Rheology

Rheology is the study of the flow and deformation of matter under various conditions of stress, strain, and temperature. This interdisciplinary field plays a crucial role in understanding and characterizing the flow properties of substances, ranging from simple homogeneous fluids like water and oils to complex materials such as polymers, colloids, and biological tissues. Rheological studies provide insights into the mechanical response of materials, helping engineers and scientists design and optimize products in industries such as pharmaceuticals, food processing, and material science.

One key aspect of rheology involves the classification of materials based on their flow behavior. Newtonian fluids, for instance, exhibit a linear relationship between stress and strain rate, maintaining a constant viscosity regardless of the applied stress. In contrast, non-Newtonian fluids display a variable viscosity under different stress conditions. Sub-categories of non-Newtonian fluids include shear-thinning, shear-thickening, and viscoelastic, depending on their response to shear forces. In upcoming posts, we will conduct a more in-depth examination of the various unique characteristics exhibited by non-Newtonian fluids. Understanding rheological behaviour is essential for tailoring material properties to specific applications, such as the design of consumer products, biomedical devices, or industrial processes.

Rheological measurements are typically conducted using specialized instruments like rheometers, which apply controlled stress or strain to a sample and measure the resulting deformation. This allows researchers to characterize a material’s flow properties, such as elasticity and viscosity, across a range of conditions. The data obtained from rheological experiments provide valuable information for quality control, process optimization, and the development of new materials with tailored properties.

Non-Newtonian liquids encompass all fluids that deviate from ideal viscous flow behavior. Pseudoplasticity is the phenomenon whereby liquids become “thinner” (i.e., less viscous) with an increasing shear rate. Examples of where this occurs include the accelerated pumping of pseudoplastic liquids through pipelines or capillaries, the rapid spraying of paints from a spray gun, and intense mixing processes.

At Coanda we utilize advanced rheometry to study materials such as emulsions, suspensions, and dispersions, which can exhibit pseudoplastic flow behavior. With the appropriate selection of sample preparation, measurement techniques, and instrument geometry, a rheometer can handle the challenges posed by complex heterogeneous liquids containing irregularly shaped particles or dispersed droplets, as well as polymer solutions with long, entangled molecular chains.

In the absence of external forces, pseudoplastic materials display high viscosity due to their irregular internal order. As the shear rate is increased, the rheometer can measure transitions in these materials. Suspended particles align in the direction of flow, and polymer chains disentangle, stretch, and orient parallel to the applied force. This alignment results in a shear-thinning effect, where viscosity decreases with an increasing shear rate. It’s important to note that this shear-thinning behavior is not uniform across the entire range of shear rates, with the transition influenced by factors such as Brownian motion at low shear rates. A high-performance rheometer allows us to investigate the nuances of pseudoplastic flow behavior at different shear rates, capturing the complexity of these materials.

Plasticity is a non-Newtonian phenomenon observed in pseudoplastic liquids which have a yield point and describes fluids displaying characteristics of both liquids and solids. Typically found in dispersions, when these substances are not flowing, they form an intermolecular/interparticle network, resulting in a solid-like character with exceptionally high viscosity. External forces smaller than those forming the network (i.e., below the yield stress) only result in a reversible elastic deformation of the material without any flow. The network collapses when external forces exceed the yield stress, initiating flow; here the stress becomes associated with a rate of deformation with an apparent viscosity as the correlating factor.

In principle, the yield stress signifies a sudden shift from a solid to a liquid state; however, in practice the measured value of the yield stress depends on the testing approach. At Coanda, we possess extensive capabilities for measuring yield stress values, and we carefully select the measurement method to suit the specific application. We employ rheometers that can produce continuous shear rate ramps, allowing the determination of dynamic yield stresses. The vane rotor method is especially effective for materials which exhibit complete structural recovery and is another common practice at Coanda. The latter has the advantage that allows for the measurement of undisturbed samples.

While rheologists debate the existence of a “yield stress”, in practice, yield stress analysis is useful for understanding complex structured fluids. A higher yield stress prevents phase separation, sedimentation, or aggregation, and reduces flow caused by shipping vibrations and gravity. Analysis aids in understanding product performance, processability, and predicting long-term stability and shelf life. Yield stress materials are common in food, pharmaceutical, and other industries, some typical examples include mayonnaise, toothpaste, drilling muds and, greases.

Thixotropy, an important rheological phenomenon in industry, refers to the transition of material from a high-viscosity gel state to a lower-viscosity sol state under prolonged high shear conditions. An essential feature of thixotropy is the reversible thickening process when the material is at rest. This transformation between gel and sol states is endlessly repeatable for true thixotropic materials.

A common method for quantifying thixotropy involves a three-step shear test that replicates behavior of the sample at near rest, during structure breakdown, and then its viscosity recovery over time. The viscosity-time curve provides a relative measure of thixotropy, where a shorter rebuild time suggests lower thixotropy compared to a sample with a longer rebuild time. The rate of transformation varies depending on the nature of the thixotropic material and its temperature. Thixotropy plays a pivotal role in industries such as paints, foods, cosmetics, and others. For instance, paint should be thick in the can during storage to prevent separation, easily thin down when stirred and applied to surfaces, and then thicken again after application to prevent drips and maintain an even coat.

Measuring the recovery rate, which influences settling, sagging, and leveling, is often more technically important than assessing the breakdown rate. Recovery involves rebuilding intermolecular structures in the sol state at rest. To assess the rest state, testing should be performed at very low shear rates. Therefore, advanced rheometers with controlled stress/controlled rate are crucial. Coanda’s advanced rheometers, featuring air-bearing rotor shafts and incremental deformation sensors, are designed for high sensitivity to determine meaningful recovery data. While thixotropy measurements can be challenging to conduct, our expertise in working with such materials enables us to identify relevant measurement techniques, variables, and material preparation for specific applications.

A unique and intriguing manifestation of non-Newtonian flow behavior is observed in dilatant (shear-thickening) flow. Dilatant materials experience a rise in viscosity when subjected to increased shear rates. This type of flow behavior is commonly observed in highly concentrated suspensions, as demonstrated by the widely recognized mixture of corn starch and water. Other notable instances of this behavior can be seen in the case of quicksand, moist beach sand, certain pharmaceuticals like penicillin suspensions, and emulsion-PVC mixed with plasticizers to produce plastisols. When subjected to high shear rates, the corn starch-water mixture displays solid-like characteristics, resisting flow. However, under low shear rates, the material undergoes a transition and starts flowing.

In these scenarios, particles are densely packed, and the quantity of liquid introduced is precisely enough to occupy the spaces between the particles. At low shear rates, the suspension assumes a liquid-like state due to the effective lubrication of particle surfaces by the plasticizer, facilitating easy repositioning of particles under applied forces. However, at higher shear rates, particles begin to displace others, resulting in a general increase in volume. During this phase, the relative liquid fraction in the overall volume diminishes, leading to an insufficient amount to fill all voids and maintain full lubrication of particles. Consequently, the suspension undergoes a transition towards increased viscosity.

The complex nature of dilatant flow underscores the importance of careful consideration and the selection of measurement techniques. Coanda, employing advanced rheometric methods, aims to investigate the complexities of dilatant flow in various materials, contributing to an improved understanding of their rheological characteristics. Material handling and preparation procedures, as well as setting appropriate measurement techniques allow us to successfully process and study these complex materials.

A lot of materials exhibit rheological behavior that places them somewhere between a liquid and a solid, classifying them as viscoelastic materials. Emphasizing the determination of elastic properties of a fluid, while acknowledging its viscous properties, provides scientists with valuable insights into its molecular structure. This understanding enables them to modify the structure to meet specific application requirements.

Many polymeric liquids possess long chain molecules that randomly loop and entangle with other molecules. When subjected to sudden high shearing forces, this chain network results in the fluid initially exhibiting a solid-like resistance. Subsequently, the chains undergo elastic stretching, and eventually disentangle and orient in the direction of the shearing force, at which point the fluid begins to behave more like a liquid. At slow rates of deformation, polymer liquids predominantly exhibit viscous flow behavior, with elasticity being less apparent. At high shear rates, a significant portion of the deforming energy is absorbed by elastic intra- and intermolecular deformation, storing energy that can be recovered during a relaxation phase.

Testing the viscous and elastic properties of materials can be crucial, particularly in production processes like coating, spraying, and extruding. Elastic behavior often takes precedence over viscous response in these processes, as it frequently governs flow anomalies that can limit production rates. Rather than subjecting viscoelastic samples to a constant stress for achieving steady-state flow, it is common practice to expose them to oscillating stresses or strains. This approach enables the rheological examination of their “at rest structure” without inducing mechanical disturbance.

At Coanda, we often perform oscillatory measurements to investigate the viscoelastic properties of different materials. Our expertise extends beyond conventional measurements, encompassing the handling and preparation of structured fluids along with more sophisticated rheological techniques to better understand material characteristics.

Mixing operations involving non-Newtonian fluids find widespread application across diverse industries, including food, pharmaceuticals, paints, and polymers. The measurement principle employed in mixer viscometry revolves around determining the torque on the impeller shaft in relation to its rotational speed. However, the complex flow dynamics within a mixing vessel typically mean that mixer viscometry only provides approximate or averaged rheology data.

When addressing the viscosities of complex fluids containing solids, such as heavy oils, emulsions, or slurries, traditional viscometer geometries like concentric cylinders or cone and plate may prove inappropriate. This limitation arises from the narrow gap or geometry being incompatible with the size of the solids, the incapacity to maintain particle suspension, or the inability to achieve the desired homogeneity. While mixer rheology can be employed to address these challenges, it also poses difficulties in translating torque and speed measurements into meaningful rheological properties, particularly when defining shear stress and shear rate for complex fluids. At Coanda, our expertise enables us to navigate various complex systems, employing mixer rheology to tackle issues related to low viscosity dispersions containing rapidly settling particles or dispersions with large particles.

The rheological behavior of a mixture is influenced by several factors such as the concentration, temperature, and shear rate. A comprehensive understanding of these parameters is essential for achieving desired product characteristics, including texture, stability, and homogeneity. In industrial applications, mixer rheology is employed to design efficient mixing systems that enhance productivity and reduce energy consumption.