Directionally Cracked Mesoporous Colloidal Films by Manipulating Notch Angles and Their Anisotropic Wicking Behavior

Wicking in porous media, such as the spreading of ink on paper or the absorption of moisture by fabric, occurs when water interacts with hydrophilic porous materials through capillary action and evaporation. The directional nature of the wicking phenomenon can be leveraged for various advanced applications, including enhanced heat transfer, colorimetric devices, energy harvesting, and microfluidics. Herein, crack generation is used to induce the anisotropic wicking of water on isolated mesoporous strips. The strips are fabricated by transforming isotropic cracks into anisotropic ones in micropyramid arrays using the Poisson effect in elastomeric blocks. Stretching an elastomeric block increases the period of a pyramid array along one direction while decreasing it in the perpendicular direction because of elastomer shrinkage. This amplifies the difference in the notch angles of pyramidal edges between parallel and perpendicular directions relative to the stretching axis. Consequently, the disparity in notch angles leads to preferential crack generation owing to elevated stress localization on the sharpened notches. Directional wicking is demonstrated using anisotropic strips of mesoporous TiO₂ colloidal films and highly anisotropic wicking of ink is illustrated by coating hydrophobic films on mesoporous strips. The anisotropic wicking observed in cracked mesoporous strips can serve as 1D microfluidic channels.