We have investigated the active motion of self-propelled colloids confined at the air–water interface and explored the possibility of enhancing the directional motion of self-propelled Janus colloids by slowing down their rotational diffusion. The two dimensional motion of micron-sized silica–platinum Janus colloids has been experimentally measured by particle tracking video-microscopy at increasing concentrations of the catalytic fuel, i.e. H2O2. Compared to the motion in the bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. The interplay of colloid self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, surface properties and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

We investigate how the microstructure of a colloidal polycrystal influences its linear viscoelasticity. We use thermosensitive copolymer micelles that arrange in water in a cubic crystalline lattice, yielding a colloidal polycrystal. The polycrystal is doped with a small amount of nanoparticles, of size comparable to that of the micelles, which behave as impurities and thus partially segregate in the grain boundaries. We show that the shear elastic modulus only depends on the packing of the micelles and does not vary neither with the presence of nanoparticles nor with the crystal microstructure. By contrast, we find that the loss modulus is strongly affected by the presence of nanoparticles. A comparison between rheology data and small-angle neutron scattering data suggests that the loss modulus is dictated by the total amount of nanoparticles in the grain boundaries, which in turn depends on the sample microstructure.

The conformation of poly(ethyl methacrylate) chains in silica–latex nanocomposites has been studied under zero average contrast conditions (ZAC) using small-angle neutron scattering (SANS). Samples have been prepared by drying colloidal suspensions of silica and polymer nanoparticles (NPs) followed by thermal annealing, for two different silica NPs (radius of 5 and 15 nm) and two chain molecular weights (17 and 100 kg/mol). By appropriate mixing of hydrogenated and deuterated polymer, chain scattering contrast is introduced, and in principle silica scattering suppressed. The silica structure consisting mostly of small fractal aggregates is characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) on the same samples. The measurement of the chain structure by SANS, however, is perturbed by unwanted silica contributions, as often reported in the literature. Here, the contribution of contrast-matched silica is evidenced as a function of system parameters, namely chain mass, silica size, and volume fraction, and a model rationalizing these contributions for the first time is proposed. On the basis of a statistical analysis, a nanometer-thick polymer shell surrounding silica NPs is shown to create contrast, which is presumably maintained by the reduced mobility of polymer close to interfaces or attractive polymer–silica interactions. This shell is proven to be quantitatively important only for the smallest silica NPs. Finally, the pure polymer scattering can be isolated, and the polymer radius of gyration is found to be independent of filler content and NP size.

We propose a simple and original way to prepare surfactant-free SWNT/hydrosoluble polymer composites with high concentrations of individual SWNT. We first disperse single-walled carbon nanotubes (SWNT) in aqueous suspensions using bile salts (BS), and we mix them with a solution of hydrosoluble polymer, poly(vinylpyrrolidone) (PVP) or poly(vinyl alcohol) (PVA). We measure the yield using visible–NIR absorption spectroscopy, and we probe both the chemical environment of the nanotubes and the effectiveness of individualization from coupled Raman/photoluminescence studies. We evidence a direct exchange of BS molecules and PVA chains at the surface of SWNT. By contrast, no direct exchange is observed with PVP. On the other hand, we show that a simple dialysis process leads to the preparation of aqueous suspensions of SWNT covered by PVP or PVA with high yields and an effective individualization of the nanotubes.

The dynamics of colloidal particles at interfaces between two fluids plays a central role in microrheology, encapsulation, emulsification, biofilm formation, water remediation and the interface-driven assembly of materials. Common intuition corroborated by hydrodynamic theories, suggests that such dynamics is governed by a viscous force lower than that observed in the more viscous fluid. Here, we show experimentally that a particle straddling an air/water interface feels a large viscous drag that is unexpectedly larger than that measured in the bulk. We suggest that such a result arises from thermally activated fluctuations of the interface at the solid/air/liquid triple line and their coupling to the particle drag through the fluctuation–dissipation theorem. Our findings should inform approaches for improved control of the kinetically driven assembly of anisotropic particles with a large triple-line-length/particle-size ratio, and help to understand the formation and structure of such arrested materials.

The dynamics of polymer and filler in simplified industrial silica-styrene-butadiene nanocomposites (silica Zeosil 1165 MP, volume fraction 0 – 21%v) have been studied with broadband dielectric spectroscopy (BDS) and nuclear magnetic resonance (NMR). The fraction of graftable matrix chains was varied from 0 – 100%D3. The introduction of silica nanoparticles is shown to leave the segmental relaxation unaffected, an observation confirmed by the measurement of only a thin (some Angstroms thick) immobilized layer by NMR. The low-frequency measurements are resolved in two distinct dielectric Maxwell-Wagner-Sillars (MWS) processes of different behavior with respect to changes of large-scale silica structures induced by variations of filler fraction and grafting. In particular, depercolation of the silica filler at constant volume fraction with increasing grafting leaves the first MWS-process unaffected, but has direct consequences on rheological reinforcement, and decreases the strength of the second MWS by about a decade. This sensitivity to large-scale reorganizations together with a characterization of local polymer dynamics provides insight over many length- and time-scales into structure and dynamics of nanocomposites, and thus the physical origin of the reinforcement effect. 

We explore the influence of particle softness and internal structure on both the bulk andinterfacial rheological properties of colloidal suspensions. We probe bulk stresses byconventional rheology, by measuring the flow curves, shear stress versus strain rate, forsuspensions of soft, deformable microgel particles and suspensions of near hard-sphere-likesilica particles. A similar behaviour is seen for both kinds of particles in suspensions atconcentrations up to the random close packing volume fraction, in agreement with recenttheoretical predictions for sub-micron colloids. Transient interfacial stresses are measured byanalyzing the patterns formed by the interface between the suspensions and their solvent, dueto a generalized Saffman–Taylor hydrodynamic instability. At odds with the bulk behaviour,we find that microgels and hard particle suspensions exhibit vastly different interfacial stressproperties. We propose that this surprising behaviour results mainly from the difference inparticle internal structure (polymeric network for microgels versus compact solid for the silicaparticles), rather than softness alone.