Ratio dependence of contact angle for droplet wetting on chemically heterogeneous substrates

In the present study, a numerical simulation of droplets wetting on surfaces with chemically heterogeneous stripes was performed by using many-body dissipative particle dynamics (MDPD). The effects of the ratio of periodicity width w to the contact length L of a 2D droplet (i.e., w/L) on the contact angle were investigated to analyse the validity of Cassie-Baxter equation. With the simulated process about the droplets wetting on chemically heterogeneous surfaces, the results show that when the ratio is smaller than 0.118 (2/17), the numerical values of the static contact angles are in good agreement with the prediction of the Cassie-Baxter equation for droplets wetting on chemically heterogeneous substrates formed by alternative stripes in mesoscopic scale. While for the ratio larger than 0.118, the Cassie-Baxter equation will not be appropriate to accurately predict the apparent contact angles. This ratio may be considered as the critical value to apply Cassie-Baxter equation on these types of chemically heterogeneous substrates. Furthermore, a comparison between rear dynamic contact angles for different ratios was found in accordant with expected.