Spreading Water on Teflon: Marangoni Influences and the Elimination of the Ring Stain
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Rendall, Clint; Majumder, Mainak; Pasquali, Matteo
In this video, a new technique of spreading water on the hydrophobic surface Teflon® is presented. Under ambient conditions, a small droplet of water placed on a Teflon® surface will form a bead with a very high contact angle; no spreading occurs, as the water seeks to minimize the surface area exposed to the hydrophobic surface. However, following the introduction of an ethanol-saturated atmosphere to the system, the situation changes dramatically and the water droplet spreads as it would on a less hydrophobic surface, such as glass or polyethylene. Particulate flow within the droplet is of interest as well. Focusing attention again on a water droplet on Teflon® in normal conditions, it is observed that particles will tend to settle and move to the edges as the droplet evaporates (a), leaving a distinctive ring stain (b). This is the so-called “coffee-stain” effect. However, with the new spreading method introduced in the video, a recirculating flow due to Marangoni influences is observed instead (c). When the droplet dries, it leaves a uniform film (d) as opposed to a ring stain. This discovery could have important applications in a number of current and future manufacturing processes. Within the compiled video, the first video clip, the overhead shot, was taken with a Fujifilm Z20fd camera in macro mode. The clip is played back at approximately 160 times faster than real time. The other two clips, the side shots of the particulate flow, were taken with a Photometrics Coolsnap cf camera and digitally processed using the commercially-available Metamorph software. The first video is presented at 60 times faster than real time, while the second is 13 times faster. The colors in these videos were inverted and contrast was enhanced for particulate clarity. The compiled video was assembled using Sony Vegas Movie Studio Platinum, Version 9.0.
fluid; dynamics; video; droplet; aps; marangoni