Mar 16 2010
A small volume of water placed on a surface does not flow forever. Molecular-level interactions between the liquid and the solid shape the water into a droplet. Scientists at the Singapore Institute of Manufacturing Technology of A*STAR and Nanyang Technological University, Singapore, have now shown that droplet formation on a polymer can be controlled by exposing the surface to short pulses of light.
A measure of how well water flows on a specific material, known as the wettability, is given by the internal angle between the droplet wall and the surface. If the water is able to spread thinly, the angle is less than 90° and the material is said to be hydrophilic. A droplet on a hydrophobic surface on the other hand acts to minimize the contact area by forming a tight droplet with an angle in excess of 90°.
In their study, the research team demonstrated that femtosecond-long pulses of near-infrared light can make polymethyl methacrylate (PMMA) either hydrophilic or hydrophobic. They scanned a small area of PMMA with a focused laser beam. The contact angle between the treated surface and a 0.5-microliter droplet of water depended on the energy of the exposure (Fig. 1), which they changed by moving the laser spot in and out of focus. PMMA was super-hydrophilic at the highest exposures investigated, but the contact angle increased as the energy decreased. An optical dose of 2.1 joules per square centimeter or less made the polymer hydrophobic, and the maximum internal angle achievable was 125°. “Laser irradiation is a promising method to control surface wettability. It offers significant flexibility because it enables selected areas to be modified at a high speed,” explains Zhong Ke Wang, the lead researcher on the study.
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