Bahadur Vaibhav, Garimella Suresh V
School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907-2088, USA.
Langmuir. 2009 Apr 21;25(8):4815-20. doi: 10.1021/la803691m.
Control and switching of liquid droplet states on artificially structured surfaces have significant applications in the field of microfluidics. The present work introduces the concept of using structured surfaces consisting of noncommunicating roughness elements to prevent the transition of a droplet from the Cassie to the Wenzel state. The use of noncommunicating roughness elements leads to a confinement of the medium under the droplet in its Cassie state. Transition to the Wenzel state on such surfaces requires expulsion of this confined medium, which offers significantly increased resistance to the Wenzel transition unlike surfaces consisting of communicating roughness elements. This enhances the robustness of the Cassie state and significantly minimizes the possibility of the Cassie-Wenzel transition. In the present work, the resistance of a surface to the Wenzel transition is measured in terms of the electrowetting (EW) voltage required to trigger this transition. It is seen that surfaces with noncommunicating roughness elements (cratered surfaces) require significantly higher voltages to trigger the Wenzel transition than corresponding surfaces with communicating roughness elements. The findings from the present work also indicate that EW-induced droplet morphology control characteristics show a strong dependence on the nature of the roughness elements (communicating versus noncommunicating). Different aspects of droplet morphology and EW-induced state transition control on surfaces with noncommunicating roughness elements are analyzed; it is seen that such surfaces offer significant possibilities for the development of robust superhydrophobic surfaces.
在人工结构化表面上控制和切换液滴状态在微流控领域有着重要应用。本工作引入了使用由不连通的粗糙度元素组成的结构化表面来防止液滴从 Cassie 状态转变为 Wenzel 状态的概念。使用不连通的粗糙度元素会导致处于 Cassie 状态的液滴下方的介质受到限制。在这样的表面上转变为 Wenzel 状态需要排出这种受限介质，这与由连通的粗糙度元素组成的表面不同，对 Wenzel 转变提供了显著增加的阻力。这增强了 Cassie 状态的稳定性，并显著降低了 Cassie - Wenzel 转变的可能性。在本工作中，通过触发这种转变所需的电润湿（EW）电压来衡量表面对 Wenzel 转变的阻力。可以看出，具有不连通粗糙度元素的表面（坑洼表面）比具有连通粗糙度元素的相应表面需要显著更高的电压来触发 Wenzel 转变。本工作的研究结果还表明，电润湿诱导的液滴形态控制特性强烈依赖于粗糙度元素的性质（连通与不连通）。分析了具有不连通粗糙度元素的表面上液滴形态和电润湿诱导的状态转变控制的不同方面；可以看出，这样的表面为开发坚固的超疏水表面提供了显著的可能性。
