Analysis of the Yielding Behavior of Electrorheological Suspensions by Controlled Shear Stress Experiments

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1 Analysis of the Yielding Behavior of Electrorheological Suspensions by Controlled Shear Stress Experiments Vladimír Pavlínek 1 *, Petr Sáha 1, José Pérez-González 2, Lourdes de Vargas 2, Jaroslav Stejskal 3 and Otakar Quadrat 3 1 Tomas Bata University in Zlín, Faculty of Technology, Zlín, Czech Republic 2 Laboratorio de Reología, Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional, C.P , Apdo. Postal , México D.F., Mexico 3 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic * pavlinek@ft.utb.cz Fax: x Received: , Final version: Abstract: The yielding behavior of two model electrorheological suspensions of uncoated silica particles and silica coated with polyaniline base in silicone oil using controlled shear rate and controlled shear stress experiments has been analyzed. The results demonstrate that unlike the uncertain dynamic yield stress values estimated from the results obtained in the former mode by extrapolation of the unsteady shear stresses to zero shear rate, the controlled shear stress measurement permits to detect sensitively the region starting from the initial rupture of particle chain structure in the electric field at rest corresponding to a static yield stress t y and ending in total breakage of suspension structure at a breaking stress t b.. The latter quantity can be detected with a good accuracy and proved to be a reliable criterion of the stiffness of ER structure. Zusammenfassung: Das Fliessverhalten von zwei elektrorheologischen Modellsuspensionen aus unbeschichteten und mit Polyanilin-Base beschichteten Silikapartikeln in Silikonöl wurde in schergeschwindigkeitskontrollierten und scherspannungskontrollierten Experimenten analysiert. Die Ergebnisse zeigen, dass im Gegensatz zu den unsicheren Werten der dynamischen Fliesspannung, die von Resultaten abgeschätzt wurde, die durch Extrapolation der nichtstationären Scherspannung auf eine Schergeschwindigkeit von Null erhalten wurde, die kontrollierte Scherspannungsmessung erlaubt, den Bereich, der mit dem anfänglichen Bruch der Teilchenkettenstruktur im elektrischen Feld bei Ruhe beginnt (entsprechend einer statischen Fliesspannung t y ) und mit dem vollständigen Aufbrechen der Suspensionsstruktur bei der Bruchspannung t b endet, zu detektieren. Die letztere Grösse kann mit hoher Genauigkeit bestimmt werden und bewährte sich als zuverlässiges Kriterium der Steifigkeit der elektrorheologischen Struktur. Résumé: Le comportement seuil de deux suspensions électro-rhéologiques modèles de particules de silice non recouvertes et de silice recouverte avec une base de polyaniline dans de l huile de silicone, a été analysé à l aide d expériences en contrôle de vitesse de cisaillement et en contrôle de contrainte. Les résultats démontrent que, contrairement aux valeurs incertaines de contrainte dynamique seuil estimées à partir des résultats obtenus par le premier mode par extrapolation des contraintes de cisaillement non permanentes à vitesse de cisaillement zéro, la mesure en contrôle de contrainte de cisaillement permet de détecter sensiblement la région qui part de la rupture initiale de la structure à l équilibre de chaîne de particules dans le champ électrique qui correspond à la contrainte seuil statique t y, et qui finit par la rupture totale de la structure de la suspension à la contrainte de rupture t b. Cette dernière quantité peut être détectée avec une bonne précision, et s avère être un critère de rigidité fiable des structures ER. Key words: electrorheology, yield stress, suspensions, polyaniline, silica Appl. Rheol. 16 (2006)

2 1 INTRODUCTION Since the discovery of the electrorheological (ER) effect by Winslow [1], many studies have been performed. A survey of the results can be found in the reviews [2-6]. Electrorheological (ER) fluids are typically composed of solid electrically polarizable particles dispersed in a non-conducting medium of a low relative permittivity. In the absence of an applied electric field, these fluids are characterized by a monotonic Newtonian or shear thinning flow curve. When an external electric field of several kv mm -1 is applied to these systems, the polarized suspension particles orient in the electric-field direction and link chains or columns spanning the gap between electrodes. This change in the suspension structure significantly affects its rheological properties; viscosity increases and the stiffness of the structure may be so high that a yield stress appears. The yielding properties have been analyzed from a micro-structural model [7]. It is assumed that the stiff ER chain structure exhibits elastic properties. The shear stress increases at shear strain loading and reaches a maximum as a static yield stress. It is obvious that the yield stress obtained by extrapolation from the controlled shear rate (CSR) results [8-10] is dubious because at low shear rates usually flow instabilities caused by deformation, destruction and reformation of the chain-like or columnar structures occur [11-19]. In contrast the controlled shear stress (CSS) experiments could provide more reliable information about yielding the ER structure. Thus, in some studies [20-26] it is mentioned the static yield stress of ER suspensions estimated as a minimum shear stress at which the flow starts. The description of the experiment, however, is lacking. In this work using two model ER suspensions of uncoated silica particle and silica coated with polyaniline base in silicone oil a detailed analysis of the breaking mechanism from the results obtained in both modes is performed. 2 EXPERIMENTAL Silica (Ultrasil VN3, Degussa-Hüls, Germany; average particle size d 50 = 15 mm) was coated with a polyaniline (PANI) overlayer during in-situ polymerization of aniline [27]. The coating of con- ducting PANI hydrochloride [28] was converted to a non-conducting PANI base in 1 M aqueous ammonia solution. The content of PANI in the coated silica was 10 wt. %. The suspensions of 10 wt. % uncoated and PANI-coated silica particles in silicone oil (Dow Corning 200 Fluid, Dow Corning, USA) were mechanically stirred before each measurement. Both components, silica particles and silicone oil, were dried under vacuo at 80 C for 8 h and then stored in a desiccator. The density and kinematic viscosity of the silicone oil were g cm -3 and 100 cst at 20 C, respectively. Rheological properties were measured at 20 C under both, CSR and CSS modes, using a Paar Physica rotational rheometer UDS 200 (Physica, Germany) equipped with a temperature controlled Couette cell (Z4 DIN) and a highvoltage generator (HVG 5000). The rotating inner cylinder of 14 mm diameter and the outer cylinder were separated by a 0.59 mm gap and the electric-field strength E ranged from 0 to 2 kv mm -1. The suspensions were placed in the Couette cell, where a DC voltage was applied for 3 min to generate the equilibrium chain-like or columnar structure of particles before shearing. Then, in CSR mode, the shear stress at the shear Figure 1: Data from controlled shearrate experiment on the suspensions of (a) uncoated silica and (b) silica coated with PANI (Electric field strength [kv mm -1 ] : 0, Û 1, Ì 2). 15

3 Figure 2: Data from controlled shearstress (full symbols) and controlled shear-rate (open symbols) experiments of suspension of uncoated silica (Electric field strength [kv mm -1 ] : 0, Û 1, Ì 2). Figure 3: Data from controlled shearstress (full symbols) and controlled shear-rate (open symbols) experiments on the suspension of silica coated with PANI (Symbols denoted as in Fig. 2). rates from 0.1 to 400 s -1 was gradually measured. After obtaining the flow curve corresponding to a given electric-field strength, the suspension was redispersed at high shear rate prior to characterization at a next electric-field value. In CSS mode the sample was stressed by an increased applied mechanical torque until the particle chain structure starts to be broken and the shear occurs. 3 RESULTS AND DISCUSSION 3.1 CONTROLLED SHEAR RATE EXPERIMENTS Figures 1a and 1b show the flow curves of suspensions of uncoated and coated particles, respectively. In the absence of electric field, the shear stresses at low shear rates in model suspension samples of uncoated particles exceed those with coated ones. This can be explained due to a high aggregation tendency of hydrophilic uncoated silica particles in the hydrophobic silicone-oil medium. After coating with PANI, the particles become more hydrophobic and the particle aggregation is reduced. In the presence of the electric field, higher shear stresses in the suspension of coated particles are a consequence of the presence of highly polarizable PANI layer. Two regions are characteristic of the flow curves at the electric-field application: the first at low shear rates, in which a more or less curly plateau is apparent and second at high shear rates, where nearly Newtonian behavior sets in. Starting from the low shear rates the transition from the former to the latter region is a consequence of gradual disintegration of organized particle structure in the electric field by shear forces, which implies that hydrodynamic forces dominate the electrostatic ones in this regime. The curly plateau-like region displaying flow instabilities is obvious so that determination of the dynamic yield stress by extrapolation of the shear stress to zero shear rate is difficult. 3.2 CONTROLLED SHEAR STRESS EXPERIMENTS The flow curves obtained in CSS mode (Figs. 2 and 3) show four different regions which, schematically sketched in Fig. 4, may be described as follows: The region A represents a solidlike behavior of the suspension structure in the electric field, where a steep increase in shear stress sets in with negligible deformation and reaches a maximum value characteristic of a static yield stress, t y. In the region B, a slight deformation related to yielding of suspension structure associated with slower increase in shear rate occurs. This region is limited by a critical shear stress, t b (breaking stress), at which a complete rupture of the chain structure appears marking the onset of a plateau region C. Then a continuous rupture-reformation process of particle chains faster than in the region B starts accompanied by stress variations found in CSR mode at low shear rates. The region C (a plateau region) in the flow curve corresponded to a jump in shear rate of several orders of magnitude. The shear forces dominate over electric ones and a complete destruction of the ER structure takes place. Consequently, in the region D, the flow becomes stable and the shear stress increases monotonically with the shear rate. Finally, the flow is nearly Newtonian regardless of the electric field or the control mode. As sown in Figs. 2 and 3, the accuracy in estimation of the static yield stress is not too high. In contrast the reliability of breaking-stress determination as a shear-stress value obtained 16

4 in this mode just before a steep increase in shear rate (flow curve discontinuity) proved to be very good. The linear plots of t b vs. E 2 for both suspensions of uncoated and coated sample (Fig. 5) confirms breaking stress as a criterion behaving in fully agreement with polarization theory [29]. 4 CONCLUSION The results demonstrate that in contrast to CSR experiments often used in ER studies, CSS mode provides more reliable way of evaluation of yielding properties of ER suspensions. Thus, the best and most accurate criterion of the original stiffness of suspension particle arrangement in the electric field proved to be the breaking stress at which the quasi-stiff structure is just completely destroyed by shear forces and the flow starts. ACKNOWLEDGEMENTS The authors wish to thank to CGPI-IPN ( , ), CONACYT (34971-U), Grant Agency of the Czech Republic (202/06/0419) and the Grant Agency of the Academy of Sciences of the Czech Republic (A ) for financial support. The experiments comply with the current laws of the Czech Republic. REFERENCES [1] Winslow WM: Induced fibration of suspensions, J. Appl. Phys. 20 (1949) [2] Block HJ, Kelly P: Electro-rheology, J. Phys. D: Appl. Phys. 21 (1988) [3] Jordan TC, Shaw MT: Electrorheology, IEEE Trans. Electric Insul. 24 (1989) [4] Parthasarathy M, Klingenberg DJ: Electrorheology: mechanisms and models, Mat. Sci. Eng. R 17 (1996) [5] Hao T: Electrorheological fluids, Adv. Mater. 13 (2001) [6] Hao T: Electrorheological suspensions, Adv. Colloid Interface Sci. 97 (2002) [7] Bonnecaze RT, Bray JF: Yield stress in electrorheological fluids, J. Rheol. 36 (1992) [8] Choi HJ, Kim JW, To K: Electrorheological characteristics of semiconducting poly(aniline-co-o-ethoxyaniline) suspension, Polymer 40 (1999) [9] Lu J, Zhao X: Electrorheological properties of a polyaniline-montmorillonite clay nanocomposite suspension, J. Mater. Chem. 12 (2002) [10] Lim YT, Park JH, Park OO: Improved electrorheological effect in polyaniline nanocomposite suspensions, J. Coll. Interface Sci. 245 (2002) [11] Choi HJ, Cho MS, To K: Electrorheological and dielectric characteristics of semiconductive polyaniline-silicone oil suspensions, Physica A 254 (1998) [12] Choi HJ, Lee JH, Cho MS, Jhon MS: Electrorheological characterization of semiconducting polyaniline suspension, Polym. Eng. Sci. 39 (1999) [13] Choi HJ, Kim JW, Yonn SH, Fujiura R, Komatsu M, Jhon MS: Synthesis and electrorheological characterization of carbonaceous particle suspensions, J. Mat. Sci. Lett. 18 (1999) [14] Cho MS, Choi HJ, Chin IJ, Ahn WS: Electrorheological characterization of zeolite suspensions, Microporous Mesoporous Mater. 32 (1999) [15] Cho MS, Kim JW, Choi HJ, Weber RM, Jhon MS: Electrorheological characteristics of polyaniline and its copolymer suspensions with ionic and nonionic substituents, Colloid Polym. Sci. 278 (2000) [16] Cho MS, Cho YH, Choi HJ, Jhon MS: Synthesis and electrorheological characteristics of Figure 4: Schematic representation of the flow curve of an electrorheological suspension measured in CSS mode. Figure 5: Breaking shear stress as a function of E 2 for suspensions of uncoated (Û) and PANI-coated silica particles ( ). 17

5 polyaniline-coated poly(methyl methacrylate) microsphere: Size effect, Langmuir 19 (2003) [17] Cho CH, Choi HJ, Kim JW, Jhon MS: Synthesis and electrorheology of aniline/pyrrole copolymer, J. Mater. Sci. 39 (2004) [18] Kim SG, Kim JE, Choi HJ, Suh MS, Shin MJ, Jhon MS: Synthesis and electrorheological characterization of emulsion-polymerized dodecylbenzenesulfonic acid doped polyaniline-based suspensions, Coll. Polym. Sci. 278 (2000) [19] Méndez-Sánchez AF, López-González MR, Rolón-Garrido VH, Pérez-González J, de Vargas L: Instabilities of micellar systems under homogeneous and non-homogeneous flow conditions, Rheol. Acta 42 (2003) [20] Choi HJ, Cho MS, Jhon MS: Electrorheological properties of poly(acene quinine) radical suspensions, Polym. Adv. Techol. 8 (1997) [21] Kim JW, Kim SG, Choi HJ, Jhon MS: Synthesis and electrorheological properties of polyaniline-na + -montmorillonite suspensions, Macromol. Rapid Commun. 20 (1999) [22] Kim JW, Noh MH, Choi HJ, Lee DC, Jhon MS: Synthesis and electrorheological characteristics of SAN-clay composite suspensions, Polymer 41 (2000) [23] Jang WH, Kim JW, Choi HJ, Jhon MS: Synthesis and electrorheology of camphosulfonic acid doped polyaniline suspensions, Coll. Polym. Sci. 279 (2001) [24] Sohn J, Sung JH, Choi HJ, Jhon MS: The effect of particle concentration of poly(p-phenylene on electrorheological response, J. Appl. Polym. Sci. 84 (2002) [25] Alanis E, Romero G, Martinez C, Alvarez L, Mechetti C: Characteristic times of microstructure formation in electrorheological fluids determined by viscosity and speckle activity measurements, Appl. Rheol. 15 (2005) [26] Souza Mendes PR, Dutra ESS: Viscosity function for yield-stress liquids, Appl. Rheol. 14 (2004) [27] Stejskal J, Gilbert RG: Polyaniline. Preparation of a conducting polymer (IUPAC technical report), Pure Appl. Chem. 74 (2002) [28] Stejskal J, Trchová M, Fedorova S, Sapurina I, Zemek J: Surface polymerization of aniline on silica gel, Langmuir 19 (2003) [29] Marshal LJ,. Zukoski IV W, Goodwin J: Effects of electric fields on the rheology of nonaqueous concentrated syspensions, J. Chem. Soc., Faraday Trans. I 85 (1989)