Transport in nanofluidic systems: a review of theory and applications

In this paper transport through nanochannels is assessed, both of liquids and of dissolved molecules or ions. First, we review principles of transport at the nanoscale, which will involve the identification of important length scales where transitions in behavior occur. We also present several important consequences that a high surface-to-volume ratio has for transport. We review liquid slip, chemical equilibria between solution and wall molecules, molecular adsorption to the channel walls and wall surface roughness. We also identify recent developments and trends in the field of nanofluidics, mention key differences with microfluidic transport and review applications. Novel opportunities are emphasized, made possible by the unique behavior of liquids at the nanoscale

Counterions and water molecules in charged silicon nanochannels: the influence of surface charge discreteness

In order to detect the effect of the surface charge discreteness on the properties at the solid-liquid interface, molecular dynamics simulation model taking consideration of the vibration of wall atoms was used to investigate the ion and water performance under different charge distributions. Through the comparison between simulation results and the theoretical prediction, it was found that, with the degree of discreteness increasing, much more counterions were attracted to the surface. These ions formed a denser accumulating layer which located much nearer to the surface and caused charge inversion. The ions in this layer were non-hydrated or partially hydrated. When a voltage was applied across the nanochannel, this dense accumulating layer did not move unlike the ions near uniformly charged surface. From the water density profiles obtained in nanochannels with different surface charge distributions, the influence of the surface charge discreteness on the water distributions could be neglected

Advances and Challenges in Computational Research of Micro and Nano Flows

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.This paper presents a collective overview of recent studies regarding the computational modelling of micro- and nano-fluidic systems. The review provides an introduction to atomistic, mesoscale and hybrid methods for simulating micro and nano-flows, as well as discusses recent applications and results from the application of such methods

Dissipative particle dynamics simulation of flow in periodically grooved three-dimensional nano- and micro-channels

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Nonequillibrium flow in three-dimensional grooved nano- and micro-channels is investigated using the Dissipative Particle Dynamics simulation method. Roughness is introduced by periodically placing rectangular protruding elements on the upper channel wall. The protrusion length and height are varied and their effect on the flow is examined. The computed macroscopic quantities of practical interest include density, velocity, pressure, and temperature profiles as well as relations between the friction factor and the Reynolds number. When compared to the smooth channel case, lower flow velocities are observed in the central part of the channel for all cases studied. This reduction of velocities becomes more pronounced as the protrusion height increases. For the micro-channel, density, pressure and temperature remain almost constant in the central part of the channel and their pattern near and inside the cavities depend on the protrusion shape. In the nanochannel case, lower temperatures and pressures are observed for all grooved channels relative to the smooth channel case. For all channel cases studied the calculated friction factor decreases as Reynolds number increases, following a power law relation

Solvo-osmotic flow in electrolytic mixtures

We show that an electric field parallel to an electrically neutral surface can generate flow of electrolytic mixtures in small channels. We term this solvo-osmotic flow, since the flow is induced by the asymmetric preferential solvation of ions at the liquid-solid interface. The generated flow is comparable in magnitude to the ubiquitous electro-osmotic flow at charged surfaces, but for a fixed surface charge density, it differs qualitatively in its dependence on ionic strength. Solvo-osmotic flow can also be sensitively controlled with temperature. We derive a modified Helmholtz-Smoluchowski equation that accounts for these effects.Comment: 11 pages, 4 figure

Charge transport in nanochannels: a molecular theory

We introduce a theoretical and numerical method to investigate the flow of charged fluid mixtures under extreme confinement. We model the electrolyte solution as a ternary mixture, comprising two ionic species of opposite charge and a third uncharged component. The microscopic approach is based on kinetic theory and is fully self-consistent. It allows to determine configurational prop- erties, such as layering near the confining walls, and the flow properties. We show that, under appropriate assumptions, the approach reproduces the phenomenological equations used to describe electrokinetic phenomena, without requiring the introduction of constitutive equations to determine the fluxes. Moreover, we model channels of arbitrary shape and nanometric roughness, features that have important repercussions on the transport properties of these systems. Numerical simulations are obtained by solving the evolution dynamics of the one-particle phase- space distributions of each species by means of a Lattice Boltzmann method for flows in straight and wedged channels. Results are presented for the microscopic density, the velocity profiles and for the volumetric and charge flow-rates. Strong departures from electroneutrality are shown to appear at molecular level

Thermo-osmosis in charged nanochannels: effects of surface charge and ionic strength

Thermo-osmosis refers to fluid migration due to temperature gradient. The mechanistic understanding of thermo-osmosis in charged nano-porous media is still incomplete, while it is important for several environmental and energy applications, such as low-grade waste heat recovery, wastewater recovery, fuel cells, and nuclear waste storage. This paper presents results from a series of molecular dynamics simulations of thermo-osmosis in charged silica nanochannels that advance the understanding of the phenomenon. Simulations with pure water and water with dissolved NaCl are considered. First, the effect of surface charge on the sign and magnitude of the thermo-osmotic coefficient is quantified. This effect was found to be mainly linked to the structural modifications of aqueous electrical double layer (EDL) caused by the nanoconfinement and surface charges. In addition, the results illustrate that the surface charges reduce the self-diffusivity and thermo-osmosis of interfacial liquid. The thermo-osmosis was found to change direction when the surface charge density exceeds $-0.03 C/m^2$. It was found that the thermo-osmotic flow and self-diffusivity increases with the concentration of NaCl. The fluxes of solvent and solute are decoupled by considering the Ludwig-Soret effect of NaCl ions to identify the main mechanisms controlling the behavior. In addition to the advance in microscopic quantification and mechanistic understanding of thermo-osmosis, the work provides approaches to investigate a broader category of coupled heat and mass transfer problems in nanoscale space.Comment: 32 pages, 11 figure