CAREER: Investigation of Flow Physics at Moving Liquid-Air Interfaces in Microfluidic Devices Using Thermochromic Liquid Crystal Particles

University Of Southern California, Los Angeles CA

Investigators

Abstract

CBET-0748294 Pottebaum This research measures the thermal gradients and the forces they produce at moving liquid-air interfaces for microfluidic devices based on thermocapillary actuation. Devices such as pumps for moving bubbles or droplets in micro-channels, and free surface flows on micro-patterned surfaces are becoming increasingly common, with applications including 'lab-on-a-chip' devices, MEMS switches, and inkjet printer nozzles. However, the underlying physics is not fully understood. The velocity fields at these moving interfaces have been measured, but the temperature gradients that drive the flows have not due to the lack of a suitable technique. Also, models of the relationship between thermal gradients and the resultant forces have not been experimentally verified. A new optical, whole-field temperature measurement technique using encapsulated thermochromic liquid crystal (TLC) particles is considered the best option. The micro-scale geometry will impose constraints on imaging and illumination configurations for TLC thermometry. The proximity of the measurements to interfaces and surfaces that scatter light creates additional challenges. The PI's expertise in TLC thermometry at the macro-scale will help develop this new technique. Circular polarization filtering will be applied to TLC thermometry for the first time. This technique will be applied to three representative thermocapillary actuated microfluidic flows: a contact line on a surface open to the air, an isolated bubble in a water-filled capillary tube, and an isolated droplet in a capillary tube. In all three cases, a temperature gradient across the setup will drive the motion. Time-dependent velocity fields will be measured at the interface, allowing the forces acting on the fluid to be determined. By improving the understanding of the forces acting on liquid-air interfaces, this research will enable the invention and refinement of new microfluidic devices used in medical testing, electronic cooling, and hazardous substance sensing. Future research using this new measurement technique will increase the impact by extending it to more micro-scale flows where temperature is important. The PI will work with local high school science teachers to develop and pilot new educational experiences to bring engineering concepts and cutting-edge applications, not just basic science, into the classroom. Some high school students may also be inspired to perform at a higher level in science courses and to consider engineering careers. The materials developed and quantitative analysis of their efficacy will be publicized widely to reach beyond the local community. Undergraduate students will also be directly involved in the research effort and will also have access to the facilities for related independent projects.