Multiphase transport : microparticle morphology and heat transfer

Abstract

The present work includes two main aspects of multiphase transport: microparticle morphology evolved from microfluidic droplets, and multiphase heat transfer involving nanofluids, dual-phase-lag heat conduction, and phase change heat transfer. The former centers on the evolution of double-emulsion droplet and phase separation of single-emulsion droplet, which are important precursors of microparticles. The latter contains the heat transport mechanism in nanofluids, thermal property in dual-phase-lag heat conduction, boiling heat transfer, and application of phase change materials. Microfluidics can produce highly-monodispersed droplets, through which microparticles could be fabricated. The present study focuses on the fabrication of non-spherical microparticles based on the evolution of microfluidic droplets. Firstly, we study the dewetting process of double-emulsion droplets. With different relationships of the interfacial tensions in double-emulsion systems, the double-emulsion could eventually behave as complete engulfing, partial engulfing and non-engulfing morphologies. We theoretically prove that the dewetting force of double-emulsion droplet is the interfacial tensions on the three-phase contact cycle. Meanwhile, the steady droplet morphology is derived based on thermodynamics, and then the time required for the evolution process is calculated numerically. Furthermore, single-emulsion droplets also could be applied to fabricate non-spherical microparticles through phase separation method. We propose a model to investigate the determinant parameters of the particle size and morphology. The results show that kinetic factors are vital for the particle formation and the particle size is determined by the capillary number and concentration of dissolved polymer. Meanwhile, the particle morphology depends on the contact angle and final volume ratio between liquid solute and polymer solute. Multiphase heat transfer involves the cross-coupling of heat transport between different phases. Nanofluids have drawn much attention because of their high thermal conductivity, while the currently proposed mechanisms cannot explain some of experimental results. In the present study, we propose a mechanism to explain the heat transport enhancement based on the cross-coupling of thermal and electric effects, which could explain many experimental results in the literatures qualitatively and is significant for the design of nanofluids. In addition, heat conduction in multiphase systems, such as nanofluids, porous media and biomaterials, is exactly equivalent to dual-phase-lag heat conduction. We analyze the temperature monotonicity with respect to the time lags and get the monotonicity transition point of the temperature development. Meanwhile, when the time lag of heat flux is greater than that of the temperature gradient, thermal waves could be generated in dual-phase-lag heat conduction. In the present work, we design an experiment to predict thermal waves by measuring the ratio between the two time lags. Phase change heat transfer is another typical multiphase heat transfer. Firstly, we fabricate different microstructures on the heating surfaces through 3D microprinting and then investigate the effect of these microstructures on boiling heat transfer. We discover that the boiling heat transfer could be weakened with appropriately-designed microstructures because of the entrapped vapor in them. Furthermore, we propose a boiling mechanism based on the surface catalytic reaction. In addition, we explore the application of liquid-solid phase change materials and develop a container with phase change material.