Tunable open-channel microfluidics on soft poly(dimethylsiloxane) (PDMS) substrates with sinusoidal grooves.

On soft poly(dimethylsiloxane) (PDMS) substrates with 1D sinusoidal wrinkle patterns, we study the anisotropic wetting behavior and fluidic transport as a function of surface energy and groove geometry. On grooved substrates with a contact angle greater than 90 degrees , liquids form dropletlike morphology, and its contact angle in the direction perpendicular to the grooves is larger than that parallel to the grooves. This wetting anisotropy, for a fixed Young's contact angle, is found to increase when the grooves become deeper. On substrates with a contact angle smaller than 90 degrees and deep grooves (aspect ratio >/=0.3), liquids form filament-like morphology. When the groove depth is further increased by compressing the PDMS film beyond a threshold value, which depends on the surface wettability, fluid starts imbibing the grooves spontaneously. The dynamics of the liquid imbibition of grooves is studied, and a square-law dependence between the length of the liquid filament and time is found, which obeys Washburn's law. Using a simple model based on force balance, we find that the capillary force is mainly responsible for groove filling in sinusoidal grooves.