arXiv subjects

Frédéric Pignon

Publications and source records attributed to Frédéric Pignon.

5 recordsLinked to original sources

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↗

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.

cond-mat.soft↗

arXiv2021

Mechanics and structure of carbon black gels under high-power ultrasound

Colloidal gels made of carbon black particles dispersed in light mineral oil are "rheo-acoustic" materials, i.e., their mechanical and structural properties can be tuned using high-power ultrasound, sound waves with submicron amplitude and frequency larger than 20~kHz . The effects of high-power ultrasound on the carbon black gel are demonstrated using two experiments: rheology coupled to ultrasound to test for the gel mechanical response and a timeresolved ultra small-angle X-ray scattering experiment (TRUSAXS) coupled to ultrasound to test for structural changes within the gel. We show that high-power ultrasound above a critical amplitude leads to a complex viscoelastic transient response of the gels within a few seconds: a softening of its storage modulus accompanied by a strong overshoot in its loss modulus. Under high-power ultrasound, the gel displays a viscoelastic spectrum with glass-like features and a significant decrease in its yield strain. Those effects are attributed to the formation of intermittent micro-cracks in the bulk of the gel as evidenced by TRUSAXS. Provided that the shear rate is not large enough to fully fluidize the sample, high-power ultrasound also facilitates the flow of the gel, reducing its yield stress as well as increasing the shear-thinning index, thanks again to the formation of micro-cracks.

cond-mat.soft↗

arXiv2020

Rheo-acoustic gels: Tuning mechanical and flow properties of colloidal gels with ultrasonic vibrations

Colloidal gels, where nanoscale particles aggregate into an elastic yet fragile network, are at the heart of materials that combine specific optical, electrical and mechanical properties. Tailoring the viscoelastic features of colloidal gels in real-time thanks to an external stimulus currently appears as a major challenge in the design of "smart" soft materials. Here we introduce "rheo-acoustic" gels, a class of materials that are sensitive to ultrasonic vibrations. By using a combination of rheological and structural characterization, we evidence and quantify a strong softening in three widely different colloidal gels submitted to ultrasonic vibrations (with submicron amplitude and frequency 20-500 kHz). This softening is attributed to micron-sized cracks within the gel network that may or may not fully heal once vibrations are turned off depending on the acoustic intensity. Ultrasonic vibrations are further shown to dramatically decrease the gel yield stress and accelerate shear-induced fluidization. Ultrasound-assisted fluidization dynamics appear to be governed by an effective temperature that depends on the acoustic intensity. Our work opens the way to a full control of elastic and flow properties by ultrasonic vibrations as well as to future theoretical and numerical modeling of such rheo-acoustic gels.

cond-mat.soft↗

arXiv2008

Structure of skimmed milk during ultrafiltration process probed by in-situ small angle X-ray scattering

The stability and mechanism underlying the formation of deposits of casein micelles during ultrafiltration process were investigated by small-angle (SAXS) and ultra small-angle X-ray scattering(USAXS) which allowed us to probe the structure of skimmed milk on an exceptionally wide range of length scales from 1 to 2000 nm. Frontal filtration cells were specially developed to probe the microstructure of deposits by SAXS during the separation process. The results revealed two characteristic length scales for the equilibrium structure with radius of gyrations Rg, about 100 nm and 5.6 nm, pertaining to the globular micelles and their non-globular internal structure respectively (Pignon et al. 2004). In-situ scattering measurements showed that the decrease of permeation flows is directly related to the deformation and compression of the micelles in the immediate vicinity of the membrane (figure 1). From absolute SAXS intensities, the concentration of the micelles in the deposit can be deduced reliably.

cond-mat.soft↗