From weak to strong non-equilibrium transport of fluids at the nanoscales – NEPTUNE

Harnessing the many possibilities of nanometer-scale transport of water and ions is an important goal of modern technology in fields as diverse as biochemical analysis, water purification and energy harvesting. Controlling liquid flow at the scale of a nanometer provides unique opportunities for technological applications, because transport properties at the molecular scale differ drastically from their bulk counterparts. At the fundamental level, however, this remains a virgin territory and a detailed understanding of nanometer-scale liquid transport remains largely elusive.

Over the past few years, there have been a number of key developments in this field. In particular, our recent experimental advances allow fabricating single nanotube nanofluidic systems, enabling a systematic exploration of water and ion transport properties at the scale of single carbon and boron-nitride nanotubes. The experiments reveal dramatically different flow properties due to nanotube confinement, such as a greatly enhanced energy conversion efficiency by diffusio-osmotic flow ((Siria et al., Nature 2013) or radius dependent superlubricity (Secchi et al., Nature 2016). In parallel, recent theoretical developments has allowed analytical solutions of the electrokinetic equations, accounting for the molecular specifics of the interface. In particular, atomistic simulations of planar solid-water interfaces, combined with analytically derived scaling laws for the transport, reveal the decisive roles of the dielectric properties, the interfacial viscosity and the adsorption of ions. For comprehensive conclusions, however, complete data sets of electrokinetics, ionic conduction and diffusio-osmosis on the same system are necessary. Therefore, a seamless interaction between theory and experiment is essential to achieve a thorough understanding of nanometer-scale transport.

Here, we propose to combine experiments on electrolyte transport through nanotubes with theoretical analysis, with atomistic and ab initio simulations. First, using state of the art nanofluidic setups, we will perform experiments of ion and water transport in single nanotubes down to a radius of ~ 1 nm. Second, based on the successful framework derived for planar surfaces, we will derive analytical scaling laws for the electrokinetic and diffusio-osmotic transport in cylindrical geometry. Third, we will perform atomistic and ab initio simulations of ions and water in carbon and boron-nitride nanotubes. The objectives of our proposed research are:
1. To perform diffusio-osmotic, electrokinetic and streaming-potential measurements in carbon and boron-nitride nanotubes and to develop a single theoretical framework to describe these different surface-driven transport modes;
2. To experimentally study nanotube transport with various diameters under various physical-chemistry conditions, and use our theoretical framework to derive the ion-specific molecular interactions inside the nanotube;
3. To find the molecular basis of radius dependent surface slip and to study the effects of surface charge and polarizability in carbon and boron-nitride nanotubes, both experimentally and in molecular dynamics simulations; and
4. To extend our study to the strongly non-equilibrium regime of ion and water transport, by exploring carbon-boron-nitride heterostructure and to investigate whether nonlinearity can be used to design ion-specific selective membranes.
Our unique collaboration between experiments and theory will provide a fundamental insight into nanometer-scale liquid transport, and its dependence on the molecular details of the nanofluidic system. This knowledge will have a large scientific and economic impact in a wide variety of fields, from the design of innovative membranes, to water desalination technology and electric power generation from osmotic differences between fresh water and sea water.