A 2-mm-diameter ethanol droplet impacts on a hot surface. The corresponding videos with TIR images showing the droplet height are below. Credit: Shirota, et al. ©2016 American Physical Society
Leidenfrost effect occurs when water droplets levitate and skid around on top of a very hot surface, rather than immediately evaporating like they do at temperatures that are not quite as hot. The effect occurs because the bottom of the droplet rapidly vaporizes as it approaches the hot surface, causing the droplet to levitate on top of its own vapor. It has industrial cooling applications.
Now researchers have filmed the 1st videos of the tiny (<100 nm) levitation space between the hot surface and the impacting liquid droplet, in this case not of water but ethanol and fluorinated heptane. The videos allow them to directly observe, for the first time, how the shape of the droplet base changes as the surface temperature approaches the dynamic Leidenfrost temperature. This temperature varies depending on the type of liquid and increases with the impact velocity of the droplet.
To film the levitation space, they used total internal reflection (TIR) imaging. By reflecting a laser beam off mirrors and into a prism at a particular angle, they could transmit the resulting laser light through the droplets. This procedure creates grayscale images of dark wet droplets that visibly stand out against the lighter dry area, allowing them to see where the droplet comes in contact with the surface, if at all.
One of the most useful pieces of info obtained from the imaging is the shape of the droplet base is not flat, as sometimes assumed, but ring-shaped. The scientists describe it as a “dimple and neck structure” in contrast to the previous “pancake model” that describes a flat bottom.
“Drop impact on a superheated surface is of key importance in various applications,” said Chao Sun, Physics Professor at the University of Twente. “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.”
From the TIR images, they could quantitatively define 3 different boiling regimes (contact, transition, and Leidenfrost) as the temperature increases at the various impact conditions. TIR imaging allows values to be calculated eg incident angle and the spreading radius of the droplet in order to generate a clearer picture and a more precise definition of the regime boundaries and associated transition temperatures.
“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,” said Detlef Lohse, Physics Professor at the University of Twente. In the future, the researchers plan to use their observations of the dimple and neck structure to develop more accurate models of the dynamic Leidenfrost temperatures for various types of liquids.
http://phys.org/news/2016-02-scientists-video-nm-space-impacting.htmljCp http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.064501
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