Thermally Actuated Pumping of a Fluid With a Free Interface Using Surface Asymmetry

An experimental study of thermally actuated pumping of a single-component fluid is presented in the context of thermal management of a heat source. The prominent feature of this pumping method is that the very heat that is to be removed from the heat source causes a net fluid motion. Surface tension is the dominant driving force for convection in this study. An asymmetry in this force is created by the use of a surface with repeated asymmetric triangular structures. Silicone oil was used as the working fluid. Independent parameters that were varied consisted of the channel surface-to-ambient temperature difference and the fluid thickness. A dye-tracking imaging method was developed to determine the fluid interfacial velocity. The flow results were corroborated with interfacial temperature measurements obtained using infrared thermography. Dye tracking experiments indicate that the direction of net fluid motion is from the less-steep side of the ratchet towards its steeper side, resulting in a clockwise flow direction in the closed loop channel for all three liquid depths of 0.5 mm, 1.0 mm and 2.7 mm. The range of the net flow velocities varies from 0.18 mm/min to 0.86 mm/min. A fluid height of 1 mm results in a maximum net fluid velocity at both surface-to-ambient temperatures studied. Interfacial temperature contour maps indicate the presence of thermal structures that are indicative of convection cells, and that an optimum thickness exists for maximum heat transfer coefficient. Difference in streamwise gradients of temperature (and hence surface tension) on either side of the thermal structures causes a net streamwise surface tension gradient in the direction of net fluid motion. An optimal fluid thickness for heat transfer as well as net interfacial fluid velocity is suggested by the results.