Gemini surfactants are ideal systems to study a wide range of rheological behaviours in soft matter, showing fascinating analogies with living polymers and polyelectrolytes. By only changing the concentration, the shear viscosity can vary by 7 orders of magnitude in the bulk when transitioning through the semidilute regime. In order to elucidate on the intrinsic shear viscosity profile at the interface in soft matter systems manifesting various concentration regimes and morphological transitions, we performed microrheology and adsorption experiments under a wide range of experimental conditions. The surface shear viscosity has been characterized by passive microrheology, tracking Brownian particles trapped at the air-solution interface, under particle wetting conditions precisely characterized by interferometry. We observe that a steep increase in bulk shear viscosity as a function of the concentration does not translate at the interface, which may show a negative surface shear viscosity. By comparing macrorheology and microrheology, we measure significant differences both at the interface and in the bulk in the semidilute regime, where wormlike micelles start to entangle. The disparity in rheological measurements can be attributed to notable depletion effects near both the air-solution and particle-solution interfaces.

To enhance the viscoelastic properties of Feruloylated Water Extractable ArabinoXylan (F-WEAX) gels, F-WEAX were covalently linked by a laccase in the presence of cellulose nanocrystals (CNC). The CNC/ F-WEAX weight ratios varied from 0 to 1.5 at a constant F-WEAX concentration of 1 % (w/v). The resulting CNC/ F-WEAX gels became stiffer with increasing CNC content with the elastic modulus scaling as [CNC]3. CNC also altered the relaxation spectrum of the CNC/ F-WEAX gels: the loss factor increased at low frequencies while remaining constant at high frequencies. The strong affinity between F-WEAX and CNC was demonstrated by in situ QCM-D analysis which showed that F-WEAX adsorb spontaneously and irreversibly onto CNC. Following laccase oxidative gelation, the rate of recovered ferulic acid (FA) in di-and trimer forms was twice as high when oxidation occurred in the presence of CNC. Due their higher covalent cross-link density, the swelling capacity of F-WEAX-CNC gels slightly decreased. The increased elasticity of the F-WEAX-CNC gels can thus be attributed to 3 factors: a higher volume fraction of CNC leading to increased entanglement, a greater number of di- or tri-ferulic acid covalent cross-links between F-WEAX chains, and the formation of multiarm physical junctions between F-WEAX and CNC.

Nous présentons dans cette étude une expérience de microscopie holographique numérique en ligne pour le suivi de bactéries en milieu confiné. Cette technique, simple de mise en œuvre, nous permet de caractériser la dynamique d'une suspension bactérienne dans un capillaire d'une centaine de micromètres. Nous présentons également le logiciel holotracker en libre accès qui permet de propager des hologrammes par la méthode du spectre angulaire et également d'utiliser différentes fonction de focus puis de tracking.

Poly(N-isopropylacrylamide) (PNIPAM) is one of the most well studied thermoresponsive polymers and its microgels undergo a reversible volume phase transition (VPT) at temperatures close to that of the human body. Coupling this property with the unique optical properties of single-walled carbon nanotubes (SWCNT) in the near infrared (NIR) leads to interesting nanohybrids that could be used as multi-responsive sensors/actuators for biological applications.We show the synthesis of thermoresponsive SWCNT/PNIPAM hybrids using two different non-covalent strategies, preserving the nanotube optical properties such as fluorescence. The first strategy involves the dispersion of the SWCNT in water with sodium dodecyl sulfate (SDS), and subsequent in situ synthesis of PNIPAM microgels inside the SDS coatings around the nanotubes. The second strategy starts with modifying the nanotubes with a pyrene derivative, which in turn is used as the starting point for the in situ polymerization of the PNIPAM microgels, thus ensuring that the polymer grows around the nanotubes. In both cases, we obtain hybrids showing a phase transition at temperatures close to that of the human body, with the absorption and fluorescence spectra of the hybrids in the NIR changing in response to the changing dielectric environment. These systems could be used as actuators/sensors in biological systems.

ContextSingle-wall carbon nanotubes (SWCNT) dispersed in water with the help of sodium dodecyl sulfate (SDS) surfactants exhibit a temperature dependent near infrared (NIR) exciton spectrum. Due to their biocompatibility and NIR spectrum falling within the transparent window for biological tissue, SWCNTs hold potential for sensing temperature inside cells. Here, we seek to elucidate the mechanism responsible for this temperature dependence, focusing on changes in the water coverage of the SWCNT as the surfactant structure changes with temperature. We compare optical absorption spectra in the UV–Vis-IR range and fully atomistic molecular dynamics (MD) simulations. The observed temperature dependence of the spectra for various SWCNTs may be attributed to changes in the dielectric environment and its impact on excitons. MD simulations reveal that the adsorbed SDS molecules effectively shield the SWCNT, with ~ 70% of water molecules removed from the first two adlayers; this coverage shows a modest temperature dependence. Although we are not able to directly demonstrate how this influences the NIR spectrum, this represents a potential pathway given the strong influence of the water environment on the excitons in SWCNTs.MethodsOptical absorption measurements were carried out in the UV–Vis-NIR range using a Varian Cary 5000 spectrophotometer in a temperature-controlled environment. PeakFit™ v. 4.06 was used as peak-fitting program in the spectral range 900–1400 nm (890–1380 meV) as a function of the temperature. Fully atomistic molecular dynamics simulations were conducted using the NAMD2 package. The CHARMM force field comprising two-body bond stretching, three-body angle deformation, four-body dihedral angle deformation, and nonbonded interactions (electrostatic and Lennard–Jones 6–16 potentials) was employed.

The addition of water to native cellulose/1-ethyl-3methylimidazolium acetate solutions catalyzes the formation of gels, where polymer chain-chain intermolecular associations act as cross-links. However, the relationship between water content (Wc), polymer concentration (Cp), and gel strength is still missing. This study provides the fundamentals to design water-induced gels. First, the sol-gel transition occurs exclusively in entangled solutions, while in unentangled ones, intramolecular associations hamper interchain cross-linking, preventing the gel formation. In entangled systems, the addition of water has a dual impact: at low water concentrations, the gel modulus is water-independent and controlled by entanglements. As water increases, more cross-links per chain than entanglements emerge, causing the modulus of the gel to scale as Gp ∼ C p^2 Wc^3.0±0.2. Immersing the solutions in water yields hydrogels with noncrystalline, aggregate-rich structures. Such water-ionic liquid exchange is examined via Raman, FTIR, and WAXS. Our findings provide avenues for designing biogels with desired rheological properties.