Department of Chemical Engineering, Birla Institute of Technology and Science Pilani, Hyderabad Campus, 500 078, Telangana, India.
Soft Matter. 2023 Jul 5;19(26):4982-4990. doi: 10.1039/d3sm00221g.
Thin polymer films can become unstable and dewet on a non-wettable substrate leading to the formation of an array of droplets. This instability-mediated drop formation lacks spatial order on flat substrates, but it can be ordered for better combinatorial studies using patterned substrates. In this work, we studied the process of dewetting polystyrene (PS) films on grating patterned substrates upon solvent vapor exposure. The PS film thickness was commensurate with the grating pattern height (). Our findings show that the initial dewetting stages follow the direction of the underlying grating pattern with the formation of directional holes in the film. During the later stages of dewetting, there was a lateral coalescence of polymer threads across the grating stripes. The final morphology comprised smaller droplets or threads confined within the pattern grooves and anisotropic large drops covering several pattern stripes. Furthermore, the larger drops show a unique behavior of shape change from anisotropic to isotropic as a function of solvent vapor concentration () inside the dewetting chamber. The drop regained its anisotropic shape with an increase in , and this transition continued, with the movement of the three-phase contact line (TPCL). While the capillary flow of the polymer causes anisotropy during high , the local orientation of the contact line and a mismatch in the value of the equilibrium contact angle can drive the drop back into an isotropic shape as reduces and capillary forces weaken. We also observed that the extent of anisotropy quantified as droplet distortion ratio () not only depends on the during dewetting but also on solvent type and . This new-found dynamic behavior of dewetted polymeric drops can be studied in greater detail and potentially leveraged for applications in sensing and microfluidics.
薄聚合物膜在非润湿基底上会变得不稳定并去湿，导致形成一系列液滴。这种由不稳定性引起的液滴形成在平坦基底上缺乏空间有序性，但可以通过使用图案化基底来有序化，以进行更好的组合研究。在这项工作中，我们研究了在溶剂蒸气暴露下，PS 薄膜在光栅图案化基底上的去湿过程。PS 薄膜厚度与光栅图案高度一致（）。我们的发现表明，初始去湿阶段遵循基底光栅图案的方向，在薄膜中形成定向孔。在去湿的后期阶段，聚合物线在光栅条带之间发生横向聚合并合。最终形态包括较小的液滴或被困在图案槽内的线以及覆盖几个图案条带的各向异性大液滴。此外，较大的液滴表现出独特的形状变化行为，从各向异性变为各向同性，这取决于去湿室内溶剂蒸气浓度（）。随着的增加，液滴恢复各向异性形状，这种转变继续进行，同时三相接触线（TPCL）移动。虽然聚合物的毛细流动在高时导致各向异性，但接触线的局部取向和平衡接触角值的不匹配可以在降低和毛细力变弱时将液滴恢复到各向同性形状。我们还观察到，各向异性程度（）不仅取决于去湿过程中的，还取决于溶剂类型和。这种新发现的去湿聚合物液滴的动态行为可以更详细地研究，并有可能应用于传感和微流控领域。
