This work reports an experimental study of the kinetics and mechanisms of gelation of carbon nanotubes (CNTs)−hyaluronic acid (HA) mixtures. These materials are of great interest as functional biogels for future medical applications and tissue engineering. We show that CNTs can induce the gelation of noncovalently modified HA in water. This gelation is associated with a dynamical arrest of a liquid crystal phase separation, as shown by small-angle light scattering and polarized optical microscopy. This phenomenon is reminiscent of arrested phase separations in other colloidal systems in the presence of attractive interactions. The gelation time is found to strongly vary with the concentrations of both HA and CNTs. Near-infrared photoluminescence reveals that the CNTs remain individualized both in fluid and in gel states. It is concluded that the attractive forces interplay are likely weak depletion interactions and not strong van der Waals interactions which could promote CNT rebundling, as observed in other biopolymer−CNT mixtures. The present results clarify the remarkable efficiency of CNT at inducing the gelation of HA, by considering that CNTs easily phase separate as liquid crystals because of their giant aspect ratio.

This study deals with the ionically-driven self-assembly of oligomeric aminosilicones, judiciously protonated with a variety of organic acids. Depending on the length of the silicone and the strength of the associated acids, (inverse) water-in-silicone emulsions, small nanoparticles, or catanionic vesicles were prepared and characterized by conventional (TEM) or original (DIC optical microscopy, DOSY NMR) techniques. For chains longer than about 40 units, a specific PEG-based sulfonic acid was synthesized and used to generate a supramolecular block-like copolymer and ensure fast and efficient emulsification. In all instances, a simple impulse such as pH increase triggered phase separation of the colloidal objects.

An un-cross-linked SBR-system filled with precipitated silica nanoparticles of radius ≈10 nm by mixing is studied as a function of the fraction of graftable matrix chains (140 kg mol-1) varying from 0% to 100%, for a low (ΦSi = 8.5 vol %) and high (16.7 vol %) silica volume fraction. The linear rheology in shear shows a strong impact of the grafting on the terminal flow regime, and a shift to longer relaxation times with increasing grafting. Simultaneously, the plateau modulus stays approximately constant for the low ΦSi, suggesting a link to the silica content. The microstructure of the silica is characterized by using a combination of transmission electron microscopy and small-angle X-ray scattering data. We apply a quantitative model of interacting aggregates, and determine the average aggregation number (decreasing from 160 to 30 with grafting), aggregate size (50 to 30 nm), and compacity (55% to 35%). While the linear rheology seems to be dominated by the matrix composition, both the mixing rheology and the structure display a saturation with increasing grafting fraction. A closer analysis of this effect indicates that a critical amount of grafting is needed to trigger structural evolution. To summarize, a quantitative study of complex nanocomposites with several features of industrial systems demonstrates that the grafting density can be used as a fine-tuning parameter of rheology and microstructure.

We report theoretical predictions and measurements of the capillary force acting on a spherical colloid smaller than the capillary length that is placed on a curved uid interface of arbitrary shape. By coupling direct imaging and interferometry, we are able to measure the in situ colloid contact angle and to correlate its position with respect to the interface curvature. Extremely tiny capillary forces down to femto-Newton can be measured with this method. Measurements agree well with a theory relating the capillary force to the gradient of Gaussian curvature and to the mean curvature of the interface prior to colloidal deposition. Numerical calculations corroborate these results.

Besides nanostructured materials, individual particles are key elements for nanosciences. The structuring properties of liquid crystals (LCs) are appealing to assemble them, to organize them on substrates or to design functional composites.We present here an overview on particles/LC systems, the size of the dispersed particles being larger than the typical LC length.We first summarize the large number of advances made these last 10 years concerning microparticles assemblies. We then discuss the evolution of the relevant interactions between nanoparticles (NPs) dispersed in LCs when their size decreases from micrometers to nanometers. This allows us to discuss the various assemblies which can be obtained, either in LC bulk, at interfaces or within LC distorted areas or topological defects. Finally, we consider the recent possibilities to use NPs as building elements of complex fluids. We discuss accordingly the LC phases, which can be obtained with pure inorganic NPs in concentrated solution, as well as the self-assemblies which can be obtained when NPs are covered by organic mesogenic ligands.

Fracture processes are ubiquitous in soft materials, even in complex fluids, subjected to stresses. To investigate these processes in a simple geometry, we use a model self-assembled transient gel and study the instability patterns obtained in a radial Hele-Shaw cell when a low viscosity oil pushes the more viscous transient gel. Thanks to an analysis of the morphology of the patterns, we find a discontinuous transition between the standard Saffman-Taylor fingering instability and a fracturing instability as the oil injection rate increases. Our data suggest that the flow properties of the gel ahead of the finger tip controls the transition towards fracturing. By analyzing the displacement field of the gel in the vicinity of the fingers and cracks, we show that in the fingering regime, the oil gently pushes the gel, whereas in the fracturing regime, the crack tears apart the gel, resulting in a strong drop of the gel velocity ahead of the crack tip as compared to the tip velocity. We find a unique behavior for the whole displacement field of a gel around a crack, which is drastically different from that around a finger, and reveals the solid-like behavior of the gel at short time. Our experiments and analysis provide quantitative yet simple tools to unambiguously discriminate a finger from a crack in a visco-elastic material.

A seminal paper [D. R. Nelson, Nano Lett., 2002, 2, 1125.] has proposed that a nematic coating could be used to create a valency for spherical colloidal particles through the functionalization of nematic topological defects. Experimental realizations however question the complex behaviour of solid particles and defects embedded in such a nematic spherical shell. In order to address the related topological and geometrical issues, we have studied micrometer-sized silica beads trapped in nematic shells. We underline the mechanisms that strongly modify the texture of the simple (particle-free) shells when colloidal particles are embedded. Finally, we show how the coupling between capillarity and nematic elasticity offers new ways to control the valence and directionality of shells.