The Microfluidics Orchestra

A friend of mine honestly believes that when scientific knowledge has been used to deconstruct something she thought was pretty, it's no longer pretty.  I disagree (of course). The ability to discern prettiness in such things lies in being able to appreciate how those things work, and how the laws of physics allow them to work that way.

With that brief introduction, let me introduce my infatuation for this week: microfluidics. I hit upon it after watching a curious and pretty demonstration on YouTube, where an arrangement of valves and capillaries were used to channel microscopic volumes of coloured liquids against an orchestral background score. What I found prettiest about it was the simplicity behind its working: how often do things get so elegant? Here's it for your viewing pleasure.

[youtube http://www.youtube.com/watch?v=UVSDRglikuM]

On the leftmost is the inlet valve which branches out into a series of smaller valves that then lead out into the horizontal chamber. As the video plays out, it becomes evident that the speeds at which liquids are being brought in correspond to high-performance pumps and liquids that are more cohesive than adhesive, i.e. low solubility. However, that may not be the case if they could somehow be made to travel in a straight line once they've been ejected.

That is effected by ensuring a few physical dimensions, and that's the essence of microfluidics: the pipes that the liquid travels through have a very low diameter, effectively making them capillaries.

When a liquid is inside a capillary, it doesn't obey the laws of gravity because then its atomic properties begin to dominate (something like how quantum mechanics takes over from classical mechanics). At the atomic level, there is an interplay of two forces brought on by two different entities. The inter-atomic interaction in the liquid, called the adhesive force, becomes more prominent because only a microscopic volume of the liquid is in play. The adhesive force gives rise to a so-called surface tension on the open surface of the liquid.

Second, the interaction between the atoms of the liquid and those of the capillary surface, manifested as the cohesive force, gives rise to the formation of a meniscus.

[caption id="" align="alignnone" width="318"] The meniscus here is concave. A common example of a liquid that has a concave meniscus is water. Mercury, on the other hand, has a convex meniscus.[/caption]

The meniscus is bent inward (concave) if the cohesive forces are stronger than the adhesive forces, i.e. the liquid's atoms are friendlier with the capillary than with others of their own kind. The meniscus is bent outward (convex) if they are more nationalist in the same context. All this while, there is a struggle between cohesion and surface tension that causes it to lift the liquid through the capillary. Once the struggle ceases, the liquid stops moving, but I don't think that happens.

Microfluidics works with this struggle, called the capillary action, and adds to it by allowing a wide variety of ways to interact with the fluids. The one that interests me most is called acoustic droplet ejection (ADE). In ADE, ultrasound pulses are shot into a liquid in a capillary. Because they have very high frequencies, ultrasounds also pack quite a bit of energy into very short pulses (however, higher the frequency after a point, lesser the energy in the pulse). This energy can be transferred, effectively allowing a human controller (working with an ultrasound-emitter) to push and pull the liquid.

This possibility reserves a de facto application in drug synthesis and delivery, where extremely small quantities of proteins and other substances can be controlled and injected into cells to study their behaviour. ADE is a gentle process, and thus works well with medical instrumentation - where there is a proliferation of systems that demand very high precision and very low error rates. Because there are no nozzles touching the liquid or the capillary, there is no chance of their composition being corrupted. The coefficient of variation—which is the measure of a system's deviation from an ideal behaviour—has also been found to be statistically low.

This video shows an ADE at work.

Other industries that work with small quantities of fluids are those that manufacture substrates for use in microscopic solid-state physics devices, micro-electromechanical systems (which also have biotechnological applications), high-resolution printers, fluid flow sensors and gauges, and any experiments that involve the study of the atomic or molecular properties of materials. Come to think of it, I can use microfluidics to determine up to nine decimal places how much alcohol can actually overpower my inhibitions. I wonder if that's what Feynman was thinking of when he said, "There's plenty of room at the bottom!"