{"id":1530,"date":"2010-07-08T13:38:51","date_gmt":"2010-07-08T17:38:51","guid":{"rendered":"https:\/\/wpmu2.mit.local\/?p=1530"},"modified":"2010-07-08T13:38:51","modified_gmt":"2010-07-08T17:38:51","slug":"manipulation-of-liquid-spreading-on-asymmetric-nanostructured-surfaces","status":"publish","type":"post","link":"https:\/\/wpmu2.mit.local\/manipulation-of-liquid-spreading-on-asymmetric-nanostructured-surfaces\/","title":{"rendered":"Manipulation of Liquid Spreading on Asymmetric Nanostructured Surfaces"},"content":{"rendered":"

The manipulation of liquid spreading is important for a broad range of microfluidic, biological, and thermal management applications [1<\/a>]<\/sup> [2<\/a>]<\/sup> [3<\/a>]<\/sup>. In this work, we investigated the ability to manipulate droplet spreading to a single direction on-demand on asymmetric nanostructured surfaces using electrowetting.\u00a0 Asymmetric nanopillar arrays were fabricated with diameters of 500 to 750 nm and deflection angles of 3 to 52 degrees.\u00a0 Figure 1 shows scanning electron micrographs (SEMs) of three representative asymmetric nanopillar arrays with deflection angles, \u03c6<\/em>, (as defined in the Figure 1 inset) ranging from\u00a07\u00b0-25\u00b0.\u00a0 A Cartesian coordinate system is defined for convenience, as shown in Figure 1, where the pillars deflect in the positive X (+X) direction.\u00a0 In the presence of asymmetric nanostructures, the spreading behavior can be dynamically controlled with electrowetting, which utilizes an electric potential, V<\/em>, across the droplet and nanostructured surface to change the surface energy (Figure 2a).\u00a0 With this approach, different droplet-spreading directionalities can be achieved based on the magnitude of the electric potential.\u00a0 If we apply V<\/em>= 1.5 V to an initially static symmetric droplet, the liquid pins in the \u2013X direction and spreads in +X, i.e.<\/em> uni-directionally (Figure 2b).\u00a0 In the case of an applied V<\/em>= 2.1 V, the liquid unpins in \u2013X and spreads bi-directionally.\u00a0 The spreading, however, is asymmetric: the rate is three times faster in +X as compared to \u2013X (Figure 2c).\u00a0 Moreover, with increasing applied V<\/em>, the asymmetry decreases.\u00a0 In the case of an applied V<\/em>> 2.5V, the liquid spreading is nearly symmetric, i.e.<\/em> the rates in +X and \u2013X are approximately equal (Figure 2d).\u00a0 The study provides design guidelines to tune the droplet\u2019s behavior from uni-directional to asymmetric or symmetric bi-directional spreading using both nanostructure design and applied electric fields for a variety of microfluidic applications.<\/p>\n