(464b) Microfluidic Passive Pumping Using Coupled Capillary/Evaporation Effects

One of the most important unit operations within microfluidic devices lies in the handling and transport of fluids. The ability to provide temporally stable flows with high precision and repeatability can improve the efficiency and reliability of bioanalytical lab-on-a-chip (LOC) devices. Traditional methods of fluid delivery, such as syringe pumps or electrokinetic phenomena, generally require bulky equipment that can be several orders of magnitude larger in size than the microfluidic network itself. Due to the inherently small size of a microchannel, capillary forces are a convenient method to passively create pressure-driven flow. Although there have been multiple studies concerning LOC devices utilizing capillary forces, none have the ability to create steady-state flows existing for periods longer than O(10) min.

In the present work, a microfluidic passive pumping mechanism is demonstrated that combines the ability to create long term steady state flows from a mm2 footprint. After sample introduction to an inlet reservoir, capillary forces drive fluid through a microfluidic network (µFN) spontaneously such that a small meniscus is created along the corner regions of an outlet reservoir. The small radius of curvature of the outlet reservoir meniscus serves to drive fluid through the system at high differential pressures, where the volumetric flow rate into the reservoir is balanced by the rate of evaporation from the meniscus. The system quickly reaches steady state and is able to provide precise flows through a µFN for periods longer than an hour. Flow rates spanning multiple orders of magnitude are accomplished via control over the dimensions of both the microchannels and outlet reservoirs.

The dynamics of fluid flow in these systems are predicted through use of (1) a geometric model to describe the shape of the outlet reservoir meniscus and (2) a computational fluid dynamics model to calculate the evaporation rate from the meniscus and (3) the Young-Laplace equation to calculate the pressure differential between the inlet and outlet reservoirs. Using this model, we show that fluid flow through µFNs can be accurately predicted to within 15% of laboratory values for systems with a variety of reservoir and microchannel dimensions.