Using light to control the flow of nano-size volumes of fluids over solid surfaces is now simpler due to advances in optoelectronics miniaturisation. For example, an advanced line of optically driven microfluidic devices capable of transferring small droplets of fluids in a reprogrammable way is already under development.
This optical technique uses lasers, or optical systems comparable to those in LCD projectors, to generate complex patterns of differing light concentrations on a flat substrate. Varying amounts of light are then absorbed by the substrate. The absorbed light, in turn, generates differing levels of heat, producing a pattern of thermal gradients on the substrate. The heat alters the surface tension, causing miniscule amounts of fluid to migrate from cooler to warmer areas through thermocapillary action.
“Such devices preclude the need to etch detailed architectures of pipes or channels into the substrate,” says Technical Insights research analyst Charles Joslin. “Other advantages include quick change and fast flow rates. Moreover, the substrate, without channels, can be easily cleaned between uses, avoiding contamination.”
Microfluidic devices based on this optoelectronic technique represent an advance over existing channel and pipe-based alternatives. “This technique could support a miniaturised lab-on-a-chip for genetic or biochemical testing in the field,” explains Joslin. “Such an easily reconfigurable system would be able to transport, merge, mix and split off streams of fluid flowing across a flat surface.”
The optically driven, dynamically reconfigurable microfluidic devices are still face challenges. These include controlling evaporation, developing interfaces to get the tiny volumes of liquid onto the surface, and choosing the right combination of substrate and heat sink to provide distinct temperature gradient patterns without overheating the fluids.