Mechanical strain induced wetting transitions between anisotropic and isotropic on polydimethylsiloxane (PDMS) films patterned by optical discs

Abstract

We introduce a simple yet robust method for fabrication of wrinkled polydimethylsiloxane (PDMS) surface exhibiting tunable wettability under applied mechanical strain. Submicrometer-scale periodic grooves were prepared by casting the optical discs such as compact disc (CD) and digital versatile disc (DVD) patterns on PDMS. On wrinkled PDMS, the liquid droplet exhibits an anisotropic wetting resulting in a higher contact angle (CA) in the direction perpendicular to the wrinkle axis. The peaks and valleys in the surface topology of PDMS lead to the wetting anisotropy of 14° for CD templated PDMS and 7° for DVD templated PDMS. The wetting anisotropy for a fixed Young's CA (110°) was found to depend on the wrinkles periodicity (λ) and amplitude (2A). A mechanical strain in the direction perpendicular to the wrinkle axis was applied on CD templated PDMS to tune λ value from 1.49μm to 1.88μm. The wetting anisotropy decrease with an increase in λ value and the water droplet becomes isotropic for 30% strain value. Comparison with existing theoretical Johnson and Dettre (J-D) model attributes the anisotropic shape of the droplet and contact angle variations to surface roughness. The multiple-cycle switching between anisotropic and isotropic wetting was achieved.