Low flow liquid flowmetry. A tutorial review

Liquid flow measurements in the sub-mL/min scale are critical in a wide range of analytical and biomedical applications. Critical demands are being made of flow sensors to measure minute flow rates. In the past three decades, innovative implementations of several flow sensing principles have been made, focused on improving performance metrics, such as limit of detection, measurement span, flow direction sensing, response speed, etc. Novel structural designs have significantly boosted performance and reduced effects of environmental parameters. New manufacturing methods have increased the development of reliable, inexpensive flow sensors that increasingly consume less power. New materials are being exploited for specialized custom applications. Further development of low-flow liquid flow meters would benefit from a review of the different flow measurement principles, rational designs based thereon, and a comparison of their strengths and weaknesses. Herein we comprehensively review liquid flow sensors operating in the sub-mL/min domain, covering nine different principles for quantitative flow sensing. We scrutinize the critical structures/designs that enhance quantitative measurement of minute liquid flow. Based on publications spanning three decades, evolving measurement challenges and the state-of-the-art, we discuss present research trends. Finally, we provide an overall comparison among the flow sensors based on different principles with a summary of performance metrics to help the choice of a sensor for a specific task. Key challenges and prospects for micro/nano/picoscale flowmetry are discussed.