Strange salty "crystal critters" help thwart fouling of metal surfaces
Pipes, instruments and surfaces that come into contact with salty water usually end up with a corrosive layer of salt and other dissolved minerals caked on, which needs to be scraped or washed off. Now, engineers at MIT have developed a new method for making those minerals so easy to remove that they often just fall off on their own – by forming “crystal critters.”
Normally, when a drop of salty water sticks to a surface, the salt forms a globe shape as the water evaporates away. That leaves a crystal with a high area of contact to the surface, and eventually you end up with a salty crust along the whole surface that’s difficult to scrub off.
So for the new study, the MIT team set out to investigate ways to change that crystallization process by tweaking the surfaces themselves. Eventually they stumbled upon an intriguing phenomenon that had never been seen before.
If a surface is hydrophobic (water repelling), heated, and has a particular nanoscale texture of low ridges, the salt crystallizes in a unique way. It starts off much the same as usual, forming a globe. But soon, strange leg-like structures begin sprouting underneath, pushing the globe upwards. Eventually they grow so long that they can no longer support the weight, and the crystal breaks off. Due to the weird animalistic shapes they form, the team dubbed them “crystal critters.”
“These legs are hollow tubes, and the liquid is funneled down through these tubes,” says Samantha McBride, lead author of the study. “Once it hits the bottom and evaporates, it forms new crystals that continuously increase the length of the tube. 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.”
The team says that the texture could be easily transferred to a range of surfaces, through etching or coating, making it relatively easy to scale up and implement in existing infrastructure. It could prove handy for a range of facilities, such as desalination plants, water distribution pipes, geothermal wells, and essentially anywhere that regularly deals with impure water.
This kind of surface could reduce fouling and the frequency of cleaning in all of these places, and could even allow for more brackish water to be used for some processes, such as cooling systems. As an added bonus, the team suggests that the salt could be collected for its own uses too.
The research was published in the journal Science Advances. The crystal critters can be seen forming and falling in the video below.