Millimeter-scale liquid metal droplet thermal switch

Abstract

Devices capable of actively controlling heat flow have been desired by the thermal management community for decades. The need for thermal control has become particularly urgent with power densification resulting in devices with localized heat fluxes as high as 1 kW/cm2. Thermal switches, capable of modulating between high and low thermal conductances, enable the partitioning and active control of heat flow pathways. This paper reports a millimeter-scale thermal switch with a switching ratio >70, at heat fluxes near 10 W/cm2. The device consists of a silicone channel filled with a reducing liquid or vapor and an immersed liquid metal Galinstan slug. Galinstan has a relatively high thermal conductivity (≈16.5 W/mK at room temperature), and its position can be manipulated within the fluid channel, using either hydrostatic pressure or electric fields. When Galinstan bridges the hot and cold reservoirs (the “ON” state), heat flows across the channel. When the hot and cold reservoirs are instead filled with the encapsulating liquid or vapor (the “OFF” state), the cross-channel heat flow significantly reduces due to the lower thermal conductivity of the solution (≈0.03–0.6 W/mK). We demonstrate switching ratios as high as 15.6 for liquid filled channels and 71.3 for vapor filled channels. This work provides a framework for the development of millimeter-scale thermal switches and diodes capable of spatial and temporal control of heat flows.

Fluidic and Mechanical Thermal Control Devices

“…This is usually achieved by using pumps [ 38,39 ] or fans [ 38,40,41 ] or by tilting (gravitational actuation). [ 42,43 ] Another possibility that remains less investigated is to replace the pump by movable pistons or membranes driven by stepper motors, linear actuators, pull‐back springs, or piezo‐transducers.…”

“…In fluidic bridge thermal switches, the heat source and the heat sink are separated by a small gap that can be closed or open to the surroundings. When closed, the gap can be empty (i.e., vacuum) [ 41,44,45 ] or be filled with another immiscible fluid (e.g., an inert gas or fluid like vapor or aqueous solution of NaOH [ 42 ] ) with low thermal conductivity. When open, the gap is normally filled with air.…”

“…[ 46 ] Typical working fluids having higher thermal conductivity than the surrounding fluid include water, [ 38,40,46 ] glycerin, [ 47 ] and liquid metals like galinstan. [ 42 ] Figure shows the general operation of a fluidic bridge thermal switch. During the off state (right), the gap is empty (i.e., vacuum) or filled with a low‐thermal‐conductivity fluid.…”

“…The highest switching ratios of SR = 71.3 (characteristic time of the order of seconds) and SR = 39 (characteristic time of the order of minutes) were respectively demonstrated by Yang et al. [ 42 ] using liquid metal galinstan and by Elsahati et al. [ 39 ] using water.…”

Liquid-Metal Enabled Droplet Circuits

“…Recently, some researches has been devoted to the study of liquid-metal droplets ( Figure 2 D), which show great potential in self-powered devices [ 23 , 24 ] and phagocytosis [ 25 ]. Yang et al introduced millimeter-scale LMDs as thermal switches, unlocking new possible solutions for thermal management [ 26 ]. Tang et al electrically controlled the size and rate of LMD formation [ 27 ].…”