| This work is a combined effort of experimental and theoretical studies toward better understanding the structural and physical properties of aqueous ferrofluids containing nano-sized magnetite (iron oxide magnetic particles) of about 10nm. Ferrofluids have attracted remarkable attention mainly because their properties can be controlled by means of an externally applied magnetic field. The dispersion of nano-sized magnets in a carrier liquid exhibits superparamagnetic behaviour while retaining its fluid properties. The interplay between hydrodynamic and magnetic phenomena has made ferrofluids an extremely promising and useful tool in wide spectra of applications, from technical applications to biomedical ones. In the presence of a magnetic field, magnetic moments of the nanomagnets suspended in the host liquid are aligned toward the field direction and begin to form microstructures such as short chains, strands and long stripes. As this process advances the microstructures may collapse into bundles and thick chains and form macrostructures. Upon the removal of the magnetic field, nanoparticles will be homogeneously redistributed throughout the sample due to thermal agitation. Zero-field structures, and especially the field-induced assembly of magnetic nanoparticles, are primarily responsible for the change in physical properties of ferrofluids, including thermophysical, optical, rheological, and magnetization properties. Because of the field-induced assembly of magnetic nanoparticles in the field direction, ferrofluids become strongly anisotropic and as a result, ferrofluids can significantly enhance directional heat transfer in a thermal system. Thermophysical properties of a ferrofluid are important in studying heat transfer processes in any thermal application, making the study of their behavior a necessity. Taking into account the influence of the formation and growth of microstructures on change in properties of ferrofluids, one can find the significance of identifying and studying the parameters by which ferrofluids' properties can be tailored for a specific need. In Chapter 2 of this dissertation, the influences of magnetic field strength and concentration of ferrofluids on the formation and growth of the chains are observed by employing cryogenic transmission electron microscopy technique. The samples are aqueous magnetite dispersions with concentrations of 0.15%, 0.48% and 0.59% (w/v%). Magnetic field strengths varies from a relatively weak strength of 51.5 mT to the strong field of 0.42 T. Cryo-TEM imaging technique is employed as it allows us to observe the near-native state of the hydrated samples. The cryo-TEM images draw a qualitative comparison basis on the relative significance of magnetic field and concentration on chaining processes. They also provide better understanding of the chains, columns and their internal structures. From a theoretical perspective, an energy equation employing an Eulerian formalism is derived in Chapter 3. Introducing the definition for isotropy and anisotropy of the medium, the equation of heat conduction can be simplified to govern each of the regimes. The equation has taken into account contributions from the important parameters (1) Brownian motion of nanoparticles, (2) magnetic field, (3) temperature, (4) particle size, and (5) volume fraction of particles. In chapter 4, change in effective heat capacity of ferrofluids is addressed and studied with the help of the derivation of the energy equation. The relative significance of the various aforementioned parameters that may have influence on heat capacity of a given medium is quantitatively studied. Lastly, a theoretical model to predict thermal conductivity of a ferrofluid is developed in Chapter 5. From the study on the micrographs, the ferrofluid that becomes anisotropic in the presence of the field is treated as a heterogeneous medium. A structural model, taken into account the anisotropy of the ferrofluid, is introduced in order to develop a theoretical model for effective thermal conductivity of ferrofluids. In order for the model to be qualitatively validated, the measured thermal conductivities of kerosenebased and water-based magnetite ferrofluids are compared to the predictions by the proposed model. The most common method that is being used for thermal conductivity measurement of the ferrofluids is the transient hot wire (THW). Thermal conductivities of the ferrofluids are also measured using a transient hot wire. Further, The accuracy of the method and reliability of the measurements are comprehensively investigated and discussed in detail. |
