McHale G, Aqil S, Shirtcliffe N J, Newton M I, Erbil H Y
School of Biomedical and Natural Sciences, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK.
Langmuir. 2005 Nov 22;21(24):11053-60. doi: 10.1021/la0518795.
The evaporation process for small, 1-2-mm-diameter droplets of water from patterned polymer surfaces is followed and characterized. The surfaces consist of circular pillars (5-15 microm diameter) of SU-8 photoresist arranged in square lattice patterns such that the center-to-center separation between pillars is 20-30 microm. These types of surface provide superhydrophobic systems with theoretical initial Cassie-Baxter contact angles for water droplets of up to 140-167 degrees, which are significantly larger than can be achieved by smooth hydrophobic surfaces. Experiments show that on these SU-8 textured surfaces water droplets initially evaporate in a pinned contact line mode, before the contact line recedes in a stepwise fashion jumping from pillar to pillar. Provided the droplets of water are deposited without too much pressure from the needle, the initial state appears to correspond to a Cassie-Baxter one with the droplet sitting upon the tops of the pillars. In some cases, but not all, a collapse of the droplet into the pillar structure occurs abruptly. For these collapsed droplets, further evaporation occurs with a completely pinned contact area consistent with a Wenzel-type state. It is shown that a simple quantitative analysis based on the diffusion of water vapor into the surrounding atmosphere can be performed, and estimates of the product of the diffusion coefficient and the concentration difference (saturation minus ambient) are obtained.
对图案化聚合物表面上直径为1 - 2毫米的小水滴的蒸发过程进行了跟踪和表征。这些表面由SU - 8光刻胶制成的圆形柱体(直径5 - 15微米)排列成方形晶格图案,使得柱体之间的中心距为20 - 30微米。这类表面为超疏水系统提供了理论上高达140 - 167度的水滴初始卡西 - 巴克斯特接触角,这比光滑疏水表面所能达到的角度要大得多。实验表明,在这些有纹理的SU - 8表面上,水滴最初以固定接触线模式蒸发,之后接触线以逐步方式从一个柱体跳到另一个柱体而后退。如果水滴在没有来自针头过多压力的情况下沉积,初始状态似乎对应于卡西 - 巴克斯特状态,水滴位于柱体顶部。在某些情况下(但不是所有情况),水滴会突然坍塌到柱体结构中。对于这些坍塌的水滴,进一步蒸发时接触面积完全固定,这与文策尔型状态一致。结果表明,可以基于水蒸气向周围大气中的扩散进行简单的定量分析,并获得扩散系数与浓度差(饱和浓度减去环境浓度)乘积的估计值。