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Sensing a picoliter for the direct measurement of liquid flow rate in nanofluidicis

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Physicists developed recently a simple sensor to measure minute liquid flow rates. This sensor, based on the accumulation of liquid within a deformable membrane, has been developed for the study of flows in nanofluidics. In particular, this sensor is devoted to the characterisation of coupled transport phenomena within a single nanopore. These phenomena, that remain partially understood, are of interest for water treatment and renewable energy conversion.

Flows and transport phenomena in confinement present specific properties dominated by surface effects. The maturation, of what is now called nanofludics, benefited in particular from the development of instruments for the characterisation of physical quantities related to transport phenomena at the vicinity of interfaces or within nanometric confinements. Flows are essential in this framework, nevertheless the measurement of flow rates is still notoriously difficult at the scale of an individual nanopore. While the electrical current induced by the transport of ions through a single nanopore can be routinely measured with commercial apparatus it turns out that the typical flow rate of interest, of the order of 10 pL/min, is three decades smaller than what can be measured with the best commercial sensor. For the measurement of such a small flow rate, indirect strategies exist that are however applicable only to specific configurations. In order to offer a generic approach, researchers came back to the direct historical method to measure a flow rate based on the accumulation of a liquid quantity during a certain time laps. In order to carry out the measurement on a time laps of a few minutes only, the difficulty is to detect a minute amount of liquid. For this purpose, the liquid is accumulated by means of the deflection of a silicon membrane a few mm wide and 10 microns thick. Taking benefit of the know-how developed for the production of pressure gauge, the membrane deflection is direcly measured with strain gauges deposited on the membrane. The sensitivity of such a measurement is such that a displacement of 1 nm at the center of the membrane can be measured this correspond to a volume variation of 1 pL. It is thus possible to measure flow rates of the order of 1 pL/min with an integration time of a few min (see Fig. 1). The dimensions of the membrane are large enough to offer an easy connection with bare hand with any macroscopic system comprising the nanofluidc circuit to be studied. The flow rate sensor has been qualified using calibrated microcapillaries. The sensor usage as been illustrated as well with the characterisation of a pressure driven flow through a single nanopore of 200 nm radius and 1 micron length. This kind of sensor can be used with any type of liquid. The presence of dye or tracers is not needed. The measurement range span over three decades from 1 pL/min to 1 nL/min. A patent has been applied for this sensor.

This work has been conducted in LIPhy and in collaboration with Institut Néel and Leti-CEA.

Contact :
Cyril Picard Université Grenoble Alpes

View online : A Direct Sensor to Measure Minute Liquid Flow Rates, P. Sharma, J.-F. Motte, F. Fournel, B. Cross, E. Charlaix, and C. Picard, Nano Letters, 2018