Pascal Hébraud's research while affiliated with University of Strasbourg and other places

The very early age flocculation of a cement based paste, in which cement is partially substituted by a calcined clay, metakaolin, is studied. We use rheological and ultrasonic reflection to monitor the evolution of the elastic properties of the paste with time, and light scattering to follow the change of its relaxation modes at small scales. We show that, at very early ages, of the order of hundreds of seconds after the paste preparation, a network of flocculated particles establishes, which manifests by a slow down of the dynamics of the relaxation modes of the paste at small scale. Then, this network consolidates and the macroscopic elastic modulus of the paste progressively increases with time, leading eventually to the setting of the paste. These observations show that, whatever the degree of replacement of clinker by metakaolin, a connected network establishes at very early times after the paste preparation, although the kinetics of flocculation slows down with clinker replacement.

The aggregation behavior of an attractive colloidal silica suspension under oscillatory flow is studied using rheological measurement. We show that the competition between the aggregation of the particles and the aggregate breakup under external stress leads to a non-monotonous evolution of the elastic modulus with time. Remarkably, under certain conditions, the elasticity is not an increasing function of time but exhibits a maximum. The value of the maximum of the elastic modulus depends on the applied shear amplitude and the ionic strength of the suspension. Scaling laws that describes the evolutions of the elastic modulus as a function of the salinity and of the deformation amplitude are proposed and discussed.

The mechanical nonlinear response of dense Brownian suspensions of polymer gel particles is studied experimentally and by means of numerical simulations. It is shown that the response to the application of a constant shear rate depends on the previous history of the suspension. When the flow starts from a suspension at rest, it exhibits an elastic response followed by a stress overshoot and then a plastic flow regime. Conversely, after flow reversal, the stress overshoot does not occur, and the apparent elastic modulus is reduced while numerical simulations reveal that the anisotropy of the local microstructure is delayed relative to the macroscopic stress.

Molecular cloning is a routine yet essential technique. Its efficiency relies on ligation and can be greatly improved when using a procedure known as Golden Gate Assembly (GGA). Essential to GGA are type IIS enzymes that have the unique property to cleave downstream their recognition sequence and thus generate any non-palindromic overhangs. Today, GGA benefits from new engineered enzymes with enhanced activity. Concomitantly, high throughput GAG assays (involving the simultaneous study of all overhangs at a time) have proposed optimal GGA substrates with high efficiencies and fidelities. Surprisingly, those assays show either no or unexpected correlation between ligation efficiencies and overhang stabilities. To explain those observations, we present here experiments involving one or two substrates (overhangs) only. When performing GGA at a stable temperature of 37°C, we found that GGA efficiency strongly correlates with overhang stability. Combining those experimental results with a kinetic model, we were able to determine how relevant parameters (time, temperature, molarity, stoichiometry, stacking energy) influence GGA. This work provides a comprehensive view of low-assembly GGA, defines the required optimal conditions and substrates (allowing the fabrication of constructs for single-molecule experiments with unprecedented yields) and gives new insights into DNA ligation that are crucial in biology.

In the last decades, in order to reduce the energy cost and the CO 2 release of cementitious pastes, substitutes of Portland cement such as latent hydraulic or supplementary cementitious materials (SCM) have been developed. Metakaolin is one of the SCM commonly used due to its friendly environmental characteristics, its abundance in nature and its high performance in combination with portland cement, improving concrete properties such as compressive strength and durability. Nevertheless, the use of metakaolin modifies the properties of fresh cement composites, reduces the workability, and increases the water demand. From the rheological point of view, metakaolin blended cement-based materials are yield stress fluids and their behavior depends on the contact surface among particles. At very short-time scale, flocculation dominates the yield stress behavior with low-level of hydration. In addition, the main forces in the system at rest involve colloidal attractive forces, Brownian motion, and gravity forces. The dynamics of the metakaolin blended cement is determining the evolution of flocculation and drives the macroscopic rheological behavior. Even though there are studies of the performance of metakaolin blended cement at early ages, publications reporting the short-time fluctuations are not common. The aim of this research is to study the dynamics of metakaolin blended cement on a very short-time scale. Multi speckle diffusing wave spectroscopy (MS-DWS) is used to measure the temporal fluctuations of a single speckle pattern, obtained the scattered light through the paste with a high-resolution camera. The motion of particles is described by the decay of the autocorrelation function at several times after cement paste mixing. Metakaolin blended cement pastes with a solid volume fraction of 0.35 and with partial replacements of white portland cement ranging 0 to 50% have been prepared. The results show fast decay time plots at a very short-time scale with an acceleration over time. We show that the higher is the metakaolin ratio the decay time is the larger than the pure cement paste, due to a faster solidification. Acknowledgment:

The mechanical non-linear response of dense Brownian suspensions of polymer gel particles is studied experimentally and by means of numerical simulations. It is shown that the response to the application of a constant shear rate depends on the previous history of the suspension. When the flow starts from a suspension at rest, it exhibits an elastic response followed by a stress overshoot and then a plastic flow regime. Conversely, after flow reversal, the stress overshoot does not occur, and the apparent elastic modulus is reduced while numerical simulations reveal that the anisotropy of the local microstructure is delayed relative to the macroscopic stress.

Controlling supramolecular polymerization is of fundamental importance to create advanced materials and devices. Here we show that the thermodynamic equilibrium of Gd3+-bearing supramolecular rod networks is shifted reversibly at room temperature in a static magnetic field of up to 2 T. Our approach opens opportunities to control the structure formation of other supramolecular or coordination polymers that contain paramagnetic ions.

We develop and characterize a wide angle static and dynamic light scattering under shear setup. The apparatus is suitable for the study of the structure and the dynamics of soft materials systems with a sub-micron characteristic length scale. The shear device consists in two parallel plates, and the optical setup allows us to perform light scattering measurements in any plane that contains the gradient of the velocity field direction. We demonstrate several capabilities of our apparatus: a measurement of the evolution with shear of the first peak of the structure factor of a concentrated suspension of spherical particles, both in the compression and extension quadrants of the shear flow, and the measurement of the velocity profile in dynamic light scattering. We present a theoretical treatment of light scattering under flow that takes into account the Gaussian character of the illumination and detection optical paths, in the case where the scattering volume extension is smaller than the gap of the flow cell, and compare with experimental measurements.

We investigate the flow of a concentrated suspension of colloidal particles at deformation rates higher than the discontinuous shear-thickening transition shear rate. We show that, under its own weight, a jet of a concentrated enough colloidal suspension, simultaneously flows while it sustains tensile stress and transmits transverse waves. This results in a new flow instability of jets of shear-thickening suspensions: the jet is submitted to rapid transverse oscillations, that we characterize.

Understanding and controlling supramolecular polymerization are of fundamental importance to create advanced materials and devices. Many stimuli have been explored in the past decades, but magnetic fields and field gradients have received little attention. This is because magnets do not provide enough magnetic energy to overcome thermal noise at the single molecule level. Here we show that significant changes in network topology of Gd³⁺-decorated supramolecular polymer rods can nevertheless be observed using magnetic fields of order 1 T at room temperature. The structure of the rod networks is influenced during a slow diffusive process over a timescale of hours by the anisotropy of the demagnetizing field. Our approach opens opportunities to control and tune structure formation of many supramolecular and coordination polymers using a variety of rare earth or other paramagnetic ions.