Influence of surface roughness frequencies and roughness parameters on lubricant wettability transitions in micro-nano scale hierarchical surfaces

In this work, the influence of surface roughness frequencies and roughness parameters on mixed wetting and its transitions are discussed. Sessile drop experiments were conducted on hierarchical engineering surfaces having multiple roughness frequencies. Results show that the net roughness Ra or Rq (RMS roughness) solely cannot dictate the wettability behavior. Fourier Band Pass Filtering (FBPF) and Power Spectral Density (PSD) analysis revealed that all roughness frequencies in a hierarchical rough surface contribute to the net Ra and Rq; however, all need not contribute to wettability transitions. Rz and LSm were found to be the roughness parameters that could predict the range of spatial roughness frequencies that dictate wettability transitions to near Wenzel and Cassie-Baxter states.