Influencing factors of droplet aggregation on hierarchical wedge-shaped functional surfaces

Spontaneous droplet aggregation has great potential applications in liquid transportation, droplet-based microfluidics and water collection in deserts. Several novel hierarchical functional surfaces have been well prepared experimentally, on which small droplets would aggregate to form large droplets. The common feature of the hierarchical surface is wedge-shaped functional zone. However, how to guarantee the stability of water collecting function in real design and application is still lack of theoretical guidance. Based on molecular dynamics simulations, numerical experiments are systematically carried out in the present paper. Several important influencing factors of droplet spontaneous aggregation are studied. It is found that three typical size-dependent moving patterns of droplets may exist on such a hierarchical functional surface, i.e. lingering, pinning and directional transporting. The physical mechanism is further revealed for each pattern in terms of potential energy, which significantly depends on the wedge angle, direction of the wedge and symmetry of the bifurcation of each level. In order to achieve spontaneous droplet aggregation effectively, three conditions need to be met: a relatively small wedge angle at each level, wedges in the same direction at different levels, and asymmetric arrangement of wedges at the same level. A three-level functional surface is further numerically designed, which can realize water collection successfully. The results in this paper should be useful for the precise design of hierarchical functional surfaces of water collection in practical application.