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Bouncing droplets - PowerPoint PPT Presentation

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Bouncing droplets
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  1. Bouncing droplets Sweden

  2. BouncingWaterdroplets • ”Investigate the motion of water dropletsfalling on a hydrophobicsurface (e. g. coated with soot or teflon).”

  3. Overview of presentation • Introduction to the phenomenon • Summary of investigations • Experimental set-up • Results • Conclusion

  4. Bouncingwaterdroplet

  5. Hydrophobic Surface • If Ѳ > 90˚ the surface is hydrophobic • This due to a difference in chemical structures • This sootedsurface has approximately a contactangle of 145˚

  6. Investigations Whatcan be changed ”Investigate the motion of water dropletsfalling on a hydrophobicsurface” Whatwill it affect • Heightof release Yet to be investigated • Radius • Density • Surface tension • Number of dropletsformed • Conservationof kineticenergy , surface tension – Weber number Yet to be investigated • Velocity • Height of bounce • Energy loss • Contact time

  7. Material • High speed camera • Spotlights • Stand • Water in a pipette • Soot coated aluminium plate

  8. Behaviour of droplet at the hydrophobicsurface • Depending on the speed and the formation of the droplet different thingswilloccurwhen the droplet hits the hydrophobicsurface. • Five major categoriescan be observed: - One cleanbounce - Bounce with strain - One large and one small droplet - One large and a few small droplets - Manydroplets, ”splash”

  9. One Clean Bounce Kineticenergydue to velocitycauses the droplet to expand Surface tension affects in the oppositedirection strives for the water molecules to be as close as possible Therefore, when the surface area gets largerenergy is stored causing the bouncewhen the molecularforces (caused by surface tension) pulls the waterdroplet back

  10. Bounce with Strain

  11. Weber Number • Comparisonbetween the kineticenergy and the surface tension of the dropletcanhelpusunderstandwhateffect the different energies and parameters willhave on the dropletwhen hitting the surface. • results in Wehaving a dimensionlessnumber, the valuewillhave no unit • Used to confirm that surface tension affectswhether it will be a bounce or a splash

  12. Weber Number Weber-Number at different Velocities 1 .Clean Bounce 2. Bounce with strain 3. One largeone small 4. One large, few small 5. Many Velocity We < 40 = BounceWe > 140 = Splash

  13. Conclusion ”Investigate the motion of water dropletsfalling on a hydrophobicsurface (e. g. coated with soot or teflon).” • Weber number is a wellfunctioningmeasurewhendetermininghow the dropletwillbehavewhen hitting the surface. We < 40 = ”Bounce”, We > 140 = ”Splash” it also provides us with a proof for the importance of the surface tension of the water.