AsianScientist (Feb. 15, 2016) – Researchers in Hong Kong and the US recently demonstrated that it is possible to exploit the Leidenfrost effect to control the direction of liquid droplets to cool surfaces more efficiently. The article was published in Nature Physics.
The discovery, said Associate Professor Wang Zuankai of the City University of Hong Kong and Professor Manoj Chaudhury of Lehigh University, has the potential to improve technologies that involve microfluidics, heat transfer, heat exchange, micro-heat exchange, water management and thermal management.
The Leidenfrost effect was named for Johann Gottlob Leidenfrost, an 18th century German physician and scientist. The phenomenon occurs when a liquid, upon approaching an object that is much hotter than the liquid’s boiling point, produces a vapor which insulates the liquid from the surface of the object. This happens when you sprinkle a teaspoon of water on a skillet that is heated up to well above the boiling point of water. Water droplets will bounce up, form spheres and scurry across the surface.
This repulsive force has two consequences, the scientists say. It prevents droplets of the liquid from making physical contact with the surface, causing them instead to hover over the surface. It also causes the droplets to boil off more slowly than they would on a surface with a lower temperature that is still above the liquid’s boiling point.
“Many applications, such as power plant reactors, require the management and control of the movement of water droplets at very high temperatures,” said Wang.
“Typically, the cooling of extremely hot surfaces has been accomplished with spray cooling. You spray a lot of water droplets onto a surface and as they boil, they take away the heat.”
“At a high temperature, however, this doesn’t work because the Leidenfrost effect prevents the droplets from making sufficient contact with the surface to cool it. Thus it takes too long to cool a surface by boiling off water.”
While scientists have learned to control the movement of liquid droplets on a solid surface, they have not yet achieved this control on surfaces heated to Leidenfrost temperatures and above, or on surfaces with extremely hot local spots.
“The Leidenfrost Effect has been extensively studied for drag reduction, while the presence of the undesired vapor layer also prevents efficient heat transfer,” says Wang.
“Thus, we came up with the idea of creating an asymmetric surface to control droplet motion at high temperatures.”
The team created topographical contrasts on a silicon wafer by etching the wafer surface with micropillars and arranging the pillars in zones that vary according to the density of the pillars and the contact angle of the pillars with the surface. This arrangement allowed them to heat single droplets to above the boiling point.
At this high temperature, the droplet showed a contrasting thermal state, with a lower contact angle in the boiling region, but a higher angle in the Leidenfrost region. The contrasting angles generated a pressure gradient through the curvature of the droplet surface.
“As the [droplet’s] viscous dissipation is minimal,” the researchers explained, “the resulting excess surface energy of the droplet is converted to kinetic energy, naturally causing it to dislodge from the surface and take flight into the air. The droplet eventually gets deposited in the contact-boiling region.”
The researchers liken this phenomenon to the action of a slingshot. They also note that something similar occurs with the filamentous mushroom, Basidiomycota.
Each spore of the fungus is part hydrophobic, with a shape like a thin film, and part hydrophilic, with a shape like a sphere. When the two regions make contact, they coalesce, and the tension between hydrophobic and hydrophilic regions creates a force that carries the entire spore into the air.
The article can be found at: Li et al. (2016) Directional Transport of High-temperature Janus Droplets Mediated by Structural Topography.
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Source: Lehigh University; Photo: Rehan Jamil/Flickr/CC.
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