Fluid transport under nanometre-scale confinement is a topic of strong and current interest, from both the fundamental and applied points of view. There is now ample experimental evidence that the continuum model, on which macroscopic fluid mechanics is based, breaks down below a certain system size threshold, leading to unexpected fluid transport phenomena. Such phenomena may have great potential to influence a wide range of practical applications—from desalination (and water treatment in general) to gas sensing and separation.
Nanostructured carbon membranes comprising carbon nanotubes or graphene are of particular interest as they can link theoretical and applications research activities. The fact that these materials can be modelled and manufactured with a high degree of control, and that modern simulation methods can produce useful insight at the length scales captured in experimental observations, means that computational and empirical techniques can positively influence each other.
This theme issue reports the novel findings presented at a Royal Society Theo Murphy International Scientific Meeting held in April 2015, where leading experts on nanostructured carbon membranes and nanoscale transport phenomena discussed the current state of the art and identified emerging opportunities. This preface attempts to summarize the main conclusions of the meeting, with the aim of serving as a guide to future research directions for the community. We thank all the meeting participants for their contributions to this summary.
First, there is a need to reconcile experimental observations with both simulation results and theoretical developments—experiment being the ultimate test and calibration benchmark. This will enable a deeper understanding of the physics of nanoscale transport. Experiments to produce detailed measurements, alongside simulations with clear predictions, need to be devised so that they can serve as directly comparable counterparts. This need is acute for pore sizes between 1 and 2 nm—the region where continuum fluid theory breaks down. Coincidentally, this size range is also of particular interest from an applications point of view, which provides further motivation to focus the community’s attention on this region.
Second, different fabrication methods can produce nanotubes and graphene with different structures, surface chemistry, defects and sizes, so all these have to be taken into account when comparing between experiments, or comparing experiment with simulation. Of particular importance is the accurate characterization of the effective pore size. Comparisons between experimental and theoretical or computational results based on the average pore size are often misleading because the ‘tail’ of larger pores in the diameter distribution has a strong influence on the experimentally measured permeability and selectivity. Experiments on single, well-characterized nanopores would circumvent the difficulties of pore diversity.
Third, just as experiments need to be well characterized, simulations need to be applied consistently and be more carefully validated. It is not clear, for example, why interatomic potentials that have been calibrated on the water contact angle, or through interactions between small organic molecules, would be relevant for the flow in the interior of a nanotube. In addition, simple simulation parameters such as temperature control algorithms and system design can affect predictions significantly.
Finally, participants at the meeting agreed on the need to measure routinely not only the flow of pure fluids but also the separation performance of these porous materials (for liquid or gas applications). In the same vein, permeability and permeance appear to be more appropriate concepts than ‘flow enhancement’ to discuss and compare results from both experiments and simulations. Such measurements will help to define the range of useful applicability of these materials.
The 2-day meeting showed that there are still significant research opportunities in understanding transport phenomena at the nanoscale, and that these may lead to the practical use of nanostructured carbon membranes with superior properties to current commercial membranes. Not only is our theoretical understanding far from complete, but there are bound to be a great number of material designs, fabrication methods and applications of nanostructured carbon membranes still to be discovered. This theme issue is an essential guide for anyone interested in these topics.
One contribution of 11 to a Theo Murphy meeting issue ‘Nanostructured carbon membranes for breakthrough filtration applications: advancing the science, engineering and design’.
- Accepted November 16, 2015.
- © 2015 The Author(s)