水辅助法（Breath Figure）是一种利用潮湿空气中的冷凝水滴为模板，通过界面动力学使其规整排列形成蜂巢结构多孔膜的方法。目前大多数对BF方法形成聚合物多孔膜的研究都是在平整基底上，而对非平面基底上的成膜研究甚少。非平面膜上的孔洞结构将有可能发展成一种新的材料和新的微结构。
本文研究了微针尖表面的BF过程，以期得到曲率对此过程的影响。同时引入了表面活性剂，试图了解在极端界面情况下的表面活性剂稳定水滴界面行为。具体使用了Poly (L-lactic acid) (PLLA)作为成膜基材，同时添加dioleoylphosphatidylethanolamine (DOPE) 作为表面活性剂。PLLA-DOPE在微针尖表面形成的多孔膜随着针尖曲率的变化而变化，同时这种变化趋势和BF过程中不同时间段的变化趋势类似：在较高的曲率上形成的孔小而稀疏，和BF过程早期类似；在较低曲率上形成的孔形成了蜂巢状的规整排列，和BF过程后期类似。特别的，我们在较高曲率范围处得到了处于“半融合”状态的孔洞，提示了表面活性剂诱导的水滴融合行为。
微针尖表面的曲率和直径变化引起了表面吸附溶液量和挥发速率差的增加，使尖端可以快速挥发，而末端挥发较慢，这可能是孔洞随曲率变化的主要原因，也是孔洞变化规律同BF不同过程相类似的原因。通常BF过程只有通过显微镜实时观测，有限的放大倍数和快速的挥发速率使得很多细节不能被观察到。本文首次实现了在聚合物表面记录不同时间段的BF成孔印迹，对BF表面动力学机理及表面活性剂稳定机理解释将提供很好的参考。
Xiaoli Jiang, Tianzhu Zhang, Lina Xu, Changling Wang, Xuefeng Zhou, and Ning Gu.
Surfactant-Induced Formation of Honeycomb Pattern on Micropipette with Curvature Gradient. Langmuir, 2011, 27 (9), pp 5410–5419
Figure 1 Characterization of the micropipettes under optical microscopy. Five sections (A-E) of the micropipette with different curvatures are chosen for the observation of the film morphologies. The corresponding curvatures of sections A-E are list in the table.
Figure 2 SEM images, average diameters (AD), entropy (S), and size distributions of pores on PLLA-2%DOPE films cast at 90% RH on (a) cover glasses and (A-E) glass micropipettes taken at different sections with increasing curvature.
Figure 3 SEM images of PLLA-2%DOPE cast on micropipettes show two transition zones of the arrangement and shapes of pores. (A-D) The transition zone at the curvature of around 4~5×10–3μm–1 represents the change between honeycomb structure and irregular hemispherical pores. (E-F) The transition zone at the curvature of around 10~15×10–3μm–1 shows the change between irregular hemispherical pores and random dispersed spherical pores.
Figure 6 The summary of A) average diameters and B) the circularity of pores on porous films cast on flat surfaces and the micropipettes from PLLA with different DOPE concentration.
Figure 7 Formation mechanism of honeycomb structure on micropipettes.
Figure 8 (A) Schematic illustration of DOPE-stabilized coalescence of water droplets. (B) The experimental observations of the coalescence on PLLA-2%DOPE, PLLA-0.5%DOPE, and PLLA-0%DOPE films at the curvature of 8×10–3μm–1, respectively.
