Microbubble actuators - an introduction to microscale thermal fluid engineering

Liquid-gas interface often appears in the microfluidic systems due to dissolved and residual gas or thermally generated vapor. Thermal expansion of bubble is the most famous way to control this interface and is used as the micro pumping engine in the thermal ink-jet printer. On the other hand, thermocapillary force of droplet is successfully applied in the micro optical switch. This force is based on the controllability of surface tension that depends on temperature. In thermal fluid engineering, a similar phenomena appears on bubbles and liquid surfaces in temperature or concentration gradient and is known generally as Marangoni effect. The contemporary microfabrication technique enables us to fabricate a microheater tiny enough to control temperature quickly and precisely in micro length scale. Consequently the Marangoni effect or thermocapillary force on microbubbles can be managed artificially and the microbubble of the order of hundreds micrometers can be driven without any movable part. Novel actuators using microbubble and their microfluidic mechanisms are explained.