Origins of Extreme Liquid Repellency on Structured, Flat, and Lubricated Hydrophobic Surfaces (original) (raw)
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Origins of liquid-repellency on structured , flat , and lubricated surfaces
2018
There are currently three main classes of high-performance liquid-repellent surfaces: micro-/nanostructured lotus-effect superhydrophobic surfaces, flat surfaces grafted with ‘liquid-like’ polymer brushes, and various lubricated surfaces. Despite recent progress, the mechanistic understanding of the differences in droplet behavior on such surfaces is still under debate. We measured the dissipative force acting on a droplet moving on representatives of these classes at different velocities U = 0.01–1 mm/s using a cantilever force sensor with sub-μN accuracy, and correlated it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal (< 0.1s) resolutions. We find that the dissipative force—due to very different physical mechanisms at the contact line—is independent of velocity on superhydrophobic surfaces, but depends non-linearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform futur...
Mapping micron-scale wetting properties of superhydrophobic surfaces
arXiv: Soft Condensed Matter, 2019
There is a huge interest in developing super-repellent surfaces for anti-fouling and heat transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be indirectly inferred from the Furmidge's relation. While easy to implement, contact angle measurements are semi-quantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an Atomic Force Microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micron-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning-depinning dynamics as a droplet detaches from or moves across the surface.
Mapping micrometer-scale wetting properties of superhydrophobic surfaces
Proceedings of the National Academy of Sciences, 2019
There is a huge interest in developing superrepellent surfaces for antifouling and heat-transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be inferred indirectly using Furmidge’s relation. While easy to implement, contact angle measurements are semiquantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micrometer-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning–depinning dynamics as the droplet detaches from or moves across the surface.
From Sticky to Slippery Droplets: Dynamics of Contact Line Depinning on Superhydrophobic Surfaces
Physical Review Letters, 2012
This study explores how surface morphology affects the dynamics of contact line depinning of an evaporating sessile droplet on micro-pillared superhydrophobic surfaces. The result shows that neither a liquid-solid contact area nor an apparent contact line is a critical physical parameter to determine the de-pinning force. The configuration of a contact line on a superhydrophobic surface is multi-modal, comprised of both two-phase (liquid-air) and threephase (liquid-solid-air). The multi-modal state is dynamically altered when a droplet recedes. The maximal three-phase contact line attainable along the actual droplet boundary is found a direct and linear parameter that decides the de-pinning force on the superhydrophobic surface.
Droplet Impingement and Wetting Hysteresis on Textured Hydrophobic Surfaces
2010 14th International Heat Transfer Conference, Volume 3, 2010
We study the wetting energetics and wetting hysteresis of sessile and impacting water droplets on superhydrophobic surfaces as a function of surface texture and surface energy. Detailed experiments tracking contact line motion simultaneously with contact angle provides new insights on the wetting hysteresis, stick-slip behavior and dependence on contact line velocity. For sessile drops, we find three wetting regimes on these surfaces: equilibrium Cassie at small feature spacing, equilibrium Wenzel at large feature spacing, and an intermediate state at medium feature spacing. We observe minimum wetting hysteresis not on surfaces that exhibit Cassie wetting but rather on surfaces in the intermediate regime. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the surface. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop to the capillary pressure associated with the wetting-energy barrier of the textured surface. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets show very good agreement with this simple pressure-balance model.
Journal of Colloid and Interface Science, 2023
Hypothesis: Contact angle and sliding angle measurements are widely used to characterize superhydrophobic surfaces because of the simplicity and accessibility of the technique. We hypothesize that dynamic friction measurements, with increasing pre-loads, between a water drop and a superhydrophobic surface is more accurate because this technique is less influenced by local surface inhomogeneities and temporal surface changes. Experiments: A water drop, held by a ring probe which is connected to a dual-axis force sensor, is sheared against a superhydrophobic surface while maintaining a constant preload. From this force-based technique, static and kinetic friction forces measurements are used to characterize the wetting properties of the superhydrophobic surfaces. Furthermore, by applying increased pre-loads to the water drop while shearing, the critical load at which the drop transitions from the Cassie-Baxter to Wenzel state is also measured. Findings: The force-based technique predicts sliding angles with reduced standard deviations (between 56-64%) compared to conventional optical-based measurements. Kinetic friction force measurements show a higher accuracy (between 35-80%) compared to static friction force measurements in characterizing the wetting properties of superhydrophobic surfaces. The critical loads for the Cassie-Baxter to Wenzel state transition allows for stability characterization between seemingly similar superhydrophobic surfaces.
Bouncing or sticky droplets: Impalement transitions on superhydrophobic micropatterned surfaces
Europhysics Letters (EPL), 2006
When a liquid drop impinges a hydrophobic rough surface it can either bounce off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel droplets). The analysis of drop impact and quasi-static "loading" experiments on model microfabricated surfaces allows to clearly identify the forces hindering the impalement transitions. A simple semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir non-wetting states. Motivated by potential applications in microfluidics and in the fabrication of self-cleaning surfaces, we finally propose some guidelines to design robust superhydrophobic surfaces.
Proceedings of the National Academy of Sciences, 2019
Significance The matchless mobility of water on superhydrophobic materials is often considered as the hallmark of water repellency. The friction of drops is indeed found to be unusual: instead of observing classical friction such as due to contact line and liquid viscosity, we show that the main source of resistance to the water motion arises from the air around it. This explains why the drop velocity becomes quasi-independent of its viscosity at low viscosity and more generally why the mobility is so high. We also discuss the case of more viscous liquids whose rolling motion induces a bulk viscous dissipation that caps the mobility.