Materials enabling nanofluidic flow enhancement
- PDF / 1,250,861 Bytes
- 5 Pages / 580.752 x 783 pts Page_size
- 26 Downloads / 190 Views
on Mass transport through nanoscale pores has been studied for many years in disciplines as diverse as membrane science, soil permeability, and cell physiology. In these fields, emphasis has been placed on the macroscopic outcome, while molecular-level effects on fluid behavior have often been neglected. In the last 10 years, however, the focus has shifted to the effects on fluid behavior of intermolecular interactions between the fluid and the walls of the channel that it flows through. Interest in this field, called nanofluidics, has dramatically increased with the widespread availability of carbon nanotubes (CNTs) and, more recently, graphene, with potential applications to filtration and separation (e.g., water desalination). Initial insights into nanoscale steady-state flow were obtained by numerical simulations,1 followed by experiments in small membranes of aligned tubes,2,3 and measurements of flow through single nanotubes.4 These results have been extensively reviewed elsewhere.5,6 A key concept in nanofluidics is flow enhancement, defined as the ratio of the measured flow to an ideal no-slip Poiseuille flow. The latter assumes that the fluid molecules closest to the channel’s surface have zero velocity, in other words, they stick to the surface.5,6 Experimental and modeling results have reported flow enhancements ranging from 10 to 100,000 for water flow inside nanotubes made of carbon and other materials.6
While a full understanding of the physical origins of flow enhancement has yet to be achieved, some aspects are now generally accepted: • A 2-nm threshold exists below which the conventional continuum fluid mechanics model can no longer be applied.7 • The presence of a reduced viscosity or depletion layer near the tube wall is due to solid–liquid molecular interactions.8 • Frictional losses are mainly confined to the entrance region of the nanochannel.9 Other aspects are still unclear and represent active areas of research, including the effective dependence of flow enhancement on the nanochannel length; whether a maximum flow enhancement value exists based on the nanochannel’s geometric characteristics and surface chemistry and structure;10–13 and the physical state of liquid molecules under nanoscale confinement.14 For example, water in nanotubes with diameters ranging from 1.1 to 2.1 nm displays strong structural anisotropy. Notably, the diffusivity and viscosity in the axial direction are much larger than in the radial direction, leading to an ordered, helical structure inside a (10,10) CNT.7
Nanochannel material and flow enhancement Experiments and simulations have both shown that the surface structure and chemistry of a nanochannel have a significant
Alan J.H. McGaughey, Carnegie Mellon University, USA; [email protected] Davide Mattia, University of Bath, UK; [email protected] doi:10.1557/mrs.2017.60
• VOLUME • APRIL 2017 Materials Research Society MRS BULLETINCore 42 use, 2017 •atwww.mrs.org/bulletin Downloaded© from https:/www.cambridge.org/core. IP address: 80.82.77.83, on 13 Apr 2017 at 13:51:01, subject to the
Data Loading...
