Researchers have discovered why droplets travel 100 times faster across heated oily surfaces than they do on bare metal. The droplets are shown in various photographs that reveal the mechanism behind the rapid motion. Credits: Edited by MIT News.You may have noticed droplets of water floating on top of hot oil when you fry something in a skillet. Researchers at MIT have now analyzed this seemingly insignificant phenomenon and are able to understand it. It may have important implications on microfluidic devices and heat transfer systems.Sometimes, a droplet of boiling water placed on a hot surface can levitate on a thin film of vapor. This phenomenon is known as the Leidenfrost effect. The droplet can glide across the surface without much friction because it is suspended on a cushion made of vapor. The hot droplet should move slower if the surface is covered with hot oil. This has a greater friction coefficient than the Leidenfrost vapor film. Contrary to what you might think, MIT experiments have shown that oil drops move much faster than bare metal.This effect propels droplets 10 to 100 times faster across heated oily surfaces than on bare steel. It could also be used to self-clean or de-icing systems or to push tiny quantities of liquid through tiny tubing in microfluidic devices that are used for chemical and biomedical research. These findings were published today in a paper in Physical Review Letters by Victor Julio Leon, a graduate student and Kripa Varanasi, a professor of mechanical engineering.Varanasi's previous research showed that this phenomenon could be harnessed for some potential applications. However, Varanasi believes the new work producing high velocities (approximately fifty times faster) could lead to even more uses.Leon Varanasi and Varanasi had to do a lot of analysis before they could determine what caused these droplets to eject quickly from the hot surface. The oil will form a thin layer on the outside of each droplet of water if it is heated up, has high oil viscosity and is thick enough. As the droplet heats, small bubbles of vapor will form at the interface of the oil and droplet. These tiny bubbles build up randomly along the base of the droplet, creating asymmetries. The droplet's surface is loosened by the lower friction and propels itself away.Varanasi explains that the oily film behaves almost like a rubber balloon and when tiny vapor bubbles burst though, they create a force. "The balloon just flies away because the balloon is going out one way, creating momentum transfer." The oil cloak would have prevented self-propulsion. However, the vapor bubbles would flow out of the droplet all directions without the cloak.Although the phenomenon may seem simple, it is actually complicated by complex interactions between events that occur at different times.The self-ejection phenomenon was recently analyzed and depends on many factors including the size of the droplets, thickness and viscosity the oil film, thermal conductivity of surface, surface tension of different liquids, type of oil and texture.To reveal details about the droplets, researchers used high-speed photography at extreme speeds. Victor Leon, a graduate student from the Massachusetts Institute of Technology, says that you can see the fluctuations on the surface. Credit: Massachusetts Institute of TechnologyThe oils that they tested had a viscosity 100 times higher than the surrounding air. It would have been expected that bubbles would move slower than the Leidenfrost air cushion. Leon states, "That is an indication of how surprising it can be that this droplet moves faster."Bubbles will form randomly from a nucleation site not at its center as boiling begins. This will cause bubble formation to increase, causing propulsion in one direction. Researchers have not been able control the direction of randomly-induced propulsion. However, they are currently working on ways to control this directionality. Leon states that they have some ideas on how to control the propulsion's direction.The tests revealed that the thin oil film on the silicon wafer surface was only 10 to 100 microns thick, about the thickness of a human hair. However, its behavior was not consistent with the equations for thin films. The film behaved like an inexhaustible pool of oil due to the vaporization. Leon states that the discovery was "astonishing". Leon says that while a thin film would have made it stick, the pool was able to give the droplet much less friction and allowed it to move faster than expected.This is due to the fact that formation of tiny bubbles takes place much faster than heat transfer through oil film. It takes about a thousand times longer for them to form, which leaves plenty of time for asymmetries in the droplet. The bubbles of vapor that form at the oil/water interface are more insulating than the liquid, which can cause significant thermal disturbances within the oil film. These vibrations cause the droplet vibrate, increasing friction and vaporization.Leon claims that extreme high-speed photography was required to capture the details of this fast effect. Leon used a 100,000 frame per second video camera. Leon states, "You can see the fluctuations on top."Varanasi said that initially, they were puzzled on multiple levels about what was happening because of the unexpected effect. This is a complex explanation for what appears to be simple, but it creates fast propulsion.The effect could be seen as a result of heating a surface with enough oil and the right kind of coating to cause corrosion scaling drops to form on a surface. The researchers will have greater control over the directionality of the system, which could allow them to replace high-tech pumps with microfluidic devices that propel droplets through the correct tubes at the right moment. This could be particularly useful in microgravity situations where ordinary pumps aren't working as well.Varanasi suggests that it may be possible to attach a payload onto the droplets to create a microscale robotic delivery system. He says that although their experiments were limited to water droplets it is possible that the payload could be applied to other liquids or sublimating solids.Continue readingMore information: Victor Julio Leon and al, Self-Propulsion Of Boiling Droplets On Thin Heated Oil Films. Physical Review Letters (2021). Information from the Journal: Physical Review Letters Victor Julio Leon and al, Self-Propulsion Of Boiling Droplets On Thin Heated Oil Films (2021). DOI: 10.1103/PhysRevLett.127.074502This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.
