Formation of wrinkled patterns in metal films deposited on elastic substrates: Tunability and wettability

We report the formation of wrinkles with tunable wavelength in Ag films and Mo films deposited on polydimethylsiloxane (PDMS) and PDMS/Si substrates by magnetron sputtering at 50 °C. The PDMS-supported Ag films have the wrinkles with μm-sized wavelength, while the sub-μm-sized wrinkles are engendered in the Ag films on PDMS/Si substrates due to the constraint from the substrates. By contrary, Mo films with a similar thickness range exhibit wrinkles with μm-sized wavelength on the PDMS/Si substrates, much larger than that of Ag films on the same substrates. This indicates that the wrinkle wavelength in metal films depends closely on the substrates and the characteristics of metal films. The possible reasons for the discrepancy in wavelength between experiments and theoretical predictions are discussed. The wettability of the wrinkled surfaces is predominantly controlled by the length scale of the wrinkles. In the Ag films with sub-μm-sized wrinkles, droplet contact angle is highly sensitive to the surface roughness, i.e., increasing quickly with the surface roughness. However, in the PDMS-supported Ag and PDMS/Si-supported Mo films both with μm-sized wrinkles, the contact angle raises slowly with the surface roughness. It is thus evident that a wettability spectrum derived from the wrinkling pattern can be artificially developed by tuning the wavelength in different length scales through controlling of substrates, metal films, and film thicknesses.