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The Thixotropic Properties of Hydrotalcite-like/ Montmorillonite Suspensions Shu-Ping Li, Wan-Guo Hou,* De-Jun Sun, Pei-Zhi Guo, and Chun-Xiao Jia Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan, Shandong 250100, People’s Republic of China
Ji-Fan Hu Physics Department, Shandong University, Jinan, Shandong 250100, People’s Republic of China Received October 10, 2002. In Final Form: January 7, 2003 The rheological properties of hydrotalcite-like compounds (HTlc)/sodium montmorillonite (MT) suspensions, which have formed gel-like structures, have been investigated. Special emphasis has been placed on the phenomenon of thixotropy. Usually, the structure recovery at rest after steady shear is considered the fundamental thixotropic process. And the recovery process was performed at the amplitude oscillatory shear measurements and the steady shear measurements. The oscillatory experiments were performed in the linear viscoelastic region so as to make the recovery process undisturbed. In the oscillatory experiments, η* increases monotonically with the time after the preshear process, and even after 3 h no equilibrium viscosity value of the suspension is reached. A single power law |η*| ∼ tn holds within the time regime from 10 s to 3 h, and the exponent n ) 0.158 ( 0.10 is not only independent of the concentration and composition of the clay and the HTlc but also independent of mechanical pretreatment of the suspension. In the oscillatory experiments, two kinds of Na-MT were used, and almost the same value of exponent n was obtained. This type of kinetics has not been reported so far for the thixotropic recovery in the mixed suspension. And the reorganization of the gel structure is interpreted as a cooperative self-delaying process similar to the aging of glassy polymers or precipitation from supersaturated solid solutions. In the steady experiments, the recovery process was monitored at the low shear rate, and η does not always increase with the time. In the recovery process, the formation of the network was disturbed by the shear rate to some extent. When the structure strength (which is indicated by the value of the viscosity) of the system is low, the disturbance of the shear rate on the recovery process is weak, which becomes distinct when the structure strength is high.
1. Introduction Hydrotalcite-like compounds (HTlc) have the general formula [MII1-xMIIIx(OH)2]x+An-x/n‚mH2O, where MIII is trivalent mental ions, MII is divalent metal ions, A is the charge compensating anions, m is the number of moles of co-intercalated water per formula weight of compounds, and x is the number of moles of MIII per formula weight of compounds, which generally ranges from 0.2 to 0.4.1 The HTlc compounds consist of layers of MII and MIII cations, which are octahedrally co-combined by six oxygen anions, as hydroxide. These layers exist with the similar structure to that exhibited by natural Mg(OH)2, also known as brucite. The substitution of MII cations into the brucite-like hydroxide layer imparts an overall positive charge on the octahedral layer,2 which was termed an isomorphic substitution. The metal cations occupy the centers of the octahedral whose vertexes contain hydroxide ion. These octahedrons are connected to each other by the shared edges to form the sheet. The cationic charges in the layers have been termed permanent positive charges. Sodium montmorillonite belongs to the 2:1 layer clay minerals, and the basic cell unit consists of an octahedral alumina sheet sandwiched between two tetrahedral silica sheets. In the tetrahedral sheets, trivalent Al sometimes * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone number: +86-0531-8564712. (1) Constantino, U.; Casciola, M.; Massinelli, L. Nocchetti, M.; Vivani, R. Solid State Ionics 1997, 97, 203-212. (2) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2001, 105, 1743-1749.
replaces tetravalent Si, whereas in the octahedral sheet there may be replacement of trivalent Al by divalent Fe or Mg. This phenomenon, which is referred as isomorphous substitution, results in a deficit of positive charges or excess of negative charges, which are compensated by adsorption of cations. Hence, the similarity between sodium montmorillonite clay and the HTlc compounds is that their particles are all platelike and they all bear permanent charges. The sodium montmorillonite bears negative charges, and the HTlc compounds bear positive charges. The suspension of HTlc/Na-montmorillonite shows lots of novel properties, which has aroused extensive attention.3 The suspension of HTlc/MT easily forms three-dimensional gel-like structures instead of flocculation or coagulation; consequently extensive studies have been reported on their rheological and electrical properties.4 This paper concentrates on an accurate experimental determination of the rheological properties of the gel-like HTlc/MT suspension, such as viscoelasticity, yield stress, and thixotropy. Special emphasis is placed on the phenomenon of thixotropy, which is typical not only for HTlc/ MT suspension but also for any natural clay gels. The recovery of the suspension at rest after steady shear is considered a fundamental thixotropic process. Small amplitude oscillatory shear measurements are used to (3) Hou, W. G.; Sun, D. J.; Han, S. H.; et al. Colloid Polym. Sci. 1998, 276 (3), 274-277. (4) Li, S. P.; Hou W. G.; Dai, X. N.; et al. Acta Chim. Sin. 2002, 60 (4), 749-752.
10.1021/la026669w CCC: $25.00 © 2003 American Chemical Society Published on Web 03/06/2003
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Table 1. Chemical Composition and Chemical Formula of the Examined HTlc Samples sample no.
sample
raw material molar ratio
chemical composition
1 2 3 4 5 6 7
Mg/Al/Fe Mg/Al/Fe Mg/Al/Fe Mg/Al/Fe Mg/Al/Fe Mg/Al Zn/Al
2:1:0.1 2:1:0.6 2:1:1 3:1:1 4.5:1:1 2:1 2:1
[Fe0.034Mg0.65Al0.32(OH)2]Cl0.14(OH)0.22 [Fe0.21Mg0.46Al0.33(OH)2]Cl0.11(OH)0.41 [Fe0.29Mg0.42Al0.29(OH)2]Cl0.10(OH)0.48 [Fe0.24Mg0.52Al0.24(OH)2]Cl0.12(OH)0.36 [Fe0.16Mg0.68Al0.16(OH)2]Cl0.18(OH)0.14 [Mg0.45Al0.55(0H)2]Cl0.43(OH)0.15 [Zn0.48Al0.52(OH)2]Cl0.18(OH)0.34
determine the kinetics of the recovery process, and the steady shear measurements are performed at low shear rate in order to monitor the recovery process. In the oscillatory experiments, various conditions of preshearing, various concentrations, and different compositions of HTlc compounds are examined in order to determine the kinetics formula of the recovery process. Moreover, the influence of MT concentration on the recovery process has also been studied; both the purified and the unpurified MT samples have been studied to determine the recovery kinetics formula. Mewis and De Bleyser originally introduced this approach to the characterization of dynamic processes in thixotropic material, and its feasibility was demonstrated using a polyamide gel as a model system.5 Moreover, this method is commonly used to study the gelation kinetics of the model system such as laponite, due to its high purity and narrow particle size distribution, but this method is for the first time used to study a mixed suspension whose particle size distribution is wide. In this paper, some important results have been reported. Different results are obtained from the linear oscillatory experiments and from the steady shear experiments. In the oscillatory experiments, the suspension structure is only slightly perturbed from the structure at rest. But in steady shear experiments, even under the low shear rates, the structure of the suspensions is disturbed or destroyed. The appearance of the disturbing force will influence the recovery process, as a result, the recovery process exhibits in different ways. 2. Experimental Section 2.1. Materials. The Preparation and the Analysis of HTlc. HTlc was synthesized by the coprecipitation method. The predesigned metal chlorides were dissolved in deioned water and then the solution of NH3‚H2O was slowly pumped into the mixed solution of metal chloride; the amount of NH3‚H2O was controlled to adjust the pH to about 9.5. The product was delicate flocs suspended in the aqueous solution of NH3‚H2O. Then the product was filtered and washed in the filter with deioned water to remove the excess NH3‚H2O, at last the filter cake was peptized at about 80 °C in oven for about 24 h to convert it into HTlc sol. The chemical composition was found with a 3080E2 type automatic X-ray fluorescent spectrometer (made in Japan), a JEM-100cx II model transmission electron microscope (made in Japan) was used to examine particle morphology. The lattice parameters of HTlc were obtained from the X-ray powder diffraction analysis (XRD) on a D/max-γA model diffractometer using Cu KR radiation (40 kV and 80 mA). All the chemical composition is listed in Table 1. The Preparation of MT Clay. In this paper, two kinds of sodium montmorillonite were used. One sample of sodium montmorillonite was transformed from commercial calcium montmorillonite (supplied by Anqiu Chemical Plant, Shandong Province),6 which was used without any treatment. This commercial material contains a small amount of impurities of organic matter and electrolytes, and no attempt was made to remove these impurities. (5) Mewis, J.; de Bleyser, R. J. Colloid Interface Sci. 1972, 40 (3), 360-369. (6) Li, S. P; Hou, W. G.; Sun, D. J.; et al. Chem. J. Chin. Univ. 2001, 22 (7) 1173-1176.
Figure 1. Apparent viscosity vs shear stress as obtained by creep tests for HTlc/MT suspension with R ) 0.17. Another sample of the sodium montmorillonite was purified as follows:7 The clay was first suspended in distilled water by stirring and then decanted leaving a residue of coarse particles. Thirty milliliters of 100 vol % H2O2 was added to 4.5 L of 10% clay to oxidize any organic matter. The dispersion was then heated to ∼70 °C to remove excess H2O2. It was then centrifuged, and the supernatant (containing the electrolyte) was replaced several times with double-distilled water until its conductivity became relatively low (∼4 × 10-5 Ω-1 cm-1) indicating that most of the electrolytes had been removed. The final concentration of the clay can be determined by evaporation. A weighed amount of the sodium montmorillonite was mixed with distilled water (doubly distilled water in an glass apparatus) using a high speed mixer (model GJ-1, Jiangyin Second Electrical Machinery Plant) for 20 min, and this procedure ensured sufficient dispersion of the powder in water. Then after aging in a sealed container for 24 h, the sodium montmorillonite can be used in the experiments. 2.2. Measurements. Oscillatory shear experiments were performed on a controlled stress instrument (RS 75, Hakke Inc., Germany) using cylindrical geometry. Creep and creep recovery experiments were also carried out using the controlled stress viscometer (RS 75, Hakke Inc., Germany), and the computer recorded all measured data. Steady shear experiments were performed on a controlled shear rate instrument with concentric cylinder viscometer (model ZNN-D6, Qingdao Camera Factory). The mixture of MT and HTlc was stirred by the high speed mixer for 20 min and then aged for about 24 h before the experiment. The mass ratio of HTlc and MT was signed as R, e.g., R ) WHTlc/WMT.
3. Experimental Results 3.1. Apparent Yield Stress. The apparent viscosity η of the HTlc/MT suspension with R ) 0.17 as a function of shear stress is shown in Figure 1 and Figure 2. The HTlc samples in Figure 1 are samples 1-3, and the content of Fe3+ in HTlc increased gradually from sample 1 to sample 3. The HTlc samples in Figure 2 are samples 3-5, and the content of Mg2+ in the HTlc sample increased gradually from sample 3 to sample 5. The weight percentage of MT in the HTlc/MT suspension was fixed at 3%. From each curve, a critical shear stress τc is observed at which the viscosity changes by more than 4 orders of magnitude. It can be seen from Figure 1 and Figure 2 that (7) Heath, D.; Tadors, TH. F. J. Colloid Surface Sci. 1983, 93 (2): 307-319.
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Figure 2. Apparent viscosity vs shear stress as obtained by creep tests for HTlc/MT suspension with R ) 0.17.
Figure 3. Dynamic moduli G′ and G′′ stress amplitude for the HTlc/MT suspension with R ) 0.17.
the value of τc for the HTlc/MT suspension increased gradually with the increasing of the content of Fe3+ in HTlc (samples 1-3) and with the increasing of Mg2+ in HTlc (samples 3-5). The value of τc to some extent can represent the structure strength of the mixed dispersions,8 then the structure strength between the HTlc and MT particles increased gradually when the content of Fe3+ in the HTlc increased gradually. Also the interaction strength between the HTlc and MT particles increased gradually with the increase of Mg2+ content in the HTlc samples. Furthermore, these data are not equilibrium values, due to the unusual thixotropic nature of the HTlc/MT suspension (see below), even in the recovery experiment, which lasts for several hours, no true steady state is reached. The creep tests considered here lasted for 15 min, and the sample were kept at rest for 20 min before the measurements started in order to reduce the influence of the mechanical treatment associated with the deposition of the sample into the rheometer. A fresh sample was used for each measurement. Similar results were obtained for the other suspensions. 3.2. Viscoelasticity. The dynamic moduli G′ and G′′ of the HTlc/MT suspension have been determined by oscillatory shear experiments. Typical results for the dependence of G′ and G′′ on shear stress and angular frequency are shown in Figure 3 and Figure 4, respectively, for the Fe-Al-Mg-HTlc/MT suspension with R ) 0.17. The weight percentage of the MT in HTlc/MT suspension was fixed at 3%. Similar results were obtained for the suspension of different R values or different concentrations of MT in the HTlc/MT suspension, respectively. When the oscillatory measurements are performed, the condition is simplified if the measurements are performed in the so-called linear viscoelastic region.9 In this region, (8) Gudrun, S.; Alan, I.; Nakatani, Charies C. Han Rheol. Acta 2002, 41, 45-54. (9) Marin, G.; Rheological Measurements; Collyer, A. A., Clegg, D. W., Eds.; Elsevier: London, 1998; p 297.
Li et al.
Figure 4. Dynamic moduli G′ and G′′ vs frequency f for the HTlc/MT suspension with R ) 0.17.
the viscoelastic response is independent of strain, and it can be assumed that no irreversible changes of the structure of the suspension take place. The linearity limit increases gradually from the suspension of HTlc sample 1 to the suspension of HTlc sample 3. Hence, it indicates that the structure strength of the network also increases gradually. At shear stress τc, G′′ decreases drastically. On the other hand, G′′ goes through a maximum at τc and dominates over G′ at τ > τc (see Figure 3). The dependence of G′ and G′′ on the frequency in the linear viscoelastic regime is shown in Figure 4. In the low-frequency regime (e.g., f < 10 Hz), the storage modulus G′ dominates over G′′ by about 1 order of the magnitude and is essentially independent of the frequency. In the high frequency (e.g., f > 10 Hz), G′′ exceeds G′ and is dependent on the frequency. The viscoelastic features reported in this section are typical for materials with a three-dimensional gel-like structure, and the results are in good agreement with the results obtained from the Laponite dispersions of the low ionic strength.10 3.3. Thixotropic Behavior in the Linear Viscoelastic Region. The re-creation of the equilibrium rest structure after shearing is considered as a fundamental thixotropic process. This process has been usually investigated by transient oscillatory shear experiments, and the kinetics of the thixotorpic recovery is usually characterized by the time dependence of |η*|. For the sake of simplicity, the oscillation frequency was set to f ) 0.1 Hz for all the experiments discussed here, as long as nothing is mentioned. The oscillatory experiments had to be carried out at sufficiently small shear stress (in the linear viscoelastic regime) in order to avoid the disturbance of the recovery process. In Figure 5, the development of |η*| after cessation of the steady shear (30 s, at γ ) 1000 s-1) is shown for different shear stress between 0.1 and 10 Pa applied to the suspension of Fe-Al-Mg-HTlc (sample 5)/MT suspension with R ) 0.17. The weight percentage of MT in the HTlc/ MT suspension was fixed at 3%. The results obtained for τ < τc, log |η*|, increase in proportion to log t, and no equilibrium viscosity value is reached, even after several hours. In the following experiments, the shear stress was set to τ ) 1 Pa. The influence of different mechanical pretreatment and preshearing procedures on the recovery of |η*| is shown (10) Norbert, W. J. Colloid Surf. Sci. 1996, 182, 501
Thixotropic Properties of Suspensions
Figure 5. Recovery of η* after cessation of steady shear under various shear stress τ in the HTlc(sample 5)/MT suspension with R ) 0.17 and τ ) 10 Pa (A), τ ) 5 Pa (B), τ ) 1 Pa (C), and τ ) 0.1 Pa (D).
Figure 6. Effect of sample history/mechanical pretreatment on the recovery of η* for R ) 0.17 suspension (A) careful placement of sample in rheometer (B) careful handling and preshearing (γ )1000 s-1, t ) 30 s) (C) intensive stiring before placement in rheometer (D) intensive stirring and preshearing (γ ) 1000 s-1, t ) 30 s).
in Figure 6 for the Fe-Al-Mg-HTlc (sample 5)/MT suspension with R ) 0.17, and the weight percentage of MT in the HTlc/MT suspension was 3%. The slope of the log |η*| vs log t curves is virtually independent of the sample history. But the viscosity level decreases as the total amount of mechanical energy brought into the material is increased. Since the different mechanical pretreatment has no influence on the linear increase of log |η*| with log t in the following experiments, the preshearing process was set to careful handling and preshearing for 30 s under the shear rate γ ) 1000 s-1. The recovery of |η*| after cessation of steady shear has been studied for various clay concentrations between 1% and 3%; see Figure 7. The HTlc sample in the HTlc/MT suspension is sample 5, and the weight ratio (R) of HTlc to MT was 0.17. When the mechanical pretreatment and R were the same for each sample, it can be seen that the concentration of the MT has no influence on the linear increase of log |η*| with log t. When the concentration of MT is fixed, the influence of the mass ratio (R) of HTlc to MT was also studied; see Figure 8. The HTlc in the HTlc/MT suspension is assigned to sample 5, and the weight percentage of MT in the HTlc/ MT suspension was 3%. From Figure 8, it can be concluded that the different values of R have no influence on the linear increase of log
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Figure 7. Recovery of η* after cessation of steady shear for the Fe-Al-Mg-HTlc/MT suspension with various concentrations of MT.
Figure 8. Recovery of η* after cessation of steady shear for HTlc/MT suspension at various HTlc/MT weight ratios (R): R ) 0.013 (A), R ) 0.051 (B), R ) 0.091 (C), and R ) 0.17 (D).
Figure 9. Recovery of η* after cessation of steady shear for Fe-Al-Mg-HTlc/MT suspension at the different MT samples: (A) unpurified 2% Na-MT; (B) purified 2% Na-MT; (C) purified 3% Na-MT.
|η*| with log t, and no equilibrium viscosity value was reached in the studied range of the time scale. In the previous experiments, the unpurified sodium montmorillonite was used to form the HTlc/MT suspension. And the purified sodium montmorillonite was also used in the following experiment to examine the influence of the purification process on the recovery process; see Figure 9. The HTlc sample in the HTlc/MT suspension is sample 5, and the R value in HTlc/MT suspension was fixed at 0.17.
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Figure 10. Recovery of η* after cessation of steady shear for the HTlc/MT suspension with R ) 0.17 at different HTlc samples: (A) sample 5; (B) sample 4; (C) sample 3; (D) sample 2; (E) sample 1.
Figure 11. Recovery of η* after cessation of steady shear for HTlc/MT suspension with R ) 0.17 at different HTlc samples: (A) sample 6; (B) sample 7; (C) sample 8.
From Figure 9, a conclusion can be obtained that the purification process has no influence on the recovery process, namely, the impurities of organic matter and electrolytes have no effect on the recovery process. In the study of the recovery process of the Laponite dispersion, the electrolytes have no influence on the recovery process.10 The influence of different HTlc samples on the recovery of |η*| is shown in Figure 10 and Figure 11. The weight percentage of the unpurified MT in HTlc/MT suspension was fixed at 3%, and the weight ratio (R) of HTlc to MT was 0.17. Hence, it can be seen from Figures 10 and 11 that different composition of HTlc samples has no influence on the recovery process, and the linear increase of log |η*| with log t is also observed. The linear relationship between log |η*| and log t shown in Figure 5 through Figure 11 corresponds to a power law
|η*| ∼ tn characterizing the kinetics of the thixotropic recovery of HTlc/MT suspensions. A single exponent is valid throughout the time regime from 10 to 10000 s. The exponent n can be determined directly from the slopes of the straight lines fitted to the experimental data in Figures 5-11 and a value of n ) 0.158 ( 0.10 is obtained, which is independent of the mechanical pretreatment, the concentration of HTlc and MT, and the composition of HTlc and MT.
Li et al.
Figure 12. Evolution of viscosity of Fe-Al-Mg-MMH/MT (sample 5) suspensions with various R values: R ) 0.013 (A), R ) 0.051 (B), and R ) 0.17 (C).
Figure 13. Evolution of viscosity of Fe-Al-Mg-HTlc/MT suspensions with R ) 0.17 at different HTlc samples: (A) sample 1; (B) sample 2; (C) sample 3; (D) sample 4; (E) sample 5.
3.4. Thixotropic Behavior under Steady Shear Experiments. Thixotropic behavior was also studied under steady shear experiments. The suspensions were first intensively stirred for 30 s before placement in the rheometer, then the recovery process was monitored under the shear rate of 10 s-1. The concentration of the unpurified Na-montmorillonite in all the examined suspensions was 3%. The different results are shown in Figures 12 and 13. From Figure 12, it can be seen that when R ) 0.013, η increased gradually with time; when R ) 0.051, η increased first and then decreased with time; and when R ) 0.17, η decreased gradually with time. From Figure 13, it can be seen that for the Fe-AlMg-HTlc/MT suspensions with R ) 0.17, which are made up of HTlc sample 1, sample 2, or sample 3, η increased first and then decreased later with time. But for the Fe-Al-Mg-HTlc/MT suspensions, which are made up of HTlc, sample 4 or 5, η decreased gradually with the time. Discussion The rheological properties of the HTlc/MT suspension forming gel-like structure have been investigated. Special emphasis has been given to the phenomenon of thixotropy. The structure recovery at rest after steady shear is considered a fundamental thixotropic process and has been characterized by small amplitude oscillatory shear measurement and the steady shear measurement. In the oscillatory shear measurement which performed under the linear viscoelastic region, a monotonic increase of |η*| with the time has been observed. Even in a long
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experiment lasting for several hours, no equilibrium viscosity value was reached, indicating that structural rearrangements are still going on even after such a long rest time. A single power law |η*| ∼ tn holds within the time regime from 10 to 104 s after cessation of steady shear. The exponent value was 0.158 ( 0.10, which is independent of the concentration of HTlc and clay, as well as the composition of the clay and HTlc. In addition, the exponent value n is also independent of the mechanical pretreatment of the HTlc/MT suspension. As far as the authors know, this type of kinetics has not been reported for the thixotropic recovery of the mixed suspension before. Note that similar power law behavior with exponent value n ) 0.18 has been reported for the thixotropic recovery of flocculated suspension of rodlike ferrite particles.11 In the laponite dispersion, a similar kinetics equation has been reported for the recovery of the gel-like laponite suspension.10 The experiment results reported here may be interpreted as follows: The three-dimensional network structure of the HTlc/ MT suspension is disintegrated into small flocs of particles or (less probably) into single platelike particles by application of shear forces. After cessation of steady shear, a microscopic structural rearrangement of the flocs and the particles takes place, setting up and reinforcing a network structure (showing up in the monotonic increase of |η*|). According to the observed power law behavior with n < 1, the rate of change for d|η*|/dt is monotonically decreasing with time, indicating a slowing down of the structural changes with increasing rigidity of the network. Therefore, the thixotropic reorganization of the HTlc/MT network can be interpreted as a cooperative, self-delaying process. Probably, the HTlc particles which are positively charged and the MT particles which are negatively charged can form a kind of “imperfect” network, at the rest time. The “imperfect” network can link together to jump into a position of lower free energy with a decrease of such “imperfections”. Finally, the equilibrium state (minimum of free energy) of the whole network is reached only after infinite time and is characterized by an infinite viscosity. A more detailed discussion of this phenomenon is not possible on the basis of our data, but it seems that the recovery process is only related to the shape of the oppositely charged particles, and the HTlc and MT particles all existed in a platelike shape. Whereas, it seems not to be appropriate, since the microscopic structure of the network, especially the physical structure and nature of the so-called flocs, is still under discussion. Similar self-delaying processes are observed for example in the aging of amorphous polymers12 and in the precipitation from supersaturated solid solutions.13 The difference between the steady shear experiments and the oscillatory experiments is that the system is disturbed during the measurements in the steady shear experiments, and this disturbance will influence the results. After cessation of the shear, the three-dimensional network was reinforced and set up gradually. When the recovery process was monitored under the low shear rate, the formation of the network was influenced to some extent.
In HTlc-clay suspensions, HTlc particles possess positive charges while clay particles possess negative charges.14 In the different HTlc/MT suspensions, the influence of the disturbance on the recovery process is different. When the structure strength (which is indicated by the value of the viscosity) of the system is low, the influence of the shear rate on the recovery process is weak. When the structure strength is high, the influence of the shear rate also becomes distinct, because the disturbance will result in the close joint of the opposite charged HTlc particles and MT particles, which exist in the form of densed floc clusters. The appearance of the densed floc clusters will destroy or inhibit the formation of the network; hence η decreased gradually with time. So, under the steady recovery process, the destroyed structures may be recovered through the following three mechanisms. First, the oppositely charged particles approach each other to form three-dimensional network structures in the whole system, then the η of the system will increase gradually with t. Second, the structure formation process in the system may be divided into two stages: stage one, forming three-dimensional network structures, then η of the system will increase gradually with t; stage two, forming dense floc units (or the dense particles cluster), then η of the system will decrease gradually with t. In a whole, η of the system will increase first then decrease later. Third, under the disturbance of the shearing, the two kinds of particles may directly form discrete dense floc units (or dense particles cluster), which decreases the viscosity of the system.3 When the concentration of HTlc in the HTlc/MT suspensions increases, the structure strength of the suspensions also increases, and the influence of the disturbance on the recovery process becomes distinct. From Figure 12, when R ) 0.013, the disturbance on the recovery process is weak, so the formation of the network is seldom disturbed and η monotonic increased with time. When R ) 0.051, the influence of the disturbance becomes distinct, and η of the system will increase first then decrease later. When R ) 0.17, the influence of the disturbance on the recovery process becomes more obvious, and η of the system will decrease with time. When the structure strength (which is indicated by the value of the viscosity) of the system is low, the influence of the shear rate on the recovery process is weak, which becomes distinct when the structure strength is high. From Figure 13, for the five different HTlc/MT suspensions with the same R value, the structure strength of the suspension increased gradually from the suspension of HTlc sample 1 to the suspension of HTlc sample 5. Then the influence of the shear rate on the recovery process becomes distinct gradually from the suspension of HTlc sample 1 to the suspension of HTlc sample 5. Hence, for the systems whose viscosity is relatively low, η of the system will increase first then decrease later. For the systems whose viscosity is relatively high, η of the system will decrease gradually.
(11) Kanai, H.; Amari, T. Rep. Prog. Polym. Phys. Jpn. 1991, 34, 71-83. (12) Struik, L.C. E. Physical Aging in Amorphous Polymers and Other Materials; Elsevier: Amsterdam, 1978. (13) Lifshitz, I. M.; Slyozov, V.V. J. Phys. Chem. Solid 1961, 19, 35.
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Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 20273041) and the Ministry of the Education.
(14) Li, S. P; Hou, W. G.; Dai, X. N. J. Dispersion Sci. Technol. 2003, 24 (1), 145.
