What tiny surfing robots teach us about surface tension

 Posted by Harrison Smith

It’s pumpkin season. The air is crisp. Leaves are falling, along with the temperature. Some may call it sweater weather; some may call it spooky season. But if you are anything like my friends, we just use it as an excuse to hit the winery. That is, those who are 21, of course. If you have ever been to a winery, I am sure you have seen a wide assortment of characters. You have your middle-aged reunions and your first dates. You have your young crowd, probably going just to drink and maybe crash a wedding (I’m definitely not speaking from experience). They don’t really know much about wine. On the contrary, you can be sure to find your wine connoisseurs, or the taste experts. You know who I’m talking about: those who taste test from nine different bottles and twirl the wine in the glass before taking a miniscule sip. We have all seen the classic wine twirl in movies, but what is it that these experts are actually looking for?

They’re looking for wine legs, or tears. Check out the picture below for a better idea!

Pretty neat right!? Who knew wine could walk? Well, that isn’t exactly what’s going on here. What we are actually looking at is the Marangoni Effect. The schematic below did a great job explaining the science behind this phenomenon.

The biggest concept here is the difference in surface tension throughout the wine. If you think of the wine as two components, the alcohol vs. the other stuff, it’s intuitive that they will have different chemical properties. Specifically, the alcohol has a lower surface tension compared to the other components. So, when the wine connoisseur spins the wine, thus thinning increasing surface area and the rate of evaporation towards the top of the glass, the liquid at the top is composed of less wine. This causes a subtle increase in surface tension towards the top of the wine. The increase in surface tension pulls mass in that direction. This net force, propelling particles from low surface tension to high surface tension, is known as the Marangoni Effect.

Dr. Hassan Masoud, at the Michigan Technological University is a Mechanical Engineer who has plenty of experience with the Marangoni Effect. A recent Science Daily article, “What tiny surfing robots teach us about surface tension,” speaks on Masoud’s work at Michigan Tech. He has been using tiny robots, just microns in size, to study liquid interfaces. Apparently, a lot of work has been published with microscopic swimming robots, but Dr. Masoud has more of a niche area of research with his surfing robots.

Recently, he made an interesting discovery. His surfing robots, with no engine or propeller mechanism, were propelled in a direction opposite of the Marangoni Effect. As previously discussed, the Marangoni Effect describes the transfer of mass and momentum in the direction of higher surface tension. Masoud’s surfing robots actually moved in the direction of lower surface area. He coined this term “reverse Marangoni propulsion.”

It was determined through further research that negative pressure suction from the low surface area was the primary propulsive force. However, by experimenting with different liquid thickness and chemical releases, Masoud and his coworkers found that particles can react in agreement with Marangoni, against Marangoni, and come to a complete stand still. Masoud noted that science has just begun to scratch the surface of understanding particle movement at the liquid interface.

Once a more concrete understanding is had, scientists can begin to learn how to control the movement of these particles. This could potentially be used in in a physiological setting to gain a better understanding of how bacteria colonize. Infection has plagued humans since we have been on Earth, and it is still a surprisingly common occurrence post-surgery. Many surgeries, particularly joint replacements requiring metal implants, result in bacteria-caused infection. If we can use Masoud’s research to gain a better understand and potentially control bacterial swarming, it would be a massive step forward in a biomedical context.

With that being said, I feel obligated to do my part to help society. I’m going to go grab a bottle and study up... in the name of science!