arXiv subjects

Thomas Gibaud

Publications and source records attributed to Thomas Gibaud.

At least 19 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 [$g_2(q, \tau)$] 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, $g_2(q, \tau)$ 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 $g_2(q, \tau)$.

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arXiv2026

Hierarchical organization governs nonlinear mechanical reversibility in $\iota$-carrageenan gels

Carrageenan gels are thermoreversible polysaccharide networks whose mechanical properties emerge from ion-mediated helix association, yet how their molecular organization controls nonlinear deformation remains poorly understood. Here, we investigate the temperature-dependent rheology of $\iota$-carrageenan gels formed in KCl solutions using linear and nonlinear oscillatory rheology combined with normal-force measurements. $\iota$-carrageenan forms homogeneous and mechanically reversible gels whose elastic modulus increases continuously with quench depth. Deep quenches generate pronounced strain stiffening before yielding, associated with the development of internal stresses revealed by negative normal forces. Remarkably, large deformations preserve the small-strain elastic modulus while progressively suppressing strain stiffening, demonstrating a partial mechanical reversibility of the network. We interpret these observations using a hierarchical network picture in which a persistent intermolecular network controls linear elasticity, while a more fragile mesoscale organization enables cooperative alignment and stress amplification under deformation. Our results highlight that nonlinear mechanics of thermoreversible polysaccharide gels are governed not only by molecular connectivity, but also by the reversible formation and eventually destruction of mechanically adaptive hierarchical structures.

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arXiv2026

Unmasking the internal structure of casein micelles through enzymatic hydrolysis: A SAXS study

Casein micelles, one of the most studied natural association colloids, are supramolecular assemblies of caseins and colloidal calcium phosphate that constitute the fundamental building blocks of dairy matrices. Despite extensive investigation, the internal structure of casein micelles remains debated. While $\kappa$-casein is known to ensure colloidal stability of casein micelle suspension, the spatial organization of casein fractions and salts is still unresolved, and several structural models coexist. Small-angle scattering is a method of choice to probe biological colloids \emph{in situ}, yet interpretation of scattering data remains challenging due to the hierarchical nature of casein micelles. Here we address this issue by probing micelle structure during enzymatic gelation induced by chymosin, which cleaves $\kappa$-casein and triggers aggregation. Using time-resolved SAXS, we probe structural changes throughout the sol-gel transition over length scales from 3 nm to 3 $\mu$m. First, we show that enzyme-driven aggregation, counterintuitively, reveals information about the internal organization of casein micelles: the reduction of specific surface area during gelation unmasks a high-q structural peak previously observed under contrast-matching conditions. Second, we report that $\kappa$-casein cleavage leads to a gradual disappearance of the structural feature at intermediate scales. Analysis of the disappearance kinetics and comparison to structural models reveal that $\kappa$-casein cleavage induces a progressive relaxation of the colloidal porous substructure, providing direct evidence for its contribution to micellar organization. More broadly, these results demonstrate that the gelation process provides unique access to the internal structure of biological colloids and offers new perspectives for interpreting scattering data in complex soft-matter systems.

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arXiv2026

Isosbestic points in time resolved SAXS: from spectroscopic analogy to model free structural markers during colloidal gelation

Gelation is the transition from a fluid state into a system-spanning, out of equilibrim soft-solid network through a hierarchical process that couples local particle interactions to mesoscopic clustering and global connectivity. In time-resolved small-angle X-ray scattering (SAXS), isosbestic points -- scattering wavevectors where scattering intensity remains invariant -- emerge during this transformation, yet their physical meaning has remained unclear. Here, we show that two isosbestic points, $q_1$ and $q_2$, observed during salt-induced gelation of Ludox colloids, reflect fundamental structural constraints rather than a two-species interconversion. The high-$q$ point $q_2$ is a universal geometric marker, determined by particle contact distances, while the low-$q$ point $q_1$ arises from Porod invariant conservation and separates rapidly arrested local clusters from the growing mesoscopic network. By decomposing the Porod invariant across the reciprocal-space regions defined by these points, we define a dimensionless parameter, $\Phi(t/t_g)$, whose sigmoidal evolution provides a simple, model-free, scale-resolved fingerprint of gelation. Together with the combined evolution of $S(q_{\min},t)$ and $S(q \rightarrow 0,t)$, these results establish a quantitative model free framework linking local structuring, global connectivity, and scattering signatures, clarifying the role of isosbestic points in soft-matter transformations.

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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 $\tau \propto q^{-1}$ at all strains, where the decorrelation velocity $v_\tau = 1/\langle q\tau \rangle$ scales linearly with the loss tangent $\tan\delta = G''/G'$, establishing $\tan\delta$ as a direct macroscopic signature of the rate of irreversible structural decorrelation; (ii)~functional form continuous evolution from compressed exponential ($\alpha \simeq 1.5$) at low strain, consistent with three-dimensional dipolar strain fields in the intact network-to stretched exponential ($\alpha \simeq 0.5$) 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.

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arXiv2026

In vivo measurements of fascia lata effective mechanics combined to a memory fiber recruitment viscoelastic modeling approach

The fascia lata plays a central role in force transmission and body mechanics, yet its in vivo mechanical behavior remains poorly characterized. Existing approaches -- shear wave elastography and direct force measurements alike -- share a fundamental limitation: none simultaneously captures both the elastic and viscous components of fascial mechanics within a single experiment. The primary aim of this study is therefore to develop an experimental and modeling framework that enables the reproducible measurement of the effective viscoelastic properties of the fascia lata in vivo. To this end, we combine controlled ramp-relaxation experiments on the human fascia lata with a constitutive model that integrates fiber recruitment and dual-timescale viscoelastic relaxation. We emphasize that this is an effective model: rather than describing intrinsic local material properties, it characterizes the mechanical response of the fascia lata complex including its coupling to the hip-thigh musculoskeletal system under controlled loading conditions. The model captures both the nonlinear stiffening during elongation and the dual decay of force during relaxation, using a minimal set of physically interpretable parameters. Repeated trials demonstrate good reproducibility, with parameter variability within 10%. Our results support the view that fascia lata behaves as a hierarchical, hydrated composite whose macroscopic mechanical response emerges from the coupled effects of collagen alignment, matrix viscoelasticity, and fluid flow. This work provides a quantitative foundation for future in vivo investigations into how training, rehabilitation, or aging influence the evolution of fascial mechanical properties.

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arXiv2026

Acoustic modulation of shear thickening transition in dense adhesive suspensions

Discontinuous shear thickening (DST) in dense suspensions leads to flow instabilities that limit processing in many systems. While high-power ultrasound has been reported to reduce the apparent viscosity of such materials, the origin of this effect remains unclear. Here, we investigate dense adhesive cornstarch suspensions, where shear thickening arises from fragile, load-bearing force networks embedded in heterogeneous density-wave structures. Using a rheo-ultrasound setup, we show that ultrasound does not directly reduce viscosity but instead shifts the shear-thickening transition toward higher shear rates. This is evidenced by the collapse of stress probability distributions onto master curves, revealing a continuous evolution toward more fluid-like states without a sharp threshold. We interpret these results through a separation of time scales, in which the suspension behaves as an effectively immobile porous medium subjected to high-frequency interstitial flows. Fluidization then arises from a combination of boundary slip, bulk destabilization of force networks by drag-force fluctuations, and localized acoustic streaming. Beyond these mechanisms, we propose that ultrasound modifies the stability of force networks by introducing fluctuating hydrodynamic forces at the pore scale. As a result, larger stresses or shear rates are required to sustain jammed states, leading to a continuous renormalization of the DST transition. These findings provide a consistent physical picture of acoustic fluidization in adhesive suspensions and establish ultrasound as a powerful tool to control discontinuous shear thickening in confined flows.

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arXiv2025

Imprinting Macroscopic Fracture during Gelation: A Mechanism for Tuning Colloidal Gels

Colloidal gels form through the sol-gel transition of attractive particle suspensions, where local aggregation leads to a space-spanning network with solid-like properties. Their microstructure and mechanical properties are highly sensitive to external perturbations, which can substantially alter the pathway of network formation. Here, we investigate how nonlinear oscillatory shear affects the sol-gel transition of colloidal silica suspensions. Using large-amplitude oscillatory shear (LAOS), we vary both the strain amplitude and the duration of oscillatory forcing, varying between one and two times the gelation time. We find that sufficiently large strain amplitudes, or prolonged exposure to oscillations in the nonlinear regime, alter irreversibly the gel properties: the storage modulus $G'$ decreases while its frequency dependence remains unchanged. In contrast, the loss modulus $G''$, which decreases monotonically with frequency under quiescent gelation, exhibits an upturn at high frequencies when the gel is formed under strong oscillatory shear. The viscoelastic spectra of gels formed under quiescent conditions are well captured by a fractional Maxwell model, while gels formed under LAOS require an additional fractional element to account for damage-induced dissipation. Rheo-imaging experiments corroborate this interpretation by revealing the growth of cracks in gels formed under LAOS. We further show that these gels display a progressively more ductile nonlinear response for prolonged exposure to LAOS during gelation. These results demonstrate that the interplay between non-linear shear and gelation can permanently imprint a macroscopic fracture pattern into colloidal gels, offering a route to tune their viscoelastic properties.

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arXiv2025

Shear-driven memory effects in carbon black gels

In recent years, significant effort has been devoted to developing smart materials whose mechanical properties can adapt under physical stimuli. Particulate colloidal gels, which behave as solids but can also flow under stress, have emerged as promising candidates. Resulting from the attractive interaction between their constituents, their network architecture exhibit solid-like properties even at very low volume fractions. This structural flexibility allows them to adopt various configurations and store structural information making them highly susceptible to memory effects. Shear flow, applied through rheometry, offers a simple and effective way to tune their properties and imprint a ``rheological memory'' of the flow history. However, the precise relationship between flow history and viscoelastic response remains elusive, largely due to the limited structural characterization of these systems during flow and after flow cessation. Here, we use ultra-small angle X-ray scattering (USAXS) to reveal a strong structural memory in the solid state, where the microstructure formed under shear is retained after flow cessation. We identify two distinct mechanisms of structural memory, as governed by the ratio of viscous to attractive forces, namely, the Mason number. Using recently developed fractal scaling laws, we show that the rheology is fully determined by the gel microstructure. Notably, these gels exhibit a double-fractal architecture, highlighting the remarkably broad range of length scales over which these disordered materials are structured. By clarifying how memory is encoded, our results offer strategies to tune shear sensitivity of colloidal gels and design smart materials.

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arXiv2025

Anti-thixotropic dynamics in attractive colloidal dispersions: a shear restructuring driven by elastic stresses

Due to rich rheological properties, dispersions of attractive colloidal particles are ubiquitous in industries. Specifically, upon experiencing a sudden reduction in shear rate, these dispersions may exhibit transient behaviors such as thixotropy-where viscosity increases over time-and anti-thixotropy, characterized by an initial viscosity decrease before reaching a steady state. While thixotropy has been described as a competition between structure buildup and disruption, the mechanisms of anti-thixotropy remain poorly understood. Here, we investigate the anti-thixotropic dynamics of carbon black particles dispersed in oil-a system known for exhibiting anti-thixotropy-through flow step-down experiments. Using a multi-technique approach combining rheology with velocimetry and structural characterizations, we show that viscosity decrease results from a decrease in wall slip concomitant to shear-induced structural rearrangements, indicating a transition from a dynamical network of fractal clusters into a network of loosely connected dense agglomerates. Additionally, after a characteristic anti-thixotropic time $\tau$, a steady flow is reached. This time $\tau$ diverges with increasing shear rate at a critical value corresponding to a Mason number of one, indicating that anti-thixotropy occurs only when colloidal attraction outweighs viscous forces. More precisely, we show that the structural rearrangement underpinning the viscosity decrease is mediated by elastic stresses $\sigma_e$, such that $\tau \propto \sigma_e^{-3}$. Finally, on long time scales, the steady state is linked to a microstructure with nearly zero yield stress, indicating a loss of flow memory. These findings provide a mechanism for anti-thixotropy and suggest pathways for controlling viscosity and yield stress in attractive colloidal dispersions.

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arXiv2024

Viologen-based supramolecular crystal gels: gelation kinetics and sensitivity to temperature

Supramolecular crystal gels, a subset of molecular gels, form through self-assembly of low molecular weight gelators into interconnecting crystalline fibers, creating a three-dimensional soft solid network. This study focuses on the formation and properties of viologen-based supramolecular crystalline gels. It aims to answer key questions about the tunability of network properties and the origin of these properties through in-depth analyses of the gelation kinetics triggered by thermal quenching. Experimental investigations, including UV-Vis absorption spectroscopy, rheology, microscopy and scattering measurements, contribute to a comprehensive and self-consistent understanding of the system kinetics. We confirm that the viologen-based gelators crystallize by forming nanometer radius hollow tube that assemble into micro to millimetric spherulites. We then show that the crystallization follows the Avrami theory and is based on pre-existing nuclei. We also establish that the growth is interface controlled leading to the hollow tubes to branch into spherulites with fractal structures. Finally, we demonstrate that the gel properties can be tuned depending on the quenching temperature. Lowering the temperature results in the formation of denser and smaller spherulites. In contrast, the gels elasticity is not significantly affected by the quench temperature, leading us to hypothesize that the spherulites densification occurs at the expense of the connectivity between spherulite.

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arXiv2024

Two-step aging dynamics in enzymatic milk gels

Colloidal gels undergo a phenomenon known as physical aging, i.e., a continuous change of their physical properties with time after the gel point. To date, most of the research effort on aging in gels has been focused on suspensions of hard colloidal particles. In this letter, we tackle the case of soft colloidal "micelles" comprised of proteins, where gelation is induced by the addition of an enzyme. Using time-resolved mechanical spectroscopy, we monitor the viscoelastic properties of a suspension of colloidal micelles through the sol-gel transition and its subsequent aging. We show that the microscopic scenario underpinning the macroscopic aging dynamics comprises two sequential steps. First, the gel microstructure undergoes rapid coarsening, as observed by optical microscopy, followed by arrest. Second, aging occurs solely through a contact-driven mechanism, as evidenced by the square-root dependence of the yield stress with the elastic modulus measured at different ages of the gel. These results provide a comprehensive understanding of aging in enzymatic milk gels, which is crucial not only for a broad range of dairy products, but also for soft colloids in general.

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arXiv2024

Attractive carbon black dispersions: structural and mechanical responses to shear

The rheological behavior of colloidal dispersions is of paramount importance in a wide range of applications, including construction materials, energy storage systems and food industry products. These dispersions consistently exhibit non-Newtonian behaviors, a consequence of intricate interplays involving colloids morphology, volume fraction, and inter-particle forces. Understanding how colloids structure under flow remains a challenge, particularly in the presence of attractive forces leading to clusters formation. In this study, we adopt a synergistic approach, combining rheology with ultra small-angle X-ray scattering (USAXS), to probe the flow-induced structural transformations of attractive carbon black (CB) dispersions and their effects on the viscosity. Our key findings can be summarized as follow. First, testing different CB volume fractions, in the high shear rate hydrodynamic regime, CB particles aggregate to form fractal clusters. Their size conforms to a power law of the shear rate, $\xi_c \propto \dot{\gamma}^{-m}$, with $m\simeq 0.5$. Second, drawing insights from the fractal structure of clusters, we compute an effective volume fraction $\phi_{\mathrm{eff}}$ and find that microstructural models adeptly account for the hydrodynamic stress contributions. We identify a critical shear rate $\dot{\gamma^*}$ and a critical volume fraction $\phi_{\mathrm{eff}}^{*}$, at which the clusters percolate to form a dynamical network.

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arXiv2023

Shear-induced reinforcement in boehmite gels: a rheo-X-ray-scattering study

Boehmite, an aluminum oxide hydroxide $\gamma$-AlO(OH), is broadly used in the form of particulate dispersions in industrial applications, e.g., for the fabrication of ceramics and catalyst supports or as a binder for extrusion processes. Under acidic conditions, colloidal boehmite dispersions at rest form gels, i.e., space-spanning percolated networks that behave as soft solids at rest, and yet yield and flow like liquids under large enough deformations. Like many other colloidal gels, the solid-like properties of boehmite gels at rest are very sensitive to their previous mechanical history. Our recent work [Sudreau et al., J. Rheol. 66, 91-104 (2022), and Phys. Rev. Material 6, L042601 (2022)] has revealed such \textit{memory effects}, where the shear experienced prior to flow cessation drives the elasticity of boehmite gels: while gels formed following application of a shear rate $\dot\gamma_{\rm p}$ larger than a critical value $\dot\gamma_{\rm c}$ are insensitive to shear history, gels formed after application of $\dot\gamma_{\rm p}<\dot\gamma_{\rm c}$ display reinforced viscoelastic properties and non-negligible residual stresses. Here, we provide a microstructural scenario for these striking observations by coupling rheometry and small-angle X-ray scattering. Time-resolved measurements for $\dot\gamma_{\rm p} <\dot\gamma_{\rm c}$ show that scattering patterns develop an anisotropic shape that persists upon flow cessation, whereas gels exposed to $\dot\gamma_{\rm p}>\dot\gamma_{\rm c}$ display isotropic scattering patterns upon flow cessation. Moreover, as the shear rate applied prior to flow cessation is decreased below $\dot\gamma_{\rm c}$, the level of anisotropy frozen in the sample microstructure grows similarly to the viscoelastic properties, thus providing a direct link between mechanical reinforcement and flow-induced microstructural anisotropy.

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arXiv2022

Three length scales colloidal gels: the clusters of clusters versus the interpenetrating clusters approach

Typically, in quiescent conditions, attractive colloids at low volume fractions form fractal gels structured into two length scales: the colloidal and the fractal cluster scales. However when flow interfere with gelation colloidal fractal gels may display three distinct length scales [Dag\`es, et al., Soft Matter 18, 6645 (2022)]. Following those recent experimental investigations, we derive two models that account for the structure and the rheological properties of such atypical colloidal gels. The gel elasticity is inferred from scaling arguments and the structure is translated into scattering intensities following the global scattering functions approach proposed by Beaucage and typically measured in small angle X-ray scattering (SAXS). In both models, we consider that the colloids condensate into fractal clusters. In the clusters of clusters model, the clusters form superagregates which then build the gel network. In the interpenetrating clusters model, the clusters interpenetrate one-another to form the gel network. Those two models are then utilised to analyse rheo-SAXS experiments carried out on carbon black gels formed through flow cessation. The results of the analysis vouch for the clusters of clusters model with a densification of the structures as the gel characteristic length scales increase.

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arXiv2022

Bonded straight and helical flagellar filaments form ultra-low-density glasses

We study how the three-dimensional shape of rigid filaments determines the microscopic dynamics and macroscopic rheology of entangled semi-dilute Brownian suspensions. To control the filament shape we use bacterial flagella, which are micron-long helices assembled from flagellin monomers. We compare the dynamics of straight rods, helical filaments, and shape diblock copolymers composed of seamlessly joined straight and helical segments. Caged by their neighbors, straight rods preferentially diffuse along their long axis, but exhibit significantly suppressed rotational diffusion. Entangled helical filaments escape their confining tube by corkscrewing through the dense obstacles created by other filaments. By comparison, the adjoining segments of the rod-helix shape-diblocks suppress both the translation and the corkscrewing dynamics, so that shape-diblocks become permanently jammed at exceedingly low densities. We also measure the rheological properties of semi-dilute suspensions and relate their mechanical properties to the microscopic dynamics of constituent filaments. In particular, rheology shows that an entangled suspension of shape rod-helix copolymers forms a low-density glass whose elastic modulus can be estimated by accounting for how shear deformations reduce the entropic degrees of freedom of constrained filaments. Our results demonstrate that the three-dimensional shape of rigid filaments can be used to design rheological properties of semi-dilute fibrous suspensions.

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arXiv2022

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation

Colloidal gels are out of equilibrium soft solids composed of attractive Brownian particles that form a space-spanning network at low volume fractions. The elastic properties of these systems result from the network microstructure, which is very sensitive to shear history. Here, we take advantage of such sensitivity to tune the viscoelastic properties of a colloidal gel made of carbon black nanoparticles. Starting from a fluidized state under an applied shear rate $\dot γ_0$, we use an abrupt flow cessation to trigger a liquid-to-solid transition. We observe that the resulting gel is all the more elastic when the shear rate $\dot γ_0$ is low and that the viscoelastic spectra can be mapped on a master curve. Moreover, coupling rheometry to small angle X-ray scattering allows us to show that the gel microstructure is different from gels solely formed by thermal agitation where only two length scales are observed: the dimension of the colloidal and the dimension the fractal aggregates. Competition between shear and thermal energy leads to gels with three characteristic length scales. Such gels structure in a percolated network of fractal clusters that interpenetrate each other. Experiments on gels prepared with various shear histories reveal that cluster interpenetration increases with decreasing values of the shear rate $\dot γ_0$ applied before flow cessation. These observations strongly suggest that cluster interpenetration drives the gel elasticity, which we confirm using a structural model. Our results, which are in stark contrast with previous literature, where gel elasticity was either linked to cluster connectivity or to bending modes, highlight a novel local parameter controlling the macroscopic viscoelastic properties of colloidal gels.

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arXiv2022

Photoredox Processes in the Aggregation and Gelation of Electron-responsive Supramolecular Polymers Based on Viologens

Viologen-based ditopic bis-pyridinyl-triazole bidentate ligands self-assemble in the presence of palladium ions into supramolecular polymers whose structure is imposed by the directed formation of coordination bonds. Light-irradiation of these electron-responsive supramolecular materials triggers a photo-induced electron transfer yielding isolated π-radicals and dimers of radicals. The photoreduction events and the associated dimerization steps trigger a large-scale reorganization occurring within the supramolecular network yielding aggregates or gels depending on the irradiation conditions (power, duration). Detailed electrochemical, spectro-electrochemical and photochemical analyses were conducted to understand the mechanisms at stakes in these light-induced aggregation and gelation.

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