Complex Thixotropy Research Articles

Effect of Electrolytes and Polymers on the Thixotropy of Mg‐Al‐Layered Double Hydroxides/Kaolinite Dispersions

AbstractThe effect of electrolytes (NaCl and CaCl2) and polymers (CPAM and HPAM) on the thixotropy of Mg‐Al‐layered double hydroxide (LDHs)/kaolinite dispersions has been investigated. It was observed that the type of thixotropy in LDH/kaolinite dispersions may be affected by NaCl, but not by CaCl2 in range of concentration of interest. The type of thixotropy in LDH/kaolinite dispersion with R=0 transformed from positive thixotropy to complex thixotropy and at last positive thixotropy again with the concentration of NaCl in range of 0.00–0.10 mol·L−1; the type of thixotropy in LDHs/kaolinite dispersions with R=0.25 transformed from complex thixotropy to positive thixotropy and then complex thixotropy again with the concentration of NaCl in range of 0.00–0.10 mol·L−1. The type of thixotropy in LDH/kaolinite dispersion with R=0 may be not affected by cationic polyacrylamide (CPAM) and hydrolyzed polyacrylamide (HPAM); but the LDHs/kaolinite dispersions with R=0.25 transformed from complex thixotropy to positive thixotropy with the both polymers concentration in range of interest, which indicated that the microstructure of the dispersion changed from weak folc sediments structure to steric network structure.

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.

Thixotropy of Mg–Al-layered double hydroxides/kaolinite dispersion

The rheological characteristics of Mg–Al-layered double hydroxide (LDHs)/kaolinite dispersions were investigated as a function of pH and mass ratio of LDHs to kaolinite ( R). The stress ramp and stress sweep experiments were used to measure the yield stress of the dispersions. In both experiments, a critical shear stress ( τ c) was obtained at which the viscosity decreased by 3–4 orders of magnitude. The τ c value decreased first and then increased with the increase of the R, showing a minimum at R = 0.05. The linear oscillatory shear experiments and the thixotropic loops experiments were performed to investigate the thixotropic behavior of the dispersions. The complex thixotropy was observed with the linear oscillatory shear experiments, and it was found that the thixotropic type of the LDHs/kaolinite dispersions may be affected by R in the range of 0–0.35 and pH value in the range of 3–12. With the increase of R or pH value, the thixotropic type of LDHs/kaolinite dispersions transformed from positive thixotropy to complex thixotropy. By comparing the results of the thixotropic loops experiments with those of the linear oscillatory shear experiments, it was demonstrated that thixotropic loop of complex thixotropy had a crossover point that divided the loop into two sections.

The Relationship between Steady Shear Viscosity and Complex Viscosity

Theoretical analysis on the relationship of complex viscosity (η*) and steady shear viscosity (η) indicates that viscous viscosity (η′) measured under linear viscoelastic region equates with steady shear viscosity at low shear rates, and the Cox‐Merz Rule applies only to the suspension whose viscous viscosity of the system dominates the whole response. From theoretical analysis, complex fluids with network structure such as hydrotalcite‐like compounds (HTlc)/montmorillonite (MT) suspensions are not suitable to the Cox‐Merz Rule, and departure from the rule has been found in all the HTlc/MT suspensions examined. The theoretical deduction and experimental results from HTlc/MT suspensions show that thixotropic types can be determined by the change of η′ in the linear viscoelastic region after the system has been presheared; the increase of η′ with time means positive thixotropy while the decrease of η′ with time means negative thixotropy. First increase and then decrease of η′ with increasing of time or vice versa means complex thixotropy. A very useful method to examine the thixotropic types in the undisturbed states has been found for the first time.

Influence of measuring conditions on the thixotropy of hydrotalcite-like/montmorillonite suspension

Abstract The influence of the measuring conditions on the thixotropy of suspensions, which were made up of hydrotalcite-like compounds (HTlc) and Na-montmorillonite (MT), was studied. Usually, the structure recovery at rest after preshear is considered the fundamental thixotropic process. And the recovery process was performed under the steady shear experiments, then the thixotropic type can be judged according to the recovery process. In this paper, the influence of rest time (ts) and the shear rate ( γ ) on the recovery process was studied. Three kinds of HTlc/MT suspension were studied, their mass ratios of HTlc to MT, which were signed as R (e.g. R=WHTlc/WMT), were 0.013, 0.051, 0.091, respectively. It is found the HTlc/MT suspension with R=0.013 showed positive thixotropy at ts=0, but it changed to negative one with the increase of ts. The suspension with R=0.051 transformed from complex thixotropy at low γ or positive thixotropy at high γ into negative one with the increase of ts. The suspension with R=0.091 behaved as complex thixotropy when γ is 1022 s−1, and it behaved as negative thixotropy when γ is 10 s−1. Whereas, the suspension transformed from negative thixotropy to the weak complex one as the increase of ts when γ is 170 or 341 s−1. For all the systems, the equilibrium viscosity decreased gradually with ts at the low γ , but the equilibrium viscosity increased with ts at high γ because of the memory effect. The mechanism has been discussed.

Influence of Shear Rate on Thixotropic Suspensions

The thixotropic type of a suspension can be specified by first shearing the suspension at high shear rate to destroy the structure of the suspension, then monitoring the change of the viscosity or stress with time under low shear (D l), i.e., to study the process of structure recovery. The shear history and D l can influence the recovery process, so they have an influence on the thixotropic type of a suspension. In this paper, the influence of D l on the thixotropy of Fe–Al–Mg‐mixed metal hydroxide (MMH for short)/Na‐montmorillonite (MT for short) suspensions is reported. The system with a weight ratio (R) of MMH/MT of 0 and with R = 0.013 both show positive thixotropy in the D 1 range of 10–1022 s−1, i.e., D l does not change the thixotropic type of the suspensions. The suspension with R = 0.051 shows complex thixotropy under lower D l values (10 and 170 s−1), but shows positive thixotropy under higher D l values (511 and 1022 s−1). The system with R = 0.091 shows negative thixotropy at lower D l values (10 and 170 s−1), but shows complex thixotropy at higher D l values (511 and 1022 s−1). With increasing D l, the viscosity of these suspensions decreases gradually, and the degree of change in η ∼ t slows down, i.e., the thixotropic strength becomes weak. The mechanism for these changes is discussed.

The influence of shear rate and rest time on the thixotropy of the complex thixotropic suspension of Fe-Al-Mg-MMH/montmorillonite

We have studied the influence of the measuringconditions on the thixotropy of the complex thixotropic system with the weight ratio of Fc-Al-Mg-MMH-to-montmorillonite(R) being 0.051, especially the effect of the shear rate(D 1 ) after high-intense shearing and the rest time (t s ) before measuring the viscosity. The influence of D 1 ) and i s was found complex, i.e. when t s = 0, the system showed complex thixotropy when D 1 was low (10. 170 s - 1 ), whereasthe thixotropic type changed to positive whenD 1 was high (340, 511, 1022 s - 1 ). The viscosity dereased with increasingD 1 . In the studiedrangeof t s t s (0-2 hl, the system changed from complex thixotropy at low D 1 (10, 170 s - 1 ) and positive thixotropy at high D 1 (340, 511, 1022 s - 1 ) all into negative thixotropy with the prolong of t s . The viscosity decreased with at low shear rate(10, 170 s - 1 ) and increased with t s at high shear rate (340, 511, 1022 s - 1 ), 1. respectively. The mechanismis discussed.

Influence of pH on the thixotropy of magnesium aluminum hydroxide-kaolinite suspension

The influence of pH on the thixotropy of pure kaolinite suspension and magnesium aluminum hydroxide (Mg-Al-MMH)-kaolinite suspension was studied. The results show that the thixotropic type of pure kaolinite suspension was not affected by pH studied in the range of 3.60– 12.00. The thixotropic type of Mg-Al-MMH-kaolinite suspension with mass ratio (R) value of MMH to kaolinite 0.029 transformed from complex thixotropy into positive thixotropy with increasing of pH in the range of pH 3.83–12.00, and the type of thixotropy of Mg-Al-MMH-kaolinite suspension withR = 0.129 transformed from positive thixotropy into complex thixotropy with increasing of pH in the range of pH 3.70–11.96.

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.