Frédéric Pignon - Academia.edu (original) (raw)
Papers by Frédéric Pignon
The sound attenuation in the THz region is studied down to T=16 K in glassy glycerol by inelastic... more The sound attenuation in the THz region is studied down to T=16 K in glassy glycerol by inelastic x-ray scattering. At striking variance with the decrease found below 100 K in the GHz data, the attenuation in the THz range does not show any T dependence. This result i) indicates the presence of two different attenuation mechanisms, active respectively in
Dans le domaine des mousses composites, l’ajout de renfort dans la matrice polymère a de nombreus... more Dans le domaine des mousses composites, l’ajout de renfort dans la matrice polymère a de nombreuses conséquences aussi bien sur la formulation que sur les propriétés mécaniques et de structure de ces matériaux alvéolaires. Des mousses polyuréthane composites, formulées à partir de suspensions constituées de carbonate de calcium (CaCO3) et de silice colloïdale (SC), ont été étudiées. Le but de ce travail a été d'explorer d'une part l'influence des fractions volumiques en charge minérales (φcc) et colloïdales (φsc) sur les propriétés rhéométriques et sur la stabilité des mélanges avant moussage. D’autre part, ce travail traite de l’influence du renforcement sur les propriétés mécaniques et sur la structure des mousses solides. Pour les suspensions sans silice colloïdale, un seuil d’écoulement et une période de stabilité sont détectés pour une fraction volumique au delà d’environ 28,2% en CaCO3. Une concentration optimale en CaCO3 a été déterminée (φcc = 46,2%) du point de ...
Cette étude a pour but d'appréhender le comportement rhéologique sous cisaillement et sous co... more Cette étude a pour but d'appréhender le comportement rhéologique sous cisaillement et sous compression des dispersions d'argile concentrées par centrifugation. Cette technique induit un gradient de concentration et de pression au sein des dépôts permettant de déterminer les seuils de contrainte en compression en fonction de la fraction volumique des dispersions. Les propriétés rhéologiques en cisaillement ont, quant à elles, été mesurées à l'aide d'un scissomètre de petites dimensions partiellement immergé dans le dépôt de centrifugation afin de minimiser les effets de déstructuration des échantillons. La réponse des dispersions soumises à un gradient de cisaillement faible est suivie dans le temps. Ainsi, les contraintes de cisaillement, les modules de cisaillement et les déformations sous cisaillement ont été déterminés en fonction de la fraction volumique. Certaines propriétés mécaniques, telles que le coefficient de Poisson, le module de compression ou encore l&#...
Journal of Materials Science, 2014
Nanofibrillated cellulose (NFC) aqueous suspensions were produced from once-dried bisulfite softw... more Nanofibrillated cellulose (NFC) aqueous suspensions were produced from once-dried bisulfite softwood pulp using enzymatic or 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO)-mediated oxidation pretreatments, followed by wet grinding, as an ultimate fibrillation technique. Two commercial enzyme solutions: cellulase, with the major activity of endoglucanase and exoglucanase, and monocomponent endoglucanase, were compared to facilitate the nanofibrils isolation from cellulose fibers. The influence of their concentrations, as well as the other processing conditions, was analyzed. The morphology of the produced NFC was characterized using optical microscopy, atomic force microscopy (AFM), field emission gun-scanning electron microscopy (FEG-SEM), and morphological fiber analyzer (MorFi). Nanofibrils with a wide size distribution were produced. The average lateral dimensions of 12 ± 7 nm for the most disintegrated enzymatically hydrolyzed NFC and 4 ± 2 nm for TEMPO-oxidized NFC were determined from the AFM height images. The degree of polymerization (DP) decreased and the crystallinity index (CI) increased with an increase of the concentration of both enzyme solutions. TEMPO-oxidation did not have a significant impact on the cellulose CI; however, the DP was strongly affected. The monocomponent endoglucanase solution was found to have a better effect on the nanofibrils isolation rather than their depolymerization.
Rheologica Acta, 2007
We study the transient response to simple shear of aqueous dispersions of Laponite clay particles... more We study the transient response to simple shear of aqueous dispersions of Laponite clay particles and poly(ethylene oxide) at concentrations for which shear induces structure in the form of a network of polymer-clay bonds. We examine the effects of shear on the structure at the micrometer length scale. Bulk rheometric measurements give the material's response to step changes in shear rate. We find that a critical value of the shear rate separates two regions with different rheological behaviors. Static small-angle light scattering shows a corresponding qualitative change in the anisotropy of the dispersion under shear at the micron scale. We interpret our results in terms of the effects of shear on the interactions between clay particles and polymer chains and on the aggregation mechanisms in the dispersion.
Physical Review E, 1997
The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal... more The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal suspension of synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light scattering. At the same time, macroscopic mechanical behavior at rest was characterized by means of rheometric measurements. Two characteristic length scales were detected in these yield stress suspensions of discotic texture. The first, measuring several tens of nanometres, is linked to a fractal dimension of 3. The second, of the order of 1 m, is linked to a fractal behavior of dimension D that increases with the particle volume fraction. Consequently, it is suggested that the structure of the dispersions at rest is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 m. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous three-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The increase of this fractal dimension with the particle volume fraction, the ionic strength, and the gelation time is correlated to a hardening of the mechanical properties of the gels at rest. The gel state is reached above a volume fraction v * for a given ionic strength and gelation time. In the gel phase, a critical volume fraction vc separates two domains. Gels belonging to the domain v *Ͻ v Ͻ vc have a fractal behavior of dimension Dϭ1Ϯ0.05, suggesting an alignment of the micrometer-sized aggregates that leads to the formation of a mechanically weak fibrous structure. Gels belonging v Ͼ vc have a fractal dimension Dϭ1.8Ϯ0.01, corresponding to a mechanically stronger structure consisting of zones of high and lower particle density. A scaling law enabled these fractal dimensions to be correlated with the effect of the volume fraction on the yield stress. In contrast to what is commonly assumed in relation to clay suspensions, it is suggested here that it is the large length scales, of the order of 1 m, associated with a fractal arrangement that governs the macroscopic mechanical behavior. ͓S1063-651X͑97͒07209-7͔
Carbohydrate polymers, Jan 25, 2015
This study investigates for the first time the behaviors of starch and cellulose nanocrystals (SN... more This study investigates for the first time the behaviors of starch and cellulose nanocrystals (SNC and CNC) suspensions which are simultaneously subjected to pressure, shear flow and ultrasound (US) during cross-flow ultrafiltration. This multi-forces process was characterized from macro-scales to nano-scales, with a custom designed "SAXS Cross-Flow US-coupled Filtration Cell". In addition, rheological behaviors of SNC samples at different concentrations/temperatures have been investigated. In both cases (ultrafiltration of SNC and CNC suspensions), better performances were observed with US. The in-situ SAXS measurements revealed that for SNC suspensions, no structure change occurred at the length scales range from 10 to 60nm in this multi-forces process, while CNC particles exhibited an ordered arrangement within the concentrated layer during the same process. SNC particles accumulated on the membrane surface forming a "fragile" concentrated layer which was remo...
Carbohydrate polymers, Jan 4, 2014
The rheological properties of enzymatically hydrolyzed and TEMPO-oxidized microfibrillated/nanofi... more The rheological properties of enzymatically hydrolyzed and TEMPO-oxidized microfibrillated/nanofibrillated cellulose (MFC/NFC) aqueous suspensions were investigated in oscillation and steady-flow modes and were compared with the morphology of the studied materials. The flow instabilities, which introduce an error in the rheological measurements, were discovered during flow measurements. A wall-slip (interfacial slippage on the edge of geometry tools and suspension) was detected at low shear rates for two types of NFC suspensions while applying cone-plate geometry. A roughening of the tool surfaces was performed to overcome the aforementioned problem. Applying to TEMPO-oxidized NFC, a stronger suspension response was detected at low shear rates with higher values of measured shear stress. However, a shear banding (localization of shear within a sample volume) became more pronounced. The use of serrated tools for enzymatically hydrolyzed NFC produced lower shear stress at the moderate...
Ultrasonics Sonochemistry, 2014
A new coupling of ultrasound device with membrane process has been developed in order to enhance ... more A new coupling of ultrasound device with membrane process has been developed in order to enhance cross-flow ultrafiltration of colloidal suspensions usually involved in several industrial applications included bio and agro industries, water and sludge treatment. In order to reduce mass transfer resistances induced by fouling and concentration polarization, which both are main limitations in membrane separation process continuous ultrasound is applied with the help of a vibrating blade (20 kHz) located in the feed channel all over the membrane surface (8 mm between membrane surface and the blade). Hydrodynamic aspects were also taking into account by the control of the rectangular geometry of the feed channel.
Ultrasonics Sonochemistry, 2014
This paper aims to illustrate the interest of ultrasound technology as an efficient technique for... more This paper aims to illustrate the interest of ultrasound technology as an efficient technique for both heat and mass transfer intensification. It is demonstrated that the use of ultrasound results in an increase of heat exchanger performances and in a possible fouling monitoring in heat exchangers. Mass transfer intensification was observed in the case of cross-flow ultrafiltration. It is shown that the enhancement of the membrane separation process strongly depends on the physico-chemical properties of the filtered suspensions.
ABSTRACT The thixotropic behavior of colloidal clay dispersions consisting of disc-shaped particl... more ABSTRACT The thixotropic behavior of colloidal clay dispersions consisting of disc-shaped particles of nanometric size, was studied by using a combination of SAXS, SANS and SLS techniques and rheometric measurements. The aim here is to examine the influence of shear flow on structural changes at various length scales, involved in the thixotropic behavior, and link these with their macroscopic mechanical properties. At rest, the structure of the dispersions is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about a micrometer. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous 3-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The gel state is reached above a volume fraction fv*, for a given ionic strength and gelation time. The influence of the volume fraction on the structure was correlated with the yield stress and the fractal dimension by means of a scaling law (Pignon et al. 1996 ; 1997 a).
ABSTRACT Extensive researches on nano-composite materials submitted to shear flow or extensional ... more ABSTRACT Extensive researches on nano-composite materials submitted to shear flow or extensional flow, have been motivated by their spectacular rheological behavior and vast industrial applications (deposition, ultrafiltration, extrusion, etc.). Indeed, these materials under extensional flow can exhibit time dependent viscoelastic properties. Instabilities such as localization, phase separation, segregation, can occur which are not well understood. The scientific objective of this research is to elucidate the relationship between the extensional flow properties (strain and stress fields) and the dynamics of the induced structures (orientation, aggregation, phase transition, etc.). The nano-composite materials studied are composed of sepiolite clay particles (rigid rods 1 micrometer long and 0.010 micrometers in diameter) in a water-soluble polymer, polyethylene oxide. The trajectory of the strain field within the sample volume has been well characterized by particle imaging velocimetry, in order to apply pure extensional flows in the desired extensional rate and viscosity domains. A new extensional flow device developed at the "Laboratoire de Rhéologie", has allowed to combine extensional flow with SAXS on ID02 at the ESRF. Time-resolved information has been obtained on the orientation dynamics of the fibers in the polymer matrix. Analysis of extensional transient and steady state regimes revealed the main characteristics of the extensional flow of these nano-composite suspensions. Under steady states, the fibers are aligned along the extensional direction. The evolution of the orientation order parameter versus strain rate has allowed defining the rate of organization of the fibers at increasing extensional flow rates. Under transient states, time evolution of the order parameter permitted to identify the time scales at which the fibers get aligned in the extensional field and the time scales over they relax in the polymer matrix.
The sound attenuation in the THz region is studied down to T=16 K in glassy glycerol by inelastic... more The sound attenuation in the THz region is studied down to T=16 K in glassy glycerol by inelastic x-ray scattering. At striking variance with the decrease found below 100 K in the GHz data, the attenuation in the THz range does not show any T dependence. This result i) indicates the presence of two different attenuation mechanisms, active respectively in
Dans le domaine des mousses composites, l’ajout de renfort dans la matrice polymère a de nombreus... more Dans le domaine des mousses composites, l’ajout de renfort dans la matrice polymère a de nombreuses conséquences aussi bien sur la formulation que sur les propriétés mécaniques et de structure de ces matériaux alvéolaires. Des mousses polyuréthane composites, formulées à partir de suspensions constituées de carbonate de calcium (CaCO3) et de silice colloïdale (SC), ont été étudiées. Le but de ce travail a été d'explorer d'une part l'influence des fractions volumiques en charge minérales (φcc) et colloïdales (φsc) sur les propriétés rhéométriques et sur la stabilité des mélanges avant moussage. D’autre part, ce travail traite de l’influence du renforcement sur les propriétés mécaniques et sur la structure des mousses solides. Pour les suspensions sans silice colloïdale, un seuil d’écoulement et une période de stabilité sont détectés pour une fraction volumique au delà d’environ 28,2% en CaCO3. Une concentration optimale en CaCO3 a été déterminée (φcc = 46,2%) du point de ...
Cette étude a pour but d'appréhender le comportement rhéologique sous cisaillement et sous co... more Cette étude a pour but d'appréhender le comportement rhéologique sous cisaillement et sous compression des dispersions d'argile concentrées par centrifugation. Cette technique induit un gradient de concentration et de pression au sein des dépôts permettant de déterminer les seuils de contrainte en compression en fonction de la fraction volumique des dispersions. Les propriétés rhéologiques en cisaillement ont, quant à elles, été mesurées à l'aide d'un scissomètre de petites dimensions partiellement immergé dans le dépôt de centrifugation afin de minimiser les effets de déstructuration des échantillons. La réponse des dispersions soumises à un gradient de cisaillement faible est suivie dans le temps. Ainsi, les contraintes de cisaillement, les modules de cisaillement et les déformations sous cisaillement ont été déterminés en fonction de la fraction volumique. Certaines propriétés mécaniques, telles que le coefficient de Poisson, le module de compression ou encore l&#...
Journal of Materials Science, 2014
Nanofibrillated cellulose (NFC) aqueous suspensions were produced from once-dried bisulfite softw... more Nanofibrillated cellulose (NFC) aqueous suspensions were produced from once-dried bisulfite softwood pulp using enzymatic or 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO)-mediated oxidation pretreatments, followed by wet grinding, as an ultimate fibrillation technique. Two commercial enzyme solutions: cellulase, with the major activity of endoglucanase and exoglucanase, and monocomponent endoglucanase, were compared to facilitate the nanofibrils isolation from cellulose fibers. The influence of their concentrations, as well as the other processing conditions, was analyzed. The morphology of the produced NFC was characterized using optical microscopy, atomic force microscopy (AFM), field emission gun-scanning electron microscopy (FEG-SEM), and morphological fiber analyzer (MorFi). Nanofibrils with a wide size distribution were produced. The average lateral dimensions of 12 ± 7 nm for the most disintegrated enzymatically hydrolyzed NFC and 4 ± 2 nm for TEMPO-oxidized NFC were determined from the AFM height images. The degree of polymerization (DP) decreased and the crystallinity index (CI) increased with an increase of the concentration of both enzyme solutions. TEMPO-oxidation did not have a significant impact on the cellulose CI; however, the DP was strongly affected. The monocomponent endoglucanase solution was found to have a better effect on the nanofibrils isolation rather than their depolymerization.
Rheologica Acta, 2007
We study the transient response to simple shear of aqueous dispersions of Laponite clay particles... more We study the transient response to simple shear of aqueous dispersions of Laponite clay particles and poly(ethylene oxide) at concentrations for which shear induces structure in the form of a network of polymer-clay bonds. We examine the effects of shear on the structure at the micrometer length scale. Bulk rheometric measurements give the material's response to step changes in shear rate. We find that a critical value of the shear rate separates two regions with different rheological behaviors. Static small-angle light scattering shows a corresponding qualitative change in the anisotropy of the dispersion under shear at the micron scale. We interpret our results in terms of the effects of shear on the interactions between clay particles and polymer chains and on the aggregation mechanisms in the dispersion.
Physical Review E, 1997
The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal... more The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal suspension of synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light scattering. At the same time, macroscopic mechanical behavior at rest was characterized by means of rheometric measurements. Two characteristic length scales were detected in these yield stress suspensions of discotic texture. The first, measuring several tens of nanometres, is linked to a fractal dimension of 3. The second, of the order of 1 m, is linked to a fractal behavior of dimension D that increases with the particle volume fraction. Consequently, it is suggested that the structure of the dispersions at rest is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 m. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous three-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The increase of this fractal dimension with the particle volume fraction, the ionic strength, and the gelation time is correlated to a hardening of the mechanical properties of the gels at rest. The gel state is reached above a volume fraction v * for a given ionic strength and gelation time. In the gel phase, a critical volume fraction vc separates two domains. Gels belonging to the domain v *Ͻ v Ͻ vc have a fractal behavior of dimension Dϭ1Ϯ0.05, suggesting an alignment of the micrometer-sized aggregates that leads to the formation of a mechanically weak fibrous structure. Gels belonging v Ͼ vc have a fractal dimension Dϭ1.8Ϯ0.01, corresponding to a mechanically stronger structure consisting of zones of high and lower particle density. A scaling law enabled these fractal dimensions to be correlated with the effect of the volume fraction on the yield stress. In contrast to what is commonly assumed in relation to clay suspensions, it is suggested here that it is the large length scales, of the order of 1 m, associated with a fractal arrangement that governs the macroscopic mechanical behavior. ͓S1063-651X͑97͒07209-7͔
Carbohydrate polymers, Jan 25, 2015
This study investigates for the first time the behaviors of starch and cellulose nanocrystals (SN... more This study investigates for the first time the behaviors of starch and cellulose nanocrystals (SNC and CNC) suspensions which are simultaneously subjected to pressure, shear flow and ultrasound (US) during cross-flow ultrafiltration. This multi-forces process was characterized from macro-scales to nano-scales, with a custom designed "SAXS Cross-Flow US-coupled Filtration Cell". In addition, rheological behaviors of SNC samples at different concentrations/temperatures have been investigated. In both cases (ultrafiltration of SNC and CNC suspensions), better performances were observed with US. The in-situ SAXS measurements revealed that for SNC suspensions, no structure change occurred at the length scales range from 10 to 60nm in this multi-forces process, while CNC particles exhibited an ordered arrangement within the concentrated layer during the same process. SNC particles accumulated on the membrane surface forming a "fragile" concentrated layer which was remo...
Carbohydrate polymers, Jan 4, 2014
The rheological properties of enzymatically hydrolyzed and TEMPO-oxidized microfibrillated/nanofi... more The rheological properties of enzymatically hydrolyzed and TEMPO-oxidized microfibrillated/nanofibrillated cellulose (MFC/NFC) aqueous suspensions were investigated in oscillation and steady-flow modes and were compared with the morphology of the studied materials. The flow instabilities, which introduce an error in the rheological measurements, were discovered during flow measurements. A wall-slip (interfacial slippage on the edge of geometry tools and suspension) was detected at low shear rates for two types of NFC suspensions while applying cone-plate geometry. A roughening of the tool surfaces was performed to overcome the aforementioned problem. Applying to TEMPO-oxidized NFC, a stronger suspension response was detected at low shear rates with higher values of measured shear stress. However, a shear banding (localization of shear within a sample volume) became more pronounced. The use of serrated tools for enzymatically hydrolyzed NFC produced lower shear stress at the moderate...
Ultrasonics Sonochemistry, 2014
A new coupling of ultrasound device with membrane process has been developed in order to enhance ... more A new coupling of ultrasound device with membrane process has been developed in order to enhance cross-flow ultrafiltration of colloidal suspensions usually involved in several industrial applications included bio and agro industries, water and sludge treatment. In order to reduce mass transfer resistances induced by fouling and concentration polarization, which both are main limitations in membrane separation process continuous ultrasound is applied with the help of a vibrating blade (20 kHz) located in the feed channel all over the membrane surface (8 mm between membrane surface and the blade). Hydrodynamic aspects were also taking into account by the control of the rectangular geometry of the feed channel.
Ultrasonics Sonochemistry, 2014
This paper aims to illustrate the interest of ultrasound technology as an efficient technique for... more This paper aims to illustrate the interest of ultrasound technology as an efficient technique for both heat and mass transfer intensification. It is demonstrated that the use of ultrasound results in an increase of heat exchanger performances and in a possible fouling monitoring in heat exchangers. Mass transfer intensification was observed in the case of cross-flow ultrafiltration. It is shown that the enhancement of the membrane separation process strongly depends on the physico-chemical properties of the filtered suspensions.
ABSTRACT The thixotropic behavior of colloidal clay dispersions consisting of disc-shaped particl... more ABSTRACT The thixotropic behavior of colloidal clay dispersions consisting of disc-shaped particles of nanometric size, was studied by using a combination of SAXS, SANS and SLS techniques and rheometric measurements. The aim here is to examine the influence of shear flow on structural changes at various length scales, involved in the thixotropic behavior, and link these with their macroscopic mechanical properties. At rest, the structure of the dispersions is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about a micrometer. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous 3-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The gel state is reached above a volume fraction fv*, for a given ionic strength and gelation time. The influence of the volume fraction on the structure was correlated with the yield stress and the fractal dimension by means of a scaling law (Pignon et al. 1996 ; 1997 a).
ABSTRACT Extensive researches on nano-composite materials submitted to shear flow or extensional ... more ABSTRACT Extensive researches on nano-composite materials submitted to shear flow or extensional flow, have been motivated by their spectacular rheological behavior and vast industrial applications (deposition, ultrafiltration, extrusion, etc.). Indeed, these materials under extensional flow can exhibit time dependent viscoelastic properties. Instabilities such as localization, phase separation, segregation, can occur which are not well understood. The scientific objective of this research is to elucidate the relationship between the extensional flow properties (strain and stress fields) and the dynamics of the induced structures (orientation, aggregation, phase transition, etc.). The nano-composite materials studied are composed of sepiolite clay particles (rigid rods 1 micrometer long and 0.010 micrometers in diameter) in a water-soluble polymer, polyethylene oxide. The trajectory of the strain field within the sample volume has been well characterized by particle imaging velocimetry, in order to apply pure extensional flows in the desired extensional rate and viscosity domains. A new extensional flow device developed at the "Laboratoire de Rhéologie", has allowed to combine extensional flow with SAXS on ID02 at the ESRF. Time-resolved information has been obtained on the orientation dynamics of the fibers in the polymer matrix. Analysis of extensional transient and steady state regimes revealed the main characteristics of the extensional flow of these nano-composite suspensions. Under steady states, the fibers are aligned along the extensional direction. The evolution of the orientation order parameter versus strain rate has allowed defining the rate of organization of the fibers at increasing extensional flow rates. Under transient states, time evolution of the order parameter permitted to identify the time scales at which the fibers get aligned in the extensional field and the time scales over they relax in the polymer matrix.