Influence of hydrophobicity and roughness on the wetting and flow resistance of water droplets on solid surface: A many-body dissipative particle dynamics study

Hydrophobicity and roughness are crucial properties for determining rock wetting and flow resistance. The wetting and flow behavior of water droplets on smooth and rough solid surfaces are investigated by performing a many-body dissipative particle dynamics simulation. The results show that, in terms of the roughness, a solid surface can be divided into three regions. In Region I, as inherent hydrophobicity increases, roughness can enhance the apparent hydrophilicity of a hydrophilic surface. In Region II, the increased hydrophobicity improves the apparent hydrophobicity of a more hydrophobic surface. In Region III, when the hydrophobicity is beyond the superhydrophobic boundary, the enhancement effect of the rough surface on the apparent hydrophobicity is weakened or even disappears. With an increase in the roughness, the boundaries among these three regions and their ranges show nonmonotonic changes, making the contact mode (Wenzel state/Cassie and Baxter state), contact angle hysteresis and drag reduction effect change nonmonotonically.