Remy Fulcrand

Giant osmotic energy conversion measured in a single transmembrane boron nitride nanotube

New models of fluid transport are expected to emerge from the confinement of liquids at the nanoscale, with potential applications in ultrafiltration, desalination and energy conversion. Nevertheless, advancing our fundamental understanding of fluid transport on the smallest scales requires mass and ion dynamics to be ultimately characterized across an individual channel to avoid averaging over many pores. A major challenge for nanofluidics thus lies in building distinct and well-controlled nanochannels, amenable to the systematic exploration of their properties. Here we describe the fabrication and use of a hierarchical nanofluidic device made of a boron nitride nanotube that pierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the detailed study of fluidic transport through a single nanotube under diverse forces, including electric fields, pressure drops and chemical gradients. Using this device, we discover very large, osmotically induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates in the anomalously high surface charge carried by the nanotube’s internal surface in water at large pH, which we independently quantify in conductance measurements. The nano-assembly route using nanostructures as building blocks opens the way to studying fluid, ionic and molecule transport on the nanoscale, and may lead to biomimetic functionalities. Our results furthermore suggest that boron nitride nanotubes could be used as membranes for osmotic power harvesting under salinity gradients.

Large apparent electric size of solid-state nanopores due to spatially extended surface conduction.

Ion transport through nanopores drilled in thin membranes is central to numerous applications, including biosensing and ion selective membranes. This paper reports experiments, numerical calculations, and theoretical predictions demonstrating an unexpectedly large ionic conduction in solid-state nanopores, taking its origin in anomalous entrance effects. In contrast to naive expectations based on analogies with electric circuits, the surface conductance inside the nanopore is shown to perturb the three-dimensional electric current streamlines far outside the nanopore in order to meet charge conservation at the pore entrance. This unexpected contribution to the ionic conductance can be interpreted in terms of an apparent electric size of the solid-state nanopore, which is much larger than its geometric counterpart whenever the number of charges carried by the nanopore surface exceeds its bulk counterpart. This apparent electric size, which can reach hundreds of nanometers, can have a major impact on the electrical detection of translocation events through nanopores, as well as for ionic transport in biological nanopores.

Sub-additive ionic transport across arrays of solid-state nanopores

Nanopores, either biological, solid-state, or ultrathin pierced graphene, are powerful tools which are central to many applications, from sensing of biological molecules to desalination and fabrication of ion selective membranes. However, the interpretation of transport through low aspect-ratio nanopores becomes particularly complex as 3D access effects outside the pores are expected to play a dominant role. Here, we report both experiments and theory showing that, in contrast to naive expectations, long-range mutual interaction across an array of nanopores leads to a non-extensive, sub-linear scaling of the global conductance on the number of pores N. A scaling analysis demonstrates that the N-dependence of the conductance depends on the topology of the network. It scales like G ∼ N/log N for a 1D line of pores, and like G∼N for a 2D array, in agreement with experimental measurements. Our results can be extended to alternative transport phenomena obeying Laplace equations, such as diffusive, thermal, or ...

Nanoscale Dynamics versus Surface Interactions: What Dictates Osmotic Transport?

The classical paradigm for osmotic transport has long related the induced-flow direction to the solute membrane interactions, with the low-to-high concentration flow a direct consequence of the solute rejection from the semipermeable membrane. In principle, the same was thought to occur for the newly demonstrated membrane-free osmotic transport named diffusio-osmosis. Using a recently proposed nanofluidic setup, we revisit this cornerstone of osmotic transport by studying the diffusio-osmotic flows generated at silica surfaces by either poly(ethylene)glycol polymers or ethanol molecules in aqueous solutions. Strikingly, both neutral solutes yield osmotic flows in the usual low to high concentration direction, in contradiction with their propensity to adsorb on silica. Considering theoretically and numerically the intricate nature of the osmotic response that combines molecular-scale surface interaction and near-wall dynamics, these findings are rationalized within a generalized framework. These elements constitute a step forward toward a finer understanding of osmotically driven flows, at the core of rapidly growing fields ranging from energy harvesting to active matter.

A simple fabrication process for an efficient constriction-based dielectrophoretic continuous flow sorter

We present a dielectrophoretic continuous flow sorter using planar micro-electrodes coupled to a SU8 channel constriction. This novel design, relying on a simple fabrication process using SU8 dry film lamination to seal fluidic channels, enables a high sorting efficiency at low voltages. Our device was used to continuously sort 10µm and 5µm latex beads with less than 2% errors at flow speeds of 100µm/s. Simulation results are in accordance with experimental data. Dielectrophoresis; SU8 lamination; bead sorting

Influence of molecular-surface interactions on osmotic flow in nanochannels

We study how solute-solid surface interaction at nanoscale influences the direction and magnitude of the induced water flow (i.e., diffusioosmosis) in the nanofluidic system, when there is a concentration gradient of solutes across the nanofluidic system. Using the velocity measurement technique based on a signal from fluorescent molecules in nanochannels, we show that the attractive nature of the electrostatic interaction is responsible for the flow from high to low solute concentration for electrolytes. However, the role of the molecular interaction for neutral species, e.g., polyethylene glycol polymers and ethanol molecules, appears to be more subtle. Despite a natural tendency of those molecules adsorbing onto the surface, we observe a water flow from low to high solute concentration, which evidences that a dynamic interaction between the solute molecules and surface, rather than a static interaction, is responsible for the generation of diffusioosmosis.

FIB Design for Nanofluidic Applications

In this chapter we briefly review the techniques available to researchers in the nanofluidic domain to fabricate nanopores and nanochannels. In this context the focused ion beam (FIB) technique will be introduced as a useful and versatile tool for nanofluidics. We illustrate it with two specific examples involving nanopores as building blocks for nanofluidics.