Dynamic charging at moving contact lines

Project Information

DynaMo

Grant agreement ID: 883631

DOI 10.3030/883631

EC signature date 2 June 2020

Start date 1 January 2021

End date 31 December 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost € 2 474 500,00

EU contribution € 2 474 500,00

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 1 - DynaMo (Dynamic charging at moving contact lines)

Reporting period: 2021-01-01 to 2022-06-30

Summary of the context and overall objectives of the project

Problem addressed
It has been observed that water drops sliding over hydrophobic surfaces leave negative electric charges for typically 5-60 sec 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 to explore if charging influences the motion of drops. Up to now, it is common belief that two interactions determine drop motion: Contact line friction (capillary force) and hydrodynamic viscous dissipation. We ask the following question: Do electrostatic forces influence drop motion? Is the Coulomb force between the negative surface charges and the oppositely charged drop strong enough to substantially retard drop motion?

Relevance for society
In general, an understanding of dynamic wetting is relevant for many daily phenomena and industrial applications. Examples include printing, painting, coating, bringing out herbicides or insecticides, fogging of glasses and mirrors, condensation and evaporation in heat exchangers and flotation. To improve printing, make heat exchangers more efficient or allow for a fast drainage of water on the windows of cars, a good understanding of dynamics wetting is essential.
A second aspect is corrosion. The charging of surfaces behind sliding water drops may enhance corrosion. We found that the surface charges neutralize within typically 5-60 sec. The chemical reactions afterwards are completely unknown. Are radicals formed? Which surface reactions are triggered by such radicals? Do these reactions lead to corrosion? How can one prevent corrosion?
A third reason to study slide electrification is to use the process for the generation of electric energy. In fact, most papers about charged drops are by far devoted to energy production. Even if slide electrification will never be a large scale substitute for hydroelectric power, wind turbines or photovoltaic, it may find its use in some niche applications.

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 and we derived a simple analytical equation to calculate them. As it turned out, electrostatic forces are substantial for drops moving over hydrophobic, electric non-conducting surfaces.
We still have three main remaining objectives. The first one is to understand how charges are deposited at the free solid surface although it is energetically unfavorable. We intend to come up with a theory for charge separation and verify it by experiments. A second objective is to 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. The third remaining objective it to design a device, which optimizes the generation of electric energy. What is the most efficient design for an electric generator?

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We verified that our previously developed model for slide electrification is applicable to all types of hydrophobic, insulating surfaces. We discovered that surfaces, which contain amino-groups, lead to a negative drop charge and a positive surface charge. These are the first experiments demonstrating a positive surface charge by drop sliding.
When drops move down inclined planes, one would expect that the dynamics solely depends on the surface chemistry and the liquid properties. We noticed, however, that on surfaces with the same surface chemistry, but different substrate conductivity, the average speeds of drops were different. Also, when the drops slide on the surface one after another, their speeds depend on the order. Thus, the surface history plays a role. The only logic explanation was that electrostatics cannot be ignored.
We developed a method based on solving the equation of motion of drops sliding down an inclined plane to measure forces acting on moving drops. We found that Coulomb interaction can be substantial. This finding will help to improve the control of drop motion in many engineer scenarios, including printing, microfluidics, water managements and the recently emerged field of triboelectric nanogenerators.
An essential part is the reliable and reproducible measurement of drop charge for different wetting situations. In addition to our tilted plate setup we developed an experimental setup to measure the charge of drops impacting and rebounding from superhydrophobic surfaces. We found that the charge of a rebounding drop is proportional to the apparent maximal contact area of the drop.
One hypothesis for the generation of surface charges is that the hydrodynamic shear applied to surfaces plays a significant role. For this reason we studied surfaces with little shear or even apparent slip. The experiments led to new insights. However, the insight did eventually not help us further in understanding slide electrification. Superhydrophobic surfaces, which show apparent slip, behave like normal flat hydrophobic surfaces. Only the real contact area is smaller.
One of the initial question was, if soft, elastic surfaces show a different charging than rigid surfaces. As it turned out: they do not. In the process, however, we noted that we can make highly stretchable superhydrophobic surfaces, much better than anything before. For this reason in the later phase of the project we focused on characterizing and optimizing the liquid repellent properties of stretchable surfaces.
Long before starting the ERC DynaMo project we already studied the effect of surfactant on the receding contact angle. Using tracer particles we were able to image the flow profile close to a moving receding contact line with unprecedented time and special resolution. In the framework of the ERC grant we studied the effect of charge surfactants.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Dynamo promoted great methodological developments. Our setup to image drops moving down inclined planes and our drop adhesion instruments turned out to be more reliable and precise than expected. I anticipate that both methods will have an impact far beyond the project on slide electrification, in particular for characterizing solid surface. Local variations in surface energy can be detected fast and with high accuracy.

A water drop sliding down a hydrophobic surface deposits negative charges