Preferential Root Tropisms in 2D Wet Granular Media with Structural Inhomogeneities.

We investigate certain aspects of the physical mechanisms of root growth in a granular medium and how these roots adapt to changes in water distribution induced by the presence of structural inhomogeneities in the form of solid intrusions. Physical intrusions such as a square rod added into the 2D granular medium maintain robust capillary action, pumping water from the more saturated areas at the bottom of the cell towards the less saturated areas near the top of the cell while the rest of the medium is slowly devoid of water via evaporation. The intrusion induces "preferential tropism" of roots by first generating a humidity gradient that attracts the root to grow towards it. Then it guides the roots and permits them to grow deeper into more saturated regions in the soil. This further allows more efficient access to available water in the deeper sections of the medium thereby resulting to increased plant lifetime.

Experimental investigation of water distribution in a two-phase zone during gravity-dominated evaporation.

We characterize the water repartition within the partially saturated (two-phase) zone (PSZ) during evaporation from mixed wettable porous media by controlling the wettability of glass beads, their sizes, and as well the surrounding relative humidity. Here, capillary numbers are low and under these conditions, the percolating front is stabilized by gravity. Using experimental and numerical analyses, we find that the PSZ saturation decreases with the Bond number, where packing of smaller particles have higher saturation values than packing made of larger particles. Results also reveal that the extent (height) of the PSZ, as well as water saturation in the PSZ, both increase with wettability. We also numerically calculate the saturation exclusively contained in connected liquid films and results show that values are less than the expected PSZ saturation. These results strongly reflect that the two-phase zone is not solely made up of connected capillary networks but also made of disconnected water clusters or pockets. Moreover, we also find that global saturation (PSZ + full wet zone) decreases with wettability, confirming that greater quantity of water is lost via evaporation with increasing hydrophilicity. These results show that connected liquid films are favored in more-hydrophilic systems while disconnected water pockets are favored in less-hydrophilic systems.

Simple analytical model of evapotranspiration in the presence of roots.

Evaporation of water out of a soil involves complicated and well-debated mechanisms. When plant roots are added into the soil, water transfer between the soil and the outside environment is even more complicated. Indeed, plants provide an additional process of water transfer. Water is pumped by the roots, channeled to the leaf surface, and released into the surrounding air by a process called transpiration. Prediction of the evapotranspiration of water over time in the presence of roots helps keep track of the amount of water that remains in the soil. Using a controlled visual setup of a two-dimensional model soil consisting of monodisperse glass beads, we perform experiments on actual roots grown under different relative humidity conditions. We record the total water mass loss in the medium and the position of the evaporating front that forms within the medium. We then develop a simple analytical model that predicts the position of the evaporating front as a function of time as well as the total amount of water that is lost from the medium due to the combined effects of evaporation and transpiration. The model is based on fundamental principles of evaporation fluxes and includes empirical assumptions on the quantity of open stomata in the leaves, where water transpiration occurs. Comparison between the model and experimental results shows excellent prediction of the position of the evaporating front as well as the total mass loss from evapotranspiration in the presence of roots. The model also provides a way to predict the lifetime of a plant.

Redispersible hybrid nanopowders: cerium oxide nanoparticle complexes with phosphonated-PEG oligomers.

Rare earth cerium oxide (ceria) nanoparticles are stabilized using end-functional phosphonated-PEG oligomers. The complexation process and structure of the resulting hybrid core-shell singlet nanocolloids are described, characterized, and modeled using light and neutron scattering data. The adsorption mechanism is nonstoichiometric, yielding the number of adsorbed chains per particle N(ads) = 270 at saturation. Adsorption isotherms show a high affinity of the phosphonate head for the ceria surface (adsorption energy DeltaG(ads) approximately -16kT) suggesting an electrostatic driving force for the complexation. The ease, efficiency, and integrity of the complexation is highlighted by the formation of nanometric sized cerium oxide particles covered with a well anchored PEG layer, maintaining the characteristics of the original sol. This solvating brushlike layer is sufficient to solubilize the particles and greatly expand the stability range of the original sol (

Organic versus hybrid coacervate complexes: co-assembly and adsorption properties.

We report the co-assembly and adsorption properties of coacervate complexes made from polyelectrolyte-neutral block copolymers and oppositely charged nanocolloids. The nanocolloids put under scrutiny were ionic surfactant micelles and highly charged 7 nm cerium oxide (CeO2) nanoparticles. Static and dynamic light scattering was used to investigate the microstructure and stability of the organic and hybrid complexes. For five different systems of nanocolloids and polymers, we first demonstrated that the electrostatic complexation resulted in the formation of stable core-shell aggregates in the 100 nm range. The microstructure of the CeO2-based complexes was resolved using cryogenic transmission electronic microscopy (Cryo-TEM), and it revealed that the cores were clusters made from densely packed nanoparticles, presumably through complexation of the polyelectrolyte blocks by the surface charges. The cluster stability was monitored by systematic light scattering measurements. In the concentration range of interest, c = 10-4-1 wt.%, the surfactant-based complexes were shown to exhibit a critical association concentration (cac) whereas the nanoparticle-polymer hybrids did not. The adsorption properties of the same complexes were investigated above the cac by stagnation point adsorption reflectometry. The adsorbed amount was measured as a function of time for polymers and complexes using anionically charged silica and hydrophobic poly(styrene) substrates. It was found that all complexes adsorbed readily on both types of substrates up to a level of 1-2 mg m-2 at stationary state. Upon rinsing however, the adsorbed layer was removed for the surfactant-based systems, but not for the cerium oxide clusters. As for the solution properties, these finding were interpreted in terms of a critical association concentrations, which are very different for organic and hybrid complexes. Combining the efficient adsorption and strong stability of the CeO2-based core-shell hybrids on various substrates, it is finally suggested that these systems could be used appropriately for coating and anti-biofouling applications.

Stability and adsorption properties of electrostatic complexes: design of hybrid nanostructures for coating applications.

We report the presence of a correlation between the bulk and interfacial properties of electrostatic coacervate complexes. Complexes were obtained by co-assembly between cationic-neutral diblocks and oppositely charged surfactant micelles or 7 nm cerium oxide nanoparticles. Light scattering and reflectometry measurements revealed that the hybrid nanoparticle aggregates were more stable through both dilution and rinsing (from either a polystyrene or a silica surface) than their surfactant counterparts. These findings were attributed to a marked difference in critical association concentration between the two systems and to the frozen state of the hybrid structures.

Adhesion of façade coating colonisers, as mediated by physico-chemical properties.

The adhesion of Klebsormidium flaccidum, Stichococcus bacillaris and Chlorella cf. mirabilis, three strains of green microalgae isolated from biofilms on façade coatings were investigated in a parallel plate flow chamber. The model surfaces tested were glass slides, and -CH(3) (mediated by octadecyltrichlorosilane [OTS] and hexamethyldisilazane [HMDZ] modification) and -NH(2) (aminopropyltriethoxysilane [APS] modification) terminated self-assembled monolayers. Algal physicochemical properties were evaluated by the microbial adhesion to solvents (MATS) assay and by contact angle measurements. The model surfaces were characterised by X-ray photoelectron spectroscopy analysis and by contact angle measurements. Predicted adhesion trends were then compared to in vitro measurements. The adhesion strength of the three algal strains followed the trend: APS > OTS > HMDZ > glass. The adhesion process thus seemed to be mediated by hydrophobic and electrostatic interactions, and was shown to be influenced by the algal culture age and the initial contact time.

Capsular polysaccharides secreted by building façade colonisers: characterisation and adsorption to surfaces.

Exopolymers secreted by algal and cyanobacterial strains isolated from building façades were imaged by microscopy techniques. They were extracted and characterised to investigate their possible contribution to interactions with solid surfaces. The polymers were polysaccharides, with anionic and hydrophobic properties varying between the various strains. Capsular polysaccharides extracted from a strain of Klebsormidium flaccidum adsorbed in higher amounts on hydrophobic than on hydrophilic surfaces. These results tend to confirm the hypothesis that exopolymers are important in the colonisation process of microorganisms to surfaces.