arXiv subjects

William Chèvremont

Publications and source records attributed to William Chèvremont.

7 recordsLinked to original sources

arXiv2026

XPCS-Echo and broad relaxation measurements using a bunch-mode data acquisition scheme

Echoes observed in the intensity-intensity autocorrelation functions [$g_2(q, τ)$] is a powerful method for probing nonaffine deformations and yielding behavior of soft viscoelastic materials subjected to an oscillatory shear. Multispeckle X-ray photon correlation spectroscopy (XPCS) measurements of large number of echoes with high time resolution impose severe constraints in terms of the computational hardware and potential degradation of the sample. These issues are alleviated by implementing a bunch-mode data acquisition scheme in which the highest resolution frames or bunches centered at the echo peaks. In addition, by placing the bunches in an aperiodic Fibonacci sequence, $g_2(q, τ)$ over a long time span can be measured with orders of magnitude lesser number of frames. The performance of these XPCS acquisition schemes is demonstrated using slowly relaxing model colloidal suspensions. Furthermore, an analytical expression is provided for the quantitative description of full $g_2(q, τ)$.

cond-mat.soft↗

arXiv2026

Tracking microscopic irreversibility during yielding of a colloidal fractal gel with Rheo-Echo-XPCS

Understanding how microscopic structural dynamics relate to macroscopic mechanical response during yielding remains a central challenge in soft matter physics. Here, we introduce rheo-echo X-ray photon correlation spectroscopy (rheo-echo-XPCS) with nonlinear acquisition synchronized to oscillatory shear, enabling direct measurement of irreversible nanoscale dynamics under strain amplitude control. Applying this to a carbon black colloidal fractal gel, we resolve time-periodic echoes in the vorticity-direction intensity autocorrelation function whose decay encodes non-affine structural rearrangements. We find: (i)~ballistic-like decorrelation with $τ\propto q^{-1}$ at all strains, where the decorrelation velocity $v_τ= 1/\langle qτ\rangle$ scales linearly with the loss tangent $\tanδ= G''/G'$, establishing $\tanδ$ as a direct macroscopic signature of the rate of irreversible structural decorrelation; (ii)~functional form continuous evolution from compressed exponential ($α\simeq 1.5$) at low strain, consistent with three-dimensional dipolar strain fields in the intact network-to stretched exponential ($α\simeq 0.5$) at high strain, reflecting a dimensional reduction from $d_f = 3$ to $d_f = 1$ as stress transmission shifts from bulk to quasi-one-dimensional filamentary backbones during network fragmentation.

cond-mat.soft↗

arXiv2025

Coherent X-rays reveal anomalous molecular diffusion and cage effects in crowded protein solutions

Understanding protein motion within the cell is crucial for predicting reaction rates and macromolecular transport in the cytoplasm. A key question is how crowded environments affect protein dynamics through hydrodynamic and direct interactions at molecular length scales. Using megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (EuXFEL), we investigate ferritin diffusion at microsecond time scales. Our results reveal anomalous diffusion, indicated by the non-exponential decay of the intensity autocorrelation function $g_2(q,t)$ at high concentrations. This behavior is consistent with the presence of cage-trapping in between the short- and long-time protein diffusion regimes. Modeling with the $δγ$-theory of hydrodynamically interacting colloidal spheres successfully reproduces the experimental data by including a scaling factor linked to the protein direct interactions. These findings offer new insights into the complex molecular motion in crowded protein solutions, with potential applications for optimizing ferritin-based drug delivery, where protein diffusion is the rate-limiting step.

cond-mat.soft↗

arXiv2025

LAOStrain response of carbon black-polymer hydrogels: insights from rheo-TRUSAXS and rheo-electric experiment

Colloid-polymer hydrogels are encountered in various applications, from flow batteries to drug delivery. Here, we investigate hydrogels composed of hydrophobic colloidal soot particles -- carbon black (CB) -- and carboxymethylcellulose (CMC), a food-grade polymer functionalized with hydrophobic groups binding physically to CB. As described in [Legrand et al., Macromolecules 56, 2298-2308 (2023)], CB-CMC hydrogels exist in two flavors: either electrically conductive when featuring a percolated network of CB particles decorated by CMC, or insulating where isolated CB particles act as physical cross-linkers within the CMC matrix. We compare these two types of CB-CMC hydrogels under Large Amplitude Oscillatory Shear (LAOS), combining rheometry with Time-Resolved Ultra-Small-Angle X-ray Scattering (TRUSAXS) and electrical conductivity measurements. Both types of hydrogels exhibit a "type III" yielding scenario, characterized by an overshoot in G'' and a monotonic decrease in G', although the underlying microscopic mechanisms differ markedly. Conductive CB-CMC hydrogels display a yield strain (6%) concomitant with a drop in DC conductivity, indicative of the macroscopic rupture of the percolated CB network at length scales larger than a few microns, beyond USAXS resolution. At larger strain amplitudes, the conductivity of the fluidized sample increases again, exceeding its initial value, consistent with shear-induced formation of a transient, dynamically percolated network of CB clusters. In contrast, insulating CB-CMC hydrogels exhibit a larger yield strain (60%), beyond which the sample flows and the average distance between CB particles decreases. This reorganization is concomitant with a more than tenfold increase in conductivity, although it remains below that of conductive hydrogels at rest.

cond-mat.soft↗

arXiv2025

Softness and Hydrodynamic Interactions Regulate Lipoprotein Transport in Crowded Yolk Environments

Low-density lipoproteins (LDLs) serve as nutrient reservoirs in egg yolk for embryonic development and as promising drug carriers. Both roles critically depend on their mobility in densely crowded biological environments. Under these crowded conditions, diffusion is hindered by transient confinement within dynamic cages formed by neighboring particles, driven by solvent-mediated hydrodynamic interactions and memory effects -- phenomena that have remained challenging to characterize computationally and experimentally. Here, we employ megahertz X-ray photon correlation spectroscopy to directly probe the cage dynamics of LDLs in yolk-plasma across various concentrations. We find that LDLs undergo anomalous diffusion, experiencing $\approx$ 100-fold reduction in self-diffusion at high concentrations compared to dilute solutions. This drastic slowing-down is attributed to a combination of hydrodynamic interactions, direct particle-particle interactions, and the inherent softness of LDL particles. Despite reduced dynamics, yolk-plasma remains as a liquid, yet sluggish, balancing dense packing, structural stability, and fluidity essential for controlled lipid release during embryogenesis.

cond-mat.soft↗

arXiv2024

Normal viscosity and Viscous resuspension of non-Brownian suspensions

Normal stresses in sheared suspensions of non-Brownian particles are obtained from numerical simulations in the viscous regime. The stresses are determined in homogeneous shear of non-buoyant particles and by analyzing shear-induced resuspension of buoyant particles in the framework of the suspension balance model (SBM). The consistency of both approaches indicates that the SBM describes the steady state properly. The results are in agreement with some earlier empirical expressions of the normal viscosity coefficients in limited ranges of particle volume fractions, but they appear to overestimate the stresses in the semi-dilute regime (solid fraction in the range 25-35\%). New expressions are proposed for. We show that these discrepancies can be due to inertial effects which appear at rather low particle Reynolds number. The results also highlight that the normal stress anisotropy depends on the volume fraction.

cond-mat.soft↗

arXiv2019

Dynamics of Particle Migration in Confined ViscoElastic Poiseuille Flows

Particles migrate in the transverse direction of the flow due to the existence of normal stress anisotropy in weakly viscoelastic liquids. We test the ability of theoretical predictions to predict the transverse velocity migration of particles in a confined Poiseuille flow according to the viscoelastic constitutive parameters of dilute polymers solutions. Firstly, we carefully characterize the viscoelastic properties of two families of dilute polymer solutions at various concentrations using shear rheometry and capillary breakup experiments. Secondly, we develop a specific 3D particle tracking velocimetry method to measure with a high accuracy the dynamics of particles focusing in flow for Weissenberg numbers Wi ranging from $10^{-2}$ to $10^{-1}$ and particle confinement $β$ of 0.1 and 0.2. The results show unambiguously that the migration velocity scales as $\text{Wi}β^2$, as expected theoretically for weakly elastic flows of an Oldroyd-B liquid. We conclude that classic constitutive viscoelastic laws are relevant to predict particle migration in dilute polymer solutions whereas detailed analysis of our results reveals that theoretical models overestimate by a few tenth the efficiency of particle focusing.

physics.flu-dyn↗