Active Heat Transfer Fluids: Liquid Coolants Containing Self-Propelled Microparticles for Heat Transfer Enhancement

Liquid coolants are critical components of many modern technologies, such as hybrid electric vehicle batteries, solar receivers, and cooling systems for electronics. Sustaining the growth of these technologies while limiting their carbon footprint requires coolants that transfer heat efficiently. Since the 1990s, significant research has been conducted into the use of suspended nanoparticles to improve coolant performance. The resulting suspensions, known as <i>nanofluids</i>, typically exhibit higher thermal conductivity than the liquids alone, since the particle material is typically more thermally conductive than the liquid. However, the improvements provided by nanoparticles are fundamentally limited by the particles' inability to move on their own through the liquid, and thus agitate convective mixing that could dramatically improve performance.<br/><br/>In this work, we explore the use of self-propelled particles (SPPs), which are colloids that can move autonomously in liquids using energy from their local environment, to accelerate heat transfer in liquids. As they move, SPPs induce disturbance flows in the surrounding liquid, and we hypothesize that this "micro-stirring" effect can enhance the heat transport rate by an amount depending on the SPPs' size, speed, and volume fraction. We demonstrated this concept experimentally by placing an SPP suspension in contact with a heated surface; for a constant heat flux, a more effective coolant should result in a lower temperature of the heated surface. By measuring the temperatures of the heater & background fluid, as well as the heat flux, we can quantify SPP-induced increases in the convective heat transfer coefficient (HTC), a widely-recognized metric of heat transfer efficiency. We present data for the HTC of various SPP suspensions under various experimental conditions. The results will inform the prospective use of coolants containing SPPs, which we refer to as "active heat transfer fluids," in a variety of cooling applications in the energy, environmental, and biomedical sectors.