Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
J Colloid Interface Sci. 2021 Dec;603:539-549. doi: 10.1016/j.jcis.2021.06.132. Epub 2021 Jun 25.
It has been verified that a surface of single micro-scale structures with certain roughness could exhibit petal effect. That is, water drops with a contact angle larger than 150° would pin on the petal effect surface. It is conjectured that the water drop could pin on the single micro-scale roughness petal effect surface by totally infiltrating into spaces (or grooves) between micro-pillars.
An inverted optical microscopy system is synchronically applied in the process of advancing/receding contact angle (ACA/RCA) measurements to directly observe the wetting behavior of water droplets on hydrophobic patterned surfaces with regular arrays of square micro-pillars.
A sequence of wetting behavior evolution, Wenzel → petal → pseudo-lotus → lotus, is observed on the hydrophobic patterned surfaces along with increasing surface roughness. It is interesting to observe a Cassie-Wenzel transition for water drops on a petal substrate during the ACA measurement (embedded needle method), leading to two ACAs, one before (in Cassie state) and one after the transition (in Wenzel state). Thus, the petal substrates have large contact angle hysteresis (CAH) (with both ACA and RCA in Wenzel state) to pin the water drop in Wenzel state. A Cassie-Wenzel transition is consistently observed during the evaporation process of water drops on pseudo-lotus substrates, leading to two RCAs: one in Cassie state and one in Wenzel state. The pseudo-lotus substrates have CAH (with both ACA and RCA in Cassie state) small enough to make water drops easily slide off.
已经验证，具有一定粗糙度的单个微尺度结构表面可以表现出花瓣效应。也就是说，接触角大于 150°的水滴会固定在花瓣效应表面上。推测水滴可以通过完全渗透到微柱之间的空间（或凹槽）而固定在单个微尺度粗糙度花瓣效应表面上。
在前进/后退接触角（ACA/RCA）测量过程中同步应用倒置光学显微镜系统，直接观察具有规则排列的正方形微柱的疏水图案表面上水滴的润湿行为。
在疏水图案表面上，随着表面粗糙度的增加，观察到一系列润湿行为的演化，Wenzel→花瓣→伪莲→莲。有趣的是，在 ACA 测量（嵌入式针方法）过程中，在花瓣基底上的水滴会发生 Cassie-Wenzel 转变，导致两个 ACA，一个在前（在 Cassie 状态），一个在后（在 Wenzel 状态）。因此，花瓣基底具有较大的接触角滞后（CAH）（在 Wenzel 状态下的 ACA 和 RCA 均为 Wenzel 状态），可将水滴固定在 Wenzel 状态。在伪莲基底上水滴蒸发过程中始终观察到 Cassie-Wenzel 转变，导致两个 RCA：一个在 Cassie 状态，一个在 Wenzel 状态。伪莲基底的 CAH（在 Cassie 状态下的 ACA 和 RCA 均为 Cassie 状态）足够小，使水滴容易滑落。
