Frédéric Pignon

Thixotropic colloidal suspensions and flow curves with minimum: Identification of flow regimes and rheometric consequences

Some colloidal suspensions with a thixotropic behavior and yield stress also display a minimum stress in their flow curves. One of these is an aqueous suspension of synthetic clay, consisting of anisotropic particles of nanometric size, forming a transparent thixotropic gel. Four flow regimes corresponding to four different states of strain field were defined by combining visualization techniques and rheometric measurements in cone‐plate geometry. Links with the microstructure of the suspension are suggested. Flow instabilities such as localized shear and stick slip were identified in the falling branch of the flow curve and in the minimum stress area. Two regimes involving the bulk properties of the sample were identified. Consequently, proper procedures for characterizing the thixotropy of the suspensions are proposed.Some colloidal suspensions with a thixotropic behavior and yield stress also display a minimum stress in their flow curves. One of these is an aqueous suspension of synthetic clay, consisting of anisotropic particles of nanometric size, forming a transparent thixotropic gel. Four flow regimes corresponding to four different states of strain field were defined by combining visualization techniques and rheometric measurements in cone‐plate geometry. Links with the microstructure of the suspension are suggested. Flow instabilities such as localized shear and stick slip were identified in the falling branch of the flow curve and in the minimum stress area. Two regimes involving the bulk properties of the sample were identified. Consequently, proper procedures for characterizing the thixotropy of the suspensions are proposed.

Thixotropic behavior of clay dispersions: Combinations of scattering and rheometric techniques

The thixotropic behavior of a colloidal dispersion of clay consisting of disk-shaped particles was studied by means of a combination of rheometric measurements, static light scattering, and small-angle neutron scattering. At rest, the structure of the gel consists of dense micrometer-sized aggregates assembled into a fractal mass of dimension D. Under shear, in the case of volume fractions with (1⩽D⩽1.2) at rest, a butterfly-type light scattering pattern is observed. This is attributed to the formation of rollers within the dispersion, which align themselves on average perpendicular to the direction of shearing. This produces a fall in resistance to flow and in viscosity. The influence of shear rate on this disaggregation process was studied and linked to the rheometric measurements. Under shear flow conditions, the fall in viscosity is due to orientation and disaggregation processes occurring at length scales on the order of 1 μm. During recovery, the two time scales identified correspond, respectively, ...

Rheological properties of micro-/nanofibrillated cellulose suspensions: wall-slip and shear banding phenomena.

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 shear rates, which was influenced by water release from the suspension.

Current Progress in Rheology of Cellulose Nanofibril Suspensions

Cellulose nanofibrils (CNFs) are produced and commonly used in the form of aqueous suspensions or gels. A number of studies have focused lately on rheological properties of CNF suspensions, which gives insight into properties of such materials and can reflect their behavior during handling. This Review summarizes the recent progress in rheological studies on CNF aqueous suspensions using rotational rheometry. Here, we discuss linear viscoelastic properties, i.e., frequency-dependent storage and loss moduli; shear flow behavior, i.e., apparent viscosity and shear stress as a function of shear rate; local flow characteristics, etc. In this Review, we point out that the rheological behavior of at least two types of CNF suspensions should be distinguished: (i) ones produced using mechanical fibrillation with or without enzymatic pretreatment (no surface chemical modification), which possess highly flocculated structure, and (ii) ones produced involving chemical modification pretreatments, e.g., carboxylation, carboxymethylation, quaternization, or sulfonation, which possess better colloidal stability and do not evidently flocculate.

Spatial and Temporal in Situ Evolution of the Concentration Profile during Casein Micelle Ultrafiltration Probed by Small-Angle X-ray Scattering

Understanding the mechanisms involved in structural development in the vicinity of membrane constitutes a considerable challenge in the improvement of ultrafiltration process in industrial applications. In situ small-angle X-ray scattering (SAXS) performed with custom-made ultrafiltration cell has permitted the structural arrangement to be probed and concentration profiles to be obtained in deposited layers during frontal filtration of casein micelle suspension. SAXS allowed the structure of the accumulated layers of casein micelles between 280 microm and 1 mm from the membrane surface to be followed at length scales from a few nanometers to about 100 nm. These results have been combined with hydrodynamic measurements (permeation flux) and rheological investigations. Under frontal filtration, the time dependence of concentration at different distances from the membrane surface has been obtained. This temporal evolution is marked by an exponential increase of the concentration followed by a slower growth which has been associated with a change in the rheological behavior of the suspension from Newtonian to shear thinning behavior.

Surface adsorption of triblock copolymer (PEO–PPO–PEO) on cellulose nanocrystals and their melt extrusion with polyethylene

Cellulose nanocrystals (CNC) have gained a lot of interest in recent years in the field of composites due to their unique mechanical properties and also because cellulose is the most abundant and renewable polymer in nature. In this work, Pluronic grade triblock copolymer was adsorbed on the surface of CNC in order to improve the thermal stability and also its dispersion from the dried state. The adsorbed cellulose nanocrystals (A-CNC) were characterized to check their thermal, functional and structural properties by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and atomic force microscopy (AFM). Interestingly, improved thermal stability was observed and also the dispersion of A-CNC in aqueous medium was much better than for unmodified CNC. The aqueous A-CNC suspensions were characterized by small angle X-ray scattering (SAXS) to evaluate the dispersion of the nanoparticles. The flow properties of A-CNC dispersions were also analyzed. Further, A-CNC was used to prepare nanocomposites by melt extrusion using linear low density polyethylene (LLDPE) as matrix. The thermo mechanical and morphological properties of the ensuing nanocomposites were characterized by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The dispersion state of A-CNC within the polymeric matrix was also characterized by SAXS.

Intensification of heat and mass transfer by ultrasound: Application to heat exchangers and membrane separation processes

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.

Tunable Aggregation and Gelation of Thermoresponsive Suspensions of Polymer-Grafted Cellulose Nanocrystals.

The colloidal stability together with the tunable aggregation and viscoelastic properties of thermoresponsive polymer-grafted cellulose nanocrystals (CNCs) were investigated. TEMPO oxidation of CNCs followed by peptidic coupling in water were used to covalently graft thermosensitive Jeffamine polyetheramine M2005 chains onto the surface of CNCs. The resulting polymer-decorated particles (M2005-g-CNCs) exhibited new colloidal properties, by their ability to perfectly redisperse in water and organic solvents such as toluene, dichloromethane or DMF after freeze-drying. In addition, they presented an enhanced thermal stability when compared to that of sulfated or TEMPO-oxidized CNCs. Dynamic light scattering experiments were used to demonstrate that the thermally induced aggregation of M2005-g-CNCs was fully reversible and reproducible over many temperature cycles and that, most interestingly, the aggregation number could be tuned by varying the ionic strength and/or the pH of the medium, making the suspension multiresponsive. This property arises from the variations of the sign (attractive or repulsive) and the range of the different types (entropic, electrostatic, hydrophobic) of interaction forces between the thermosensitive polymer-decorated nanoparticles. The variation of the viscoelastic properties of M2005-g-CNCs suspensions as a function of temperature, probed by oscillatory rheology measurements of more concentrated suspensions, revealed a reversible temperature-triggered liquid-to-gel transition. Such enhanced functionalities pave the way to the design of advanced CNC-based materials benefiting both from the intrinsic characteristics of these biosourced particles and the new properties imparted by the stimuli-sensitive grafted chains.

Single-cell adhesion probed in-situ using optical tweezers: A case study with Saccharomyces cerevisiae

A facile method of using optical trapping to measure cell adhesion forces is presented and applied to the adhesion of Saccharomyces cerevisiae on glass, in contact with solutions of different compositions. Trapping yeast cells with optical tweezers (OT) is not perturbed by cell wall deformation or cell deviation from a spherical shape. The trapping force calibration requires correction not only for the hydrodynamic effect of the neighboring wall but also for spherical aberrations affecting the focal volume and the trap stiffness. Yeast cells trapped for up to 5 h were still able to undergo budding but showed an increase of doubling time. The proportion of adhering cells showed the expected variation according to the solution composition. The detachment force varied in the same way. This observation and the fact that the detachment stress was exerted parallel to the substrate surface point to the role of interactions involving solvated macromolecules. Both the proportion of adhering cells and the removal force showed a distribution which, in our experimental conditions, must be attributed to a heterogeneity of surface properties at the cell level or at the subcellular scale. As compared with magnetic tweezers, atomic force microscopy, and more conventional ways of studying cell adhesion (shear-flow cells), OT present several advantages that are emphasized in this paper.

New capillary rheometer allowing for small-angle x-ray scattering experiments inside the die. Application to the extrusion of block copolymers, their macroscopic defects, and their structure

Extrusion rates are often limited by the surface cracks originating at the die exit. These cracks can be so severe in the case of certain polymers that the phenomenon known as “flow split” occurs. Previous work by the same authors explains the macroscopic mechanisms leading to flow split observed during the extrusion of polystyrene-block-poly(ethylene-co-buylene)-block-polystyrene block copolymers in their microphase separated state. The present paper looks at the effects of extrusion on the structure of these block copolymers at regimes showing the different characteristic defects of the fluids. The structure of extruded samples is examined by small-angle x-ray scattering (SAXS). A comparison between the relaxation times characteristic of the copolymers and the time necessary to prepare the samples shows that quenching of samples the flow of which is stable at the die exit is practically impossible with a standard capillary rheometer. Hence, in situ experiments inside the die are necessary to understand ...