be interpreted based on the proposed theory. This fraction, or “transfer coefficient”, is dependent on the electric potentials of surface and drop. To explain these results, we theorize that some fraction of the charge in the Debye layer is transferred to the surface rather than being neutralized as the drop passes. These charge saturations indicate a limited “storage capacity” of the system, as well as a gradual discharging of the surface. We observe charge saturation in three variables: increasing drop number, increasing interval between drops, and increasing drop-sliding length. On this surface, sliding drops gain a positive charge.
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As an experimental system, we choose water drops moving down an inclined plane of glass hydrophobized with perfluoro octadecyltrichlorosilane (PFOTS). We reproducibly measure the charge gained by water drops sliding down a substrate, and we outline an analytical theory to describe this charging process.
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In this work, we address both the experimental and theoretical sides of this problem. Although the phenomenon of liquid charging has been consistently reported, these reports are primarily observational, results are difficult to reproduce, and no quantitative theory has been developed. We investigate the charge separation caused by the motion of a water drop across a hydrophobic, insulating solid surface.