© 2014 The Korean Society of Rheology and Springer 105 Korea-Australia Rheology Journal, Vol.26, No.1, pp.105-116 (February 2014) DOI: 10.1007/s13367-014-0011-7 www.springer.com/13367 Experimental study on the rheology of anisotropic, flocculated and low volume fraction colloids Burcu Genc Ozel, Aslihan Orum, Mehmet Yildiz* and Yusuf Z. Menceloglu § Faculty of Engineering and Natural Sciences, Advanced Composites and Polymer Processing Laboratory (AC2PL), Sabanci University, Istanbul, 34956, Turkey (Received August 26, 2013; final revision received January 24, 2014; accepted January 27, 2014) In this work, we have investigated rheological behavior of colloids with a low particle volume fraction, and anisotropic and flocculated microstructures through measuring their viscosity and electrical resistance under varying shear rates together with utilizing several relevant characterization methods (i.e., Dynamic Light Scattering, Transmission Electron Microscopy, Atomic Force Microscopy, and Capacitance and Electrical resistance measurements). The colloids are formed through the suspension of hydrophilic/phobic fumed sil- ica particle with attractive/repulsive interaction in polyethylene glycol and/or ethylene oxide-propylene oxide copolymer. It is observed that studied suspensions display shear thickening/thinning flow behavior depending on their microstructure (the spatial distribution and arrangements of particles in continuous media) and associated changes in cluster sizes, which are controlled by the break down of densified clusters (due to the shear induced mechanical and hydrodynamical forces) and the interaction forces among particle- particle and particles-polymers (owing to physicochemical effects). The detailed evaluation of the exper- imental results indicates that the shear thickening phenomena in low volume fraction, anisotropic and floc- culated systems can be mainly attributed to the increase in the effective volume fraction of particles due to both hydrodynamic and physicochemical forces. Keywords: colloids, hydrophobic/philic surface, non-Newtonian flow, interparticle interaction. 1. Introduction The rheological properties of suspensions have been attracting the attention of scientist and engineers for use in many important industrial applications since these fluids exhibit complex non-Newtonian flow behavior (either shear thinning or shear thickening) when subjected to external shear forces. Shear thinning is a familiar phe- nomenon in polymer solutions and molten polymers, which facilitates the transport of these types of fluids through processing equipment since the pressure drop at the walls is reduced due to the decrease in the fluid vis- cosity (Eesa and Barigou, 2009). Contrary to the shear thinning, the shear thickening phenomenon is not a pre- ferred fluid behavior in certain industrial processes (Bar- nes, 1989) such as the manufacturing of nano-particles integrated polymers and nano-composites since it adversely affects the performance of the process as well as the process-ability of material. Nevertheless, the shear thickening behavior of colloidal suspensions can be advantageous for some other specific applications and thus, have resulted in a tremendous amount of industrial and commercial innovations in many areas such as bio- medical, sportswear, damping devices, shock absorbers for automotive industry and ballistic protection, among others (Lee et al., 2003). In this work, the term suspension is used to refer to a biphasic system where a solid phase is present in a con- tinuous/fluid phase. The term dispersion is reserved for a mixture which is the special case of a dispersed (or sta- bilized) suspension wherein particles are kept apart from each other due to the action of either shear forces, or repulsive (or dispersive) interparticle forces. The solid particles are referred to as discrete phase rather than the dispersed phase since in suspensions, particles may tend to flocculate if attractive forces are dominant. Three different types of forces can be present in suspensions; namely, hydrodynamic, Brownian, and colloidal forces. The hydrodynamic force only exists in flowing suspensions due to the relative motion of constituents while the Brownian force is present owing to the thermal fluctu- ations in the suspension, and acts as randomizing force on particles. Colloidal forces are elastic interparticle forces, which can be repulsive and attractive in nature. The repul- sive interparticle forces keep the discrete particles apart leading to dispersed or stabilized particles. The attractive force on the other hand promotes the formation of col- loidal suspensions with aggregated or flocculated fractal clusters. For particles with the size in the range of 10 -3 m- 10 -1 m, the microstructure of the suspension is influenced by all these three forces to various degrees. The total force (hydrodynamic (or viscous) forces, Brownian forces, and interparticle or colloidal forces) affects the deformation *Corresponding author: meyildiz@sabanciuniv.edu § Corresponding author: yusufm@sabanciuniv.edu