Dynamic polygonal spreading of a droplet on a lyophilic pillar-arrayed surface

We experimentally investigated the dynamic polygonal spreading of droplets on lyophilic pillar-arrayed substrates. When deposited on lyophilic rough surfaces, droplets adopt dynamic evolutions of projected shapes from initial circles to final bilayer polygons. These dynamic processes are distinguished in two regimes on the varied substrates. The bilayer structure of a droplet, induced by micropillars on the surface, was explained by the interaction between the fringe (liquid in the space among the micropillars) and the bulk (upper liquid). The evolution of polygonal shapes, following the symmetry of the pillar-arrayed surface, was analysed by the competition effects of excess driving energy and resistance which were induced by micropillars with increasing solid surface area fraction. Though the anisotropic droplets spread in different regimes, they obey the same scaling law S ~ t2/3 (S being the wetted area and t being the spreading time), which is derived from the molecular kinetic theory. These results may expand our knowledge of the liquid dynamics on patterned surfaces and assist surface design in practical applications.

本研究通过实验探究了液滴在亲液柱阵列基底（lyophilic pillar-arrayed substrates）上的动态多边形铺展行为。当液滴沉积于亲液粗糙表面时，其投影形貌会经历从初始圆形到最终双层多边形的动态演化过程。该动态过程可根据基底差异分为两种模式。由表面微柱（micropillars）诱导形成的液滴双层结构，可通过边缘相（填充于微柱间隙内的液体）与体相（上层液体）之间的相互作用进行阐释。遵循柱阵列表面对称性的多边形形貌演化过程，可通过随固体表面积占比提升的微柱所诱导的过剩驱动能与阻力之间的竞争效应进行分析。尽管各向异性液滴在不同模式下的铺展行为存在差异，但它们均遵循统一的标度律S ~ t^(2/3)（其中S为润湿面积，t为铺展时间），该标度律源自分子动理论（molecular kinetic theory）。本研究结果可加深我们对图案化表面上液体动力学的认知，并可为实际应用中的表面设计提供参考。