Abstract
The transport of small amounts of liquids on solid surfaces is fundamental for microfluidics applications. Technologies allowing control of droplets of liquid on flat surfaces generally involve the generation of a wettability contrast. This approach is however limited by the resistance to motion caused by the direct contact between the droplet and the solid. We show here that this resistance can be drastically reduced by preventing direct contact with the help of dual-length scale micro-structures and the concept of “liquid-surfaces”. These new surfaces allow the gentle transport of droplets along defined paths and with fine control of their speed. Moreover, their high adhesion permits the capture of impacting droplets, opening new possibilities in applications such as fog harvesting and heat transfer.
Highlights
The transport of small amounts of liquids on solid surfaces is fundamental for microfluidics applications
Motion can be externally driven through gradients in electric fields1,2, temperature3–5, light[6,7] and pressure[8], or structural topography combined with vibration or phase c hange[9,10], but these all inconveniently involve the input of external energy
We demonstrate the self-propulsion of droplets on a shaped-liquid surface, where a lubricant layer is combined with a heterogeneous topography
Summary
The transport of small amounts of liquids on solid surfaces is fundamental for microfluidics applications. We show here that this resistance can be drastically reduced by preventing direct contact with the help of dual-length scale micro-structures and the concept of “liquid-surfaces” These new surfaces allow the gentle transport of droplets along defined paths and with fine control of their speed. Gradients in physical shape and wettability—the conical shape of cactus spines[11] and the wettability of butterfly wings12—occur naturally and can be engineered into surfaces to create self-propelled m otion[7,13,14,15,16,17,18] Such self-propelled motion to date has limited success in overcoming the inherent static resistance to motion of the liquid contact with the solid. These design principles are highly beneficial for droplet transport in microfluidics, self-cleaning surfaces, fog harvesting and in heat transfer
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