Probing the nanohydrodynamics at liquid-solid interfaces using thermal motion

We report on a new method to characterize nano-hydrodynamic properties at the liquid/solid interface relying solely on the measurement of the thermal motion of confined colloids. Using Fluorescence Correlation Spectroscopy (FCS) to probe the diffusion of the colloidal tracers, this optical technique --equivalent in spirit to the microrheology technique used for bulk properties-- is able to achieve nanometric resolution on the slip length measurement. It confirms the no-slip boundary condition on wetting surfaces and shows a partial slip b=18 +/- 5 nm on non-wetting ones. Moreover, in the absence of external forcing, we do not find any evidence for large nano-bubble promoted slippage on moderately rough non-wetting surfaces.Comment: 4 pages, 3 figure

Hydrodynamics within the Electric Double Layer on slipping surfaces

We show, using extensive Molecular Dynamics simulations, that the dynamics of the electric double layer (EDL) is very much dependent on the wettability of the charged surface on which the EDL develops. For a wetting surface, the dynamics, characterized by the so-called Zeta potential, is mainly controlled by the electric properties of the surface, and our work provides a clear interpretation for the traditionally introduced immobile Stern layer. In contrast, the immobile layer disappears for non-wetting surfaces and the Zeta potential deduced from electrokinetic effects is considerably amplified by the existence of a slippage at the solid substrate.Comment: accepted for publication in Physical Review Letter

Large permeabilities of hourglass nanopores: From hydrodynamics to single file transport

In fluid transport across nanopores, there is a fundamental dissipation that arises from the connection between the pore and the macroscopic reservoirs. This entrance effect can hinder the whole transport in certain situations, for short pores and/or highly slipping channels. In this paper, we explore the hydrodynamic permeability of hourglass shape nanopores using molecular dynamics (MD) simulations, with the central pore size ranging from several nanometers down to a few Angstr{\"o}ms. Surprisingly, we find a very good agreement between MD results and continuum hydrodynamic predictions, even for the smallest systems undergoing single file transport of water. An optimum of permeability is found for an opening angle around 5 degree, in agreement with continuum predictions, yielding a permeability five times larger than for a straight nanotube. Moreover, we find that the permeability of hourglass shape nanopores is even larger than single nanopores pierced in a molecular thin graphene sheet. This suggests that designing the geometry of nanopores may help considerably increasing the macroscopic permeability of membranes

Colloidal motility and pattern formation under rectified diffusiophoresis

In this letter, we characterize experimentally the diffusiophoretic motion of colloids and lambda- DNA toward higher concentration of solutes, using microfluidic technology to build spatially- and temporally-controlled concentration gradients. We then demonstrate that segregation and spatial patterning of the particles can be achieved from temporal variations of the solute concentration profile. This segregation takes the form of a strong trapping potential, stemming from an osmotically induced rectification mechanism of the solute time-dependent variations. Depending on the spatial and temporal symmetry of the solute signal, localization patterns with various shapes can be achieved. These results highlight the role of solute contrasts in out-of-equilibrium processes occuring in soft matter

Vegetation stability in the Brazilian littoral during the late holocene - anthracological evidence

Anthracological studies carried out on six sambaquis from Rio de Janeiro State (southeastern Brazilian coast) and one sambaqui from Santa Catarina State (southern Brazil) showed that coastal vegetation presents a marked resilience to climatic changes. Charcoal samples were collected from vertical profiles along the entire sambaqui height and examined under reflected light microscope. In the southeastern Brazilian coast, the anthracological spectra is essentially the same between 5500 and 1400 yrs BP. Coastal sandy beach ridges were already colonized by the resting vegetation since at least the Middle Holocene. Depending on the site, the open resting or the resting forest are dominant. Forest and mangrove elements are present in all sites. In southern Brazil, no significant variation of the vegetal environment is recorded between 2500 and 1800 yrs BP. The studied site was established in the resting environment, and the inland Atlantic Forest was probably situated quite far away. Mangrove vegetation was probably already absent from this region, which is presently under subtropical climatic conditions. No major changes of the vegetal ecosystem has taken place during the second part of the Holocene, notwithstanding many centuries of human occupation, pointing out to the fact that the vegetation was not greatly affected either by climatic or by anthropogenic perturbations. However, significant oscillations recorded in the mangrove vegetation from the Cabo Frio region (southeastern Brazil) might be related to climatic variations, is probably due to the edaphic character of coastal environments, particularly the resting ecosystem, which is related to sandy soils and to the geomorphologic nature of sandy beach ridges