Enhanced transport phenomena involving droplets with nanoparticles

Microand nano uidics are experienceing an increase in research interest as the size of microelectronics and lab-on-chip appliances continuously decreases. The underlying reason is, that an increase in power density makes the quick removal of process heat more relevant and challenging. In this context multiphase ows, as well as engineered nano uids (nanoinks) are investigated, as they possess enhanced thermal properties. Further, the understanding of nanoink behavior plays a vital role in the manufacturing of nanoand microelectronic systems. Despite the large range of potential applications, the unique thermophysical aspects of multiphase micro uidics, especially surface e ects, are still not su ciently understood. These include capillary and Marangoni e ects at phase interfaces as well as the transfer of mass, momentum and energy across interfaces. Moreover the convective and di usive nanoparticle transport and its in uence on the thermal transport needs further investigation. The present research work aims to theoretically and numerically investigate physical aspects connected to micro uidics by focusing on three di erent scenarios: The di usion controlled shrinking of micron sized nanoink droplets, the dissolution of gas bubbles into a surrounding liquid and the thermal transport in liquid-liquid ow of nano uid droplets through microchannels. In the rst part of the thesis a theoretical investigation of the di usion-dominated shrinking of a nanoink droplet into a surrounding liquid of a di erent substance is presented. The shrinking is controlled by thermally enhancing the solubility of the droplet into the surrounding medium. The spherically symmetric model is based on the mass, momentum and energy transport equations for both phases, taking all important thermophysical phenomena such as particle transport and di usion processes within the droplet and the surrounding medium into account. Analytical solutions for the interfacial velocity and the velocity elds are presented. Parametric studies reveal the strong in uence of temperature on the shrinking process. In a second step, the derived methodology is applied to investigate the dissolution of micron-sized oxygen bubbles into a surrounding blood medium. The collapse of the bubbles is controlled by di usion, which is facilitated by the oxygen uptake reaction between the dissolved gas and the hemoglobin in the red blood cells. Results for several physiological parameters and bubble sizes are obtained, showing that the collapse time may vary from ten seconds to two or three hours. Finally, the thermal transport and the pressure drop in a two phase liquid ow of nano uid droplets through microchannels is investigated. The calculation of the multiphase-multispecies ow problem is performed applying a Front Tracking Method, additionally extended to account for nanoparticle transport in the suspended phase. The in uence of di erently shaped droplets on the thermal transport iv and the friction losses in the channel are investigated. The e ect of Marangoni ow at the liquid interfaces is further taken into account and quanti ed. It is shown that the use of a second suspended liquid (with or without nanoparticles) is an e cient way to signi cantly increase the thermal performance while avoiding large pressure drop.