NANOFLUIDICS IN VAN DER WAALS CHANNELS: FROM ION TRANSPORT TO CAPILLARY PHENOMENA

Abstract

Low dimensional nano-objects such as nanopores, nanotubes and nanochannels allow the investigation of fluid confinement below the nanometre scale, revealing new mechanisms relevant for biology and energy science. In this work, we first employed atomically flat nanochannels, 0.7-30 nm nanometre high, embedded in van der Waals heterostructures based on graphene and other two-dimensional materials. We fabricated such channels by stacking two thick (~100 nm) graphite crystals separated by a patterned graphene flake which acts as a spacer, defining a slit-like cavity. The channels were filled with electrolytes, and ion currents driven by voltage, concentration and pressure gradients were measured. Despite a very low surface charge, we found evidence that the interaction of ions with the graphitic surface leads to apparent enhanced ion mobilities. Our pressure-driven ion transport experiments suggest near-frictionless water flow. Next, we investigated capillary phenomena in nanochannels. We resolved the drying flow and the capillarity-induced deformations down to sub-10 nanometre confinement using a high-speed camera. By tuning the channel shape, we studied the balance between their mechanical stiffness and the forces at play: adhesion and capillarity. Interestingly, flexible channels can reversibly cave in under capillary forces and relax, leading to 2D devices with switchable, engineered strain: “capillary zippers.”