In this article, we present a simple and fast optical method based on transmission microscopy to study the stochastic wetting transitions on micro- and nanostructured polymer surfaces immersed in water. We analyze the influence of immersion time and the liquid pressure on the degree of water intrusion in individual microcavities on these surfaces as well as the lifespan of their superhydrophobicity. We show that transitions among the three wetting states (Cassie, Cassie-impregnating, and Wenzel) occur with a certain pressure threshold (300 mbar for a microcavity diameter of 7.5 μm). Below this threshold, the transitions between the Cassie and the Cassie-impregnating states are reversible, whereas above this threshold, irreversible transitions to the Wenzel state start to occur. The transitions between the different wetting states can be explained by taking into account both the Young-Laplace equation for the water menisci in the cavities and the diffusion of dissolved gas molecules in the water. In addition, the wetting transitions had a stochastic nature, which resulted from the short diffusion distance for dissolved gas molecules in the water between neighboring cavities. Furthermore, we compared the contact angle properties of two polymeric materials (COC and PP) with moderate hydrophobicity. We attributed the difference in the water repellency of the two materials to a difference in the wetting of their nanostructures. Our experimental observations thus indicate that both the diffusion of gas molecules in water and the wetting properties of nanostructures are important for understanding the sustainability of superhydrophobicity of surfaces under water and for improving the structural design of superhydrophobic surfaces.
We study water drop roll-off at superhydrophobic surfaces with different surface patterns. Superhydrophobic microcavity surfaces were fabricated in silicon and coated with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS). For the more irregular surface patterns, the observed increase in roll-off angles is found to be caused by a decrease of the receding contact angle, which in turn is caused by an increase of the triple phase contact line of the drops for those more irregular surfaces. To understand the observation, we propose to treat the microdrops as rigid bodies and apply a torque balance between the torque exerted by the projected gravity force and the torque exerted by the adhesion force acting along the triple line on the receding side of the drop. This simple model provides a proper order of magnitude estimate of the measured effects.
Contact angle measurements is a fast and simple way to measure surface properties and is therefore widely used to measure surface energy and quantify wetting of a solid surface by a liquid substance. In common praxis contact angle measurements are done with sessile drops on a horizontal surface fitted to a drop profile derived from the Young-Laplace equation. When measuring the wetting behaviour by tilting experiments this is not possible since it involves moving drops that are not in equilibrium. Here we present a fitting technique capable of determining the contact angle of asymmetric drops with very high accuracy even with blurry or noisy images. This we do by splitting the trace of a drop into a left and right part at the apex and then fit each side to an ellipse. KeywordsContact angle, wetting behaviour, roll-off angle, drop shape analysis, axisymmetric drop shape analysis (ADSA), triple-line, and contact angle hysteresis.
We present the fabrication process for injection molded multi-height surface structures for studies of wetting behavior. We adapt the design of super hydrophobic structures to the fabrication constrictions imposed by industrial injection molding. This is important since many super hydrophobic surfaces are challenging to realize by injection molding due to overhanging structures and very high aspect ratios. In the fabrication process, we introduce several unconventional steps for producing the desired shapes, using a completely random mask pattern, exploiting the diffusion limited growth rates of different geometries, and electroforming a nickel mold from a polymer foil. The injection-molded samples are characterized by contact angle hysteresis obtained by the tilting method. We find that the receding contact angle depends on the surface coverage of the random surface structure, while the advancing contact angle is practically independent of the structure. Moreover, we argue that the increase in contact angle hysteresis correlates with the concentration of pinning sites among the random surface structures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.