This issue of MRS Bulletin focuses on materials that enable nanofluidic systems with unusually high mass fluxes, termed “enhancement factor” or “slip flow.” There is now ample evidence of such flow enhancement in nanochannels, with sizes ranging from subnanometer to a few nanometers. Most of the studies to date, both experimental and modeling, have focused on carbon nanotubes and, more recently, on graphene. Different fabrication methods result in different structures, surface chemistries, and defects, with a significant effect on flow enhancement. As new one-dimensional and two-dimensional nanomaterials are synthesized, a deeper understanding of the nanoscale transport physics is needed, particularly in the relationship between material properties and flow behavior. Herein, authors at the forefront of experimental, modeling, and theoretical developments in nanofluidic flow describe the state of the art in materials development and characterization.

Carbon materials exist in a large number of allotropic forms and exhibit a wide range of physical and chemical properties. From the perspective of fluidics, particularly within the confines of the nanoscale afforded by one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene structures, many unique properties have been discovered. However, other questions, such as the link between electronic states and hydrodynamics and accurate model predictions of transport, remain unanswered. Theoretical studies, experiments in large-scale ensembles of CNTs and stacked graphene sheets, and precise measurements at the single-pore and single-molecule level have helped in our understanding. These activities have led to explosive growth in the field, now known as carbon nanofluidics. The ability to produce membranes and devices from fluid phases of graphene oxide, which retain these special properties in molecular-scale flow channels, promises realization of applications in the near term.

Transport properties of fluids in nanopores are of both fundamental as well as practical interest. Water flow in carbon nanotubes (CNTs) has received significant attention since the early 2000s for technological applications of CNTs. In this article, we provide a brief overview of modeling the slip and flow enhancement of water in CNTs. A number of experimental and computational studies have found water to flow very fast in CNTs, but the measured flow rates, which are high compared to classical hydrodynamics predictions, are scattered over 2–5 orders of magnitude. Slip lengths of 1 to 500,000 nm, resulting in almost zero to 500,000 flow enhancement, are reported for water in CNTs with diameters of 0.8 to 10 nm. We highlight some challenges in modeling fluid flow in nanopores and outline a few research directions that may resolve the order of slip and flow enhancement of water in CNTs in computational studies.

This article discusses the modeling of liquid flow inside nanotube membranes. Applying known simplifications to the classical fluid model leads to the so-called Hagen–Poiseuille equation, which predicts no flow for diameters up to 1 nm, and very modest flows in nanochannels up to 100 nm. The main feature of classical fluid dynamics that negates the possibility of high flow is the assumption that fluid molecules closest to the channel wall stick to it, the no-slip boundary condition. In the past 10 years, a wealth of experimental evidence has, on the contrary, demonstrated significant water flow in nanotubes with diameters equal to or smaller than 1 nm, opening the possibility of nanotube membranes capable of high flows and fine separation. These high flows have also been observed in molecular dynamics simulations, particularly for water flowing through carbon nanotubes, showing the presence of strong water slip near the walls of the nanotubes. The term “flow enhancement” has been introduced to refer to the ratio of predicted (or measured) flows and the no-slip Hagen–Poiseuille equation. Both experimental and modeling results point to a strong effect on flow enhancement of the interaction between the fluid and the tube’s wall, particularly the wall surface chemistry and structure.

Nanotubes (NTs) with diameters less than 2 nm have been proposed for next-generation reverse osmosis membranes. At this molecular scale, the NTs are narrow enough to block salt ions and other contaminants, but still wide enough to allow water to flow along the NTs at seemingly unprecedented rates. Simulations for design of NT membranes can be challenging. On the one hand, the standard equations for water flow through pipes are not applicable at sub-2-nm scales due to the dominance of non-continuum phenomena; on the other hand, full molecular simulations are computationally intractable for flows up to laboratory or prototype scales. This article describes recent multiscale approaches to simulating flows through aligned NT membranes of various materials. These multiscale techniques offer a unique and economical solution that can shed light on sometimes conflicting experimental results and point the way to future engineering design of nanostructured membranes.

Technologies for detecting and analyzing a single molecule help us understand and engineer numerous phenomena observed in nature. Carbon nanotubes (CNTs) are highly efficient molecular conduits due to their atomically smooth surface. Because of their small diameters, comparable to the size of a single molecule, even a single blocking molecule can obstruct CNT fluidic channels. Analyzing these pore-blocking events in CNTs therefore enables single-molecule studies. The high-aspect ratios of CNT channels, which extend the time scale of transport, allow for studying molecular transport that is too fast to record in other systems. Both theoretical studies and ensemble experimental measurements have verified the enhanced flow of various ions and molecular species in CNTs. Experimental measurements of a single-CNT fluidic channel, however, have only recently begun, demonstrating the detection of individual DNA, polymer, and alkali-metal ions. This article reviews recent advances in single-nanotube fluidic channels with a focus on experimental measurements.

Carbon nanostructures, especially carbon nanotubes and graphene nanopores, have been suggested for use in a wide range of purification and separation applications, from the desalination of seawater to the separation of liquids and gases. However, achieving the required high degree of selectivity among the molecules passing through the pores while maintaining rapid transport is a difficult challenge. Here, we examine the physical mechanisms by which nanopores distinguish between small ions and reject salts while passing water, as examples of how selectivity and purification can be achieved. The simple principles described can be utilized to design novel nanoporous materials for the separation of a wide range of gases, liquids, and solutes.

Metal additive manufacturing (AM) processes, such as selective laser melting (SLM), enable powdered metals to be formed into arbitrary three-dimensional shapes. For aluminum alloys, which are desirable in many high-value applications for their low density and good mechanical performance, SLM is regarded as challenging due to the difficulties in laser melting aluminum powders. However, a number of recent studies have demonstrated successful aluminum processing, and have gone on to explore its potential for use in advanced AM componentry. In addition to enabling the fabrication of highly complex structures, SLM produces parts with characteristically fine microstructures that yield distinct mechanical properties. Research is rapidly progressing in this field, with promising results opening up a range of possible applications across scientific and industrial sectors. This article reports on recent developments in this area of research and highlights key topics that require further attention.