, , James R. Hands, ,
The ultimate performance of flow-based measurements in microfluidic systems is limited by their accuracy at the nanoliter-per-minute scale. But improving such measurements (especially in contexts that require continuous monitoring) is challenging because of constraints associated withshrinking system geometries and limitations imposed by making precise measurements of smaller quantities in real time. A particularly interesting limit is the uncertainty associated around zero flow, which typically diverges for most measurement methods. To address these problems, we have developed an optofluidic measurement system that can deliver and record light in a precise interrogation region of a microfluidic channel. The system utilizes photobleaching of fluorophore dyes in the bulk flow and can identify zero flow with sub-nL/min absolute accuracy. The technique also provides an independent method for determining non-zero flow-rates based on a robust scaling relationship between the fluorescence emission and flow. Together, these two independent approaches enable precise flow control to within 5 % uncertainty down to the 1 nL/min scale, a regime that we can only measure to 5 % accuracy at 10 nL/min based on existing flow meters. Further, we demonstrate how our approach enables dynamic measurement of flows to regimes well below the uncertainty limits of an existing flow measurement technique, effectively extending the performance of the meter to nearly a factor of 10 lower flows while maintaining the relative uncertainties that would have otherwise expanded significantly in this regime.
Flow Metrology, Optofluidics, Fluorescence, Zero Flow