Surfactant droplets on hydrophobic microstructures

Surfactants—amphiphilic molecules—can easily adsorb at interfaces. Their presence can destroy the useful, gas-trapping (Cassie–Baxter, CB) wetting state of a drop sitting on a superhydrophobic surface. Here, we examine how surfactants alter the wetting state and contact angle of aqueous drops on hydrophobic microstructures of different surface roughness ($r$) and solid fraction ($\phi$). Experimentally, at low surfactant concentrations ($C_s$), some drops attain a homogeneous wetting state (Wenzel, W), while other drops are in the CB state. In contrast, all of our high $C_s$ drops attain the Wenzel state. To explain this observed transition, we develop a thermodynamic free energy analysis and find that, theoretically, for our surfaces the W state is always thermodynamically preferred, while the CB state is metastable at low C, consistent with experimental results. We further provide a beneficial blueprint for stable CB, gas-trapping states for applications exploiting superhydrophobicity.