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Concept: Rheometer


The rheological properties of wet powder masses used in the preparation of pharmaceutical pellets by extrusion/spheronization were evaluated utilizing capillary and rotational rheometers. A ram extruder was used as a capillary rheometer to construct flow and viscosity curves for each wet mass under different extrusion rates and die geometry. As a result, shear thinning behavior was observed for all wet masses. Among the considered rheological models Power Law and Herschel-Bulkley models fitted well with the experimental results. For the majority of the wet masses, water separation and migration occurred during extrusion which led to uneven water content in the extrudate. The effect of extrusion condition including extrusion speed, die geometry and water content on the occurrence of water separation was investigated and the surface quality of the extrudates was compared. In addition, dynamic rheometry tests were done by a parallel plate rheometer to investigate the viscoelastic properties of the wet masses. The frequency sweep tests showed that as water content of the wet masses decreases storage (G') and loss modulus (G″) increase. The storage modulus values were much higher than those of the loss modulus showing dominated elastic rather than viscous behavior for the wet masses at low deformation rates.

Concepts: Continuum mechanics, Fluid mechanics, Viscosity, Liquid, Viscoelasticity, Rheology, Rheometry, Rheometer


PURPOSE: In the present study we evaluated a novel processing technique for the continuous production of hotmelt extruded controlled release matrix systems. A cutting technique derived from plastics industry, where it is widely used for cutting of cables and wires was adapted into the production line. Extruded strands were shaped by a rotary-fly cutter. Special focus is laid on the development of a process analytical technology by evaluating signals obtained from the servo control of the rotary fly cutter. The intention is to provide a better insight into the production process and to offer the ability to detect small variations in process-variables. MATERIALS AND METHODS: A co-rotating twin-screw extruder ZSE 27 HP-PH from Leistritz (Nürnberg, Germany) was used to plasticize the starch; critical extrusion parameters were recorded. Still elastic strands were shaped by a rotary fly-cutter type Dynamat 20 from Metzner (Neu-Ulm, Germany). Properties of the final products were analyzed via digital image analysis to point out critical parameters influencing the quality. Important aspects were uniformity of diameter, height, roundness, weight and variations in the cutting angle. Stability of the products was measured by friability tests and by determining the crushing strength of the final products. Drug loading studies up to 70% were performed to evaluate the capacity of the matrix and to prove the technological feasibility. Changes in viscosities during API addition were analyzed by a Haake Minilab capillary rheometer. X-ray studies were performed to investigate molecular structures of the matrices. RESULTS: External shapes of the products were highly affected by die-swelling of the melt. Reliable reproducibility concerning uniformity of mass could be achieved even for high production rates (>2500 cuts/min). Both mechanical strength and die swelling of the products could be linked to the ratio of amylose to amylopectin. Formulations containing up to 70% of API could still be processed. Viscosity measurements revealed the plasticizing effect caused by API addition. Dissolution data proved the suitability of extruded starch matrices as a sustained release dosage form. Monitoring of consumed energies during the cutting process could be linked to changes in viscosity. The established PAT system enables the detection of small variations in material properties and can be an important tool to further improve process stability.

Concepts: Viscosity, Strength of materials, Compressive strength, Work hardening, Industry, Extrusion, Rheometer, Starch gelatinization


When hagfish (Myxinidae) are attacked by predators, they form a dilute, elastic, and cohesive defensive slime made of mucins and protein threads. In this study we propose a link between flow behavior and defense mechanism of hagfish slime. Oscillatory rheological measurements reveal that hagfish slime forms viscoelastic networks at low concentrations. Mucins alone did not contribute viscoelasticity, however in shear flow, viscosity was observed. The unidirectional flow, experienced by hagfish slime during suction feeding by predators, was mimicked with extensional rheology. Elongational stresses were found to increase mucin viscosity. The resulting higher resistance to flow could support clogging of the attacker’s gills. Shear flow in contrast decreases the slime viscosity by mucin aggregation and leads to a collapse of the slime network. Hagfish may benefit from this collapse when trapped in their own slime and facing suffocation by tying a sliding knot with their body to shear off the slime. This removal could be facilitated by the apparent shear thinning behavior of the slime. Therefore hagfish slime, thickening in elongation and thinning in shear, presents a sophisticated natural high water content gel with flow properties that may be beneficial for both, defense and escape.

Concepts: Fluid dynamics, Continuum mechanics, Fluid mechanics, Viscosity, Viscoelasticity, Non-Newtonian fluid, Rheology, Rheometer


In this study, a new magnetorheological (MR) grease was made featuring plate-like carbonyl iron (CI) particles, and its magnetic field-dependent rheological properties were experimentally characterized. The plate-like CI particles were prepared through high-energy ball milling of spherical CI particles. Then, three different ratios of the CI particles in the MR grease, varying from 30 to 70 wt% were mixed by dispersing the plate-like CI particles into the grease medium with a mechanical stirrer. The magnetic field-dependent rheological properties of the plate-like CI particle-based MR grease were then investigated using a rheometer by changing the magnetic field intensity from 0 to 0.7 T at room temperature. The measurement was undertaken at two different modes, namely, a continuous shear mode and oscillation mode. It was shown that both the apparent viscosity and storage modulus of the MR grease were heavily dependent on the magnetic field intensity as well as the CI particle fraction. In addition, the differences in the yield stress and the MR effect between the proposed MR grease featuring the plate-like CI particles and the existing MR grease with the spherical CI particles were investigated and discussed in detail.

Concepts: Magnetic field, Mode, Electric field, Viscosity, Physical quantities, Solid mechanics, Rheology, Rheometer


Rheological modeling as a function of temperature is a useful tool for describing products undergoing thermal processing. The rheological behavior of a range of dairy-based (4%, w/w) protein beverages was investigated for applicability to semi-empirical temperature-dependent viscosity equations. The viscosity at 16.8 rad/s of the beverages was measured during heating, holding, and cooling over a temperature range of 25 to 90oC using a rheometer with starch pasting cell geometry. Five established fitting methods were applied based on the Arrhenius and Williams-Landel-Ferry (WLF) equations using nonlinear regression analysis. A two-parameter WLF (WLF2) model, using viscosity at a reference temperature of 25oC resulted in high R2values (0.974 to 0.988) and a statistically superior fit compared to the Arrhenius, Generalized Arrhenius, and exponential equations (P < 0.001). Deviation from the WLF2modeled equation was used to describe and investigate the effect formulation had on the changes in viscosity during thermal heating. This study successfully applied the WLF equation to a liquid protein system, proving that a consistent and close fit can be achieved across a range of formulations. A rapid, quantitative method for viscosity-temperature profile evaluation is presented, which can ease product development and optimization of product processing stability.

Concepts: Scientific method, Regression analysis, Temperature, Heat, Viscosity, Rheology, Polynomial, Rheometer


Commercial interest on the application of starch nanoparticles (SNP) in food is rapidly growing. Since gums are commonly used in food processing for their texturizing functionality, it is important to understand the influence of SNP on gum rheology. λ-Carrageenan and xanthan gum were selected in this study due to their wide range of applications in food systems. SNP isolated from waxy (0% amylose) and high amylose maize (71% amylose) were used. Binary blends consisting of 0.5% (w/v) gum and different concentrations of SNP were prepared and their flow behaviors evaluated using a rheometer. The starch source, concentration and thermal stability of the SNP influenced the viscosity and viscoelastic properties of the gum-SNP blends, in particular in the low elasticity λ-carrageenan. The blends containing high concentrations of high amylose maize SNP, when compared to waxy SNP, showed higher viscoelastic stability at temperatures up to 90°C, possibly due to their higher thermal stability.

Concepts: Starch, Continuum mechanics, Viscosity, Viscoelasticity, Rheology, Gums, Rheometer, Polysaccharides


This study aimed at determining the composition and rheology changes during the ripening (60days) of Manchego-type cheeses prepared with Lactococcus lactis subsp. lactis and Lactococcus lactis subsp. cremoris in the free status and microencapsulated in gellan gum. The composition indicated that microencapsulation had a greater influence than the period of ripening since discrepancies were statistically significant (p<0.05) regarding the levels of protein, moisture, total solids and pH between the two cheeses both at the beginning and end of the ripening. Both cheeses presented predominantly elastic characteristics (G'>G″). The moduli and the viscosity significantly increased with the presence of microcapsules and, ripening time. Conversely, the recovery percentages on the creep curve decreased in both cheeses with an increase in ripening as a result of the degradation of proteins. The retarded time ranged between 2.32×10-2and 2.38×10-2s and the Jovalues were 29 times higher than the Jivalues in the studied cheeses, indicating the loss of elasticity of the cheeses as they ripen.

Concepts: Time, Statistical significance, Fluid mechanics, Viscoelasticity, Rheology, Lactococcus lactis, Lactococcus, Rheometer


When surfactants adsorb at liquid interfaces, they not only decrease the surface tension, they confer rheological properties to the interfaces. There are two types of rheological parameters associated to interfacial layers: compression and shear. The elastic response is described by a storage modulus and the dissipation by a loss modulus or equivalently a surface viscosity. Various types of instruments are available for the measurements of these coefficients, the most common being oscillating pendent drops instruments and rheometers equipped with bicones. These instruments are applicable to systems with large enough interfacial tensions, typically above a few mN/m. We use a new type of instrument based on spinning drop oscillations, allowing to extend the interfacial rheology studies to low and ultralow interfacial tension systems. We present examples of measurements with systems of high and low tension, discuss the possible artifacts and demonstrate the capability of this new technique. We emphasize that the data shown for low interfacial tensions are the first reported in the literature. The instrument is potentially interesting for instance in enhanced oil recovery or demulsification studies.

Concepts: Fluid mechanics, Viscosity, Liquid, Surface tension, Drop, Rheology, Type system, Rheometer


The influence of concentration on the shear and extensional rheology properties of aqueous solutions of xanthan and guar gums was studied in this work. Shear rheology involved small amplitude oscillatory shear (SAOS), flow curves and transient flow, while the extensional rheology was analyzed using hyperbolic contraction flow. In addition, the mechanical properties during solutions manufacture were monitored in situ through the evolution of torque with processing time by mixing rheometry. The results showed that the hydrocolloids exert a great influence on the process rheokinetics and on the resulting rheological response. SAOS tests showed that the xanthan gum solutions behaved as weak gels, whereas guar gum solutions suggest the presence of entanglement and the formation of a viscoelastic, gel-like structure. All the systems exhibited shear-thinning behaviour. Guar gum solutions obeyed the Cox-Merz rule, with some divergence at high rates for the more concentrated solutions, while the Cox-Merz rule was not followed for xanthan gum in the range of concentration studied. The extensional viscosity exhibited an extensional-thinning behaviour within the strain range used and all solutions were characterized by a high Trouton ratio.

Concepts: Concentration, Continuum mechanics, Fluid mechanics, Viscosity, Rheology, Rheometry, Rheometer, Guar gum


It is known that additives like glycerol and sucrose lead to the swelling of aqueous bilayer Lα phases. The swelling of the Lα phases can be explained by the increase of the refractive index of the mixed solvent, which lowers the van der Waals attraction between the bilayers. Afterwards, the undulation forces between the bilayers can push them apart. This hypothesis was previously extended to wormlike micelles (WLM) of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). These types of self-assembly structures have viscoelastic properties, and the zero shear viscosity of these solutions is dependent on the molar ratio NaSal/CTAB, R. At R = 0.6, R = 1.0 and R ≈ 2.6 the viscosity goes, respectively, through a maximum, a minimum and another maximum. These viscosities can be explained by differences in relaxation mechanisms predominant in each region. Similarly to what is observed to bilayer Lα phases, the additives would change the interaction between the WLM, affecting the relaxation processes of each region, altering the profile from two maxima and one minimum to a single maximum in viscosity. In the present manuscript, it is investigated whether it is only the refractive index, other solvent properties, or a combination of several factors that induce these changes in WLM. For this, several additives, forming binary mixtures with water, were studied, through rheology of CTAB/NaSal and calorimetry of tetradecyltrimethylammonium bromide (TTAB)/NaSal.

Concepts: Fundamental physics concepts, Fluid mechanics, Viscosity, Shear stress, Viscoelasticity, Maxima and minima, Rheology, Rheometer