Scanning Drop Friction Force Microscopy
Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 μL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the
samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant super-hydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick−slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick−slip−stick−slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip−stick−slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces. more
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
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
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
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
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
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 Θa and receding Θr contact angles, depending on how the drop is placed. Θa − Θr 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
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
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
(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
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
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.
One‐Step Synthesis of a Durable and Liquid‐Repellent Poly(dimethylsiloxane) Coating
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
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