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

THANK YOU

g.kumaraswamy@ncl.res.in