振动诱导微结构粗糙表面水滴Wenzel-Cassie状态转变特性

以聚二甲基硅氧烷（PDMS）基底采用光刻蚀技术制备了微方柱结构粗糙表面。采用高速摄影对液滴在垂直振动作用下的动态浸润状态进行了图像采集。通过对水滴振动过程中的动态浸润特性分析，研究了粗糙表面水滴的Wenzel-Cassie浸润状态转变特征。结果表明，对于一定尺寸的Wenzel状态水滴，只有当施加的振动能量超过某一阈值时，微方柱粗糙表面Wenzel状态液滴才可以发生向Cassie状态的完全转变，且存在发生Wenzel-Cassie浸润转变的阈值范围；此外，当外加振动频率和液滴固有频率一致时，即在共振频率时，液滴发生Wenzel-Cassie状态转变需要的能量最小。外加振动频率偏离液滴固有频率越远，发生Wenzel-Cassie状态转变需要的能量最大。基于表面化学和振动力学理论，建立了液滴发生Wenzel-Cassie转变时的物理模型。

Vibration-induced Wenzel-Cassie wetting transition on rough patterned surface

Superhydrophobic surfaces have aroused great attention for promising applications, e.g., enhanced heat transfer. The rough surface of square-shaped pillars was prepared from the polydimethyl-siloxane (PDMS) substrate by using photolithography technique. Based on the analysis of dynamic wetting characteristics of water droplets during vertical vibration, the Wenzel-Cassie wetting transition on the rough surface was studied with experimental and theoretical techniques. The experimental results showed that the Wenzel state droplets on the square-shaped pillars rough surface could change to the Cassie state when forced vibration frequency and amplitude were in the threshold range. When the eigenfrequency of the droplet was in accordance with forced vibration frequency, that is to say, at the resonance frequency, the forced vibration amplitude for Wenzel-Cassie wetting transition reached the lowest value. When forced vibration frequency was far from eigenfrequency, vibration amplitude was greater than the amplitude corresponding to resonance frequency. In the end, using the theory of surface chemistry, combining with vibration mechanics, a physical model was proposed to explain the Wenzel-Cassie wetting transition mechanism. This study could be potentially used to improve and control the heat transfer performance of dropwise condensation.