dscienceparent
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Hydrophilic vs hydrophobic

Postby dscienceparent » Mon Mar 16, 2020 8:46 pm

My child obtained these results for Which Materials can catch a bubble. We are trying to understand the explanation in terms of the hydrophilic and hydrophobic discussion. We tried droplets of water, but couldn't really tell if they were spreading out or beading up. I also don’t understand whether a hydrophilic or hydrophobic substance would be more likely to catch the bubble. Do you have any recommendations for us to help us explain the results of the study?

Felt 5 bubbles caught
Foil 2
Scrubber sponge 2
Sandpaper 2
Glass 0
Brick 0

[Project: https://www.sciencebuddies.org/science- ... tchbubbles]

Darin11
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Re: Hydrophilic vs hydrophobic

Postby Darin11 » Wed Mar 18, 2020 2:32 pm

So you are examining behavior of soap bubbles as they land on these different surfaces?

bvionis
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Re: Hydrophilic vs hydrophobic

Postby bvionis » Wed Mar 18, 2020 10:02 pm

Hello,

Firstly, it is important to understand that a bubble consists of an outer and inner layer of soap surrounding a central layer of water. The reason a bubble pops is because the layer of water lying between the two thin layers of soap dries up; thus, a dryer surface will cause the bubble to pop sooner than it would on a wet surface.

After doing a little research, I came to the conclusion that a surface could be considered hydrophilic (attracted to water) if the bubble pops, as the hydrophilic surface would cause the water within the bubble to be attracted to itself, causing the water to be absorbed, dried up, etc. by that surface and as a result popping the bubble (as explained by the above note). Following this logic, it can also be concluded that any material would be hydrophobic if the bubble does not pop, as there is no attraction between the material's surface and the water within the bubbles. This would enable the bubble to remain on that surface (at least before the water within it naturally dries up), as the thin layer of water within it would not be under threat of being absorbed/exposed by the surface.

To help you understand whether or not a water droplet is spreading out (coming into contact with a hydrophilic surface) or beading up (coming into contact with a hydrophobic surface), the contact angle between that surface and the droplet must be considered. A hydrophilic surface will cause the contact angle between that surface and the water to be less than 90 degrees, while a hydrophobic surface will cause the contact angle to be greater than 90 degrees. This of course cannot practically be measured, but a close observation of the droplet can determine with enough accuracy whether or not the surface contacting the water droplet is hydrophobic or hydrophilic. Here is a simple visual of the above explanation that may help:

https://blog.iglcoatings.com/wp-content ... urface.jpg

Note: Because of their delicate nature, a bubble will most likely pop by the time the contact angle becomes less than 90 degrees, as by then the thin inner layer of water would most likely come into contact with the surface and as a result dry up or even get absorbed. This further explains why a surface is most likely hydrophilic when a bubble easily pops on it (although there are other factors that can affect this, such as the dryness/wetness of the surface).

I hope that this will help you to properly interpret the results of your experiment. If you have any other questions/concerns, I will be happy to answer them for you.

EmmaGriffiths
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Re: Hydrophilic vs hydrophobic

Postby EmmaGriffiths » Fri Apr 03, 2020 2:32 pm

Hello!

This response may be slightly delayed, but I would like to add some input that may be valuable to others. There is already a very well-spoken response by bvionis which explains why bubbles can land on hydrophobic surfaces but not on hydrophilic ones.

One other thing that may be misleading to someone trying this experiment is that some materials are hydrophilic but have other properties that may allow them to catch bubbles. Felt, for example, is a hydrophilic material -- we know this because it can absorb water quite easily. So why does it catch bubbles? The surface of felt has very small fibers that may not pop a bubble if it lands on many fibers at once.

Have you ever seen a balloon pressed into many nails without popping? When a balloon lands on one nail, it bursts. When it lands on many nails, however, the pressure is distributed across the surface of the balloon, preventing it from popping.

The felt is similar in this way. One fiber would probably pop a bubble, but many fibers have the ability to hold a bubble. A fuzzy sponge could yield the same result.

Hopefully this all makes sense. Above all, have fun with this project... that's what science is all about!

Take care,
Emma :)


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