Capillarity is a common feature of everyday life. It is used by plants to transport liquids, and capillarity research is applied to oil extraction, textiles, paper and printing, and in weightless environments, for example. Capillary uptake is most simply observed when a liquid is drawn into a thin tube. For example, when a glass tube comes into contact with a pool of water, the water moves up the tube. In this case, the uptake is driven by the wetting interaction between the water and the glass – the water forms a contact angle of less than 90º with glass at the meniscus. If the interaction is non-wetting – forming a contact angle greater than 90º – then the liquid does not move from a pool into a capillary. This is the case for a teflon tube in contact with water.

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Figure (from [1]): Upper, a drop immobilized on a superhydrophobic surfaces, journal cover image. Lower, three sequential photos showing a teflon capillary (inner and outer diameters 0.31 mm and 0.76 mm respectively) brought into contact with a water droplet. In the third photo, the arrow indicates the height of the meniscus after uptake.

Our research concerns capillary uptake when the liquid is drawn up from a small droplet, rather than from a pool. The Laplace pressure due to the curvature of the drop enhances uptake, and can even drive uptake in the non-wetting case of teflon and water  ([1,2], see Figure). Our interest in microfluidic non-wetting capillary uptake stems from a MacDiarmid Institute collaboration, in which researchers have observed uptake of a non-wetting metal catalyst into carbon nanotubes as they are grown.

Superhydrophobic surfaces have especially interesting wetting properties – they have water contact angles of at least 150-160° and they are therefore extremely water-repellent. To be superhydrophobic, a surface must have some rough microstructure, and it must be chemically water-repellent (hydrophobic). On such a surface, water drops stay on top of ‘peaks’, and therefore easily skate across the surface.

There are many superhydrophobic surfaces in nature, such as leaves of the lotus plant and butterfly wings. Man-made superhydrophobic surfaces may be useful for applications such as condensation management, ice-prevention, or as self-cleaning surfaces. One easy way to make a superhydrophobic surface is to deposit microstructured silver crystals on a copper surface by electroless deposition, then attach hydrophobic thiol groups to this surface. We have used this type of surface to produce suitable droplets for our capillary uptake experiments (see Figure) and in drop splash experiments. We are also interested in making superhydrophobic surfaces which consist a micrometer-scale array of polymer posts, defined and fabricated using soft lithography.

References:

[1] Willmott, G. R., Neto, C. and Hendy, S. C., “Uptake of Water Droplets by Nonwetting Capillaries,” Soft Matter 7, 2357-2363 (2011).

[2] Willmott, G. R., Neto, C. and Hendy, S. C., “Dynamics of Drop Size-Dependent Uptake in Non-Wetting Capillaries,” Faraday Discuss. 146, 233-245 (2010).

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