Fig. 1.

Directionally pumping droplets by capillary force on slippery surface with immobilized meniscus. (A) Schematic shows surface-piercing vegetation capturing floating seeds by capillary force. (B) Inspired by this, capillary pumping of microdroplets on slippery surface with immobilized meniscus is developed, whereby a meniscus-immobilizing hydrogel dot captures nearby droplets by capillary force. (C) A hydrogel dot was printed on PDMS surface. (D) After infusing the surface with silicone oil, the hydrogel dot raises and immobilizes oil meniscus. (E and F) Mediated by the meniscus, a water microdroplet can be pumped to the hydrogel dot against gravity, whether the slippery surface is tilted by 45° (E) or 90° (F). (G and H) In contrast, microdroplets cannot be driven without the oil meniscus. (I) Optical images showing a microdroplet (on the left) being driven toward the hydrogel dot (on the right), bridged by the oil meniscus. (J) The driving force is due to the net surface tension force projected on the horizontal direction (γcosβ−γcosα) exerted on the opposite sides of the droplet. (K) cos β−cos α and the velocity of the droplet follow a nearly linear relationship.