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 ~ t^2/3 (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.