Nanofluidics has firmly established itself as a new field in fluid mechanics, as novel properties have been shown to emerge in fluids at the nanometric scale. Thanks to recent developments in fabrication technology, artificial nanofluidic systems are now being designed at the scale of biological nanopores. This ultimate step in scale reduction has pushed the development of new experimental techniques and new theoretical tools, bridging fluid mechanics, statistical mechanics, and condensed matter physics. This review is intended as a toolbox for fluids at the nanometer scale. After presenting the basic equations that govern fluid behavior in the continuum limit, we show how these equations break down and new properties emerge in molecular-scale confinement. A large number of analytical estimates and physical arguments are given to organize the results and different limits.

Fluid and ionic transport at the nanoscale has recently demonstrated a wealth of exotic behaviours1,2,3,4,5,6,7,8,9,10,11,12,13,14. However, artificial nanofluidic devices15,16,17,18 are still far from demonstrating the advanced functionalities existing in biological systems, such as electrically and mechanically activated transport19,20. Here, we focus on ionic transport through 2-nm-radius individual multiwalled carbon nanotubes under the combination of mechanical and electrical forcings. Our findings evidence mechanically activated ionic transport in the form of an ionic conductance that depends quadratically on the applied pressure. Our theoretical study relates this behaviour to the complex interplay between electrical and mechanical drivings, and shows that the superlubricity of the carbon nanotubes4,5,6,7,8,21 is a prerequisite to attaining mechanically activated transport. The pressure sensitivity shares similarities with the response of biological mechanosensitive ion channels19,20, but observed here in an artificial system. This paves the way to build new active nanofluidic functionalities inspired by complex biological machinery

We report a numerical study of the diffusiophoresis of short polymers using non-equilibrium molecular dynamics simulations. More precisely, we consider polymer chains in a fluid containing a solute that has a concentration gradient and examine the variation of the induced diffusiophoretic velocity of the polymer chains as the interaction between the monomer and the solute is varied. We find that there is a non-monotonic relation between the diffusiophoretic mobility and the strength of the monomer–solute interaction. In addition, we find a weak dependence of the mobility on the length of the polymer chain, which shows clear difference from the diffusiophoresis of a solid particle. Interestingly, the hydrodynamic flow through the polymer is much less screened than for pressure driven flows.

This is a turning point for nanofluidics. Recent progress allows envisioning both fundamental discoveries for the transport of fluids at the ultimate scales, and disruptive technologies for the water–energy nexus

Electro- and diffusio-phoresis of particles correspond respectively to the transport of particles under electric field and solute concentration gradients. Such interfacial transport phenomena take their origin in a diffuse layer close to the particle surface, and the motion of the particle is force free. In the case of electrophoresis, it is further expected that the stress acting on the moving particle vanishes locally as a consequence of local electroneutrality. But the argument does not apply to diffusiophoresis, which takes its origin in solute concentration gradients. In this paper we investigate further the local and global force balance on a particle undergoing diffusiophoresis. We calculate the local tension applied on the particle surface and show that, counter-intuitively, the local force on the particle does not vanish for diffusiophoresis, in spite of the global force being zero, as expected. Incidentally, our description allows us to clarify the osmotic balance in diffusiophoresis, which has been a source of debate in recent years. We explore various cases, including hard and soft interactions, as well as porous particles, and provide analytic predictions for the local force balance in these various systems. The existence of local stresses may induce deformation of soft particles undergoing diffusiophoresis, hence suggesting applications in terms of particle separation based on capillary diffusiophoresis.

Noninvasiveness, minimal handling, and immediate response are favorable features of fluorescence readout for high-throughput phenotyping of labeled plants.Yet, remote fluorescence imaging may suffer from an autofluorescent background and artificial or natural ambient light. In this work, the latter limitations are overcome by adopting reversibly photoswitchable fluorescent proteins (RSFPs) as labels and Speed OPIOM (out-of-phase imaging after optical modulation), a fluorescence imaging protocol exploiting dynamic contrast. Speed OPIOM can efficiently distinguish the RSFP signal from autofluorescence and other spectrally interfering fluorescent reporters like GFP. It can quantitatively assess gene expressions, even when they are weak. It is as quantitative, sensitive, and robust in dark and bright light conditions. Eventually, it can be used to nondestructively record abiotic stress responses like water or iron limitations in real time at the level of individual plants and even of specific organs. Such Speed OPIOM validation could find numerous applications to identify plant lines in selection programs, design plants as environmental sensors, or ecologically monitor transgenic plants in the environment.

Fluorescence has become a ubiquitous observable in biology. Yet it encounters limitations, which may originate from optical interferences such as ambient light, autofluorescence, and spectrally interfering fluorophores. In this review, we first report on dynamic contrast which can overcome these limitations. Then we specifically describe out-of-phase imaging after optical modulation, which proved relevant for multiplexed fluorescence imaging even under adverse optical conditions with several optical setups.

Interrogating living cells requires sensitive imaging of a large number of components in real time. The state-of-the-art of multiplexed imaging is usually limited to a few components. This review reports on the promise and the challenges of dynamic contrast to overcome this limitation.

Nucleus centering in mouse oocytes results from a gradient of actin-positive vesicle activity and is essential for developmental success. Here, we analyze 3D model simulations to demonstrate how a gradient in the persistence of actin-positive vesicles can center objects of different sizes. We test model predictions by tracking the transport of exogenous passive tracers. The gradient of activity induces a centering force, akin to an effective pressure gradient, leading to the centering of oil droplets with velocities comparable to nuclear ones. Simulations and experimental measurements show that passive particles subjected to the gradient exhibit biased diffusion toward the center. Strikingly, we observe that the centering mechanism is maintained in meiosis I despite chromosome movement in the opposite direction; thus, it can counteract a process that specifically off-centers the spindle. In conclusion, our findings reconcile how common molecular players can participate in the two opposing functions of chromosome centering versus off-centering.

The fast-developing field of synthetic biology enables broad applications of programmed microorganisms including the development of whole-cell biosensors, delivery vehicles for therapeutics, or diagnostic agents. However, the lack of spatial control required for localizing microbial functions could limit their use and induce their dilution leading to ineffective action or dissemination. To overcome this limitation, the integration of magnetic properties into living systems enables a contact-less and orthogonal method for spatiotemporal control. Here, we generated a magnetic-sensing Escherichia coli by driving the formation of iron-rich bodies into bacteria. We found that these bacteria could be spatially controlled by magnetic forces and sustained cell growth and division, by transmitting asymmetrically their magnetic properties to one daughter cell. We combined the spatial control of bacteria with genetically encoded-adhesion properties to achieve the magnetic capture of specific target bacteria as well as the spatial modulation of human cell invasions.