Dynamic Measurement of Nanoflows: Realization of an Optofluidic Flow Meter to the Nanoliter-per-Minute Scale

Abstract

The ultimate performance of flow-based measurements in microfluidic systems is currently limited by their accuracy at the nanoliter-per-minute scale. Improving such measurements (especially in contexts that require continuous monitoring) is challenging because of constraints associated with shrinking system geometries and limitations imposed by making precise measurements of smaller quantities in real time. A particularly interesting limit is the relative uncertainty as flow approaches zero, which 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 to better than 1 nL/min absolute accuracy. The technique also provides an independent method for determining nonzero flow rates based on a robust scaling relationship between the fluorescence emission and flow. Together, these two independent approaches enable precise measurement of flow to within 5% accuracy down to 10 nL/min and validation of flow control to within 5% uncertainty down to 2 nL/min. We also demonstrate that our technique can be used to extend a calibrated flow meter well below its specified range (e.g., 500 nL/min) and to make dynamic measurements of similar relative uncertainties to the calibrated meter, which would have otherwise expanded significantly in this regime.

Dynamic Measurement of Nanoflows: Analysis and Theory of an Optofluidic Flowmeter

“…based on the height of the source water column) to provide reference flow rates, which are accurate to 5% down to 10 nL/min and 10% at 5 nL/min. [28] Next, we used the master curve and Eq. (25) to measure flow rates using our optofluidic flowmeter and then computed the relative errors…”

Impact of inertia and channel angles on flow distribution in microfluidic junctions

“…They would require the use of direct measurements of the flow with the use of: flow-metres, pressure sensors (Kim et al 2006); or indirect methods, e.g. : utilizing photobleaching (Cooksey et al 2019), micro-PIV measurements (Santiago et al 1998;Blonski et al 2007) or weighting the output flows (Zeitoun et al 2013).…”

Optofluidic flow meter for sub-nanoliter per minute flow measurements

“…We empirically determined that a nonlinear correction of 1.26 was necessary to correct excitation power such that fluorescence emissions matched for all conditions having equivalent dosages. 15 …”

“…We previously developed a model wherein these assumptions were used to define a scaling relationship that quantifies the flow rate in terms of the dosage of light received. 15 – 17 The key idea of this approach relies on fluorescence efficiency , i.e., the fluorescence power measured per unit laser power, being a one-to-one function of the dosage, i.e., . As depends directly on the laser power and is inversely proportional to the flow rate, there is a universal reference curve such that, for any flow rate, measurements of can be mapped onto this reference by appropriately scaling the laser power.…”

“…The method utilizes photophysics of photobleaching to calculate the flow rate from a relationship between fluorescence efficiency and dosage of light sustained by fluorophores as they pass through an interrogation region. 15 By itself, this efficiency-dosage relationship only allows flow measurement on a relative scale and must be calibrated using a reliable absolute method. Previously, we transferred calibration to our system from a thermal flow meter calibrated to 5% relative uncertainty at .…”