Thixotropic Phenomenon Research Articles

PERSPECTIVE: Analysis of thixotropic timescale

Thixotropy is characterized by a time-dependent rise in viscosity under no or weak flow conditions and a decrease in viscosity over time when subjected to strong flow conditions. The characteristic timescale associated with the thixotropic phenomenon, particularly how the viscosity increases with time, has been termed the thixotropic timescale. Several approaches have been suggested in the literature for estimating the thixotropic timescale. The most prominent approach, however, infers it from a specific form of a kinetic expression for structure parameter evolution. In this paper, we study the various kinds of structural kinetic models. By carefully analyzing the same, we propose a parameter for the thixotropic timescale associated with the most generic form of kinetic expression for structure parameter evolution. We observe that increasing the thixotropic timescale weakens the thixotropic character of a system when the viscosity of the structural kinetic model continuously increases over time and eventually diverges under quiescent conditions. We also propose a new phenomenological measure of the thixotropic timescale: τthix=(dlnη/dt)−1, where η is viscosity and t is time. The proposed definition allows a straightforward and unique way to determine the thixotropic timescale through experiments and agrees well with the conventional notion of thixotropy.

Effect of temperature on dynamic and steady-state shear rheological properties of Jambolan (Syzygium cumini) pulp

Jambolan is an exotic fruit rich in phenolic compounds with many health benefits. The study of fruit's rheological properties plays key roles in its industrial processing and obtainment of products. The present work evaluated the time-dependent and steady-state shear rheological properties of Jambolan pulp at several temperatures. The Jambolan pulp presented thixotropy as a time-dependent property and also showed shear thinning behaviour with yield stress in the steady shear measurements. The Figoni-Shoemaker and Hershel-Buckley models adequately described both the product rheological phenomena. During the oscillatory analysis, the pulp showed some viscoelastic properties, increasing when the oscillatory frequency (ω) increased. All previous parameters also verified the impact of the temperature. The increment in temperature resulted in a reduction in consistency levels. In the temperature range studied, the viscoelastic solid component presented higher values than the viscous component. The thixotropic phenomenon and the presence of the yield stress can be related to this. Viscoelastic moduli decreased with the increase in temperature. When relevant, the Arrhenius’ Activation Energy of some model parameters were calculated. The data obtained are applicable for future studies, to improve industrial process design, and to evaluate food properties.

Chemical and thixotropic contribution to the structural build-up of cementitious materials

The structural build-up of fresh cement paste is often considered as a purely thixotropic phenomenon in literature even though cementitious materials undergo a non-reversible hydration process that can have an influence on the structuration process. In the current paper a method is proposed to validate the impact of the non-reversible structural build-up. It is shown that fresh cement paste samples lose their structural gain almost completely due to thixotropy while the structural build-up due to hydration can be observed but occurs in a significantly lower order of magnitude over the course of the first hours of hydration. In addition, it is shown, that the chemical component of the structural build-up accelerates with the onset of the acceleration period of hydration, while its contribution in the entire structural build-up remains constant.

Synthesis and thixotropic characterization of micron-nanometer magnetic grease with high suspension stability

The grid structure formed by the ionic bonding between bentonite and carrier liquid could support and uniformly disperse the micro-nanometer magnetic particles. In order to overcome the sedimentation of magnetorheological fluid the bi-dispersed magnetic grease with excellent magnetism suspension stability and thixotropy was prepared for the first time. The suspension stability was accurately measured by the L-C vibration detection. By observing the time dependence of the viscosity under coupling effect shear rate and magnetic field the thixotropy phenomenon was found and analyzed. The results showed that the addition of bentonite increased the suspension stability by 17.873%. A surprising thixotropy with changes regularly was found even the viscosity increased by an order of magnitude. Moreover the rate of viscosity changes was accelerated with the decrease of shear rate and the increase of magnetic field. Under the action of shear rate the formation and destruction of the magnetic response of the particle cloud structure were promoted by the grid structure with a large internal surface area. This work has improved the suspension stability of rheological fluid and found the thixotropic phenomenon with regular changes which is of great significance for the precise control and stable operation of damping braking and transportation systems.

Time dependent stress–strain behavior of bentonite slurries; effect of thixotropy

Thixotropy phenomenon in soils is usually defined as the gain of shear strength with time developed by some fine grained soils after remolding. This work reports the results of a study focused on the thixotropic phenomenon developed in sodium bentonite slurries saturated with sodium chloride solutions at different electrolyte concentrations. Changes in fabric and structure of the soil during the thixotropic process were monitored indirectly by measuring wave velocity, matric suction, electrical resistance and capacitance, and shear strength. The slurries were placed in a water bath to keep the samples at constant temperature during remolding and subsequent thixotropic hardening. The obtained results show that there is no variation in water tension during thixotropic behavior. Results also suggest that fabric changes occur during the process, which reveals the influence of unbalanced electrical forces at double layer levels. Rising in temperature and lowering in electrical resistance during remolding is attributed to the effect of breaking of bonds between particles. Increase in salt concentration makes double layer shrinks and particles tend to accommodate closer in a more parallel structure. The effect of salt concentration, and consequently the soil structure, becomes relevant in the shear strength of the pastes but it has negligible effect on stiffness.

Possible Further Evidence for the Thixotropic Phenomenon of Water

In this work we review the literature for possible confirmation of a phenomenon that was proposed to develop when water is left to stand for some time undisturbed in closed vessels. The phenomenon has been termed thixotropy of water due to the weak gel-like behaviour which may develop spontaneously over time where ions and contact with hydrophilic surfaces seem to play important roles. Thixotropy is a property of certain gels and liquids that under normal conditions are highly viscous, whereas during mechanical processing their viscosity diminishes. We found experiments indicating water’s self-organizing properties, long-lived inhomogeneities and time-dependent changes in the spectral parameters of aqueous systems. The large-scale inhomogeneities in aqueous solutions seem to occur in a vast number of systems. Long-term spectral changes of aqueous systems were observed even though the source of radiation was switched off or removed. And water was considered to be an active excitable medium in which appropriate conditions for self-organization can be established. In short, the thixotropic phenomenon of water is further indicated by different experimental techniques and may be triggered by large-scale ordering of water in the vicinity of nucleating solutes and hydrophilic surfaces.

틱소몰딩 공정을 이용한 AZ91D Mg합금의 기계적 성질 증대

Thixomolding of Mg-alloy is a semi-solid injection molding process utilizing thixotropic phenomenon. Using this process, higher strength, thinner wall section and tighter tolerance without porosity are obtained. It has been applied for production of near-net-shape magnesium component. To design optimal thixomolding process of Mg-alloy part, molding conditions such as slurry temperature, mold temperature and injection time should be determined properly. Selection of these parameters has been dependent upon engineers' experience and intuitiveness. In this paper, to improve mechanical properties of the thixomolded product, optimal selection of process variables such as injection velocity, barrel temperature and die temperature in the process has been studied through microstructural analysis and Taguchi method. Performance of the process is verified through experiments.

Properties of very soft clays: A study of thixotropic hardening and behavior under low consolidation pressure

In a study on the properties of very soft clays, bender element testing was used to evaluate thixotropic hardening behavior; that is, to measure the stiffness with resting time under constant volume and water content. A laboratory vane test, which measures the undrained shear strength of the materials, was also carried out for comparison purposes. To investigate the mechanism of the thixotropic phenomenon, a consolidation test with very low pressure was also performed in a cell equipped with bender elements. The most important findings from this study are as follows: (1) regardless of soil types, the effect of thixotropy was significant around the liquid limit state and less remarkable at the lower and higher ranges; (2) the shear modulus at the liquid limit after 24h resting is around 200kPa; (3) the correlation between the shear modulus and the undrained shear strength of very soft clays is similar to that of cement-treated soil proposed by Seng and Tanaka (2011); (4) the increment of the shear modulus developed in the thixotropy process appears to be noticeably higher than that in the secondary consolidation process. It is believed that these findings are very useful to establish a new theory for the consolidation of ground filled by very soft clays or dredged soils with extremely high water content as well as to understand the effects of ageing on the consolidation properties of natural soils.

Thixotropy of Aqueous Suspensions Containing Mg-Al Hydrotalcite-like Compound and Low-substituted Cationic Starch: Comparison between Oscillatory Shear and Thixotropic Loop Measurements

The rheological properties of aqueous suspensions consisting of positively charged aluminum magnesium hydrotalcite-like compound (HTlc) and low-substituted cationic starch (LCS) were investigated. Special emphasis was placed on the thixotropic phenomenon. Thixotropic behavior was investigated by two thixotropic methods: thixotropic loop and oscillatory shear measurements. LCS molecules could be adsorbed onto HTlc particles due to the hydrogen bonding between ether groups or hydroxyl groups of LCS and hydroxyl groups of HTlc. The elastic dynamic response of the HTlc/LCS suspension increased with increasing mass ratio (R) of HTlc and a three-dimensional network structure could be formed in the suspension with higher R value. The thixotropic type of the HTlc/LCS suspension transformed from negative to positive and then to complex thixotropy when R changed from 0 to 0.5. By comparing between the thixotropic results obtained by thixotropic loop and oscillatory shear measurements, it was validated that the thixotropic loop for the suspension showing complex thixotropy had a crossover point.

Thixotropic phenomenon in flocculated aqueous dispersions of acrylate methacrylate copolymers

Aqueous gels of two analogous, water-insoluble copolymers A and B have been formed by addition of excess water to concentrated ethanol solutions of the polymers. A and B differed only in their content of cationic groups in a ratio 2:1 (A:B). The gels were converted permanently to fluids (i.e. gel-sol transformation) at high shear rates. Stability of the resulting polymer dispersions depended on the presence and mutual repulsion of the polymer cations. Polymer B dispersions were less stable to the flocculating effect of an electrolyte (sodium chloride). At certain critical concentrations, 0.2 M NaCl (for polymer A), or 0.1 M NaCl (for polymer B) the electrolyte flocculated the otherwise stable dispersions to a gel structure. The electrolyte-flocculated gels were readily redispersed to fluids by shaking but reverted to gels on standing (thixotropic). In contrast the original coacervated gels (without electrolyte) could not be redispersed easily with manual shaking. Lower polymer-polymer interaction in the thixotropic system relates possibly to increases in particle size and irregularity of particle shape during flocculation.

Thixotropic properties of aqueous suspensions containing cationic starch and aluminum magnesium hydrotalcite-like compound

The rheological properties of aqueous suspensions consisting of cationic starch (CS) and positively charged aluminum magnesium hydrotalcite-like compound (HTlc) were investigated. Special emphasis was placed on the thixotropic phenomena. With the increase of mass ratio ( R) of HTlc to CS, the equilibrium viscosity ( η eq ) and the consistency coefficient ( m) values of the suspensions increase in the range of neutral and alkaline pH (higher than 6.5) while decrease in the range of acid pH (lower than 6.5). With the increase of pH value, the η eq and m values of the suspensions in the R range of 0–0.08 studied increase initially and then decrease, appearing a maximum value at about pH 7.41 ± 0.25 . The CS/HTlc suspensions display viscid character and the yield point of the suspensions was not observed except the suspension with R = 0.08 in the pH range of 7.66–9.70, which showed a yield point and viscoelasticity. The CS/HTlc suspensions may display different thixotropic types: negative, complex or positive thixotropy, depending on pH and R value. The thixotropic type of the CS/HTlc suspension may be transformed from negative (pure CS solution), through complex ( R = 0.02 ), into positive thixotropy ( R = 0.05 and 0.08) with the increase of R in the studied R range of 0–0.08, and the thixotropic strength of the suspensions increases initially and then decreases with pH value in the pH range studied. The mechanism of the thixotropic phenomenon is discussed.

Evaluation on Accuracy of the Rheological Data of PIM Feedstocks.

The rheological characterization with capillary rheometer requires three major corrections named as the Rabinowitsch, Bagley and Mooney corrections. Due to the amount of time and effort needed to complete the correction procedure, some engineers may avoid the corrections. However, engineers who want to avoid it should understand how much the theological data is affected by influencing factors. In this article, the correction values made by the Rabinowitsch and Bagley corrections were analyzed for PIM feedstocks in terms of particle shape and size, powder concentration, binder composition, temperature, shear rate and capillary length. Two types of SUS powders with irregular and round particle shapes and a zirconia powder were used with wax-based binders. As results, particle shape and size were the major influencing factor followed by powder concentration. Also a longer capillary is recommended for less error as long as it does not cause thixotropic phenomenon. Without extensive experiment, a feasible result could be obtained only by the Rabinowitsch correction for feedstocks with round particle shape at a relatively low powder concentration. However, the correction is strongly recommended for feedstocks with irregular particle shape at a relatively high powder concentration.

Study on the thixotropy of aluminum magnesium hydroxide- Na-montmorillonite suspension

In this paper, the thixotropy of the aqueous suspension consisting of aluminum magnesium hydroxide (AMH) possessing permanent positive charges and Na-montmorillonite possessing permanent negative charges was studied. Besides positive thixotropy and negative thixotropy, a novel thixotropic phenomenon was found, i.e., a given system studied may display, early and late, positive M28.8nthixotropic character and negative thixotropic character, we describe it as “complex thixotropy”. The content of the suspension and electrolyte may influence the forms of thixotropy of the suspension studied.

Low temperature rheological behavior of Umbarka waxy crude and influence of flow improver

The low temperature flow properties (pour point and rheological parameters) have been determined for untreated and additive-treated Umbarka waxy crude. The rheological characteristics of the crude showed non-Newtonian yield-pseudoplastic flow behavior and a pronounced thixotropic effect. Viscometric data are fitted to Bingham, Casson and Herschel-Bulkley plastic flow models using linear and non-linear computer regression analysis. Herschel parameters showed inconsistent results, whereas the Casson equation fitted the data reasonably well. Treatment of the crude with flow improver ameliorated both rheological properties and pour point, but in a different manner. The influence of the additive on the rheology of the crude is affected greatly by shear rate. The thixotropic phenomenon of the crude is likely to be temperature-dependent in addition to the other controlling factors.

Low Temperature Rheological Behavior of Umbarka Waxy Crude and Influence of Flow Improver

The low temperature flow properties; pour point and rheological parameters; have been determined for untreated and additive treated Umbarka waxy crude. The rheological characteristics of the crude showed non-Newtonian yield pseudoplastic flow behavior and pronounced thixotropic effect. Viscometric data are fitted to Bingham, Casson and Herschel-Bulkley plastic flow models using linear and non-linear computer regression analysis. Herschel parameters showed inconsistent results whereas Casson equation fitted the data reasonably well. Treatment of the crude with flow improver ameliorated both rheological properties and pour point but in a different manner. The influence of the additive on the rheology of the crude is greatly affected by shear rate. The thixotropic phenomenon of the crude is likely to be temperature-dependent in addition to the other controlling factors.

Unusual Thixotropic Properties of Aqueous Dispersions of Laponite RD

The rheological properties of aqueous dispersions of the synthetic hectorite clay Laponite RD that form gel-like structures have been investigated. Special emphasis has been laid on the phenomenon of thixotropy. Structural recovery at rest after steady shear is considered a fundamental thixotropic process and has been characterized by small amplitude oscillatory shear measurements. A special sealing technique was used to prevent the evaporation of water during these experiments. After cessation of steady shear |η*| increases monotonically with time and even after 16 days no equilibrium viscosity value is reached. A single power law |η*| ∼tnholds within the time regime from 10 to 106s. The exponentn= 0.13 ± 0.02 is independent of clay concentration and mechanical pre-treatment of the material. This type of kinetics has not been reported so far for the thixotropic recovery of any clay dispersion and it does not fit the common definition of thixotropy. The reorganization of the gel structure determining this thixotropic phenomenon is interpreted as a cooperative self delaying process similar to the aging of glassy polymers or precipitation from supersaturated solid solutions.

CJ89-02. Design and study of lathe tools of heat pipe type: By Qiao Zhongfu and Liu Jiannan (Northeast University of Technology, Shenyang, China). Chinese J. Mech. Engng25, No. 1 (March, 1989), pp. 8–12

The low temperature flow properties (pour point and rheological parameters) have been determined for untreated and additive-treated Umbarka waxy crude. The rheological characteristics of the crude showed non-Newtonian yield-pseudoplastic flow behavior and a pronounced thixotropic effect. Viscometric data are fitted to Bingham, Casson and Herschel-Bulkley plastic flow models using linear and non-linear computer regression analysis. Herschel parameters showed inconsistent results, whereas the Casson equation fitted the data reasonably well. Treatment of the crude with flow improver ameliorated both rheological properties and pour point, but in a different manner. The influence of the additive on the rheology of the crude is affected greatly by shear rate. The thixotropic phenomenon of the crude is likely to be temperature-dependent in addition to the other controlling factors.

Mechanism of thixotropic behavior at relaxed joints in the rat.

When a relaxed joint is subjected to a small sinusoidal torque, the amplitude of the steady-state displacement response is increased up to severalfold by a transient larger perturbation. The original state, in which the relaxed joint is unexpectedly stiff, is restored by several seconds of inactivity. This thixotropic phenomenon has previously been observed in a variety of human joints. We have now investigated the mechanism of thixotropic behavior at relaxed joints in rats anesthetized with pentobarbital sodium, by using a series of preparations including the intact ankle joint, a blood-perfused soleus muscle preparation, an isolated soleus muscle, and ankle joint isolated by severing all muscular attachments. Thixotropic behavior was observed in all intact, isolated muscle, and isolated joint preparations. The contribution of the joint to thixotropic behavior was comparable to, and at times exceeded, the contribution of muscle. We also analyzed the short-range stiffness properties of relaxed, blood-perfused soleus muscles and found them to be similar to thixotropy with respect to range of action (0.2-0.3% of muscle length), elastic modulus (approximately 4 kg/cm2), and time course for redevelopment (time constant = 2.5 s at 34 degrees C). Thus thixotropic behavior at a relaxed joint may be attributed both to the joint structures and to short-range stiffness of muscles acting at the joint.

Structure formation in heated clay dispersions

The results of an investigation of structure formation processes in mineral dispersions are given. It is shown that the thixotropic phenomenon has an important meaning in the strengthening of mineral systems without additional pressure. The conditions for achieving optimum structure under drying of mineral dispersions were revealed. The theological investigations of some minerals under high temperature was introduced, and the role of crystalline transformations of kaolinite and montmorillonite was shown affect the greatest plastic visconsity η 1 of their dispersions.

Semisolid bases contaning hydroxypropyl cellulose.

Intending to explore useful ointment bases containing hydroxypropyl cellulose (HPC), an investigation was made on the spreadability, washing-out time and drug release of combined mixtures of HPC with aqueous glycerine (Aq. G1), propylene glycol (PG) and polyethylene glycol (PEG). The decrease of the spreadability due to the increase of concentration of HPC was remarkable in the low concentration, declining above 10% (w/v) in HPC/PG and above 14% (w/v) in HPC/PEG. HPC/PEG containing above 12% (w/v) lost the fluidity to take a gel-like state and this gel-like mixture indicated a typical thixotropic phenomenon. The washing-out time increased with the concentration of HPC, and it seemed to be related to the increase of viscosity. With the addition of 30% of zinc oxide as an insoluble drug, the decrease of spreadability was 23% at the largest, which was observed for HPC/Aq. G1. The spread radius at 2 min on the measurement for the sample of HPC/PEG (10 : 100, w/v) after heating at 150° for 1 hr was 54% larger than that of the intact one, indicating the structure relating to the thixotropy of the mixture might be crushed by heating. The drug release from each mixture decreased with the increase of concentration of HPC. HPC/PEG showed the fastest drug release at the given spreadability below 4.1 cm. As a result, it was shown that HPC would provide a promising material as bases for ointments and other semisolid preparations combining properly with Aq. G1, PG and PEG.