Controlled pumping of fluids through microfluidic networks is a critical step in many lab-on-a-chip applications. Traditional methods of fluid delivery, such as syringe pumps or electrokinetic phenomena, generally require bulky equipment that at a minimum is several orders of magnitude larger in size than the microfluidic network itself. Due to the inherently small size of the microchannel, capillary forces are a convenient method to create pressure-driven flow. Historically, passive microfluidic pumping via surface tension forces have required either large sample volumes (external capillaries) or large arrays of microchannels (several square centimeters area) for precise flow control.

Here we present a simple method of fluid delivery via control of capillary size differences present between the inlet and outlet reservoirs of a microchannel. The driving capillary fills the corner region present in the outlet reservoir of a microchannel, retaining the benefits of a small internal capillary while taking up an area of only several square millimeters. Precise fluid delivery is accomplished through control of multiple parameters describing the system, such as the geometries of the reservoirs and microchannel, volume of the inlet drop, and the surface characteristics of the channel itself. We show that controlled, constant flow rates can be delivered through a microchannel for periods of up to several hours, with required sample volumes of less than a microliter.