Frédéric Renou

Yielding processes in a colloidal glass of soft star-like micelles under large amplitude oscillatory shear (LAOS)

The understanding of yielding and flow of a colloidal glass under large amplitude oscillatory shear (LAOS) represents a motivating challenge. Monitoring the higher harmonics in the stress signal by Fourier-transform (FT) rheology may provide useful insight on the progressive transition from linear to nonlinear viscoelastic response. However, the physical interpretation of FT-rheology data is still not obvious. Here we study the process of yielding in a colloidal glass formed by star-like block copolymer micelles with LAOS experiments and interrogate the nonlinear intracycle stress response by FT analysis and decomposition to an orthogonal set of Chebyshev polynomials [Ewoldt, R. H., et al. J. Rheol. 52(6), 1427–1458 (2008)]. Such approach provides a robust framework enabling us to map out a rich phenomenology of intracylce nonlinearities that may relate to distinct physical mechanisms. We find that the nonlinearities during yielding are represented by intracylce shear thickening/thinning and strain harden...

Hydrophobically modified xanthan: an amphiphilic but not associative polymer.

Hydrophobic octyl moieties have been grafted in various densities onto the carboxylic acid functions of xanthan under its ordered conformation. The outcoming amphiphilic and associative properties were studied by fluorescence spectroscopy and rheology. Results showed that the conformation of xanthan is not affected by the chemical modification and remains the same as the native one. Additionally, xanthan derivatives do not show any viscoelastic enhancement; nevertheless, their dynamics is strongly slowed down: the higher the grafting density, the slower the relaxation. We proved that hydrophobically modified xanthan, even being amphiphilic, does not exhibit any additional associating properties compared to the unmodified xanthan. The high stiffness of xanthan helices does not allow the derivatives to adopt the organization usually observed for flexible amphiphilic polymers. On the basis of these observations, a model depicting such a singular behavior is proposed.

Effect of salt on turbulent drag reduction of xanthan gum

The turbulent flow of an aqueous KCl solution driven by a rotating disc in a closed chamber showed significant drag reduction (DR) when a small amount of xanthan gum (XG) was added. The effects of the experimental parameters (XG and KCl concentrations, and time) on the drag reduction efficiency were examined. While the DR efficiency of XG decreased with increasing salt (KCl) concentration, the time-dependent DR efficiency was found to be fitted well using Brostow model equation.

Hydrophobically modified xanthan: Thickening and surface active agent for highly stable oil in water emulsions

Oil-in-water (O/W) emulsions were prepared using exclusively a hydrophobically modified xanthan (HMX) under ordered conformation without any surfactant. A series of HMX differing in grafting density were utilized at different concentrations. It was demonstrated that HMX stabilizes O/W emulsions where pristine xanthan do not. Indeed, in some cases, emulsions with HMX proved to be homogeneous and stable over months. Emulsions microstructure and rheological properties were investigated over time as a function of HMX concentration and grafting density in order to determine the mechanisms governing this efficient stability. The higher the HMX concentration is, the better the emulsions stability. However, unexpectedly, increasing the HMX grafting density leads to faster destabilization. The whole set of results demonstrate that the stability of the emulsions against coalescence and creaming is explained by partitioning of HMX molecules between bulk and interface, thus split between viscosifying the aqueous phase and stabilizing the interface.