Thermal Conductivity of a Fluid-Filled Nanoporous Material: Underlying Molecular Mechanisms and the Rattle Effect.

Nanoporous materials are central to the energy and environmental crisis, with key applications in adsorption, separation, and catalysis. While confinement and surface effects on fluids severely confined in their porosity are well documented, the thermal behavior of nanoporous solids subjected to fluid adsorption remains puzzling in many aspects. With striking phenomena such as the so-called rattle effect, through which fluid/solid collisions decrease the overall thermal conductivity, the solid thermal conductivity and, more generally, heat transfer and dispersion in these complex systems challenge classical approaches (e.g., mixing rules including effective medium approaches fail to capture such effects as shown here). In particular, a robust molecular framework to describe the crossover between the decrease in thermal conductivity through the rattle effect in very narrow pores and the increase in thermal conductivity when replacing vacuum with a fluid phase in larger pores is still missing. Here, using a prototypical model of fluid-filled nanoporous materials (a Lennard-Jones phase confined in an all-silica zeolite), we perform a molecular simulation study to shed light on the parameters that govern the rattle effect in nanoporous solids. First, by varying the fluid/fluid, fluid/solid, and solid/solid interaction strengths as well as the fluid number density and mass density, we unravel the ingredients that lead to the essential coupling between fluid adsorption and phonon transport. Second, despite this complex interplay, inspired by pioneering molecular approaches on the rattle effect, we show that all data obey a simple statistical physics model that relies on the change in the speed of sound due to the fluid adsorbed density and the decrease in phonon lifetime due to scattering by fluid molecules. This framework, which provides a simple formalism to rationalize the thermal behavior of this class of solid/fluid composites, points to a decrease in thermal conductivity upon fluid confinement (up to 30% in some cases). Such an effect paves the way for the design of novel applications involving fluids in interaction with nanoporous materials.

Self-supported MOF/cellulose-nanocrystals materials designed from ultrafiltration.

Metal-organic-frameworks (MOFs) are promising materials for addressing critical issues such as petrochemical separation, water purification, energy storage and drug delivery. Their large-scale deployment, however, is hampered by a limited processability due to their powdery nature. Recently, the hybridization of MOFs with biopolymers has emerged as a greener, biocompatible strategy to shape MOFs composites into more processable membranes, films, and porous materials. In this work, cellulose nanocrystals (CNCs) were used in combination with ZIF-8 (a widely used synthetic zeolite) to produce hybrid composites through ultrafiltration. Results showed that small quantities of CNCs (1 to 20 CNC:ZIF-8 volume ratio) were sufficient to form a self-supported, dense deposit with high ZIF-8 loadings. Compared to classical MOF in situ growth strategies, this approach allowed the tuning of the composition of the final nanocomposite by controlling the nature and quantities of particles in the suspension. The fabrication of the deposit was strongly dependent on the physiochemical properties of the suspension, which were fully characterized with a set of complementary techniques, including in situ SAXS. This technique was employed to investigate the filtration process, which exhibited a homogeneous deposition of ZIF-8 particles mediated by CNC self-assembly. Finally, the available pore volume and integrity of the internal porosity of ZIF-8 were characterized by water porosimetry, demonstrating that the presence of CNCs did not alter the properties of the supported ZIF-8.

Bowtie-Shaped Deformation Isotherm of Superhydrophobic Cylindrical Mesopores.

Deformation of superhydrophobic cylindrical mesopores is studied during a cycle of forced water filling and spontaneous drying by in situ small-angle neutron scattering. A high-pressure setup is put forward to characterize the deformation of ordered mesoporous silanized silica up to 80 MPa. Strain isotherms of individual pores are deduced from the shift of the Bragg spectrum associated with the deformation of the hexagonal pore lattice. Due to their superhydrophobic nature, pore walls are not covered with a prewetting film. This peculiarity gives the ability to use a simple mechanical model to describe both filled and empty pore states without the pitfall of disjoining pressure effects. By fitting our experimental data with this model, we measure both the Young's modulus and the Poisson ratio of the nanometric silica wall. The measurement of this latter parameter constitutes a specificity offered by superhydrophobic nanopores with respect to hydrophilic ones.

Carbon dioxide as a line active agent: Its impact on line tension and nucleation rate.

By considering a water capillary bridge confined between two flat surfaces, we investigate the thermodynamics of the triple line delimiting this solid-liquid-vapor system when supplemented in carbon dioxide. In more detail, by means of atom-scale simulations, we show that carbon dioxide accumulates at the solid walls and, preferably, at the triple lines where it plays the role of a line active agent. The line tension of the triple line, which is quantitatively assessed using an original mechanical route, is shown to be driven by the line excess concentrations of the solute (carbon dioxide) and solvent (water). Solute accumulation at the lines decreases the negative line tension (i.e., more negative) while solvent depletion from the lines has the opposite effect. Such an unprecedented quantitative assessment of gas-induced line tension modifications shows that the absolute value of the negative line tension increases upon increasing the carbon dioxide partial pressure. As a striking example, for hydrophilic surfaces, the line tension is found to increase by more than an order of magnitude when the carbon dioxide pressure exceeds 3 MPa. By considering the coupling between line and surface effects induced by gaseous adsorption, we hypothesize from the observed gas concentration-dependent line tension a nontrivial impact on heterogeneous nucleation of nanometric critical nuclei.

Probing the concept of line tension down to the nanoscale.

A novel mechanical approach is developed to explore by means of atom-scale simulation the concept of line tension at a solid-liquid-vapor contact line as well as its dependence on temperature, confinement, and solid/fluid interactions. More precisely, by estimating the stresses exerted along and normal to a straight contact line formed within a partially wet pore, the line tension can be estimated while avoiding the pitfalls inherent to the geometrical scaling methodology based on hemispherical drops. The line tension for Lennard-Jones fluids is found to follow a generic behavior with temperature and chemical potential effects that are all included in a simple contact angle parameterization. Former discrepancies between theoretical modeling and molecular simulation are resolved, and the line tension concept is shown to be robust down to molecular confinements. The same qualitative behavior is observed for water, but the line tension at the wetting transition diverges or converges toward a finite value depending on the range of solid/fluid interactions at play.

Reminiscent capillarity in subnanopores.

Fluids in large and small pores display different behaviors with a crossover described through the concept of critical capillarity. Here we report experimental and simulation data for various siliceous zeolites and adsorbates that show unexpected reminiscent capillarity for such nanoporous materials. For pore sizes D exceeding the fluid molecule size, the filling pressures p are found to follow a generic behavior kBT ln p ∼ γ/ρD where γ and ρ are the fluid surface tension and density. This result is rationalized by showing that the filling chemical potential for such ultra-small pores is the sum of an adsorption energy and a capillary energy that remains meaningful even for severe confinements. A phenomenological model, based on Derjaguin's formalism to bridge macroscopic and molecular theories for condensation in porous materials, is developed to account for the behavior of fluids confined down to the molecular scale from simple parameters.

Large slippage and depletion layer at the polyelectrolyte/solid interface.

The slippage of polymer solutions on solid surfaces is often attributed to a depletion layer whose origin, thickness, and interaction with the flow are poorly understood. Using a Dynamic Surface Force Apparatus we report a structural and nanorheological study of the interface between hydrolyzed poly-acrylamide solutions and platinum surfaces. Polyelectrolyte chains adsorb on the surfaces in a thin charged layer, acting as a nonattractive wall for the bulk solution. We investigate the flow of the visco-elastic solution on the adsorbed layer from the nanometer to 10 micrometers, bridging microscopic to macroscopic properties. At distances larger than 200 nanometers, the flow is well described by an apparent slip boundary condition. At smaller distance the apparent slip is found to decrease with the gap. In contrast to the apparent slip model, we show that a 2-fluids model taking into account the finite thickness of depletion layers at the non-attractive wall describes accurately the dynamic forces over 4 spatial decades of confinement. Depletion layers are found to be an equilibrium property of the interface, independent on the flow and on the confinement. Their thickness is phenomenologically described by ξ + 2lD with ξ the correlation length of the semi-dilute solutions and lD the Debye length. We interpret this result in terms of screened repulsion between the charged adsorbed layer and the bulk polyions.

A Direct Sensor to Measure Minute Liquid Flow Rates.

Nanofluidics finds its root in the study of fluids and flows at the nanoscale. Flow rate is a quantity that is both central when dealing with flows and notoriously difficult to measure experimentally at the scale of an individual nanopore or nanochannel. We show in this letter that minute flow rate can be directly measured accumulating liquid over time within the compliant membrane of a commercial piezoresistive pressure sensor. Our flow rate sensor is versatile and can be operated independently of the nature of the liquid, flow profile, and type of nanochannel. We demonstrate this method by measuring the pressure-driven flow of silicon oil in a single nanochannel of average radius 200 nm. This approach gives reliable measurement of the flow rate up to 1 pL/min. Unlike other nanoscale flow measurements methods based, for instance, on particle tracking, our sensor delivers a direct voltage output suitable for nanoflow control applications.

A micro-nano-rheometer for the mechanics of soft matter at interfaces.

We present a nano-rheometer based on the dynamic drainage flow between a sphere and a plane from bulk regime to highly confined regime. The instrument gives absolute measurements of the viscosity of simple liquids in both regimes. For complex fluids, the measurements involve the viscosity and the elastic modulus. The device operates on distances ranging over four orders of magnitude from 1 nm to 10 µm, bridging rheological properties from the macroscopic to the molecular scale. This allows to measure an hydrodynamic or visco-elastic boundary condition and to explore the causes of the boundary condition at the microscopic level.

Giant Osmotic Pressure in the Forced Wetting of Hydrophobic Nanopores.

The forced intrusion of water in hydrophobic nanoporous pulverulent material is of interest for quick storage of energy. With nanometric pores the energy storage capacity is controlled by interfacial phenomena. With subnanometric pores, we demonstrate that a breakdown occurs with the emergence of molecular exclusion as a leading contribution. This bulk exclusion effect leads to an osmotic contribution to the pressure that can reach levels never previously sustained. We illustrate, on various electrolytes and different microporous materials, that a simple osmotic pressure law accounts quantitatively for the enhancement of the intrusion and extrusion pressures governing the forced wetting and spontaneous drying of the nanopores. Using electrolyte solutions, energy storage and power capacities can be widely enhanced.

Long-term, inducible gene loss-of-function in the chicken embryo.

The use of shRNAmir to down-regulate the expression of genes of interest is a powerful tool for studying gene function during early chick development. However, because of the limitations of electroporation-mediated transgenesis, the down-regulation of genes expressed at late stages of development in specific tissues is difficult to perform. By combining electroporation of a doxycycline-inducible, miR30-based shRNA plasmid with the Tol2 genomic integration system, we are now able to down-regulate the expression of any gene of interest at defined stage of chicken development.

Two distinct muscle progenitor populations coexist throughout amniote development.

During embryonic and fetal life, skeletal muscle growth is dependent upon the proliferation and the differentiation of a population of resident muscle progenitors, from which derive the muscle stem cells of the adult, the satellite cells. Under poorly defined extrinsic and intrinsic influences, muscle progenitors proliferate, differentiate or enter a quiescent state to become reserve satellite cells. Despite their primordial role, surprisingly little is known on the homeostasis of resident progenitors during embryogenesis. Preliminary studies in chick and mouse describing the key progenitor populations contributing to muscle growth during embryogenesis have led to differing results that could be due to technical issues or to fundamental differences between animal models. To address this question, we have undertaken a comprehensive analysis of the state of differentiation and proliferation of muscle progenitor cells from the time of their emergence within the dermomyotome until late fetal life, when they adopt a satellite cell-like position under the basal lamina. This was done by immunostaining against key players of myogenic differentiation, in muscles chosen from different regions of the body in two model organisms, the chick and mouse. This study identified two co-existing populations of progenitors during embryonic and fetal life in both chick and mouse: a minor, slow-cycling pool of undifferentiated resident progenitors which express Pax7, co-existing with a major fast-cycling population that co-express Pax7 and the early myogenic differentiation marker Myf5. We found that the overall proliferation rate of both progenitors drastically decreased with embryonic age, as an increasingly large portion of slow and fast-cycling progenitors entered quiescence during development. Together, this data suggests that the cellular strategies that drive muscle growth during embryonic and fetal life are remarkably conserved in amniotes throughout evolution. They rely on the tight regulation of proliferation, entry in quiescence, and modulation of the cell cycle's length for both of the co-existing populations of muscle progenitors to maintain the homeostasis of growing muscles during development.

A micro-incubator for cell and tissue imaging.

A low-cost micro-incubator for imaging dynamic processes in living cells and tissues has been developed. This micro-incubator provides a tunable environment that can be altered to study responses of cell monolayers for several days as well as relatively thick tissue samples and tissue-engineered epithelial tissues in experiments lasting several hours. Samples are contained in a sterile cavity closed by a gas-permeable membrane. The incubator can be positioned in any direction and used on an inverted or upright microscope. Temperature is regulated using a Peltier module controlled by a sensor positioned close to the sample, enabling compensation for any changes in temperature. Rapid changes in a sample's surrounding environment can be achieved due to the fast response of the Peltier module. These features permit monitoring of sample adaptation to induced environmental changes.

Transient aging of a liquid-gas interface stretched by standing waves: on the interplay of chemical kinetics.

This article discusses the aging of a liquid surface enriched with surface active species and suddenly perturbed by standing capillary waves. Special attention is paid to deriving an accurate initial condition of the surface elevation. Due to the arising of the waves, the sub-phase and surface concentrations, C and Gamma, which were initially uniform and controlled by thermodynamic equilibrium, are modified by a transient oscillatory regime. A complete analytical description of the time-dependent carrier wave and oscillating chemical modulations associated with these concentrations is proposed for: (1) a small surface elevation, (2) a weak coupling with momentum transport, (3) but a strong coupling between all chemical transport phenomena which might be involved during transient regime: surface adsorption/desorption, 3-D diffusion within the sub-phase as well as near the liquid surface, and 2-D chemical diffusion along the surface. Analytical expressions resulting from regular perturbation series are compared to the limit aging regimes most commonly invoked in the literature, namely, diffusion- or sorption-limited surface aging. Finally, the (surface) compositional elasticity due to the arising of surface tension gradients is derived.

Microsatellites for disentangling underground networks: strain-specific identification of Glomus intraradices, an arbuscular mycorrhizal fungus.

The underground network of arbuscular mycorrhizal (AM) fungi is decisive for the above-ground diversity of many plant ecosystems, but tools to investigate the population structure of AM fungi are sorely lacking. Here, we present a bioinformatics approach to identify microsatellite markers in the AM fungus Glomus intraradices. Based on 1958 contigs of this fungus, assembled from public databases, we identified 842 microsatellites. One hundred of them were subjected to closer scrutiny by designing flanking primers and performing an extensive screen to identify polymorphic loci. We obtained 18 polymorphic microsatellite markers, and we found that seven out of eight individual single-spore cultures of G. intraradices could readily be identified by at least five allelic differences, as compared to all other strains. Two single-spore cultures, however, nominally originating from completely different locations, displayed identity at all 18 loci, suggesting with 99.999999% probability that they represent a single clone.

A web application to manage a database for liquid nitrogen tanks.

Summary We have developed a software, Cell Database, for archiving records about cells stored in liquid nitrogen tanks. Once installed on a web server, the database is accessed through a standard web browser. This user-friendly and self-explanatory application is independent of computer platform and periodic upgrades of a commercial software. Our web application allows import of data from other database programs and adaptation to different tank formats, types of samples, and archiving needs.