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Sigurdur T. Thoroddsen (King Abdullah University of Science and Technology (KAUST), Saudi Arabia)

“An Impacting Drop” probed with nanoscale high-speed interferometry

Title: Probing the nanoscale with high-speed interferometry of an impacting drop
Author: Sigurdur T. Thoroddsen
Affiliation: King Abdullah University of Science and Technology (KAUST), Saudi Arabia
Email: sigurdur.thoroddsen[at]kaust.edu.sa
Abstract:
(a) Breakup of a submicron layer of an air bowl between a drop and a pool of viscous fluid (b) 5 Mfps interferometry of the air dimple under an impacting drop.
(a) When a drop hits a pool, a very thin air layer is sustained at the interface for a very short time, and suddenly breaks up to create tiny bubbles, in this case, like a pearl necklace around the drop and a relatively large bubble climbing up along the tailing stem of the drop. (b). The thickness of the lubricating air-layer under an impacting drop is measured with ultra-high-speed interferometry.  The instance of the breakup can be seen in the image as the outermost dark ring.
The simple phenomenon of a water drop falling onto a glass plate, may seem like a trivial fluid mechanics problem.  However, detailed imaging has shown that this process is highly complex and a small air-bubble is always entrapped under the drop when it makes contact with the solid.  This bubble can interfere with the uniformity of spray coatings and degrade inkjet fabrication of displays etc.   We will describe how we use high-speed interferometry at 5 million frames per second to understand the details of this process.   The air-layer can be highly compressed and the deceleration of the bottom of the drop can be as large as 300,000 g. We will describe how the thickness-evolution of the lubricating air-layer is extracted from following the interference fringes.  Two-color interferometry is also used to extract absolute layer thicknesses.   Finally, we identify the effects of nanometric surface roughness on the first contact of the drop with the substrate.  Here we need to resolve 200 ns changes in the 100 nm thin air layer, requiring state of the art high-speed cameras.
Biographical Sketch: Siggi Thoroddsen is a Professor in the Mechanical Engineering Program of the Physical Sciences and Engineering Division at King Abdullah University of Science and Technology (KAUST) since 2009. From 2001-2008, he was an Associate Professor in ME at National University of Singapore. Before that, he was an Assistant Professor in Theoretical and Applied Mechanics at the University of Illinois, Urbana-Champaign. Thoroddsen has also been a visiting professor at Kinki University in Osaka.

Thoroddsen obtained his bachelor’s degree in Mechanical Engineering from his native country of Iceland, at the University of Iceland. Subsequently, he continued graduate studies in the United States, receiving a master’s in Civil Engineering from Colorado State University and his doctorate in Applied Mechanics from UC, San Diego.

Prof. Thoroddsen has authored over 110 publications in international scientific journals, describing his work on turbulence, coating flows and granular jets, in addition to his high-speed studies of the dynamics of drops and bubbles. His images and videos of flow phenomena have six times received the Gallery of Fluid Motion Award given at the American Physical Society Meeting of the Division of Fluid Dynamics. This award is granted based both on the novelty and the aesthetic value of the images, which he uses extensively in his teaching.  Thoroddsen was elected a fellow of the American Physical Society in 2012.

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