Bubble time-of-flight: A simple method for measuring microliter per minute flows without calibration

Monitoring liquid flow at microliter per minute (μL/min) rates is important for many lab-on-a-chip applications. Several technologies have been investigated to achieve this resolution, most of which require expensive detection systems and extensive calibration. Those technologies also rely on phenomena impacted by diffusion (e.g., heat pulses) that quickly lose accuracy as flow rates decrease. An alternative method, bubble time-of-flight, surmounts the limitations of diffusion by tracking the flow with bubbles. This method also has the advantage of not requiring calibration or complex sensing circuitry to accurately measure flow rates. We demonstrate this method with a device that uses thermoresisitive sensors to detect the passage of bubbles between two points. This device is able to measure flows from 100 to 1 μL/min. The accuracy of the system increases as flow rates decrease such that the low end uncertainty is 1% (±0.01 μL/min). We also demonstrate the feasibility of measurements in the sub 100 nL/min range by using a single sensor.