Effects of a chemically heterogeneous island on the dynamic contact angles of droplets

Droplets moving on surfaces are affected by the heterogeneity of the substrates. In the present study, the effects of partial heterogeneity of the substrates on the dynamic contact angles are numerically investigated by using a two-dimensional many-body dissipative particle dynamics (MDPD)model. The results show that for a droplet moving over a chemically heterogeneous island, the effects of the heterogeneity on the advancing and receding contact angles are largely different, although both the advancing and receding contact angles increase firstly when the front contact point reaches the heterogeneous island. In the meantime, the front contact point gets pinned and de-pinned alternately with the shape of the droplet deforming periodically while it is not so apparent for the rear contact point. Both the advancing and receding contact angles experience drastic fluctuation when the front contact point passes the island and it is different for the tail of the droplet passing the island. Also, the kinetic energy of the droplet decreases rapidly. In addition, the energy dissipation increases with the increasing number of stripes in the island region. The results illustrate that the advancing contact angle has higher sensitivity to the heterogeneity of the substrate than the receding one. Our findings can bring an insight on the mechanism of micro-droplets moving on heterogeneous substrates.