Brown School of Engineering, Brown University, 182 Hope Street, Providence, Rhode Island 02912, United States.
Langmuir. 2012 Sep 4;28(35):12771-8. doi: 10.1021/la301901m. Epub 2012 Aug 23.
We investigated the deposition and accumulation of droplets on both smooth substrates and substrates textured with square pillars, which were tens of micrometers in size. After being coated with a hydrophobic monolayer, substrates were placed in an air flow with a sedimenting suspension of micrometer-sized water droplets (i.e., fog). We imaged the accumulation of water and measured the evolution of the mean drop size. On smooth substrates, the deposition process was qualitatively similar to condensation, but differences in length scale revealed a transient regime not reported in condensation experiments. Based on previous simulation results, we defined a time-scale characterizing the transition to steady-state behavior. On textured substrates, square pillars promoted spatial ordering of accumulated drops. Furthermore, texture regulated drop growth: first enhancing coalescence when the mean drop size was smaller than the pillar, and then inhibiting coalescence when drops were comparable to the pillar size. This inhibition led to a monodisperse drop regime, in which drop sizes varied by less than 5%. When these monodisperse drops grew sufficiently large, they coalesced and could either remain suspended on pillars (i.e., Cassie-Baxter state) or wet the substrate (i.e., Wenzel state).
我们研究了液滴在光滑基底和具有数十微米尺寸的方形柱体纹理基底上的沉积和积累。在基底涂覆疏水单分子层后，将其置于含有微米级水液滴（即雾）的沉降悬浮液的气流中。我们对水的积累进行成像，并测量平均液滴尺寸的演化。在光滑基底上，沉积过程在定性上类似于凝结，但由于尺度的差异，在凝结实验中没有报道过瞬态阶段。根据先前的模拟结果，我们定义了一个时间尺度来描述向稳态行为的转变。在纹理化基底上，方形柱体促进了积累液滴的空间有序排列。此外，纹理调节了液滴的生长：当平均液滴尺寸小于柱体时，首先促进聚并，然后当液滴与柱体尺寸相当时，抑制聚并。这种抑制导致了单分散液滴状态，其中液滴尺寸的变化小于 5%。当这些单分散液滴足够大时，它们聚并，要么悬停在柱体上（即 Cassie-Baxter 状态），要么润湿基底（即 Wenzel 状态）。
