Microthermal Devices for Fluidic Actuation by Modulation of Surface Tension.

Abstract

Fluid manipulation at the micrometer scale has traditionally involved the use of batch-fabricated chips containing miniature channels, electrodes, pumps, and other integrated structures. This dissertation explores how liquids on non-patterned substrates can be manipulated using the Marangoni effect. By placing miniature heat sources above a liquid film, it is possible to generate micro-scale surface temperature gradients which results in controlled Marangoni flow. A variety of useful flow patterns can be designed by tailoring the geometry of the heat source.

As a surface tension-based phenomenon, the Marangoni effect is an efficient actuation mechanism at submillimeter dimensions. With optimized liquid carriers, flow velocities >10 mm/s can be generated with only small perturbations in surface temperature (<10 K). Thermally efficient microfabricated heat sources, such as polyimide thermal probes, can produce >1700 µm/s flow velocity in mineral oil while consuming <20 mW of power. In water films, the probes can generate surface doublets with linear velocities up to 5 mm/sec and rotational velocities up to 1300 rpm, making them potentially useful for active mixing.

The utility of Marangoni flows is demonstrated within the context of digital microfluidic systems. In contrast to conventional microfluidics, where samples are flowed through microchannels, digital microfluidic systems contain liquid samples in micro and nanoliter-sized droplets suspended in an immiscible oil layer. Marangoni flows generated in the oil layer can manipulate droplets without any physical structures, thus avoiding surface contamination. By using point, linear, annular, and tapered heat source geometries, it is possible to engineer Marangoni flows which mimic the functionality of droplet channels, mixers, size-selective filters, and pumps. Arbitrary, two-dimensional actuation of droplets (Ф=400-1000 µm) can also be achieved using an array of heaters suspended above the oil layer. The 128-pixel heater array incorporates addressing logic and a software interface which allows it to programmatically transport and merge multiple droplets through the sequential activation of heaters.

The appendices outline other aspects of thermal probes, including i) the structure, fabrication, and operational characteristics of single probes and probe arrays, and ii) scanning thermal lithography, a technique for nanoscale patterning of thin films with heat.