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# Surface tension

## by Then Murugeshwari, Webdesigner at mspvl on Jul 21, 2011

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This presentation has the details about the surface tension of the water

This presentation has the details about the surface tension of the water

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## Surface tensionPresentation Transcript

• Surface Tension
• Water striders are light (like ants) thus don’t “break” surface
• Ooh! Look at me! I have hydrophobic feet and I weigh less than Fritz does! I’m soooo great!
• Even a piece of steel can do this trick if it is small (steel  ~ 8x water)
• 4 H 2 O molecules
• separated in space from each other
• have partial + and – charges
• what would they do???
but what’s surface tension, really?
• 4 H 2 O molecules
• they clump together
• + and – charges snuggle up close
• potential energy of system has dropped
• Surface Tension
• water in bulk has many binding partners
• water at surface has less, has exposed charges left over
• potential energy of water at surface is higher
• deforming droplet to increase surface area takes work
• Surface Tension
• E = FX, energy = force * distance
• dE = F dX
• F = dE/dX
• e.g. spring energy = ½ kx 2 , dE/dX = kx = F
• Surface Tension
• creating surface area in 20  C water droplet takes
• 73 ergs/cm^2
• droplet thus seems springy
• if mg <<  l it dominates and you can walk on water (Vogel pp 72, 104-109)
• Surface Tension
• surface area in 20  C water costs
• 73 ergs/cm 2
• = “  ”
• Surface Tension
• surface area in 20  C water costs
• 73 ergs/cm 2
• F = dE/dX
• can get  from F in this apparatus
• if film is w by w cm, how much area has been created?
• Surface Tension
• 2 W 2
• OK so remember this? (steel  ~ 8x water)
• Floating without floating – The SECRET OF THE STRIDERS REVEALED!!!
•  = 73 ergs/cm 2 = 73 dyne-cm/cm 2 = 73 dynes/cm
• 73 dynes/cm is also like a tear strength
• if we stacked poker chips on water it might look like below
• area of chip doesn’t matter so much as the edge (vertical contributions)
• lift = perimeter *  * sin 
• wait, why sin  ? why not pull them all at 90 degrees?
• Floating without floating -
•  is constant of water / air interface, so can’t just “choose” to pull less
• surface fails when tension along perimeter of chips exceeds 73 dynes/cm
• after that, the water does something else more energetically profitable –
• Incidentally – Scaling tie-in - Why droplets are droplet-sized - mass increases faster than length or area, so above about 1 cm diameter, water droplet mg >  l, so more likely to get torn apart by its own weight
• Floating without floating -
• anyway so if the outlines of your feet are long enough for  L to add up to more than your weight (and your contact angle is high) you too can walk on water
• Contact Angles
• here’s a droplet on a surface -
• Contact Angle
• here’s a slice of it –
• tangent to droplet edge is “contact angle”
• why is theta theta?
• Contact Angle
• balance of forces
• surface tension pulls up
• gravity & adhesion pulls down
• what are the other two?
• Contact Angle
• F = dE/dX
• surface/air & surface/water interfaces also have “surface tension”, in ergs/cm 2
• moving water edge back and forth incurs energy costs/profits
• but units of F are energy/distance, not area?! what’s the deal?
• Contact Angle
• problem is 3-D
• surface tension is force per length
• each dL of perimeter contributes  dL force
• F = dE/dX =>  dL
• dE =  dL dX =  dA
• back to ergs/cm 2
• Obtuse contact Angles
• hydrophobic surface
• “ gravity & adhesion” is now “gravity & repulsion”
• if no gravity, drop leaves
• Contact Angle
• why doesn’t drop pull or push itself along the surface?
• it did when initially set down, it distorted itself until equilibrium reached
• edge equilibrium is one thing
• equilibrium between  (roundness) & gravity (flatness) & surface coverage (adhesion/repulsion) is another factor...