Liquid Flow Rates Measurements Through Graphene Nanopores by Micro-PIV
Abstract
Separation membranes are widely used in a variety of industrial and environmental applications, including water purification and desalination. However, these membranes can see their performance limited because of the inherent trade-off between permeance (rate of flow of a fluid through a membrane) and selectivity (ability of the membrane to separate different components of a fluid mixture) where high permeance typically means low selectivity and vice versa. Nanoporous atomically thin membranes (NATM), such as graphene membranes, have the potential to overcome the permeance-selectivity trade-off due to their atomic thickness and the capacity to be fabricated with pores of very small size and uniform geometry. While permeance is a key membrane performance metric, liquid flow rate measurements through single graphene nanopores have never been reported due to lack of a suitable measurement technique. In this study, we develop a method to measure flow rates through micro and nanopores in membranes by analyzing velocity fields obtained through micro particle image velocimetry (micro-PIV). We validate our method and determine the Reynolds number limit for which it is valid by conducting experiments with micropores of defined sizes (50, 6, 5, 3 μm) in transmission electron microscope (TEM) grids and comparing the results with values obtained by calibrated flow sensors. The technique is then applied to measure flow rates through pores of 500 and 200 nm in graphene, obtaining flow rates as low as 2 nl/min, which is lower than the minimum detectable flow rate of 10 nl/min for commercial sensors. We also report permeation coefficients ranging from 7.0x10-19 to 1.5x10-18 m3/s-Pa for the 500 nm pore and 8.5x10-20 to 2.0x10-19 m3/s-Pa for the 200 nm pore. Furthermore, we demonstrate that our method is valid regardless of the geometry of the pore, showing its importance for applications involving irregularly shaped pores. This research makes a significant contribution to the NATM field by enabling non-invasive flow quantification through graphene nanopores, which could not be measured by conventional sensors.