Metallic nanoemulsion with galinstan for high heat-flux thermal management

Abstract

Galinstan is a eutectic alloy of gallium, indium, and tin, of which thermal conductivity is ∼27 times higher than that of water, while the dynamic viscosity is only twice. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the direct use of galinstan can suffer from practical issues such as oxidation and low specific heat. Therefore, galinstan is mixed with a coolant as an additive to form a colloidal fluid; i.e., dispersion of nanoscale galinstan droplets in a coolant to enhance the thermal conductivity. It is expected that this “metallic nanoemulsion” can contribute to substantial improvement in heat transfer capability. Also, the common issues with colloidal fluids such as rapid sedimentation, erosion, and clogging, can be minimized by the “fluidity” of the liquid metal.It was shown that ultrasonic emulsification can yield nanodroplets with a few hundreds of nanometer or smaller. However, the long exposure of galinstan to oxygen in water inevitably results in severe oxidation of the droplets. Theoretical analysis was also conducted to examine the feasibility of the metallic nanoemulsion as a microchannel heat-sink working fluid. Effective medium theory was used to evaluate the thermal conductivity of the mixture. The viscosity change was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Under one-dimensional laminar flow assumption, mass, momentum, and energy conservation equations were analytically solved. The effect of high thermal conductivity of galinstan was evident; the convection heat transfer capability was greatly enhanced, while the high viscosity due to the nanoscale blending and the low specific heat of galinstan reduce the flow rate and thus increase the caloric thermal resistance.