Microfluidics is a technology used to move and distribute liquids in quantities that could sit on the end of a pin. Could hydronics and plumbing have a future with applications on a tiny scale? At a minimum, microfluidics has lessons to teach about Reynolds numbers in fluid distribution.
Elveflow.com defines this technology: “Microfluidics is both the science that studies the behavior of fluids through micro-channels and the technology of manufacturing microminiaturized devices containing chambers and tunnels through which fluids flow or are confined.” If you take the word “micro” out of that quote, we are talking about plumbing and hydronics.
How small are we talking? Elveflow continues, “Microfluidics deals with very small volumes of fluids, down to femtoliters (fL), which is a quadrillionth of a liter. Fluids behave very differently on the micrometric scale than they do in everyday life: these unique features are the key for new scientific experiments and innovations.”
The microfluidic applications currently lean more toward plumbing than hydronics. An inkjet is a common example of this technology. If the distribution of the liquid inks in your printer wasn’t at such a small scale, everything you printed would look like someone drew your PDF with bottles of ketchup and mustard. The micro dots of mixed ink colors let you print a photo quality with small plumbing and color gradient mixing.
Fluids can act differently at a micro level, compared to water in a ¾-inch pipe. The reason relates to the Reynolds number. This number can tell you what the molecules inside a length of pipe will look like. In some cases, the Reynolds number is low, and the molecules look like a military march down a street, known as laminar flow. If the Reynolds number is high, it may be more like a group of caffeinated kids running to recess down the hallway, turbulent flow.
Where does the Reynolds number come from? Osborne Reynolds was the pioneer of this research of flow characteristics. According to a biography by the University of Houston, “His Reynolds number is a ratio that shows the effect of viscosity in a flow. Compute it, and you'll know the nature of the flow before you see it. The Reynolds number for a BB sinking in honey is less than one. For a reed in the breeze it might be a 100. For a golf ball in flight, it’s over 200,000.” The Reynolds number for many microfluidics applications is below 1.
Issue 16 of Caleffi idronics has a good explanation of the Reynolds numbers to shoot for in a hydronics system: “If the Reynolds number is 4,000 or higher, the flow will be turbulent. If the Reynolds number is 2,300 or lower, the flow will be laminar. If the Reynolds number is between 2,300 and 4,000, the flow could be either turbulent or laminar. It may even transition back and forth between turbulent and laminar. This unpredictable flow condition should be avoided, and thus the minimum acceptable condition that ensures turbulent flow is Reynolds number ≥ 4,000.”
In a hydronics system, you want to keep your fluid laminar when distributing it and turbulent when transferring heat. Inside the heat exchanger of a boiler, if your flow is laminar, you won’t exchange as much heat to the walls, which is why heat exchangers are goofy shapes inside.
If you want to see an example of something changing from a low Reynolds number to a high one, laminar to turbulent, grab a candle and a big glass or vase. Light the candle and put the glass over the top of it to snuff it out. When the flame burns out, the smoke will be pencil straight to the top and then become turbulent and mix when it runs out of vertical space.
Could we use microfluidics in hydronics? Currently, microfluidic technologies favor a very structured, laminar flow. As with the heat exchanger example, we need to make a heating system more turbulent at certain points to exchange heat energy more effectively. A micro radiator would only be effective if you could make the flow more turbulent than the ink coming out of your printer.
If you did want to build a micro radiator, you could create interesting shapes with micro distribution channels. If you could print the path of water like a circuit board, your imagination would be the only limit. You could even 3D print your own radiator, with some planning and the proper materials.
Imagine if you wanted to renovate your bathroom in the future and you could buy a 1/8-inch tile that had a circuit of water passageways built in. You would have to approach supply and return piping from a new angle and your head loss calculations may be completely different from a traditional project. However, you could open up more retrofit opportunities if the radiant was part of, not below the flooring surface.
A micro radiator tile may be fairly brittle, and a broken tile could mean a flooded bathroom. The installation flexibility may not outweigh the other negative possibilities for this particular example. However, microhydronics would open up a whole new world of radiant options.
In the plumbing world, the possibilities seem more of a natural fit for microfluidics. You could potentially monitor water quality levels and adjust micro amounts of additives to mix water with the perfect softness, pH and chlorine level in real time with microinjections. This general application is being pursued already in the pharmaceutical world.
With microfluidics, you could mix drugs on demand. Essentially, you would be able to build your own gel cap pill on the fly. If you are an EMT in the middle of nowhere and you need a perfect dose of a drug, you could build your own. The pharmacy of the future could come out of an inkjet printer of sorts.
I’m not sure what microfluidic technology is production-ready for plumbing or hydronics today, but it is an interesting new world of moving fluids. Who better to figure out those logistics and applications than the readers of this magazine — what would you build?