Nanoporous Capillary Gripper for Ultragentle Micro-Object Manipulation

Surfaces become "sticky" at the micro/nano length scale as the gravitational force is no longer effective. Ultragentle, high-contrast switching of interfacial adhesion is the key to reliable small-scale object manipulation. Here, a novel approach is presented for surface adhesion control utilizing a liquid-permeable nanoporous surface, which can switch from off-state adhesion (< 0.002 kPa) to on-state attraction (0.8 kPa) without preload. The surface of the gripper is composed of vertically aligned composite nanowires with an average diameter of 79 nm. When a liquid is injected into the nanoporous membrane, capillary adhesion occurs, allowing the object to be picked up. As the liquid evaporates, the object can be released by extremely sparse contact. The off-state adhesion of a millimeter-scale gripper is even lower than the gravitational force of thin polymer films (0.18 mN cm^-2), enabling the solid-contactless release of lightweight materials. We characterize and model the mechanism across length scales and provide pick-and-place demonstrations including LED chips, micro-architected materials, and thin-film electronics.