Effective thermal conductivity of dispersions with a liquid continuous phase

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Dispersions with a liquid continuous phase are heterogeneous systems which consist of a liquid dispersion medium and a dispersed phase of colloidal particles. These particles can be solid particles as given in nanofluids, liquid particles, i.e. droplets forming emulsions, or gaseous particles in the form of bubbles which are present in foams. A key property characterizing dispersions is their effective thermal conductivity (ETC). Many literature studies report that by adding a small amount of solid nanoparticles to liquids, the ETC of nanofluids can be increased extraordinarily relative to the base fluid, while others do not observe any significant enhancement. Until today, debate has continued on the relevant mechanisms affecting the effective thermal conduction in nanofluids where Brownian motion, interfacial layering, and aggregation related to the particles or a combination of such effects are considered. This is reflected in related modeling approaches for the ETC of nanofluids providing often strong variations in the predicted values. The ongoing controversy about the ETC of dispersions is also connected to the reliability of the experimental methods. The main objective of the present project is to contribute to a fundamental understanding of the ETC of dispersions with a liquid continuous phase. For that, the relevant influences should be investigated systematically. Here, the focus lies on the thermal conductivities of the dispersed and the continuous phase, the morphology of the dispersed particles, and particle aggregation. Furthermore, conclusions about the roles of an interfacial thermal resistance and of Brownian motion of the particles regarding the ETC should be drawn. For addressing these effects, selected dispersions of solid or liquid particles on the nanometer scale in a liquid continuous phase with different physical and chemical characteristics should be investigated theoretically and experimentally by using a steady-state guarded parallel-plate instrument. Together with critically evaluated literature data, the experimental results should serve as a reliable database for studying the aforementioned influences on the ETC of such systems. They should also help to answer the question whether the findings obtained for dispersions with solid particles are transferable to those with liquid particles. For the characterization of the dispersions with respect to particle size and shape as essential parameters for the ETC, information about the translational and rotational diffusivity of the particles should be accessed by dynamic light scattering. Based on the experimental results, theoretical considerations, and exisiting modelling approaches, a generalized prediction method for the ETC of dispersions with a liquid continuous phase should be developed.