Paper-based dual-mode liquid manipulation system: Oil/water separation and time-lapse droplet switch

Multi-mode liquid handling system integrated different types of liquid behaviors on super-wetting surfaces has attracted great interest from both academia and industries due to their promising applications in the field of surface science. Herein, a paper-based dual-mode droplet manipulation system that combines oil/water sepa-ration and time-delay droplet switch was designed by spray-coating alpha-cellulose 10-undecenoyl ester (CUE) particles on filter paper with further "reverse sizing" by polydimethylsiloxane (PDMS) based on capillary action and "like dissolve like" principle. Such novel binding technique leaded to a remarkably robust super-hydrophobicity of the as-fabricated PDMS@CUE-coated paper (PDMS@CCP), including outstanding mechanical resistance and environmental durability. In particular, the front side of PDMS@CCP displayed excellent sepa-ration efficiency (higher than 99.89%) for light oil/water mixture, heavy oil/water mixture, and even surfactant-stabilized water-in-oil emulsion. Interestingly, based on the hydrophobicity of the back side of PDMS@CCP and the intrinsic hygroscopicity of paper itself, a time-delay droplet switch was simply achieved in between by the Laplace pressure derived from the curvature difference of the droplet on two paper surfaces with surface energy gradient. By adjusting the width of the filter paper and the volume of the droplet, the switch time could be rationally modulated between 0.8 and 202.1 s. The designed functional paper would enrich the application field of the liquid manipulation system.