Observing surface wetting under water

A Leica confocal microscope and profilometer has been used by scientists to better understand the wetting properties of hydrophobic (water repellant) surfaces. They recorded images of the surfaces with water placed on top of them to study the roughness and how it changes over time. The aim is to contribute to the development of materials with self-cleaning surfaces for diverse applications.

Water repellant surfaces are often used in nature by plants and insects to keep themselves clean, free of debris, and reduce water loss. Normally, we see this when water forms drops while in contact with a material’s surface. Examples are wax or hydrocarbon polymers. The ability of a material to repel water is known as hydrophobicity. Superhydrophobic surfaces, like those of plant leaves and insect exoskeletons, repel water immensely.

Self-cleaning surfaces would be very practical for electronic devices (panel displays, circuit boards, and cases), vehicles and buildings (seats, furniture, windows, and walls), as well as, textiles (clothing and fabric). Such technology can help maintain surfaces contamination free, simplifying cleaning. It would reduce greatly the adhesion of dust and other particles, as well as, water, oil, and other liquids.

Superhydrophicity is a central theme in the research group of Prof. Boaz Pokroy at the Technion Israel Institute of Technology. Using a DCM confocal microscope/profilometer, the group is the first to image the bottom of a water drop on a hydrophobic surface at the water−air interface. This image data can help to improve understanding of interfacial wetting phenomena. In addition, the group recorded a series of images of a thin waxy layer on gold with a drop of water on top of it. The waxy layer showed a change in surface roughness over time. The surface transformed from hydrophobic to superhydrophobic over hours due to strain in the layer which induced recrystallization.

Perhaps one day superhydrophobic surfaces will be widely used for tables, windows, doors, and even clothing. Superhydrophobic surfaces would make the job of keeping things clean much easier.

Line profile measured along the x dimension of the confocal image shown above. The profile is of the water−air interface below the water drop resting on the substrate surface. The maximum height of the profile is indicated by the Rt value.

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