Macroscopic Rheological Measurements Research Articles

Effect of nanocomposite pour point depressant EVAL/CNT on flow properties of waxy crude oil

The nanocomposite EVAL-CNT was produced by chemical grafting in the solution system through the esterification of ethylene-vinyl alcohol copolymer (EVAL) and carboxylated multi-walled carbon nanotubes (O-MWCNT), and its structural properties were characterized. The improvement of the rheological properties of the waxy oil system by the novel pour point depressant was investigated using macroscopic rheological measurements and microscopic observations. The results showed that EVAL-CNT nanocomposite pour point depressant (PPD) could significantly reduce the pour point and improve the low temperature fluidity of crude oil and had better performance than EVAL-GO at the same addition level. The best effect was achieved at the dosing concentration of 400 ppm, which reduced the pour point by 13 °C and the low-temperature viscosity by 85.4%. The nanocomposites dispersed in the oil phase influenced the precipitation and crystallization of wax molecules through heterogeneous crystallization templates, which led to the increase of wax crystal size and compact structure and changed the wax crystal morphology, which had a better effect on the rheological properties of waxy oil.

Rheology of a 2D granular film.

We study experimentally the rheology of a macroscopic particle-laden soap film, designated as a "Granular Film", in the simple shear configuration. Macroscopic particles are dispersed in a soap film, while being large enough that they bridge both fluid interfaces. We simultaneously perform macroscopic rheological measurements with a classical rheometer and investigate interactions at the particle scale with a camera underneath the film. The determination of the velocity field of the grains reveals the presence of an inhomogeneous shear within the granular film. Trying to correlate both measurements unveils the non-locality of the rheology of the granular film: similar to what has been observed in a dry granular material, we find an highly-sheared zone close to the moving wall contrasting with a large quasistatic area. This behavior can be accounted for through extended kinetic theory and correlated with a transition in the dominant component of the stress.

Enhanced microscopic dynamics in mucus gels under a mechanical load in the linear viscoelastic regime

Mucus is a biological gel covering the surface of several tissues and ensuring key biological functions, including as a protective barrier against dehydration, pathogen penetration, or gastric acids. Mucus biological functioning requires a finely tuned balance between solid-like and fluid-like mechanical response, ensured by reversible bonds between mucins, the glycoproteins that form the gel. In living organisms, mucus is subject to various kinds of mechanical stresses, e.g., due to osmosis, bacterial penetration, coughing, and gastric peristalsis. However, our knowledge of the effects of stress on mucus is still rudimentary and mostly limited to macroscopic rheological measurements, with no insight into the relevant microscopic mechanisms. Here, we run mechanical tests simultaneously to measurements of the microscopic dynamics of pig gastric mucus. Strikingly, we find that a modest shear stress, within the macroscopic rheological linear regime, dramatically enhances mucus reorganization at the microscopic level, as signaled by a transient acceleration of the microscopic dynamics, by up to 2 orders of magnitude. We rationalize these findings by proposing a simple, yet general, model for the dynamics of physical gels under strain and validate its assumptions through numerical simulations of spring networks. These results shed light on the rearrangement dynamics of mucus at the microscopic scale, with potential implications in phenomena ranging from mucus clearance to bacterial and drug penetration.

Super-resolution microscopy to visualize and quantify protein microstructural organization in food materials and its relation to rheology: Egg white proteins

Nowadays, food materials can be well characterized at their molecular level (food chemistry) and macroscopic level (texture and rheology) thanks to the advances in these areas in the last decades. However, there is a gap in the understanding of how food materials organize at microscopic level. This article shows how an optical super-resolution microscopy technique can be used to gain more information about food microstructure compared to conventional confocal microscopy. Egg-white is used as a protein aggregation model system, it was mixed with a fluorescent dye and was cooked for 40 min at temperatures with low, intermediate, or high albumin denaturation (69 °C, 72 °C, or 75 °C). Samples were observed using confocal and super-resolution stimulated emission depletion (STED) microscopy, and their elastic modulus was recorded with rheological amplitude sweeps. The different cooking temperatures generated materials with very different textural attributes. STED microscopy was able to resolve protein microstructures with a 5-fold increase in resolution compared to the confocal system. This gain in resolution allowed a more precise quantification of the structure, with quantification of 7–13 times more particles in the solid area of the materials compared to confocal images. The increase of particle count and particle density measured in the STED images shows a possible correlation with the logarithmic increase of the elastic modulus and the brittleness of the egg-white cooked at the three different temperatures. This shows how this extra information from STED microscopy can be correlated with macroscopic rheological measurements for a more complete understanding of protein aggregation in food matrices.

Network structure and gelation mechanism of kappa and iota carrageenan elucidated by multiple particle tracking

Network structure and gelation mechanism of kappa carrageenan (KC) and iota carrageenan (IC) were investigated using multiple particle tracking. Based on bulk rheology measurements, KC gels exhibit characteristics of a hard and brittle gel, whereas IC gels form a soft and so-called “weak-gel”. Particle tracking revealed the differences in the local physical properties of the two gel systems, which are not accessible with the macroscopic rheological measurements. The mean square displacement (MSD) of the probe particles were investigated to characterize changes in the gel network structure on cooling and storage. On cooling, the MSD of the probe particles in the KC solution decreased drastically at around the gelling temperature, demonstrating the trapping of particles within the network structures of KC gels. The MSD of the probe particles in the IC solution exhibited diffusive behavior even far below the gelling temperature on cooling, although the MSD decreased on storage. These results suggest that KC solutions formed a permanent gel network structure of KC chain aggregates that restricted the motion of particles on cooling. For IC at low temperatures, in contrast, the results suggested two possible structures: 1. clusters of IC chain aggregates, or, 2. a loose network with large pores, which allow the diffusion of particles and lead to weak-gel behavior. The aggregates further aggregate to form a more permanent gel network structure during storage.

Retraction of “Coupled Effect of Orientation, Stretching and Retraction on the Dimension of Entangled Polymer Chains during Startup Shear”

The theoretical description of how entangled polymers undergo large deformation has been at the heart of polymer dynamics. Over the years, the polymer physics community has adopted the tube model as the standard paradigm for describing polymer deformation and flow in both the linear and nonlinear regimes. The tube model envisions an ensemble of chains that undergo reptation and fluctuation inside smooth confining tubes on time scales of the Rouse relaxation time. It provides a reasonable and appealing description of the equilibrium and near equilibrium dynamics of entangled polymer melts and concentrated solutions. Some of the predictions based on this paradigm are also in apparent agreement with macroscopic rheological measurements for large deformations. However, some discrepancies with the tube model have been noted in the literature. For example, the time for the onset of time-strain superposibility in experiments has been shown to be much longer than the Rouse time anticipated based on the tube model. More recently, S.-Q. Wang’s group using particle-tracking velocimetry reported a number of nonlinear rheological phenomena that questioned the validity of the tube-model assumptions. The theoretical implications of the reported phenomena have been controversial.

Structure and Dynamics of a Model Polymer Mixture Mimicking a Levan-Based Bacterial Biofilm of Bacillus subtilis

In this paper, we report on the structure and dynamics of biologically important model polymer mixtures that mimic the extracellular polymeric matrix in native biofilm of Bacillus subtilis. This biofilm is rich in nonionic polysaccharide levan, but also contains other biopolymers such as DNA and proteins in small concentrations. Aiming to identify the contribution of each component to the formation of the biofilm, our investigations encompassed dynamic rheology, small-angle X-ray scattering, dynamic light scattering, microscopy, densitometry, and sound velocity measurements. As it turned out, this very powerful combination of techniques is able to provide solid results on the dynamical and structural aspects of the microbiologically and chemically complex biofilm formations. Macroscopic rheological measurements revealed that the addition of DNA to levan solution increased the viscosity, pseudoplasticity, and elasticity of the system. The addition of protein contributed similarly, but also increased the rigidity of the system. This confirms that the presence of minor biofilm components is essential for biofilm formation. DNA and proteins appear to confine levan molecules within their supramolecular structure and, in this way, restrict the role of levan to merely a filling agent. These findings were complemented by small-angle X-ray scattering data, which provided insight into the structure on a molecular scale. One of the essential goals of this work was to compare the structural properties of the native biofilm and synthetic biofilm mixture.

External and internal gelation of pectin solutions: microscopic dynamics versus macroscopic rheology

Pectin is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non-covalent cross-links. We investigate and compare the kinetics of formation and aging of pectin gels obtained either through external gelation via perfusion of free Ca2+ ions, or by internal gelation due to the supply of the same ions from the dissolution of CaCO3 nanoparticles. The microscopic dynamics obtained with photon correlation imaging, a novel optical technique that allows obtaining the microscopic dynamics of the sample while retaining the spatial resolution of imaging techniques, is contrasted with macroscopic rheological measurements at constant strain. Pectin gelation is found to display peculiar two-stage kinetics, highlighted by non-monotonic growth in time of both microscopic correlations and gel mechanical strength. These results are compared to those found for alginate, another biopolymer extensively used in food formulation.

Coupled Effect of Orientation, Stretching and Retraction on the Dimension of Entangled Polymer Chains during Startup Shear

The theoretical description of how entangled polymers undergo large deformation has been at the heart of polymer dynamics. Over the years, the polymer physics community has adopted the tube model as the standard paradigm for describing polymer deformation and flow in both the linear and nonlinear regimes. The tube model envisions an ensemble of chains that undergo reptation and fluctuation inside smooth confining tubes on time scales of the Rouse relaxation time. It provides a reasonable and appealing description of the equilibrium and near equilibrium dynamics of entangled polymer melts and concentrated solutions. Some of the predictions based on this paradigm are also in apparent agreement with macroscopic rheological measurements for large deformations. However, some discrepancies with the tube model have been noted in the literature. For example, the time for the onset of time-strain superposibility in experiments has been shown to be much longer than the Rouse time anticipated based on the tube model. More recently, S.-Q. Wang’s group using particle-tracking velocimetry reported a number of nonlinear rheological phenomena that questioned the validity of the tube-model assumptions. The theoretical implications of the reported phenomena have been controversial.

Small-angle static light scattering of concentrated silica suspensions during in situ destabilization

The aggregation of concentrated aqueous silica suspensions is characterized by means of static light scattering. We use an in situ destabilization mechanism based on the enzyme-catalyzed hydrolysis of urea. This method enables us to continuously and homogeneously change the interparticle potential from repulsive to attractive without disturbing the aggregation process. Moreover, our electrostatically stabilized suspensions can be destabilized by two different methods. In the first method, the pH is shifted toward the isoelectric point of the particles (ΔpH method), thereby leading to a decrease of their surface charge. In the second method, the ionic strength is continuously increased at constant pH (Δ I method), leading to a compression of the electrical double layer around the charged particles. A laboratory-built flat-cell light-scattering instrument is used, which allows fast data acquisition and an adjustment of the sample cell thickness. To circumvent multiple scattering effects, we use a very small sample thickness (≈13 μm). In addition, the refractive index difference between the aqueous phase and the particles is reduced by adding sucrose to the liquid phase of our suspensions. We are able to characterize the structural changes at the very early stages of the destabilization process, where no significant effects are yet detected in macroscopic rheological measurements. While during the ΔpH destabilization, the scattering curve shows significant changes only after some characteristic delay time, it changes continuously during the Δ I destabilization. The latter is attributed to the formation of a weak pre-gel structure in the suspensions, as a shallow secondary minimum appears in the interparticle potential. Data are evaluated by using a HMSA square-well structure factor model. Results are in good agreement with those predicted from DLVO theory.