Rheological behavior of fumed silica suspension in polyethylene glycol

Vol.13 No. 1J. CENT. SOUTH UNIV. TECHNOL.Feb. 2006Article ID: 1005 - 9784(2006)01 - 0001 - 05Rheological behavior of fumed silica suspension in polyethylene glycolWU Qiu-mei(伍秋美) , RUAN Jian-ming(阮建明) ,HUANG Bai-yun(黄伯云),ZHOU Zhong -cheng(周忠诚), ZOU Jian- peng(邹俭鹏)(State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China)Abstract: The rheological behavior of fumed silica suspensions in polyethylene glycol(PEGand oscillatory shear stress using AR 2000 stress controlled rheometer. The systems show reversible shear thicken-ing behavior and the shear-thickening behavior can be explained by the clustering mechanism. The viscosity and thedegree of shear thickening of the systems strongly depend on the mass fraction of the silica, the molecular weigh ofPEG and the frequency used in the rheological measurement. The silica volume fraction of the systems is 1. 16%-3. 62%, corresponding to the mass fraction of 4%- 9%. The shear thickening taking place in the low volume frac-tion may contribute to the fractal nature of the silica. At oscillatory shear stress, when the shear stress is less thanthe critical stress, the storage modulus decreases significantly, meanwhile the loss modulus and the complex viscosi-ty almost remain unchanged; when the shear stress is larger than the critical stress, the storage modulus, the lossmodulus and the complex viscosity increase with the increase of shear stress. The loss modulus is larger than thestorage modulus in the range of stress studied and both moduli depend on frequency.Key words : fumed silica suspension; polyethylene glycol; shear thickening; storage modulus; loss modulusCLC number: O648Document code: A1 INTRODUCTIONring is a more accurate and generalized model, withthe ODT being a possible concomitant effect inFumed silica is an extremely versatile materialmany cases C8;9].that can be used to modify the rheological behaviorSome researchers in USA and Singapore haveof systems. It can function as an effective thicken-used the shear thickening properties of silica sus-ing and thixotropic agent in low molecular weightpensions in ethylene glycol, polyethylene glycolsolvent. Several novel applications are being found(PEG) and water to make advanced body armorfor these suspensions, for example, gels of fumedmaterials[10-12]. These materials can offer equiva-silica in mineral or silicone oils are used as fillinglent ballistic performance of existing body armorcompounds in fiber-optic cables. Suspensions ofmaterials, but with significantly more compactnessfumed silica are being used as polymer electrolytesand flexibility. However, there is little informa-in rechargeable lithium batteries[2].tion on the rheological behavior of the silica sus-The rheological behavior of the silica suspen-pensions in PEG. In this paper, the controlledsion in different continuous fluid has been studiedstress rheometer was used to study the steady andbecause of their practical importance. Raghavan etoscillatory rheological behavior of the fumed silicaal[3], Leet al4], Maranzano et a1[5] did some re-suspensions in PEG.searches on the silica suspensions in polypropyleneglycol(PPG), water and tetrahydrofurfuryl alcohol2 EXPERIMENTAL(THFFA) by steady and oscillatory shear, andfound that the suspensions showed shear-thicken-2. 1 Materialsing behavior. Although two microstructure modelsThe fumed silica was Aerosil A200 (Degussahave been offered to explained the phenomena: oneCorporation). The fumed silica was prepared byis the order disorder transition(ODT) theory pres-the flame hydrolysis of silicon tetrachloride (SiCl)ented by Hoffmanl6, the other is clustering theoryin a flame of H2 and Oz. The particle diameter andbased on Stokesian dynamics simulations， thesurface area are respectively about 12 nm and 200shear-thicken state is due to the clustering of parti-m2/g. These particles fuse to irreversibly aggre-cles7]，the microstructure mechanism for thegates because of high free energy and these aggre-shear-thickening still remains unresolved. Howev-gates are hydrophilic due to the presence of hy-er, there is an increasing indication that the clustedrox中国煤化工e sufacela.YHCNMHG①Foundation item: Project (50174059) supported by the National Natural Science Foundation of ChinaReceived date: 2005 -07- 20; Accepted date: 2005-10 -10Correspondence : WU Qiu mei, Doctoral candidate; Tel: + 86-731-8876644; E mail: wuqiumei20073@ sina. com. cnJournal CSUT Vol.13 No. 12006A polar liquid, viz. polyethylene glycol(PEG)with an average molecular weight of 200 (PEG1)and 400 (PEG2) was used as the continuous phaserespectively. PEG1 and PEG2 are Newtonian liq-uids, with low viscosity of 0. 03 and 0.01 Pa. s。After the fumed silica was dispersed in PEG，thesilica aggregates preferentially interact with thePEG molecules through hydrogen bonds, whichleads to non-flocculated suspensions.2.2 Sample preparationThe suspensions were prepared by adding thesilica powder into the liquid in a blender and mix-100 nming for approximately 30 min. Samples were placedin vacuum at room temperature for 24 h to removeFig. 1 TEM image of fumed silica powderair bubbles. The mass fraction of silica in the sus-pensions was between 4% and 9%.2.3 Rheological behavior measurement1The rheological measurements were conductedusing a stress controlled rheometer AR2000 ( USA:TA company). Experiments were conducted underboth steady and dynamic oscillatory shear stress atthe temperature of 25 C. The steady shear ratewas ranged from0. 1 to 100. 0s-'. Dynamic exper-iments were stress sweeps, where the frequencywas fixed and the shear stress ranged from 0. 1 to1000. 0 Pa. In all these experiments, cone plate0.11.00.0100.0fixture (diameter 40 mm, angle 1°) was used.i/s!Fig.2 Apparent viscosity η as function of2.4 Morphology of fumed silicashear rate i of fumed silica suspensionThe morphology of the fumed silica was ob-with 9% silica in PEG1served by H-800 transmission electron microscopy(TEM) after the fumed silica' was dispersed in eth-anol with ultrasonic treatment for 30 min and thennonflocculated nature of the system. At the shearrate () close to 15.06s-1，the viscosity begins toplaced on copper foil dryness.increase steeply until j≈156.43s-'. At the incipi-3 RESULTS AND DISCUSSIONent point of shear thickening, the shear rate wasrecorded as critical shear rate (i。).The change in the viscosity of the silica dis-3.1 Morphology of fumed silicaTEM image of the fumed silica is shown inpersion is assumed to be the result of the. micro-Fig. 1. The fumed silica primary particles do notstructural change. As was mentioned in the intro-exist individually and irreversibly fuse into largerduction, there are two microstructural models thatstructures called aggregates because of high freehave been used to explain the shear thickening be-energy. The silica aggregates with a chain-likehavior. The key feature of the ODT mechanism isstructure and the aggregates are found to be self-that the particle should exist in an ordered arrange-similar (i. e. showing the fractal nature) the samement before shear thickening takes place and theas the suspensions in PPG whose fractal dimensionordered arrangement needs the particles with regu-lar shapes, but the silica aggregates have irregularequals 1. 71士0.05 in the 3-D space C[3]shapes in this system as shown in Fig. 1, so thesesystems can not be explained by the ODT theory.3.2 Steady rheological behaviorThe steady apparent viscosity as function ofCor中国煤化工ry can be used to ex-the shear rate for the suspension in PEG1 with theplaiavior preferably. Acsilica mass fraction of 9% is shown in Fig. 2. It cancordCN M H Gy, the shear thicken-be seen from Fig. 2 that at low shear rate, theing state is characterized by the presence of flow-slight shear thinning can be observed. The lowinduced clusters [7.14]. At low shear rates, the sili-level of viscosity at these shear rates reflects theca aggregates in suspension are prevented from fur-WU Qiu-mei, et al; Rheological behavior of fumed silica suspension in polyethylene glycolther aggregation by weak steric repulsive andagain when the shear stress is decreased. That isBrownian forces. When the shear rate is increased, .to say the shear-thickening behavior is reversible.the hydrodynamic forces increase, and at the criti-cal transition, their levels exceed those of the re-pulsive interactions. At this point, clusters of ag-●一Ascendinggregates form and block the flow of the fluid, soo- Descendingthe viscosity increases.3.3 Dynamic rheological behaviorThe dependence of storage modulus G'，lossmodulus G" and the complex viscosity η" on the os-cillatory stress σ at the constant frequency of20 rad/s of fumed silica suspension with 9% silicain PEG1 is shown in Fig.3. It can be seen that G'，0.11001 000G" and η" depend strongly on the stress. There is aσ/Pacritical stress 0.=15.85 Pa, when o>oc，G', G"and η° increase as σ increasing due to the formationFig. 4 Reversible shear-thickening behavior ofof the clusters; when σ< > o。, the degree of the shearto theG"，which also indicates that the η" is main-thickening is also increased as the silica mass frac-ly composed of viscous composition.tion increases from 4% to 9%. The increase in vis-10cosity may contribute to the change of the numberof the hydrogen bonds between the silica and the-ηPEG as the mass fraction increases. The increasein the degree of the shear-thickening behavior maybe explained as follows: the more the silica parti-台1...cles, the bigger and the more the clusters.w(H2O):■- 9%5%- 4%Fig.3 Storage modulus G'，loss modulus G"and complex viscosity η° as functionof oscillatory stress σ of fumed silicasuspension with 9% silica in PEG1 at 20 rad/s01000To confirm the reversibility of the shear thick-ening behavior, the η”was measured for both as-中国煤化工cending and descending stress sweeps as shown inMHCNMHGy'asfunctionofFig. 4. The good agreement of n”measured by theoscillatory stress σ of fumed silicadifferent ways shows that the clusters leading tosuspensions containing different mass fractionthe shear thickening are unstable and will decom-of silica in PEG1 at 50 rad/spose and disperse into the continuous phase PEG1Journal CSUT Vol.13 No.1 2006Another point must be noticed, that is, the .suspensions with shear thickening property usually[(@)PEG1:.-G;o-G"have high volume fraction[lts.16] ，while the volumePEG2: *fraction of silica particles P。in our suspensions wasvery low (as calculated from bulk density, it wasg1(0.0161- 0. 0362，corresponding to the 4% - 9%)and the shear thickening behavior still takes place.This puzzling aspect could be explained by assum-ing that the effective disperse phase volume frac-tion Petr is much higher. The higher value of Petover P。is due to a significant amount of liquid be0.111001000ing occluded in the loosely packed silica aggre-o /Pagates. The open, loose packed structure of silicaaggregates is a consequence of their fractal nature.100 F0)PEG1:o- G';o- G"In the fractal aggregates, these holes are occupiedPEG2:*by the liquid phase, and the entire structure is de-formed into a single entity, therefore the effectivevolume fraction increases.3.5 Effect of molecular weight of PEG and frequencyThe dependences of storage G and loss modu-lus G" on the stress σ at the constant frequency of20，50 and 80 rad/s of fumed silica suspensionswith 9% silica in different PEG are shown in1 000Fig. 6. The circle plots present the suspensions ofσ/Pathe silica in the PEG1，while the pentacle plots(C)present the PEG2. The solid plots present G'，while the empty plots present G". It can be seenPEG2:二G;t=G .PEG1:●that G' and G" are affected by both frequency andthe molecular weight of the PEG，but show thesimilar trends regardless of the frequency.Comparing the plots of the two systems con-taining different PEG in Fig.6, it can be seen thatthe varying trends of both moduli are the same as10 |that in Fig. 2, but when σ<σ，G' and G" of thesuspension in PEG1 are larger than those in PEG2，and the degree of the decreasing behavior of G' forthe suspensions in PEG1 is higher than that for thesuspension in PEG2; when σ > >o。, the degrees ofFig.6 Storage G' and loss G" modulus ofthe shear-thickening behavior of G' and G" for thesilica suspension in PEG with different molecularsuspensions in PEG1 is lower than those for theweight as functions of oscillatory stress σsuspension in PEG2. The differences in the G' andFrequency/(rad. s~I): (a)- -20; (b)- -50; (c)- 80G" between the two systems may be explained bythe different molecular weight of the PEG. Com-studied. When σ> σ。，though the increasing de-paring PEG1 with PEG2, PEG1 normally has moregrees of the moduli of the systems in PEG2 are-OH when with the same mass and comparativesharper than those in PEG1, the differences in thedensity, then the interaction between the silica anddegrees of these systems are larger with the fre-PEG is strengthened, which leads to higher viscos-quency increasing. For example, while o>oε, G'ity before σ。. After the shear-thickening behaviorand G" of the system in PEG2 show comparativetakes place, for the system in PEG2，the interac-values to those of the system in PEG1 when thetion between the silica and the PEG is weaker,freq中国煤化工the frequency is 2which makes the formation of the clusters easier ，andhe systems in PEG2so the viscosity increases steeply.shoH.CNMHGthanthoseinPEG1From Fig. 6, it can be observed that with therespectively, which illustrates that the systems infrequency increasing from 20 to 80 rad/s, o。increa-higher molecular weight PEG2 may be influencedses and the two moduli increase in the stress rangeby the frequency more easily. When σ< < 0。，the .WU Qiu-mei, et al; Rheological behavior of fumed silica suspension in polyethylene glycol●5.higher frequency make the silica dispersed morethickening colloidal suspensions [J]. 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