Surface Tensionand
Wetting
Guruswamy Kumaraswamy
CSIR-National Chemical Laboratory, Pune
and
Arun Banpurkar
Physics, University of Pune
Let’s start with some chemistry
What is?
Let’s start with some chemistry
What is?
Let’s start with some chemistry
What is?
Let’s start with some chemistry
What is?
Let’s try some more chemistry
What is?
What are the properties of H2O?
Let’s try some more chemistry
What is?
What are the properties of H2S?
We expect S and O to have similar properties...
H2S a gas while H2O is a liquid . WHY?
The difference between a gas and a liquid?
Molecules in a liquid are STUCK to each other
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Water forms hydrogen bonds
attractive force that holds water molecules tightly together in liquid phase
This happens in the bulk
What happens at the surface of water?
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ONE molecule thick layer of water where bonds are very different from the bulkInterface water molecules can’t H-bond with airDANGLING BONDS
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Can think of this as a SKIN for waterThe force that holds the skin together is the
surface tension
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Due to strong H-bonding, water has a really high surface tension (72 mN/m at RT)
Surface tension – how much is 72 mN/m?
- Water “skin” can hold up insects…- Can water “skin” support the weight of dense
metal objects? Can metal objects float?
Is 72 mN/m sufficient to hold up a paper clip?
But, this has nothing to do with surface tension
Remember Archimedes?
Surface tension“skin” force
Surface tension“skin” force
Net force
Can the skin of water pull itself up, into a tube?
https://upload.wikimedia.org/wikipedia/commons/6/66/CurvedSurfaceTension.png
Some creepy crawlies have learnt some really cool ways of using surface tension effects
Multimedia Fluid Mechanics: Cambridge University Press
Where do we encounter capillarity?
Chromatography
Wet hair, fibers sticktogether
Wet sand sticks together
Plant uptake Is this the only mechanism for water uptake?
Does this set a limit on how high plants can grow?
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Question:Why does adding soap to stagnant water help control dengue?
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Covering the skin – Changing surface tension
What happens when we add soap to water?
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Tails don’t stick so stronglySkin force decreases a lot
Making surface tension anisotropic
Marangoni effect
Camphor boatexperiment
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Temperature also changes surface tension
Multimedia Fluid Mechanics: Cambridge University Press
Now, let’s talk about WETTINGht
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The contact angle tells us about wetting
Hydrophobic: > 110o
SUPER-hydrophobic: > 160o
WETTING depends on whether the solid likes the liquid
Oil and water hate to mix
Solids that like water, don’t like oilandSolids that like oil, don’t like waterCan we use this to separate oil and water?
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Sponges that love oil but hate water
We’ve made
- HYDROPHOBIC sponges that are capable of absorbing hexane (dyed red) underwater
- OMNIPHILIC sponges: absorb > 10X their weight of water OR oil
Pesticide use can contaminate food, soil, water
Problem: How do we efficiently get hydrophobic pesticides onto leaves?
Water on rose leaf tilted at 30o
In general, leaf surfaces are HYDROPHOBIC
Leaf surfaces have a waxy coating. Therefore, they hate water.
We’ve done some work on this problem
Food grade nanoparticles
NP/water on rose leaf at 30o
We’ve done some work on this problem
We’ve done some work on this problem
We have discovered that nanoparticles (prepared from sunflower oil) can solve this problem
These nanoparticles are non-toxic. In fact, they are food-grade (viz. you can eat them)
SUPERHYDROPHOBICITYNatural superhydrophobic surface: Lotus leaf
High speed imaging of water drop falling on lotus leaf
Drops of water/our nanoparticle dispersions on Lotus leaves