A study titled "Dynamic Leidenfrost Effect: Relevant Time and Length Scales" and published in the journal Physical Review Letters by researchers from the University of Twente reveals the 100-nm space underneath the levitated droplet of the Leidenfrost effect, further shedding light on how this phenomenon occurs.
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The Leidenfrost effect occurs when water droplets levitate and move around on a very hot surface instead of evaporating on surfaces not as hot; the effect occurs due to the fact that the bottom of the droplet vaporizes as it gets to the hot surface, causing the droplet to levitate on top of its own vapor - Phys.org reports.
Researchers had never caught this phenomenon on video but they have managed to do that now, videoing the space between the levitated droplet and the hot surface – in this case using ethanol and fluorinated heptanes instead of water. In the video, the researchers were able to fully watch the droplet as it changed shape and neared the Leidenfrost hot heat. This temperature varies depending on the type of liquid and increases with the impact velocity of the droplet.
The researchers used the total internal refection (TIR) imaging technique to video the levitation space.
The researchers observed that rather than the base of the droplet being flat, as would have been thought, it was ring-shaped – prompting the research team to term it a “dimple and neck structure” against the “pancake model” that reflects a flat bottom.
"Drop impact on a superheated surface is of key importance in various applications," said coauthor Chao Sun, Physics Professor at the University of Twente. "A quantitative physical understanding of the phenomenon is, however, lacking. In this work, we show that the whole impact dynamics is determined by the dimple and neck formation beneath the droplet at the very beginning of the impacting process."
The team could see from the images that the droplet has three levels of heating up – contact, transition, and Leidenfrost – changing as the temperature rises after the droplet makes an impact on the hot surface.
Detlef Lohse, a physics professor from Twente and co-author of the study stated that “Heat transfer from solid surfaces to impacting liquid droplets plays an important role in many industrial technologies such as power electronics cooling, spray cooling, engine performance, and pollutant emissions.”
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Researchers Sun and Lohse finally submitted that they are still committed to measuring the quantitative and theoretical dynamics of Leidenfrost temperature, and understanding how surrounding pressure impacts on this phenomenon.