Lee Yaerim, Amberg Gustav, Shiomi Junichiro
Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Bunkyo-ku, Hongo, Tokyo 113-8656, Japan.
Department of Mechanics, Linné Flow Centre, The Royal Institute of Technology, SE-100 44 Stockholm, Sweden.
PNAS Nexus. 2022 Mar 16;1(2):pgac027. doi: 10.1093/pnasnexus/pgac027. eCollection 2022 May.
Droplet spreading and transport phenomenon is ubiquitous and has been studied by engineered surfaces with a variety of topographic features. To obtain a directional bias in dynamic wetting, hydrophobic surfaces with a geometrical asymmetry are generally used, attributing the directionality to one-sided pinning. Although the pinning may be useful for directional wetting, it usually limits the droplet mobility, especially for small volumes and over wettable surfaces. Here, we demonstrate a pinning-less approach to rapidly transport millimeter sized droplets on a partially wetting surface. Placing droplets on an asymmetrically structured surfaces with micron-scale roughness and applying symmetric horizontal vibration, they travel rapidly in one direction without pinning. The key, here, is to generate capillary-driven rapid contact-line motion within the time-scale of period of vibration. At the right regime where a friction factor local at the contact line dominates the rapid capillary motion, the asymmetric surface geometry can induce smooth and continuous contact-line movement back and forth at different speed, realizing directional motion of droplets even with small volumes over the wettable surface. We found that the translational speed is selective and strongly dependent on the droplet volume, oscillation frequency, and surface pattern properties, and thus droplets with a specific volume can be efficiently sorted out.
液滴铺展和传输现象无处不在，并且已经被具有各种形貌特征的工程表面所研究。为了在动态润湿中获得方向偏向，通常使用具有几何不对称性的疏水表面，将方向性归因于单侧钉扎。尽管钉扎对于定向润湿可能有用，但它通常会限制液滴的流动性，特别是对于小体积液滴以及在可湿润表面上。在此，我们展示了一种无钉扎方法，可在部分润湿表面上快速传输毫米大小的液滴。将液滴放置在具有微米级粗糙度的不对称结构表面上并施加对称水平振动，它们会在不发生钉扎的情况下沿一个方向快速移动。这里的关键是在振动周期的时间尺度内产生由毛细作用驱动的快速接触线运动。在接触线处局部摩擦系数主导快速毛细运动的合适状态下，不对称表面几何形状可以诱导接触线以不同速度来回平滑连续移动，即使在可湿润表面上小体积液滴也能实现定向运动。我们发现平移速度具有选择性，并且强烈依赖于液滴体积、振荡频率和表面图案特性，因此具有特定体积的液滴可以被有效地分选出来。
