Roughness dependent wettability of sputtered copper thin films: The effect of the local surface slope

Here, we investigated the static and the dynamic wetting behaviors of copper (Cu) thin films deposited by DC magnetron sputtering. The deposited films have random rough surfaces for which the rms roughness amplitude σ, the lateral correlation length ξ, and the roughness exponent α were obtained from the analysis of height topography images acquired by atomic force microscopy. The time-dependent height-height correlation functions indicated anomalous kinetic roughening with roughness exponents α ≈ 0.9 and evolving roughness parameters σ and ξ with deposition time. The latter yields a nonstationary local surface slope σ / ξ that has a crucial impact on the surface wettability. Indeed, static and dynamic contact angles’ (CAs) measurements revealed two wetting regimes associated with different growth stages leading to a transition from a metastable Cassie-Baxter to a Wenzel-like state for the roughest films. Moreover, the increasing roughness with well distributed peaks and valleys leads to increasing CAs due to trapped air in surface cavities, while after some point the larger surface features lead to a decrement of the CAs that vary only slightly with further roughening. Although the apparent wetting transition with increasing surface roughness is not favored by the local Laplace pressure estimation, the energy of the system decreases with surface roughening, or equivalently increasing local surface slope, favoring energetically a Wenzel state. Under these conditions, the water droplet can spontaneously fill the surface cavities once the impregnation is initiated by the hydrophilic nature of the surface, in agreement with our experiments for significantly large local surface slopes ρ (>0.1) and large roughness exponents α ∼ 1.