arXiv subjects

Thibaut Divoux

Publications and source records attributed to Thibaut Divoux.

At least 19 recordsLinked to original sources

arXiv2026

Visualizing shear-induced structures in carbon black gels by tomo-rheoscopy

Suspensions of attractive particles form space-spanning networks that endow the suspension with solid-like behavior at rest. The microstructure of these colloidal gels depends sensitively on the shear history and on the path followed across the sol-gel transition, resulting in viscoelastic properties that can be tuned by shear. Here, we report in situ X-ray tomo-rheoscopy experiments on carbon black gels whose elastic properties exhibit a non-monotonic dependence on the shear intensity applied prior to flow cessation. By directly imaging the gel microstructure under a well-controlled rheological protocol, we reveal the emergence of pronounced structural heterogeneities extending from tens to hundreds of microns -- length scales far larger than those accessible by conventional scattering techniques such as Ultra-Small Angle X-ray Scattering. In particular, we show that only the low-shear reinforcement of elasticity correlates with a growing mesoscale correlation length, while high-shear strengthening occurs without detectable mesoscale reorganization. These observations demonstrate that flow memory in colloidal gels is not solely governed by local particle rearrangements, but is also encoded in a mesoscale structural organization extending up to 100 times the particle size. More broadly, this work highlights the power of X-ray tomo-rheoscopy to uncover large-scale structural signatures of flow history in soft materials, opening new perspectives to tailor their mechanical properties.

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arXiv2026

Gelation dynamics of charged colloidal rods: critical behaviour and time-connectivity superposition principle

Charged colloidal particles can self-assemble into gel networks upon screening of electrostatic repulsion by added salt. While gelation of spherical colloids has been extensively studied, much less is known about the gelation dynamics of anisotropic colloids. Here, we focus on cellulose nanocrystals (CNCs) as prototypical rigid, highly charged rod-like colloids. In aqueous solution with salt, CNCs display a rich phase diagram ranging from gel at low solid content to glassy phases at higher concentrations. Building on our previous work [Morlet-Decarnin et al., ACS Macro Lett., 2023, 12, 1733], we present an extensive study of the mechanical recovery dynamics of CNC suspensions following a strong shear. Time-resolved mechanical spectroscopy reveals a liquid-to-solid transition characterized by a well-defined critical gel point. The evolving viscoelastic spectra can be rescaled onto master curves, demonstrating a time-connectivity superposition principle and critical dynamics on both sides of the gel point. By varying the CNC weight fraction and salt concentration, we identify a boundary between gel and attractive glass states marked by clear changes in rheological observables, including the elastic and viscous moduli at the gel point and their high-frequency power-law exponents. Analysis of dynamic critical exponents and hyperscaling reveals pronounced asymmetry between pre-gel and post-gel dynamics and non-universal values of the dynamic exponent. These findings highlight gelation mechanisms specific to highly charged rod-like colloids and call for complementary microstructural characterization and theoretical modeling.

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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

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.

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arXiv2025

Impact of early-age exposure to power ultrasound on the micromechanical properties of hardened cement paste

Cement paste serves as the universal binder in concrete, formed by mixing water with Ordinary Portland Cement (OPC). In its fresh state, cement paste is a suspension in which the initial particle inventory continuously dissolves, while simultaneously, a growing population of colloidal calcium silica-hydrate (C-S-H) particles forms and progressively builds a percolated structure that densifies and hardens through hydration. Like most colloidal gels, cement paste is sensitive to external stimuli such as mechanical vibrations and temperature variations. Here, we show that power ultrasound (PUS) applied to OPC paste during the very early stages of hydration significantly alters the mechanical properties of the hardened material. Freshly prepared cement pastes are exposed to PUS at varying durations and intensities before curing for 28 days. Micro-indentation testing at the scale of tens of microns reveals that increasing PUS amplitude and prolonged PUS exposure degrade the micro-mechanical properties of hardened cement paste, making it more ductile. This softening behavior evolves continuously with exposure conditions and is consistent with an increase in porosity, likely caused by micro-cracks induced by PUS. This scenario is further supported by micro-scratch testing, which shows higher levels of acoustic emission in PUS-exposed samples. Finally, nano-indentation confirms that the properties of the individual phases composing the hardened paste -- low-density and high-density C-S-H -- remain largely unaffected. These findings offer valuable experimental insights into novel pathways for modifying the mechanical properties of reactive colloidal gels.

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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

Delayed recovery in a dense suspension of core-shell attractive particles

Soft particulate glasses are dense suspensions of jammed particles that flow like liquids under external shear and recover their solid-like properties almost instantly upon flow cessation. Here, we consider a dense suspension of core-shell attractive particles whose polymer brush allows for a delayed recovery that we monitor by time-resolved mechanical spectroscopy. Viscoelastic spectra recorded upon flow cessation show a striking power-law behavior and can be rescaled onto a master curve that hints at a time-connectivity superposition principle. Additionally, the non-linear response of this soft glass, measured at various aging times, shows a ductile-to-brittle transition, which suggests that the interactions between the particles display a progressive repulsive-to-attractive transition. These results depict an original scenario for the delayed recovery involving a time-dependent interaction potential in which attractive hydrophobic forces are only activated when neighboring particles deform their polymer shell and come in close contact.

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arXiv2024

Understanding Polymer-Colloid Gels: A Solvent Perspective Using Low-Field NMR

The present work emphasizes the relevance of low-field NMR relaxometry to investigate colloid-polymer hydrogels by probing water dynamics across a wide range of formulations between $\rm 10^{\circ}C$ and $\rm 80^{\circ}C$. By examining the temperature dependence of the transverse relaxation time $T_2$, we demonstrate a clear link between the NMR response and the rheological behavior of the hydrogels. In particular, we show that NMR relaxometry targeting the solvent provides reliable insights into the hydrogel microstructure and allows the detection of phase transitions and aging processes. Our findings suggest that this solvent-focused technique could greatly benefit the soft matter community, complementing other experimental methods in the study of gels.

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arXiv2024

Rheological properties of acid-induced carboxymethylcellulose hydrogels

Cellulose ethers represent a class of water-soluble polymers widely utilized across diverse sectors, spanning from healthcare to the construction industry. This experimental study specifically delves into aqueous suspensions of carboxymethylcellulose (CMC), a polymer that undergoes gel formation in acidic environments due to attractive interactions between hydrophobic patches along its molecular chain. We use rheometry to determine the linear viscoelastic properties of both CMC suspensions and acid-induced gels at various temperatures. Then, applying the time-temperature superposition principle, we construct master curves for the viscoelastic spectra, effectively described by fractional models. The horizontal shift factors exhibit an Arrhenius-like temperature dependence, allowing us to extract activation energies compatible with hydrophobic interactions. Furthermore, we show that acid-induced CMC gels are physical gels that display a reversible yielding transition under external shear. In particular, we discuss the influence of pH on the non-linear viscoelastic response under large-amplitude oscillatory shear. Overall, our results offer a comprehensive description of the linear and non-linear rheological properties of a compelling case of physical hydrogel involving hydrophobic interactions.

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arXiv2024

Discrepancies in dynamic yield stress measurements of cement pastes

The dynamic yield stress associated with the flow cessation of cement pastes is measured using a rheometer equipped with various shear geometries such as vane, helical, sandblasted co-axial cylinders, and serrated parallel plates, as well as with the mini-cone spread test. Discrepancies in yield stress values are observed for cement pastes at various volume fractions, with one to two orders of magnitude difference between vane, helical and mini-cone spread measurements on the one hand, and co-axial cylinder and parallel plate measurements on the other hand. To understand this discrepancy, the flow profile of a cement paste in the parallel-plate geometry is investigated with a high-speed camera, revealing the rapid formation of an un-sheared band near the static bottom plate. The width of this band depends upon the rotational velocity of the top plate, and upon the shear time. Recalculation of shear stress shows that the reduced sheared gap alone cannot explain the low measured yield stress. Further exploration suggests the formation of zones with lower particle content, possibly linked to cement particle sedimentation. Here, we argue that the complex nature of cement pastes, composed of negatively buoyant non-Brownian particles with attractive interactions due to highly charged nano-size hydration products, accounts for their complex rheological behavior.

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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

Textural properties of dense granular pastes produced by kneading

The efficiency of supported catalysts depends on the porous microstructure of their solid support, which regulates mass transfer, exposure to the active phase, and mechanical strength. Here, we focus on the manufacturing of a specific type of catalytic support, $\gamma$-alumina extrudates, by a kneading-extrusion process. In this process, a paste is initially formed by blending and subsequently kneading a boehmite powder, an aluminum oxide hydroxide precursor of $\gamma$-alumina, with various liquids before undergoing extrusion. The crucial step in this process is the kneading step, which allows control over the textural and mechanical properties of the extrudate. The present experimental study aims to measure the impact of the kneading step on boehmite pastes prepared using a pilot kneader. The pastes are obtained by mixing boehmite powder with an acid and a basic solution in a two-step process known as peptization and neutralization. In this study, kneading is conducted at various mixing speeds and durations while monitoring the torque exerted by the boehmite paste on the kneader blades. Moreover, samples are extracted from the pilot kneader at various stages of the kneading process, and their textural properties are determined by both nitrogen sorption and mercury intrusion porosimetry. Our findings show that, at fixed composition, the textural properties of boehmite pastes are controlled by the overall deformation accumulated during kneading. Conversely, for a fixed accumulated deformation, the pH sets the textural properties. Finally, we identify an empirical control parameter that captures the combined effects of pH and accumulated deformation on the key textural attributes of boehmite pastes. These results set the stage for a systematic design approach of boehmite-based catalyst supports.

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arXiv2023

Ductile-to-brittle transition and yielding in soft amorphous materials: perspectives and open questions

Soft amorphous materials are viscoelastic solids ubiquitously found around us, from clays and cementitious pastes to emulsions and physical gels encountered in food or biomedical engineering. Under an external deformation, these materials undergo a noteworthy transition from a solid to a liquid state that reshapes the material microstructure. This yielding transition was the main theme of a workshop held from January 9 to 13, 2023 at the Lorentz Center in Leiden. The manuscript presented here offers a critical perspective on the subject, synthesizing insights from the various brainstorming sessions and informal discussions that unfolded during this week of vibrant exchange of ideas. The result of these exchanges takes the form of a series of open questions that represent outstanding experimental, numerical, and theoretical challenges to be tackled in the near future.

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arXiv2023

Acid-induced gelation of carboxymethylcellulose solutions

The present work offers a comprehensive description of the acid-induced gelation of carboxymethylcellulose (CMC), a water-soluble derivative of cellulose broadly used in numerous applications ranging from food packaging to biomedical engineering. Linear viscoelastic properties measured at various pH and CMC contents allow us to build a sol-gel phase diagram, and show that CMC gels exhibit broad power-law viscoelastic spectra that can be rescaled onto a master curve following a time-composition superposition principle. These results demonstrate the microstructural self-similarity of CMC gels, and inspire a mean-field model based on hydrophobic inter-chain association that accounts for the sol-gel boundary over the entire range of CMC content under study. Neutron scattering experiments further confirm this picture and suggest that CMC gels comprise a fibrous network crosslinked by aggregates. Finally, low-field NMR measurements offer an original signature of acid-induced gelation from the solvent perspective. Altogether, these results open avenues for precise manipulation and control of CMC-based hydrogels.

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arXiv2023

Critical-like gelation dynamics in cellulose nanocrystal suspensions

We use time-resolved mechanical spectroscopy to offer a detailed picture of the gelation dynamics of cellulose nanocrystal (CNC) suspensions following shear cessation in the presence of salt. CNCs are charged, rodlike colloids that self-assemble into various phases, including physical gels serving as soft precursors for biosourced composites. Here, a series of linear viscoelastic spectra acquired across the sol-gel transition of CNC suspensions are rescaled onto two master curves, that correspond to a viscoelastic liquid state prior to gelation and to a soft solid state after gelation. These two states are separated by a critical gel point, where all rescaling parameters diverge in an asymmetric fashion, yet with exponents that obey hyperscaling relations consistent with previous works on isotropic colloids and polymer gels. Upon varying the salt content, we further show that these critical-like dynamics result in both time-connectivity and time-concentration superposition principles.

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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

The hidden hierarchical nature of soft particulate gels

Soft particulate gels include materials we can eat, squeeze, or 3D print. From foods to bio-inks to cement hydrates, these gels are composed of a small amount of particulate matter (proteins, polymers, colloidal particles, or agglomerates of various origins) embedded in a continuous fluid phase. The solid components assemble to form a porous matrix, providing rigidity and control of the mechanical response, despite being the minority constituent. The rheological response and gel elasticity are direct functions of the particle volume fraction $\phi$: however, the diverse range of different functional dependencies reported experimentally has, to date, challenged efforts to identify general scaling laws. Here we reveal a hidden hierarchical organization of fractal elements that controls the viscoelastic spectrum, and which is associated with the spatial heterogeneity of the solid matrix topology. The fractal elements form the foundations of a viscoelastic master curve, which we construct using large-scale 3D microscopic simulations of model gels, and can be described by a recursive rheological ladder model over a range of particle volume fractions and gelation rates. The hierarchy of the fractal elements provides the missing general framework required to predict the gel elasticity and the viscoelastic response of these ubiquitous complex materials.

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arXiv2022

Dual origin of viscoelasticity in polymer-carbon black hydrogels: a rheometry and electrical spectroscopy study

Nanocomposites formed by mixing nanoparticles and polymers offer a limitless creative space for the design of functional advanced materials with a broad range of applications in materials and biological sciences. Here we focus on aqueous dispersions of hydrophobic colloidal soot particles, namely carbon black (CB) dispersed with a sodium salt of carboxymethylcellulose (CMC), a food additive known as cellulose gum that bears hydrophobic groups, which are liable to bind physically to CB particles. Varying the relative content of CB nanoparticles and cellulose gum allows us to explore a rich phase diagram that includes a gel phase. We investigate this hydrogel using rheometry and electrochemical impedance spectroscopy. CB-CMC hydrogels display two radically different types of mechanical behaviors that are separated by a critical CMC-to-CB mass ratio $r_c$. For $r r_c$ CB-CMC gels display a power-law viscoelastic spectrum that depends strongly on the CMC concentration. These relaxation spectra can be rescaled onto a master curve that exhibits a power-law scaling in the high-frequency limit, with an exponent that follows Zimm theory, showing that CMC plays a key role in the gel viscoelastic properties for $r>r_c$. Our results offer a characterization of CB-CMC dispersions that will be useful for designing nanocomposites based on hydrophobic interactions.

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