Topographical length scales of hierarchical superhydrophobic surfaces

•Hydrophobic CF4 plasma fluorinated polybutadiene surfaces has been characterised using AFM.•Micro, Nano, and Micro+Nano topographies generated by altering plasma power and duration.•Dynamic scaling theory and FFT analysis used to characterize these surfaces quantitatively.•Roughnesses are different for different length scales of the surfaces considered.•Highest local roughness obtained from scaling analysis for shorter length scales of about 500 nm explains the superhydrophobicity for the Micro+Nano surface. The morphology of hydrophobic CF4 plasma fluorinated polybutadiene surfaces has been characterised using atomic force microscopy (AFM). Judicious choice of the plasma power and exposure duration leads to formation of three different surface morphologies (Micro, Nano, and Micro+Nano). Scaling theory analysis shows that for all three surface topographies, there is an initial increase in roughness with length scale followed by a levelling-off to a saturation level. At length scales around 500nm, it is found that the roughness is very similar for all three types of surfaces, and the saturation roughness value for the Micro+Nano morphology is found to be intermediate between those for the Micro and Nano surfaces. Fast Fourier Transform (FFT) analysis has shown that the Micro+Nano topography comprises a hierarchical superposition of Micro and Nano morphologies. Furthermore, the Micro+Nano surfaces display the highest local roughness (roughness exponent α=0.42 for length scales shorter than ∼500nm), which helps to explain their superhydrophobic behaviour (large water contact angle (>170°) and low hysteresis (