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.

X-ray microscopy with soft X-rays is well suited for investigations of aqueous samples of some microns thickness when the resolution is required to be better than in visible light microscopy an not better than 30 nm. Sample preparation is as simple as for light microscopy, i.e. no fixation or metal coating is needed.In Aarhus, an X-ray microscope is used for investigations in fields as biology, medicine and soil sciences. A ray diagram of the Aarhus X-ray microscope is shown in Fig.l. Synchrotron radiation at a wavelength of 2.4 nm from the Aarhus Storage Ring is focused by a condenser zone plate onto an object. Another zone plate as an objective behind the object forms the image on a CCD camera. Objects are located under atmospheric pressure. For dry samples, almost any kind of holder can be mounted in the microscope. Wet samples are placed between thin silicon foils in a sealed chamber. With typical liquid layer thickness of 5-15 µm, samples can be kept in the chamber for many hours without drying out.

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