Colloids and Surfaces A: Physicochemical and Engineering Aspects
Effect of field strength and temperature on viscoelastic properties of electrorheological suspensions of urea-modified silica particles
Introduction
Electrorheological (ER) fluids first described by Winslow [1] more than 50 years ago has been the object of many studies. The origin of this phenomenon and the main results obtained so far are discussed in several review articles [2], [3], [4], [5], [6], [7], [8]. While a majority of studies deals with steady-state viscosity or shear stress behaviour, viscoelastic properties of ER fluids rank among little investigated problems. It is well demonstrated that in suspensions of electrically polarizable particles in a non-conducting medium, a fibrous chain structure of polarized particles is formed on electric field strength application and, consequently, an increase in viscosity of several orders of magnitude occurs. In this process a transition from liquid to quasi-solid state sets in accompanied by a yield stress appearance and a steep increase in elasticity of the system. At this transition point, which shows much similarity to physical gelation of polymers, the storage modulus, G′, which is significantly lower at zero or low electric field strength than the loss modulus, G″, begins to dominate, steeply increasing with the field strength. It has been assumed that at the beginning of the liquid-to-solid transition, when gelation starts, the same frequency dependences of dynamic moduli G′ and G″ are observed [9]. As expected, the experiments with monodisperse silica particle suspensions at increasing electric field strengths showed that the slope of loss tangent, tan δ = G″/G′, originally negative turns to positive values in the course of continuing gel solidification [10]. This suggests that the sol–gel transition starts before the chains of polarized particles appear in the suspension. In the suspensions of polydisperse irregular particles, the case of G′ = G″ did not occur in the whole frequency range and the frequency dependences of the moduli cross at different field strengths. In this case, the field strength for G′ = G″ at the adopted frequency as the criterion characterizing the sol–gel transition has been proposed. Using the assumption, the dynamic characteristics of silicone oil/polyaniline suspensions in electric field revealed that the field strength corresponding to the transition gel point decreases with the particle concentration and viscosity of suspension [9].
In our previous studies [11], [12], the effect of coating of nanosilica particles with urea on the electrorheological behaviour of their suspensions has been investigated. Due to strong interparticle forces in silica, a much higher field-off suspension viscosity appeared than that of the urea-modified particles. In this work the influence of temperature on viscoelastic and dielectric properties of urea-modified silica particles in the electric field was examined.
Section snippets
Materials
Nanosilica particles (average particle size ≃10 nm, Aerosil A 200, Degussa, Germany) have been used. To modify silica with urea, 10 g of the nanoparticles was immersed in 500 ml of distilled water at 40–50 °C. After 1 h stirring, 10 ml of 10 wt.% aqueous solution of urea (Aldrich, USA) was added and the suspension stirred for another 16 h. Then the suspension particles were filtered off, washed with ethanol and dried at 60 °C in vacuum to constant weight. The amount of urea deposited on silica particles
Field off behaviour
It is generally accepted that the field-off hydrodynamic properties of suspensions depend on the size and shape of suspended particles or their aggregates, in addition to viscosity of the medium, and on their interaction, which is controlled by compatibility of the components. Thus, a close relation of association of poly(ethylene oxide)/organoclay nanoparticles to viscoelastic properties have been observed [13]. We found that in suspension of silica particles, a quasi-solid structure with high
Conclusion
Apart from particle loading, compatibility of particles with the liquid medium crucially affects the field-off dynamic properties of electrorheological suspensions. It is understandable that the character of the primary structure of particle arrangement in the absence of electric field controls an increase in both dynamic moduli with electric field strength and the transition from liquid to quasi-solid state. Also temperature is a significant factor influencing this behaviour. It is clear that
Acknowledgement
The authors wish to thank the Ministry of Education, Youth and Sports of the Czech Republic (MSM 7088352101) for financial support.
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