MIT engineers have developed a new salt crystallization process to help reduce fouling of heat exchanger surfaces.
Heat exchangers are used in a wide variety of different processes, and their efficiency is strongly affected by surface fouling caused by salts and other dissolved minerals.
This fouling lowers the efficiency of multiple industrial processes – water distribution systems, geothermal wells, desalination plants – and often requires expensive countermeasures such as water pretreatment.
Now an MIT team consisting of postdocs Samantha McBride and Henri-Louis Girard, led by professor Kripa Varanasi, have discovered that by combining hydrophobic surfaces with heat, dissolved salts can crystallize in a way that makes it easy to remove them from the surface, and in some cases by gravity alone.
These crystallized salts grow ‘legs,’ then tip over and fall away, potentially helping to prevent fouling of metal surfaces.
When McBride and Girard first began studying the way salts crystallize on such surfaces, they found that the precipitating salt would initially form a partial spherical shell around a droplet.
Unexpectedly, this shell would then suddenly rise on a set of spindly leg-like extensions grown during evaporation.
The process repeatedly produced multi-legged shapes, resembling elephants and other animals, and even sci-fi droids – leading Varanasi’s team to dub these formations as ‘crystal critters.’
The engineers’ work was motivated by the desire to limit or prevent the formation of scaling on surfaces – including inside pipes where such scaling can lead to blockages.
“Samantha’s experiment showed this interesting effect where the scale pretty much just pops off by itself,” notes Varanasi.
“These legs are hollow tubes, and the liquid is funneled down through these tubes. Once it hits the bottom and evaporates, it forms new crystals that continuously increase the length of the tube,” says McBride.
“In the end, you have very, very limited contact between the substrate and the crystal, to the point where these are going to just roll away on their own.”
McBride further adds that while many different length scales of patterning can yield hydrophobic surfaces, only patterns at the nanometer scale achieve this self-ejecting effect.
The self-ejecting process has many uses. It could especially be a boon to marine metal structures as these suffer the most from scaling and corrosion on account of being exposed to seawater.
Apart from this, it could also enable the use of salty or brackish water for cooling systems that otherwise require valuable and limited fresh water.
Image and content: MIT News