Aspects of electromagnetic transport characteristics of nanocolloidal complex fluids

Abstract

The aim of this thesis is to understand the external field induced (electric and magnetic) transport behaviors of the complex fluids. In pursuit of this, the present study explores the governing roles of concentration regimes of the inclusions and frequency and voltage of the imposed electric field on the dynamic dielectric responses of the nanocolloidal non-polar suspensions. For a given voltage, frequency dependent dielectric behaviors have been evaluated in the band of 1 to 107 Hz at constant temperature via dielectric spectroscopy. In addition to prevalant literature measurements, influence of field amplitude has also been performed to investigate the role of electric field intensity upon the relaxation behaviors. The experimental observations reveal that variation of frequency, as well as choice of particle material significantly influences the dielectric responses of nanocolloids. Further, the amplitude of field has also been found to induce diverse dielectric responses. Moreover, the relaxation responses of concentrated dispersions are grossly different from the dilute suspensions. The concentration regimes predominantly exhibit convoluted relaxation responses; whereas the diluted colloids conform to the classical Debye type relaxations. Good degree of quantitative agreement has been found between the experimental observations and the generic Havriliak-Negami relaxation model. It has been shown that for a given nanomaterial, reduction of relaxation time scale is a direct consequence of enhancement in concentration and the colloids transit from predominantly storage to leaky systems with concentration regimes. Additionally, the volume diffusion mechanism (VDM) is found to be in compliance with the experimental findings. Further, the concept of improving the AC dielectric breakdown strength of insulating mineral oils by the addition of graphene or CNTs in the form stable dispersions has been addressed. Experimental observations of graphene and CNT nano-oils show that not only improved average breakdown voltage, but also significantly improved reliability and survival probabilities of the oils under AC high voltage stressing is achieved. Improvement of the tune of ~ 70-80 % in the AC breakdown voltage of the oils has been obtained. The study examines the reliability of such nano-colloids using two parameter Weibull distribution and the oils show greatly augmented electric field bearing capacity. The fundamental mechanism responsible for such observed outcomes is reasoned to be delayed streamer development and reduced streamer growth rates due to effective electron scavenging. A mathematical model based on the principles of electron scavenging is proposed to quantify the amount of electrons scavenged by the nanostructures. This is followed by the investigations of static and dynamic electrorheological (ER) and electro-viscoelastic behaviors of the colloidal dispersions. The study aims at describing the ER performances of the complex fluids containing dielectric particles and later compares their responses with same nanomaterial, grafted with conducting polymers. Nanocrystalline meso-nanoporous zeolite has been prepared by chemical synthesis and subsequently polyaniline (PANI) coating has been implemented. Electrorheological (ER) suspensions have been formed by dispersing the nanoparticles in silicone oil and their steady and dynamic behaviors are inspected to understand the nature of such complex colloidal systems under electric fields. PANI-Zeolite based ER fluids demonstrate higher static electroviscous effects and yield stress potential than the untreated Zeolite, typically studied in literature. Transient electro-viscous characterizations show a stable and negligible hysteresis behavior when both the fluids are exposed to constant as well as time varying electric field intensities. Further oscillatory shear experiments of frequency and strain sweeps exhibit predominant elastic behavior in case of Zeolite based ER suspensions as compared to PANI systems. Detailed investigations reveal Zeolite based ER suspensions display enhanced relative yielding as well as electro-viscoelastic stability than the PANI-Zeolite. The steady state viscous behaviors are scaled against the non-dimensional Mason number to model the system behavior for both fluids. Experimental data of flow behaviors of both the ER fluids are compared with semi-classical models and it is found that the CCJ model possesses a closer proximity than traditional Bingham model, thereby revealing the fluids to be generic pseudo-linear fluids. The analysis reveals that the PANI based fluids have improved yield responses. However, in case of selecting the right material to be employed in dynamic environments under oscillatory conditions, the traditional Zeolite based fluids exhibit superior ER potential. Finally, the magnetorheological (MR) performances of ferrite based suspensions have been examined, where the magnetic materials have been prepared by substituting doping atoms. The study deals with the preeminent influence of substituting the M2+ site in nanoscale MFe2O4 spinel ferrites by different magnetic metals (Fe/Mn/Co/Ni) on magnetorheological and magneto-elastoviscous behaviors of the corresponding magnetorheological fluids (MRFs). Different doped MFe2O4 nanoparticles have been synthesized using the polyol-assisted hydrothermal method. Detailed steady and oscillatory shear rheology have been performed on the MRFs to determine the magneto-viscoelastic responses. The MRFs exhibit shear thinning behaviors and augmented yield characteristics under influences of magnetic fields. The steady state magnetoviscous behaviors are scaled against the governing Mason number and self-similar responses from all the MRFs have been noted. The MRFs conform to an extended Bingham plastic model under field effect. Transient magnetoviscous responses show distinct hysteresis behaviors when the MRFs are exposed to time varying magnetic fields. Oscillatory shear studies using frequency and strain amplitude sweeps exhibit predominant solid like behaviors under field environment. However, the relaxation behaviors and strain amplitude sweep tests of the MRFs reveal that while the fluids show solid-like behaviors under field effect, they cannot be termed as typical elastic fluids. Comparisons show that the MnFe2O4 MRFs have superior yield performances among all. However, in case of dynamic and oscillatory systems, CoFe2O4 MRFs have the highest caliber. The viscoelastic responses of the MRFs are noted to correspond to a three element viscoelastic model.