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Bouncing droplets. Sweden. Bouncing Waterdroplets. ” Investigate the motion of water droplets falling on a hydrophobic surface (e. g. coated with soot or teflon).”. Overview of presentation. Introduction to the phenomenon Summary of investigations Experimental set-up Results

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Bouncing waterdroplets
BouncingWaterdroplets

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


Overview of presentation
Overview of presentation

  • Introduction to the phenomenon

  • Summary of investigations

  • Experimental set-up

  • Results

  • Conclusion


Bouncing waterdroplet
Bouncingwaterdroplet


Hydrophobic surface
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˚


Investigations
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


Material
Material

  • High speed camera

  • Spotlights

  • Stand

  • Water in a pipette

  • Soot coated aluminium plate


Behaviour of droplet at the hydrophobic surface
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”


One clean bounce
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



Weber number
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


Weber number1
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


Conclusion
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.


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