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 [Formula: see text], contact width [Formula: see text], contact length [Formula: see text], advancing [Formula: see text], and receding contact angle [Formula: see text] 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 ([Formula: see text]) ranging from 10-3 to 1 [Formula: see text] can empirically be described by [Formula: see text] for a velocity range up to 0.7 ms-1. The dimensionless friction coefficient [Formula: see text] 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 [Formula: see text] and [Formula: see text], for dynamic wetting the friction coefficient is additionally necessary. 

Kinetic drop friction

Wetting plays a major role in the close interactions between liquids and solid surfaces, which can be tailored by modifying the chemistry as well as the structures of the surfaces' outermost layer. Several methodologies, such as chemical vapor deposition, physical vapor deposition, electroplating, and chemical reactions, among others, have been adopted for the alteration/modification of such interactions suitable for various applications. However, the fabrication of low-contact line-pinning hydrophobic surfaces via simple and easy methods remains an open challenge. In this work, we exploit one-step and multiple-step silicone oil (5-100 cSt) grafting on smooth silicon substrates (although the technique is suitable for other substrates), looking closely at the effect of viscosity as well as the volume and layers (one to five) of oil grafted as a function of the deposition method. Remarkably, the optimization of grafting of silicone oil fabrication results in non-wetting surfaces with extremely low contact angle hysteresis (CAH) below 1° and high contact angles (CAs) of ∼108° after a single grafting step, which is an order of magnitude smaller than the reported values of previous works on silicone oil-grafted surfaces. Moreover, the different droplet-surface interactions and pinning behavior can additionally be tailored to the specific application with CAH ranging from 1 to 20° and sliding angles between 1.5 and 60° (for droplet volumes of 3 μL), depending on the fabrication parameters adopted. In terms of roughness, all the samples (independent of the grafting parameters) showed small changes in the root-mean-square roughness below 20 nm. Lastly, stability analysis of the grafting method reported here under various conditions shows that the coating is quite stable under mechanical vibrations (bath ultrasonication) and in a chemical environment (ultrasonication in a bath of ethanol) but loses its low-pinning characteristics when exposed to saturated steam at T ∼ 99 °C. The findings presented here provide a basis for selecting the most appropriate and suitable method and parameters for silicone oil grafting aimed at low pinning and low hysteresis surfaces for specific applications. 

Silicone Oil-Grafted Low-Hysteresis Water-Repellent Surfaces

Inhibition of Condensation-Induced Droplet Wetting by Nano-Hierarchical Surfaces

Triboelectric nanogenerators (TENGs) are cutting-edge technology that converts mechanical energy into electrical energy through contact electrification (CE). The CE process involves the chemical, physical, and mechanical properties of material surfaces and interfaces. Studying CE from the perspective of interfacial science can provide a comprehensive understanding of the charging mechanism and improve the performance of TENGs through structural design and technical optimization. Therefore, this paper reviews the research progress in CE from the perspective of surface and interface science. Specifically, we introduce the static and dynamic behavior of the liquid-solid (L-S) interaction during CE separately. Then, the interface phenomenon involved in gas-liquid-solid CE, such as surface wettability and ionic effect on liquid film drainage between the bubble and solid surface in water is discussed. This new interdisciplinary area of surface science of TENGs provides many research opportunities, while it is essential to explain critical phenomena in the CE process. 

Liquid-solid Contact Electrification Through the Lens of Surface and Interface Science

Heat exchangers are made of metals because of their high heat conductivity and mechanical stability. Metal surfaces are inherently hydrophilic, leading to inefficient filmwise condensation. It is still a challenge to coat these metal surfaces with a durable, robust, and thin hydrophobic layer, which is required for efficient dropwise condensation. Here, we report the nonstructured and ultrathin (∼6 nm) polydimethylsiloxane (PDMS) brushes on copper that sustain high-performing dropwise condensation in high supersaturation. Due to the flexible hydrophobic siloxane polymer chains, the coating has low resistance to drop sliding and excellent chemical stability. The PDMS brushes can sustain dropwise condensation for up to ∼8 h during exposure to 111 °C saturated steam flowing at 3 m·s-1, with a 5-7 times higher heat transfer coefficient compared to filmwise condensation. The surface is self-cleaning and can reduce the level of bacterial attachment by 99%. This low-cost, facile, fluorine-free, and scalable method is suitable for a great variety of heat transfer applications.

pdf/durable-ultrathin-and-antifouling-polymer-brush-coating-for-1zlid024g5.pdf

Durable, Ultrathin, and Antifouling Polymer Brush Coating for Efficient Condensation Heat Transfer.

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 performance 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. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/dro2.31

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

Due to poor chemical robustness, superhydrophobic surfaces become susceptible 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 polydimethylsiloxane (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 O2 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.

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

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.

Self-generated electrostatic forces of drops rebounding from hydrophobic surfaces

Abstract Slide electrification is the spontaneous separation of electric charges at the rear of water drops sliding over solid surfaces. This study delves into how surfaces treated with a low-pressure plasma impact water slide electrification. Ar, O 2 , and N 2 plasma treatment reduced the drop charge and contact angles on glass, quartz, and SU-8 coated with 1H,1H,2H,2H-perfluoroctyltrichlorosilane (PFOTS), and polystyrene. Conversely, 64% higher drop charge was achieved using electrode-facing treatment in plasma chamber. Based on the zeta potential, Kelvin potential, and XPS measurements, the plasma effects were attributed to alterations of the topmost layer's chemistry, such as oxidation and etching, and superficially charge deposition. The surface top layer charges were less negative after electrode-facing and more negative after bulk plasma treatment. As a result, the zeta potential was less negative after electrode-facing and more negative after bulk plasma treatment. Although the fluorinated layer was applied after plasma activation, we observed a discernible impact of plasma-glass treatment on drop charging. Plasma surface modification offers a means to adjust drop charges: electrode-facing treatment of the fluorinated layer leads to an enhanced drop charge, while plasma treatment on the substrate prior to fluorination diminishes drop charges, all without affecting contact angles or surface roughness. 

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

Drop Splitting on Hydrophobic Wedge-shaped Tips after Central Impact: Effect of Sharpness and Wetting Properties

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