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International audience; Discontinuous shear thickening is usually observed in very dense suspensions of particles and manifests by a jump of shear stress at a given shear rate that we call the critical shear rate or by a decrease of the shear rate during a ramp of stress. This transition is believed to be created by the sudden formation of a percolating network with frictional contacts between the particles and happens when the shear forces dominate the repulsive forces which keep the particles apart. These repulsive forces can be due to ionic or to polymer layers adsorbed on the surfaces of the particles. Our aim here was to be able to control this transition thanks to the application of a magnetic field

The detection of protein-protein interactions by imaging techniques often requires the overexpression of the proteins of interest tagged with fluorescent molecules, which can affect their biological properties and, subsequently, flaw experiment interpretations. The recent development of the proximity ligation assays (PLA) technology allows easy visualization of endogenous protein-protein interactions at the single molecule level. PLA relies on the use of combinations of antibodies coupled to complementary oligonucleotides that are amplified and revealed with a fluorescent probe, each spot representing a single protein-protein interaction. Another application of this technique is the detection of proteins posttranslational modifications to monitor their localization and dynamics in situ. Here, we describe the use of PLA to detect protein SUMOylation, a posttranslational modification related to ubiquitination, as well as interaction of SUMOylated substrates with other proteins, using both adherent and suspension cells.

SUMO is a ubiquitin-like protein that is covalently conjugated to numerous cellular proteins to modify their function and fate. Although large progresses have been made in the identification of SUMOylated proteins, the molecular consequences of their SUMOylation are generally unknown. This is, most often, due to the low abundance of SUMOylated proteins in the cell, usually less than 1 % of a given protein being modified at steady state. To gain insights into the role of specific SUMOylation targets, SUMO conjugation can be reconstituted in vitro using purified proteins. However, for most substrates, the efficiency of in vitro SUMOylation is too low to obtain sufficient amounts of their SUMOylated forms for biochemical studies. Here, we describe a detailed protocol to purify large amounts of recombinant SUMOylated proteins using bacteria modified to express His-tagged SUMO as well as the SUMO-activating and -conjugating enzymes.

International audience; IIn this work, we have studied the magnetorheological (MR) fluid rheology in the magnetic field parallel to the fluid vorticity. Experimentally, the MR fluid flow was realized in the Couette coaxial cylinder geometry with the magnetic field parallel to the symmetry axis. The rheological measurements were compared to those obtained in the cone-plate geometry with the magnetic field perpendicular to the lower rheometer plate. Experiments revealed a quasi-Bingham behavior in both geometries with the stress level being just a few dozens of percent smaller in the Couette cylindrical geometry at the same internal magnetic field. The unexpectedly high MR response in the magnetic field parallel to the fluid vorticity is explained by stochastic fluctuations of positions and orientations of the particle aggregates. These fluctuations are induced by magnetic interactions between them. Once misaligned from the vorticity direction, the aggregates generate a high stress independent of the shear rate, and thus assimilated to the suspension apparent (dynamic) yield stress. Quantitatively, the fluctuations of the aggregate orientation are modeled as a rotary diffusion process with a diffusion constant proportional to the mean square interaction torque. The model gives a satisfactory agreement with the experimental field dependency of the apparent yield stress and confirms the nearly quadratic concentration dependency , revealed in experiments. We also discuss a possible mechanism to explain the enhancement of the magnetic field-induced yield stress when nonmagnetic particles are added to magnetic particulate suspensions –i.e., bi-component suspensions. Our main hypothesis is that the nonmagnetic particles collide with the field-induced magnetic aggregates under shear flow. Consequently, supplementary fluctuations of the orientations of the magnetic aggregates occur, resulting in an effective rotary diffusion process, which increases the dynamic yield stress of the suspension. Furthermore, the collision rate and the rotary diffusivity of the aggregates should increase with the concentration of nonmagnetic particles. Rheological measurements in plate-plate and cylindrical Couette geometries confirm the increase of the yield stress with the volume fraction of nonmagnetic particles. In addition, such an effect appears to be more important in Couette geometry, for which orientation fluctuations of the magnetic aggregates play a more significant role. Finally, a theoretical model based on this rotary diffusion mechanism is developed, providing with a quantitative explanation to the experimentally-observed trends.

International audience; The fluidizer molecules which are used in cement industry allow to increase the maximum volume fraction of mineral particles in order to improve the mechanical properties of the concrete, still keeping a good fluidity for facilitating the molding of the cement slurries. We study both experimentally and theoretically the effect of fluidizer molecules on the critical shear rate where an abrupt shear thickening transition occurs. A detailed analysis of the forces between polymer molecules adsorbed on the surface of the particles versus their gyration radius allow to predict the value of the critical shear rate. These predicted values are in quite good agreement with the experimental ones obtained in microgravity to rule out the effect of sedimentation. Interestingly, a kind of very regular stick-slip instability where ,at constant imposed stress, the shear rate increases slowly and then falls to zero abruptly before starting again, is observed before the jamming transition. This instability is interpreted through a model taking into account the proportion of frictional contacts between particles as a variable depending on the local stress.