Dynamic Measurements of sub-nL/min flows with an optofluidic flowmeter

With the increasing prevalence and miniaturization of microfluidic systems, there is a pressing need for reliable flow rate measurements at the nL/min level. State-of-the-art flowmeters can measure 100 nL/min flows with 1% uncertainty but face fundamental limits at lower flow rates due to evaporation and lack of detailed knowledge of system geometry. We have developed an optofluidic flowmeter which directs laser light into a microfluidic channel and monitors the photobleaching of a fluorophore in the fluid. The amount of photobleaching scales inversely with the flow rate. We demonstrate that this device can measure sub-nL/min flows in real-time with less than 5% uncertainty without any detailed knowledge of the channel geometry or photobleaching physics. The precision of the flowmeter is limited only by the requirement that the Peclet number be >> 1, so increasingly low flows can be measured by reducing the channel dimensions or increasing the diffusion coefficient of the fluorophores. We study the dynamic response of the flowmeter to characterize its time resolution under a variety of conditions. Finally, we use the flowmeter to measure flows induced by electrospray ionization from glass capillaries with different tip diameters, demonstrating that a distinct emission mechanism is responsible for the current generated from nanoscale capillaries.