Evaporation based micro pump integrated into a scanning force microscope probe

A micro pump was integrated into a scanning force microscope probe for circulating liquid through its hollow cantilever and tip. The interior cross section of the cantilever was 2.25 μm × 3.75 μm. All fluidic parts were made of SiO₂, while the tip apex was made of Si₃N₄. The key fabrication techniques were silicon wafer bonding and wet-oxidation. The pumping mechanism was relying on the enhanced evaporation at an enlarged water/air interface at the exit of the microchannel. Capillary forces continuously wetted this interfacial area, thus drawing the liquid through the system. At room temperature, a pump rate of 11 pl s₋₁ was experimentally evaluated. The observed temperature dependence of the pump rate could be qualitatively understood by a plain model calculation.