Nanofluidics has emerged recently in the footsteps of microfluidics, following the quest for scale reduction inherent to nanotechnologies. By definition, nanofluidics explores transport phenomena of fluids at nanometer scales. Why is the nanometer scale specific? What fluid properties are probed at nanometric scales? In other words, why does ‘nanofluidics’ deserve its own brand name? In this critical review, we will explore the vast manifold of length scales emerging for fluid behavior at the nanoscale, as well as the associated mechanisms and corresponding applications. We will in particular explore the interplay between bulk and interface phenomena. The limit of validity of the continuum approaches will be discussed, as well as the numerous surface induced effects occurring at these scales, from hydrodynamic slippage to the various electro-kinetic phenomena originating from the couplings between hydrodynamics and electrostatics. An enlightening analogy between ion transport in nanochannels and transport in doped semi-conductors will be discussed (156 references).

In this Letter, we explore experimentally the phase behavior of a dense active suspension of self-propelled colloids. In addition to a solidlike and gaslike phase observed for high and low densities, a novel cluster phase is reported at intermediate densities. This takes the form of a stationary assembly of dense aggregates\char22{}resulting from a permanent dynamical merging and separation of active colloids\char22{}whose average size grows with activity as a linear function of the self-propelling velocity. While different possible scenarios can be considered to account for these observations\char22{}such as a generic velocity weakening instability recently put forward\char22{}we show that the experimental results are reproduced mathematically by a chemotactic aggregation mechanism, originally introduced to account for bacterial aggregation and accounting here for diffusiophoretic chemical interaction between colloidal swimmers.

The pressure-driven flow rate through individual carbon nanotubes is precisely determined from the hydrodynamics of emerging water jets, revealing unexpectedly large and radius-dependent surface slippage. Carbon nanotubes are of interest for applications such as desalination, because of the exceptional speed with which water moves through them. But the underlying mechanism for water transport has not been well characterized, reflecting the difficulty of unambiguously measuring the permeability of single nanotubes. Lydéric Bocquet and colleagues now show that permeability can be accurately determined by observing the hydrodynamics of water jets as they emerge from single nanotubes into a surrounding fluid. The results reveal large, radius-dependent surface slippage in carbon nanotubes and no slippage in boron nitride nanotubes. This contrast is attributed to subtle, atomic-scale differences between the solid–liquid interfaces of the two systems, illustrating that nanofluidics is the frontier at which the continuum picture of fluid mechanics meets the atomic nature of matter. Measurements and simulations have found that water moves through carbon nanotubes at exceptionally high rates owing to nearly frictionless interfaces1,2,3,4. These observations have stimulated interest in nanotube-based membranes for applications including desalination, nano-filtration and energy harvesting5,6,7,8,9,10, yet the exact mechanisms of water transport inside the nanotubes and at the water–carbon interface continue to be debated11,12 because existing theories do not provide a satisfactory explanation for the limited number of experimental results available so far13. This lack of experimental results arises because, even though controlled and systematic studies have explored transport through individual nanotubes7,8,9,14,15,16,17, none has met the considerable technical challenge of unambiguously measuring the permeability of a single nanotube11. Here we show that the pressure-driven flow rate through individual nanotubes can be determined with unprecedented sensitivity and without dyes from the hydrodynamics of water jets as they emerge from single nanotubes into a surrounding fluid. Our measurements reveal unexpectedly large and radius-dependent surface slippage in carbon nanotubes, and no slippage in boron nitride nanotubes that are crystallographically similar to carbon nanotubes, but electronically different. This pronounced contrast between the two systems must originate from subtle differences in the atomic-scale details of their solid–liquid interfaces, illustrating that nanofluidics is the frontier at which the continuum picture of fluid mechanics meets the atomic nature of matter.

It is well known that, at a macroscopic level, the boundary condition for a viscous fluid at a solid wall is one of “no slip.” The liquid velocity field vanishes at a fixed solid boundary. We consider the special case of a liquid that partially wets the solid (i.e., a drop of liquid, in equilibrium with its vapor on the solid substrate, has a finite contact angle). Using extensive molecular dynamics simulations, we show that when the contact angle is large enough, the boundary condition can drastically differ (at a microscopic level) from a no-slip condition. Slipping lengths exceeding 30 molecular diameters are obtained for a contact angle of 140°, characteristic of mercury on glass. This finding may have important implications for the transport properties in nanoporous media under such “nonwetting” conditions.