Publications

How Charges Separate when Surfaces Are Dewetted

Phys. Rev. Lett. 2024, 132, 224002

Three decades ago, it was found that moving droplets generate electric charges, leaving opposite charges on surfaces such as glass, polymers, and plant leaves. So far, the mechanisms of charge separation remained unclear. Here, we unveil the mechanism and introduce a simple yet effective model that not only elucidates but also accurately predicts experimental outcomes. Charging intensifies on hydrophobic surfaces and decreases unexpectedly with higher sliding velocities. This new understanding allows for precise control of charging during semiconductor manufacturing, where excess charging can damage wafers. Moreover, it enhances the design of energy harvesting technologies by optimizing charge generation. This research offers valuable insights for reducing industrial risks and advancing sustainable technologies.
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Surface charge density and induced currents by self-charging sliding drops

Soft Matter 2024, 10.1039/d4sm00205a

Spontaneous charge separation in drops sliding over a hydrophobized insulator surface is a well-known phenomenon, and many efforts have been made to utilize this effect for energy harvesting. For maximizing the efficiency of such devices, a comprehensive understanding of the dewetted surface charge is required to quantitatively predict the electric current signals, particularly for drop sequences. Here, we use a method based on mirror charge detection to locally measure the surface charge density after drops move over a hydrophobic surface. For this purpose, we position a metal electrode beneath the hydrophobic substrate to measure the capacitive current induced by the moving drop. Furthermore, we investigate drop-induced charging on different dielectric surfaces, along with the surface neutralization processes. The surface neutralizes over a characteristic time, which is influenced by the substrate and the surrounding environment. We present an analytical model that describes the slide electrification using measurable parameters such as the surface charge density and its neutralization time. Understanding the model parameters and refining them will enable a targeted optimization of the efficiency in solid–liquid charge separation. more

Control of spontaneous charging of sliding water drops by plasma-surface treatment

Scientific Reports 2024, 14, 10640 more

Thickness of Nanoscale Poly(Dimethylsiloxane) Layers Determines the Motion of Sliding Water Drops

Adv. Materials 2024, 10.1002/adma.202311470

A team led by Hans-Jürgen Butt at the Max Planck Institute for Polymer Research has discovered that altering the thickness of nano-scale polymer layers, even within a range of 10 nm, can significantly influence drop motions. They found a 'magic thickness' of 5 nm for polydimethylsiloxane (PDMS) which results in the lowest interfacial friction between a sliding water drop and the PDMS layer. This finding is of great significance for promoting the application of fluoride-free nano coatings in liquid environments. more

How Surface and Substrate Chemistry Affect Slide Electrification

J. Am. Chem. Soc. 2024, 146, 10073–10083

When water droplets move over a hydrophobic surface, they and the surface become oppositely charged by what is known as slide electrification. The chemical processes causing droplet charging are still poorly understood. We show that the charge of the first droplet on a hydrophobically coated substrate depends on both, coating and substrate chemistry. We found that charge separation can be considered as an acid base reaction between the ions of the water and the surface. By exploiting the acidity (Pearson hardness) of elements such as aluminum, magnesium, or sodium a positive saturated charge can be obtained by the counter charge remaining on the surface. more

The influence of ions and humidity on charging of
solid hydrophobic surfaces in slide electrification

Soft Matter, 2024, 20, 558–565

The influence on slide electrification of added non-hydrolysable salt, acid, or base in the sliding water drops as well as the surrounding humidity on surface electrification and charge formation is studied. Here, we measure the charging on hydrophobic solid surfaces (coated with PFOTS or PDMS) by sliding drops with varying concentration for different types of solutions as NaCl, CaCl₂, KNO₃, HCl, and NaOH. The effect of humidity on the measured charge was tested over the range from 10% to 90% of humidity. more

Chemically robust superhydrophobic surfaces
with a self‐replenishing nanoscale liquid coating

Droplet 2024; 3:e103

Due to poor chemical robustness, superhydrophobic surfaces become suscepti-
ble to failure, especially in a highly oxidative environment. To ensure the long‐term efficacy of these surfaces, a more stable and environmentally friendly coating is required to replace the conventional salinization layers. Here, soot‐templated surfaces with re‐entrant nanostructures are precoated with poly-dimethylsiloxane (PDMS) brushes. An additional nanometer‐thick lubricant layer of PDMS was then applied to increase chemical stability. The surface is superhydrophobic with a nanoscale liquid coating. Since the lubricant layer is thin, ridge formation is suppressed, which leads to low drop sliding friction and fast drop shedding. By introducing a bottom “reservoir” of a free lubricant as an oil source for self‐replenishing to the upper layer, the superhydrophobic surface becomes more stable and heals spontaneously in response to alkali erosion and O 2 plasma exposure. This design also leads to a higher icing delay time and faster removal of impacting cooled water drops than for uncoated surfaces, preventing icing at low temperatures. more

Surface Charge Deposition by Moving Drops Reduces Contact Angles

Phys. Rev. Lett. 2023, 131, 228201

Slide electrification—the spontaneous charge separation by sliding aqueous drops—can lead to an electrostatic potential in the order of 1 kV and change drop motion substantially. To find out how slide electrification influences the contact angles of moving drops, we analyzed the dynamic contact angles of aqueous drops sliding down tilted plates with insulated surfaces, grounded surfaces, and while grounding the drop. The observed decrease in dynamic contact angles at different salt concentrations is attributed to two effects: An electrocapillary reduction of contact angles caused by drop charging and a change in the free surface energy of the solid due to surface charging. more

Self-generated electrostatic forces of drops rebounding from hydrophobic surfaces

Physics of Fluids 2023, 35, 017111

We study the charge separation of drops rebounding from hydrophobic surfaces. Based on high-speed video imaging and the deflection of drops by electric fields, we reproducibly detected the amount of charge. Here, we show that the charge separation of bouncing drops can be 2 orders of magnitude higher on hydrophobic than superhydrophobic surfaces. We observed the existence of self-generated electrostatic forces between the drop and the surface. These forces affect the maximum rebounding height and slow down the retraction motion of drops. We additionally calculated the electrostatic forces using an energy conservation approach. Our results indicate that electrostatic forces on hydrophobic surfaces can be even stronger than gravity, reducing the restitution coefficients up to 50%. This new approach becomes advantageous compared with other methods that require more complicated setups for drop charge detection. more

High Voltages in Sliding Water Drops

J. Phys. Chem. Lett. 2023, 14, 49, 11110–11116

Water drops on insulating hydrophobic substrates can generate electric potentials of kilovolts upon sliding for a few centimeters. We show that the drop saturation voltage corresponds to an amplified value of the solid–liquid surface potential at the substrate. The amplification is given by the substrate geometry, the drop and substrate dielectric properties, and the Debye length within the liquid. Next to enabling an easy and low-cost way to measure surface- and zeta- potentials, the high drop voltages have implications for energy harvesting, droplet microfluidics, and electrostatic discharge protection. more

Sustainable and Practical Superhydrophobic Surfaces via Mechanochemical Grafting

Adv. Materials, Vol. 10, Issue15, 2023, 2300069

We report mechanochemical approach for practical and solvent-free manufacturing of superhydrophobic surfaces. This approach enables solvent-free and ultra-rapid preparation of superhydrophobic surfaces in a single-step. Thereis no need for any washing, separation, and drying steps leading to superhydrophobic surfaces with a water contact angle of >165° and a sliding angle of <2°. The resulting superhydrophobic surfaces are highly biocompatible as demonstrated by fibroblast cells using two different assays.
Monolith materials fabricated from silicone-grafted nanoparticles exhibit bulk and durable superhydrophobicity. The presented approach offers tremendous potential with sustainability, scalability, cost-effectiveness, simplicity, biocompatibility, and universality. more

Kinetic drop friction

Nature Communications, Vol. 14, 2023, 4571

Liquid drops sliding on tilted surfaces is an everyday phenomenon and is important for many industrial applications. Still, it is impossible to predict the drop’s sliding velocity. To make a step forward in quantitative understanding, we measured the velocity (U), contact width (w), contact length (L), advancing (θ_a), and receding contact angle (θ_r) of liquid drops sliding down inclined flat surfaces made of different materials. We find the friction force acting on sliding drops of polar and non-polar liquids with viscosities (η) ranging from 10⁻³ to 1 Pa*s can empirically be described by F_f(U) = F₀ + βwηU for a velocity range up to 0.7 ms⁻¹ . The dimensionless friction coefficient (β) defined here varies from 20 to 200. It is a material parameter, specific for a liquid/surface combination. While static wetting is fully described by θ_a and θ_r , for dynamic wetting the friction coefficient is additionally necessary. more

Tuning static drop friction

Droplets, Vol. 2, Issue 1, 2023, e42

The friction force opposing the onset of motion of a drop on a solid surface is typically considered to be a material property for a fixed drop volume on a given surface. However, here we show that even for a fixed drop volume, the static friction force can be tuned by over 30% by preshaping the drop. The static friction usually exceeds the kinetic friction that the drop experiences when moving in a steady state. Both forces converge when the drop is prestretched in the direction of motion or when the drop shows low contact angle hysteresis. In contrast to static friction, kinetic friction is independent of preshaping the drop, that is, the drop history. Kinetic friction forces reflect the material properties. more

Spontaneous charging of drops on lubricant-infused surfaces

Langmuir, 38, 41, 2022, 12610–12616

When a drop of a polar liquid slides over a hydrophobic surface, it acquires a charge. As a result, the surface charges oppositely. For applications such as the generation of electric energy, lubricant-infused surfaces (LIS) may be important because they show a low friction for drops. However, slide electrification on LIS has not been studied yet. Here, slide electrification on lubricant-infused surfaces was studied by measuring the charge generated by series of water drops sliding down inclined surfaces. As LIS, we used PDMS-coated glass with micrometer-thick silicone oil films on top. For PDMS-coated glass without lubricant, the charge for the first drop is highest. Then it decreases and saturates at a steady state charge per drop. With lubricant, the drop charge starts from 0, then it increases and reaches a maximum charge per drop. Afterward, it decreases again before reaching its steady-state value. This dependency is not a unique phenomenon for lubricant-infused PDMS; it also occurs on lubricant-infused micropillar surfaces. We attribute this dependency of charge on drop numbers to a change in surface conductivity and depletion of lubricant. These findings are helpful for understanding the charge process and optimizing solid–liquid nanogenerator devices in applications. more

Vapor Lubrication for Reducing Water and Ice Adhesion on Poly(dimethylsiloxane) Brushes

Adv. Mater. 34, 2022, 2203242

Fast removal of small water drops from surfaces is a challenging issue in heat transfer, water collection, or anti-icing. Poly(dimethylsiloxane) (PDMS) brushes show good prospects to reach this goal because of their low adhesion to liquids. To further reduce adhesion of water drops, here, the surface to the vapor of organic solvents such as toluene or n-hexane is exposed. In the presence of such vapors, water drops slide at lower tilt angle and move faster. This is mainly caused by the physisorption of vapor and swelling of the PDMS brushes, which serves as a lubricating layer. Enhanced by the toluene vapor lubrication, the limit departure volume of water drop on PDMS brushes decreases by one order of magnitude compared to that in air. As a result, the water harvesting efficiency in toluene vapor increases by 65%. Benefits of vapor lubrication are further demonstrated for de-icing: driven by gravity, frozen water drops slide down the vertical PDMS brush surface in the presence of vapor. more

Deep Learning to Analyze Sliding Drops

Langmuir, 39, 3, 2023, 1111–1122

State-of-the-art contact angle measurements usually involve image analysis of sessile drops. The drops are symmetric and images can be taken at high resolution. The analysis of videos of drops sliding down a tilted plate is hampered due to the low resolution of the cutout area where the drop is visible. The challenge is to analyze all video images automatically, while the drops are not symmetric anymore and contact angles change while sliding down the tilted plate. To increase the accuracy of contact angles, we present a 4-segment super-resolution optimized-fitting (4S-SROF) method. We developed a deep learning-based super-resolution model with an upscale ratio of 3; i.e., the trained model is able to enlarge drop images 9 times accurately (PSNR = 36.39). In addition, a systematic experiment using synthetic images was conducted to determine the best parameters for polynomial fitting of contact angles. Our method improved the accuracy by 21% for contact angles lower than 90° and by 33% for contact angles higher than 90°. more

Mechanically Robust and Flame-Retardant Superhydrophobic Textiles with Anti-Biofouling Performance

Langmuir, 38, 42, 2022, 12961–12967

The attachment of bio-fluids to surfaces promotes the transmission of diseases. Superhydrophobic textiles may offer significant advantages for reducing the adhesion of bio-fluids. However, they have not yet found widespread use because dried remnants adhere strongly and have poor mechanical or chemical robustness. In addition, with the massive use of polymer textiles, features such as fire and heat resistance can reduce the injuries and losses suffered by people in a fire accident. We developed a superhydrophobic textile covered with a hybrid coating of titanium dioxide and polydimethylsiloxane (TiO₂/PDMS). Such a textile exhibits low adhesion to not only bio-fluids but also dry blood. Compared to a hydrophilic textile, the peeling force of the coated textile on dried blood is 20 times lower. The textile’s superhydrophobicity survives severe treatment by sandpaper (400 mesh) at high pressure (8 kPa) even if some of its microstructures break. Furthermore, the textile shows excellent heat resistance (350 °C) and flame-retardant properties as compared to those of the untreated textile. These benefits can greatly inhibit the flame spread and reduce severe burns caused by polymer textiles adhering to the skin when melted at high temperatures. more

Enhanced condensation heat transfer by water/ethanol binary liquids on polydimethylsiloxane brushes

Droplets, Vol.1, Issue 2, 2022, 214-222

Enhancing heat transfer efficiency by liquid condensation plays a critical role in recovering and utilizing low‐grade heat. However, overall heat transfer efficiency is commonly limited by the inefficient vapor–liquid phase transition flux and enthalpy during liquid condensation. Here, we report that by introducing small amount of water into the phase‐change process of ethanol on a liquid‐like polydimethylsiloxane (PDMS) brush surface, the heat transfer coefficient is significantly enhanced, in particular, by more than one order of magnitude compared to the pure ethanol condensation. Such enhanced thermal erformance is primarily due to the elaborate balance between promoting condensation, that is, nucleation and growth, and increasing latent heat by regulating components of water and ethanol, as well as the
rapid droplet removal by condensing on the PDMS brushes. Note that the more stabilized dropwise condensation of the binary liquids, retained by accelerating the droplet coalescence velocity, beyond filmwise condensation ensures its significant effectivity under high heat flux. more

Charging of Dielectric Surfaces in Contact with Aqueous Electrolytes -- the Influence of CO2

Charging of Dielectric Surfaces in Contact with Aqueous Electrolytes -- the Influence of CO₂

J. Am. Chem. Soc., 144, 46, 2022, 21080–21087

The charge state of dielectric surfaces in aqueous
environments is of fundamental and technological importance.
Here, we study the influence of dissolved molecular CO2 on the charging of three chemically different surfaces (SiO2, Polystyrene, Perfluorooctadecyltrichlorosilane). We determine their charge state from electrokinetic experiments. We compare an ideal, CO2-free reference system to a system equilibrated against ambient CO2
conditions. In the reference system, the salt-dependent decrease of the magnitudes of ζ-potentials follows the expectations for a constant charge scenario. In the presence of CO2, the starting potential is lower by some 50%. The following salt-dependent decrease is weakened for SiO2 and inverted for the organic surfaces. We show that screening and pH-driven charge regulation alone cannot explain the observed effects. As an additional cause, we tentatively suggest dielectric regulation of surface charges due to a diffusively adsorbed thin layer of molecular CO2. The formation of such a dynamic layer, even at the hydrophilic and partially ionized silica surfaces, is supported by a minimal theoretical model and results from molecular simulations. more

Spontaneous charging affects the motion of sliding drops

Nature Phys. 18, 2022, 713–719

Water drops moving on surfaces are not only an everyday phenomenon seen on windows but also form an essential part of many industrial processes. Previous understanding is that drop motion is dictated by viscous dissipation and activated dynamics at the contact line. Here we demonstrate that these two effects cannot fully explain the complex paths of sliding or impacting drops. To accurately determine the forces experienced by moving drops, we imaged their trajectory when sliding down a tilted surface, and applied the relevant equations of motion. We found that drop motion on low-permittivity substrates is substantially influenced by electrostatic forces. Our findings confirm that electrostatics must be taken into consideration for the description of the motion of water, aqueous electrolytes and ethylene glycol on hydrophobic surfaces. Our results are relevant for improving the control of drop motion in many applications, including printing, microfluidics, water management and triboelectric nanogenerators. more

Tuning the Charge of Sliding Water Drops

Langmuir, 38, 19, 2022, 6224–6230

When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (N-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (−NH₂ with H⁺ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification. more

Fabrication of Stretchable Superamphiphobic Surfaces with Deformation-Induced Rearrangeable Structures

Adv. Materials, Vol. 34, Issue 10, 2022, 2107901

Stretchable superamphiphobic surfaces with a high deformation resistance are in demand to achieve liquid-repellent performance in flexible electronics, artificial skin, and textile dressings. However, it is challenging to make mechanically robust superamphiphobic coatings, which maintain their superliquid repellency in a highly stretched state. Here, a stretchable superamphiphobic surface is reported, on which the microstructures can rearrange during stretching to maintain a stable superamphiphobicity even under a high tensile strain. The surface is prepared by spray-coating silicone nanofilaments onto a prestretched substrate (e.g., cis-1,4-polyisoprene) with poly(dimethylsiloxane) (PDMS) layer as a binder. After subsequent fluorination, this surface keeps its superamphiphobicity to both water and n-hexadecane up to the tensile strain of at least 225%. The binding PDMS layer and rearrangeable structures maximize the deformation resistance of the surface during the stretching process. The superamphiphobicity and morphology of the surface are maintained even after 1000 stretch–release cycles. Taking advantage of the mentioned benefits, a liquid manipulation system is designed, which has the potential for fabricating reusable and low-cost platforms for biochemical detection and lab-on-a-chip systems. more

Contact angle hysteresis

Current Opinion in Colloid & Interf. Sci., Vol. 59, 2022, 101574

In thermodynamic equilibrium, the contact angle is related by Young's equation to the interfacial energies. Unfortunately, it is practically impossible to measure the equilibrium contact angle. When for example placing a drop on a surface its contact angle can assume any value between the advancing Θ_a and receding Θ_r contact angles, depending on how the drop is placed. Θ_a − Θ_r is called contact angle hysteresis. Contact angle hysteresis is essential for our daily life because it provides friction to drops. Many applications, such as coating, painting, flotation, would not be possible without contact angle hysteresis. Contact angle hysteresis is caused by the nanoscopic structure of the surfaces. Here, we review our current understanding of contact angle hysteresis with a focus on water as the liquid. We describe appropriate methods to measure it, discuss the causes of contact angle hysteresis, and describe the preparation of surfaces with low contact angle hysteresis. more

Charging of drops impacting onto superhydrophobic surfaces

Soft Matter, 18, 2022, 1628-1635

When neutral water drops impact and rebound from superhydrophobic surfaces, they acquire a positive electrical charge. To measure the charge, we analyzed the trajectory of rebounding drops in an external electric field by high-speed video imaging. Although this charging phenomenon has been observed in the past, little is known about the controlling parameters for the amount of drop charging. Here we investigate the relative importance of five of these potential variables: impact speed, drop contact area, contact line retraction speed, drop size, and type of surface. We additionally apply our previously reported model for sliding drop electrification to the case of impacting drops, suggesting that the two cases contain the same charge separation mechanism at the contact line. Both our experimental results and our theoretical model indicate that maximum contact area is the dominant control parameter for charge separation. more

Flow profiles near receding three-phase contact lines: influence of surfactants

Soft Matter, 17, 2021, 10090-10100

The dynamics of wetting and dewetting is largely determined by the velocity field near the contact lines. For water drops it has been observed that adding surfactant decreases the dynamic receding contact angle even at a concentration much lower than the critical micelle concentration (CMC). To better understand why surfactants have such a drastic effect on drop dynamics, we constructed a dedicated setup on an inverted microscope, in which an aqueous drop is held stationary while the transparent substrate is moved horizontally. Using astigmatism particle tracking velocimetry, we track the 3D displacement of the tracer particles in the flow. We study how surfactants alter the flow dynamics near the receding contact line of a moving drop for capillary numbers in the order of 10⁻⁶. Even for surfactant concentrations c far below the critical micelle concentration (c ≪ CMC) Marangoni stresses change the flow drastically. We discuss our results first in a 2D model that considers advective and diffusive surfactant transport and deduce estimates of the magnitude and scaling of the Marangoni stress from this. Modeling and experiment agree that a tiny gradient in surface tension of a few μN m⁻¹ is enough to alter the flow profile significantly. The variation of the Marangoni stress with the distance from the contact line suggests that the 2D advection–diffusion model has to be extended to a full 3D model. The effect is ubiquitous, since surfactant is present in many technical and natural dewetting processes either deliberately or as contamination. more

Wetting-regulated gas-involving (photo)electrocatalysis: biomimetics in energy conversion

Chem. Soc. Rev., 50, 2021,10674-10699

(Photo)electrolysis of water or gases has drawn tremendous attention. Aiming to bridge the fields of wetting and catalysis, we review the cutting-edge design methodologies of both gas-evolving and gas-consuming (photo)electrocatalytic systems. more

Optical Manipulation of Liquids by Thermal Marangoni Flow along the Air–Water Interfaces of a Superhydrophobic Surface

Langmuir, 37, 29, 2021, 8677–8686

A method to control liquid flow by temperature gradients on superhydrophobic surfaces was investigated experimentally and via numerical modelling. We gained insight into the physics of Marangoni flow and light-driven liquid transport systems. more

Clathrate Adhesion Induced by Quasi-Liquid Layer

J. Phys. Chem. C,125, 38, 2021, 21293–21300

The adhesive force of clathrates to surfaces is a century-old problem of pipeline blockage for the energy industry. Here, we provide new physical insight into the origin of this force by accounting for the existence of a quasi-liquid layer (QLL) on clathrate surfaces.
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One‐Step Synthesis of a Durable and Liquid‐Repellent Poly(dimethylsiloxane) Coating

Adv. Materials, Vol. 33, Issue 23, 2021, 2100237

A non-stick coating with autophobicity was fabricated by a one-step grafting-from approach for polydimethylsiloxane (PDMS) brushes on surfaces. Independent of surface tension, liquid droplets can easily slide on such surface. more