Periodic Reporting for period 2 - DynaMo (Dynamic charging at moving contact lines)
Reporting period: 2022-07-01 to 2023-12-31
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 to find out how charges are separated and 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. The key question was: Do electrostatic forces influence drop motion?
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.
A second 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.
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 insultaing 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?
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 to find out how charges are separated and 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. The key question was: Do electrostatic forces influence drop motion?
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.
A second 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.
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 insultaing 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?
Main results achieved so far
Negatively charged water drops. 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.
In the last decade, lubricant-infused surfaces have attracted attention because they let water drops slide even at very low tilt angles of typically 1-3°. We demonstrated that on lubricant-infused surfaces no slide electrification occurs. No charges are deposited and the drops remain uncharged. However, once a certain number of drops has moved over such a surface the lubricant is depleted and charges are again separated. We demonstrated that drop charging can be used to monitor depletion of lubricant.
Slide electrification affects the motion of sliding drops. In cooperation with colleagues from the Technical University of Darmstadt, 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 addition to contact line friction and viscous dissipation, electrostatic retardation is third channel for energy dissipation in moving drops.
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 water drops impacting and rebounding from (super)hydrophobic surfaces. Our previously developed theory described drop charging for impacting drops as well.
To better understand the charging process we studied the influence of dissolved CO2. In water, CO2 is converted to carbonate which is negatively charged. In cooperation with Thomas Palberg and Peter Vogel (University Mainz) we could indeed demonstrate that CO2 changes the surface potential by roughly 30%.
Effect of slip on slide electrification. 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 with respect to flow of water over superhydrophobic surfaces. 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.
We identified three effects which sintantially drop motion: The direct Coulomb force, electrocapillary reduction of contact angles and a change in the surface energy of the free solid surface behind the sliding drop.
Negatively charged water drops. 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.
In the last decade, lubricant-infused surfaces have attracted attention because they let water drops slide even at very low tilt angles of typically 1-3°. We demonstrated that on lubricant-infused surfaces no slide electrification occurs. No charges are deposited and the drops remain uncharged. However, once a certain number of drops has moved over such a surface the lubricant is depleted and charges are again separated. We demonstrated that drop charging can be used to monitor depletion of lubricant.
Slide electrification affects the motion of sliding drops. In cooperation with colleagues from the Technical University of Darmstadt, 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 addition to contact line friction and viscous dissipation, electrostatic retardation is third channel for energy dissipation in moving drops.
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 water drops impacting and rebounding from (super)hydrophobic surfaces. Our previously developed theory described drop charging for impacting drops as well.
To better understand the charging process we studied the influence of dissolved CO2. In water, CO2 is converted to carbonate which is negatively charged. In cooperation with Thomas Palberg and Peter Vogel (University Mainz) we could indeed demonstrate that CO2 changes the surface potential by roughly 30%.
Effect of slip on slide electrification. 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 with respect to flow of water over superhydrophobic surfaces. 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.
We identified three effects which sintantially drop motion: The direct Coulomb force, electrocapillary reduction of contact angles and a change in the surface energy of the free solid surface behind the sliding drop.
Establishing electrostatic effects on drop motion as a third channel of energy dissipation after more than 50 years of intense research will change our view of kinetic wetting. We are still working on a better estimation and prediction of the three dissipation channels (contact line friction, viscous dissipation and electrostatics). The aim is to be able to predict under which circumstances which effect contributes and how dissipation depends on the drop velocity.
DYNAMO promoted great methodological developments. Our setup to image drops moving down inclined planes turned out to be reliable and precise. I anticipate that the methods will have an impact far beyond the project on slide electrification.
DYNAMO promoted great methodological developments. Our setup to image drops moving down inclined planes turned out to be reliable and precise. I anticipate that the methods will have an impact far beyond the project on slide electrification.