Although liquids typically flow downwards, the capillary effect — a gravity-defying process exploited by the roots and stems of plants — also allows them to be drawn up through narrow tubes. The phenomenon relies on the adhesion of liquid to the walls of a narrow vessel, pulling the edges of the liquid up to form a concave meniscus. Surface tension then causes the entire column of liquid to be drawn upwards — a process commonly called wicking.
Now, A. Y. Vorobyev and Chunlei Guo from the Institute of Optics at the University of Rochester, USA, have demonstrated that laser pulses can be used to treat the surface of silicon so that it becomes superhydrophilic and exhibits the capillary effect (Opt. Express 18, 6455–6460; 2010). The pulsed beam from an amplified Ti:Sapphire femtosecond laser was used to create a 22 mm × 11 mm array of parallel microgrooves on a 25 mm × 25 mm × 0.65 mm single-crystal phosphorus-doped silicon sample. The laser generated 65 fs pulses at a central wavelength of 800 nm and with 1.5 mJ of energy per pulse, fabricating each microgroove at a scanning speed of 1 mm s−1 and with a focused laser spot diameter of 100 μm. The resulting grooves had an average depth of 40 μm.
The researchers tested the wetting properties of the samples by applying 1–5 μL drops of various liquids, including distilled water, acetone and methanol, to their silicon surfaces. Using a camera to capture the spreading dynamics of the liquids at a speed of five frames per second, they were able to compare the wicking behaviours of liquids on both laser-treated and untreated samples of silicon. The results showed that the laser treatment caused the silicon to become superhydrophilic, and that the spreading distance of the liquids, regardless of chemical composition, followed the classical square root of time dependence — a characteristic of the Washburn equation, which governs the motion of liquids in a closed capillary. The same results were achieved when the silicon samples were stood vertically, with the liquid rising upwards against gravity.
Vorobyev and Guo believe that the capillary properties of their ultrafast-laser-treated silicon may find applications in a number of areas, including in microfluidics, lab-on-a-chip technology, biomedicine, and chemical and biological sensors.
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Heat and Mass Transfer (2011)