Xavier Chateau

In highly filled particulate composites, the particle-size distribution and the resulting packing state limit how much reinforcement a formulation can carry and strongly influence its macroscopic stiffness, yet neither is easily incorporated into predictive micromechanical models. Classical schemes treat inclusions as abstract phases, missing both the maximum-packing (crowding) effect and the influence of particle-size polydispersity on the effective modulus; existing Morphologically Representative Pattern (MRP)-based extensions that do incorporate a finite packing fraction remain restricted to a single particle size. This paper extends the two-pattern MRP framework of [Marcadon, Hervé and Zaoui (2007)] to particle-size polydispersity by introducing a second composite pattern with a size-ratio parameter γ that captures the geometrical interaction between differently sized particles, linking measurable descriptors of the particle-size distribution and packing state directly to the effective stiffness within a single analytical formulation. The resulting scheme reduces exactly to the monodisperse MRP model when γ = 0, and to the Einstein dilute estimate as the particle volume fraction tends to zero. For bidisperse rigid particles in a yield-stress fluid, it reduces the linear-space mean squared error against experimental data from 0.0613 (best-fit empirical power law) to 0.0316; for bidisperse diamond particles in a metal matrix it achieves a mean squared error of 0.0014 against a small dataset, with predicted stiffness comparatively robust to the closure relation's calibration constants. The formulation thus provides an analytical link between particle-size distribution, packing state and effective stiffness, evaluable before more detailed microstructural simulations.

This work numerically explores the anisotropy, impact phase wave propagation, buckling resistance, and natural vibration of ultra-high performance concrete (UHPC) and UHPC-steel interpenetrating phase composite (IPC) with triply periodic minimal surfaces (TPMSs), including sheet and solid gyroid, primitive, diamond, and the Schoen I-graph-wrapped package (I-WP). The experiment is conducted to verify the accuracy of the numerical model in terms of Young's modulus of polylactic acid (PLA)-based TPMS lattices and PLA-cement IPCs with TPMS cores, with the highest percent difference of 15% found for IPCs and 17% found for lattice. The results indicate that UHPC material with sheet gyroid exhibits the least extreme anisotropy in response to the varying orientation among other lattices regardless of the change of solid density, making it the ideal candidate for construction materials. Interestingly, compared to UHPC-based TPMS lattice, IPCs possess a much smaller anisotropy and exhibit almost isotropy regardless the variation of solid density and TPMS topology, offering a free selection of TPMS type to fabricate IPCs without much care of anisotropy. The phase wave velocity and buckling resistance of UHPC- and IPC-based beams with TPMSs nonlinearly decrease with a drop of TPMS solid density, but it is the almost linear pattern for the case of natural vibration frequency. UHPC material and IPC with sheet gyroid lattice are found to possess the lowest phase wave velocity and exhibit the least anisotropy of wave propagation, showing it as an ideal candidate for UHPC material to suppress the destructive energy induced by the external impact.

The homogenization method is used to investigate temperature effects on the water content-capillary pressure relationship and on the poro-mechanical behavior of unsaturated porous media. Two different phenomena have been considered: the temperature variations of surface tensions and the thermal dilatation of the solid phase in the framework of micromechanical models that are proposed in order to highlight the influence of the deformability of the solid phase on the macroscopic behavior. The result show that taking into account the coupling between the deformation of the porous space and the capillary effects can radically modify the prediction of the temperature influence on the capillary curve.

We investigate the origin of yield stress aging in semidense, saline, and turbid suspensions in which structural evolution is rapidly arrested by the formation of thermally irreversible roll-resisting interparticle contacts. By performing optical tweezer three-point bending tests on particle rods, we show that these contacts yield by overcoming a rolling threshold, the critical bending moment of which grows logarithmically with time. We demonstrate that this time-dependent contact-scale rolling threshold controls the suspension yield stress and its aging kinetics. We identify a simple constitutive relation between the contact-scale flexural rigidity and rolling threshold, which transfers to macroscopic scales. This leads us to establishing a constitutive relation between macroscopic shear modulus and yield stress that is generic for an array of colloidal systems.

The ageing behaviour of dense suspensions or pastes at rest is almost exclusively attributed to structural dynamics. Here, we identify another ageing process, contact-controlled ageing, consisting of the progressive stiffening of solid–solid contacts of an arrested colloidal suspension. By combining rheometry, confocal microscopy and particle-scale mechanical tests using laser tweezers, we demonstrate that this process governs the shear-modulus ageing of dense aqueous silica and polymer latex suspensions at moderate ionic strengths. We further show that contact-controlled ageing becomes relevant as soon as Coulombic interactions are sufficiently screened out that the formation of solid–solid contacts is not limited by activation barriers. Given that this condition only requires moderate ion concentrations, contact-controlled ageing should be generic in a wide class of materials, such as cements, soils or three-dimensional inks, thus questioning our understanding of ageing dynamics in these systems.

From direct observations and MRI measurements we demonstrate that during the drying of a direct (oil in water) emulsion the whole system essentially concentrates homogeneously, which leads to shrinkage, without air penetration. The structure and mechanical strength (i.e. the elastic modulus) of this concentrated bulk are not significantly different from those of an emulsion directly prepared at this higher concentration. Despite this phenomenon, the drying rate continuously and rapidly decreases as the water content decreases, in contrast with the drying of a simple granular packing. This results from a concentration gradient which develops towards the free surface of the sample where the oil droplets finally coalesce, ultimately forming an oil layer covering the sample through which the water molecules have to diffuse before evaporating. Moreover, as during the process, the liquid is transported towards the free surface where it evaporates, surfactants accumulate and tend to form a thin solid layer below the oil layer, which tends to further reduce the drying rate.

Shear viscosity of bubbly liquids is known to depend on both the gas volume fraction and the capillary number. Here, we study the impact of confinement on their behavior by investigating the viscosity of semi-dilute bubbly liquid layers confined between two plates and characterized by a ratio of the undeformed bubble diameter to the layer thickness equal to or larger than unity. For all the studied confinement ratios, viscosity is shown to be smaller than the viscosity of the suspending liquid for capillary numbers larger than 0.1. Measurements of bubble deformations show that this behavior is related to bubble stretching in the direction of shear induced flow. In the limit of high capillary numbers, viscosity reaches values predicted for unconfined bubbly liquids. On the other hand, our results for smaller capillary numbers, i.e. within the range 0.001–0.1, reveal a non-monotonic variation of the viscosity as a function of the confinement ratio, exhibiting a well-defined maximum value for the ratio close to 1.8. This behavior differs strongly from the reference case of unconfined bubbly liquid, and it is shown to result from both bulk and wall drag forces on the squeezed bubbles.