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Rheological Principles
Published in B. R. Gupta, Rheology Applied in Polymer Processing, 2023
The time dependent fluids are those liquids, which show either a decrease or increase in the viscosity with time at a particular rate of shear i.e. if these fluids are subjected to a constant shear rate then the shear stress will either decrease or increase. These fluids are respectively known as thixotropic fluids and rheopectic fluids (Fig. 2.16). Examples of thixotropic systems are filled polymers, bentonite clay suspended in water, drilling muds, cement slurries, crude oils, coal-water slurries, yoghurt, salad dressing, mayonnaise, xanthan gum solutions, aqueous iron oxide gels, gelatin gels, pectin gels, hydrogenated castor oil, montmorillonite clay suspensions, carbon black suspensions in molten tire rubber, colloidal suspensions etc.
Exploiting the rheology of mine tailings for dry disposal
Published in A.A. Balkema, Tailings and Mine Waste 2000, 2022
Thixotropy is the result of structural breakdown under shear and manifests itself as a decrease in the viscosity and yield stress with time for a given, constant shear rate. As time of shear elapses, the rate of breakdown will decrease, as fewer structural bonds are available for breakdown. Structural reformation may take place and the rate of this process will increase with time of shear due to the increasing number of bonding sites available.
Hydrogels: Characteristics and Properties
Published in Claudio Migliaresi, Antonella Motta, Scaffolds for Tissue Engineering, 2014
Rolando Barbucci, Daniela Pasquia
Thixotropy is a characteristic of some pseudo-plastic materials that leads to a progressive decrease in their viscosity when these materials are subjected to a specific mechanical stress, followed by the recovery of its rheological properties after a more or less prolonged period of rest.22 Under stress conditions, an infinitesimal layer of fluid slides onto the adjacent layer, leading to the settling of a laminar flux. Hence, the fluid transforms from the gel state to the
Pulsatile flow of thixotropic blood in artery under external body acceleration
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2023
Louiza Cheffar, Abdelhakim Benslimane, Djamel Sadaoui, Adel Benchabane, Karim Bekkour
Whereas many studies have considered Newtonian, non-Newtonian and viscoelastic assumptions, studies regarding the time-dependent thixotropic effect exhibited by physiological systems such as blood are fewer despite its importance in blood flow modeling. However, the concept of thixotropy is well established and the related rheological experimentation has already been extensively investigated (Barnes 1997; de Souza Mendes et al. 2012; Pritchard et al. 2020) given the widespread nature of thixotropic fluid applications. Thixotropic fluids are commonly used in the construction industry (e.g. liquid cements, liquid concrete, drilling fluids), industrial applications (e.g. muds, paints) and the food industry (e.g. liquid dairy products, ketchup). Related applications include some forms of mud flows and debris flows, pasty sewage sludges and some wastewater treatment residues (Benslimane et al. 2016a, 2016b).
Factors influencing the structure build-up of fresh cement asphalt emulsion paste
Published in Road Materials and Pavement Design, 2018
A temperature-controlled rheometer with a coaxial cylinder geometry is employed to do the thixotropy test. The tested temperature is controlled at 23°C. The rheometer and rotor are shown in Figure 1. The radius of the rotor is 10 mm. The gap between rotor and cylinder is 0.85 mm. After mixing, the sample is immediately loaded to the cylinder for testing, a process that takes 1 min. Then, the shear protocol shown in Figure 2 is performed to study the structure build-up of fresh CA paste. First, a 2 min pre-shear at 300 s−1 is intended to create a uniform condition for samples before testing. Then, samples are rested for certain time (1 min, 5 min, 10 min, 15 min, 20 min). Finally, samples are sheared by a constant shear rate at 50 s−1 for 30 s. In the third step, data are used to analyse the thixotropy of CA paste. In all, 50 s−1 is chosen in the third step according to the flow curve of CA pastes. The apparent viscosity of CA paste decreases firstly and then increases slightly with the increasing shear rate due to the dilatancy effect (Barnes, 1989). The corresponding shear rate of the minimum apparent viscosity is near 50 s−1 (Ouyang et al., 2015; Ouyang & Tan, 2015). Thus, all CA pastes do not have a pronounced dilatancy effect at 50 s−1. This test is performed thrice for each sample.
Bigels as novel systems for the delivery active compounds from Centella asiatica
Published in Soft Materials, 2023
Agnieszka Kulawik-Pióro, Emilia Osak, Mariola Mendrycka, Zuzanna Trześniewska-Ofiara
The investigated bigels belong to the group of non-Newtonian liquids that are shear thinning with the yield stress. The rheograms of the analyzed bigels presented in the figure have a visible hysteresis loop; moreover, the hysteresis phenomenon does not disappear, which means that none of the received preparations rebuilds the system structure after the disappearance of shear. This confirms the results of Singh and co-workers[59] that bigels have thixotropic properties. A thixotropic system reveals a loss in apparent viscosity over time at a constant shear rate, while the shear stress is removed. Subsequently, the apparent viscosity constantly increases and returns to its primary value. It is assumed that under shearing stress, the bonding between polymer chains is broken down, resulting in a more fluid-like low-viscosity system, while in the absence of mechanical forces, polymer particles rejoin again, and the three-dimensional network rebuilds, restoring the primary structure of the material.[60,61] The thixotropy behavior leads to relatively low viscosity of the product at the time of its application, but also prevents it from running off the skin surface. Also, their pseudoplastic tendency results in the formation of a coherent film covering the skin surface. This property is valuable and critical for a better fortification of the skin surface.[62] Furthermore, Wróblewska et al.[61] claim that the shear thinning phenomenon is considered a relevant parameter for the local application of the semisolid formulation. This facilitates the formation of a thin layer of the gel preparation when it is applied locally, which results in the efficient delivery of active agents at the site of application. Bigels had a similar energy dissipated, which means that the changes to the bigels structure due to the shear rating used were the same in all samples.