Authors:	Díaz, Diego
Advisor:	Butt, Hans-Jürgen
Title:	Charge separation of bouncing water drops on hydrophobic and superhydrophobic surfaces
Online publication date:	12-Jun-2023
Year of first publication:	2023
Language:	english
Abstract:	In this thesis, I investigate experimentally and theoretically the mechanism behind the charge separation of neutral water drops rebounding from \textit{hydrophobic} and \textit{superhydrophobic} surfaces. So far, charging by drop impact has not been systematically quantified and it is not clear which are the main parameters that control the process. In addition, a quantitative theoretical model to describe the phenomenon is missing. To address these issues, I designed a method for drop charge detection based on electric fields and high-speed video imaging. The method allows to analyze the trajectory of bouncing drops deflected in an external lateral electric field upon impact. Coatings made of superhydrophobic silicone nanofilaments and hydrophobic Teflon were used as impact targets. From the lateral deflection of drops rebounding from \textit{superhydrophobic} surfaces, the following outcome was observed: when neutral water drops impact and rebound from both hydrophobic and superhydrophobic surfaces, they acquire a positive electrical charge, since the deflection is always in the direction of the applied electric field. Here, to understand the underlying physics of the phenomenon, I study the relative importance of five of these potential variables: impact speed, drop contact area, contact line retraction speed, drop size, and type of surface. I additionally derived a theoretical model based on a previously reported sliding drop electrification model. The model assumes that drops gain progressively positive charge during the drop retraction motion. Both theoretical and experimental results reveal that the maximum contact area is the main controlling parameter of the charging mechanism. Drop charging on \textit{hydrophobic} surfaces confirms the relevant influence of the contact area, with values of charge up to two order of magnitude higher than the observed for \textit{superhydrophobic} surfaces. From the spontaneous charging on these surfaces, one important result was observed: Self-generated charges lead to significant electrostatic forces between the drop and surface. These forces affect the drop motion in two ways. First, the electrostatic forces slow down the drop retraction motion. This motion takes place when drops, prior the rebound, convert back the excess of surface energy into kinetic energy after spreading on the surface. Second, the electrostatic forces reduce the maximum rebounding height. I calculated the electrostatic forces using an energy conservation approach. My results indicate that electrostatic forces on hydrophobic surfaces can be even stronger than gravitational forces, allowing to estimate the drop charge by energy conservation even in the absence of external electric fields. The experimental method presented in this thesis is advantageous compared with other methods that require additional electronic devices for charge detection. More importantly, it is based on very fundamental physics. Furthermore, the findings could be useful for the control of drop charging in energy harvesting applications. From a fundamental viewpoint, the results of this thesis serve to describe drop impact not only in terms of surface energy, viscous dissipation energy, gravitational potential energy and kinetic energy, but also including the energy dissipated by electrostatic forces.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-9143
URN:	urn:nbn:de:hebis:77-openscience-614d612b-e4fc-4757-92f3-d67a08fc8a606
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	vii, 117 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen