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Yesterday when I was taking my bath, I scratched my soap bar a bit and threw that bit on the shower floor. There was a little amount of water on the floor where that soap bit fell. To my surprise, I noticed that the the water around that soap bit surrounding it had moved away. I'm clueless why this happened. I tried to reproduce the results and every time the water would surround the soap bit without touching it. The water around would sort of encircle the soap bit.

Is this because of soap micelles? I know that the hydrophilic head of the micelles always point outwards so shouldn't the water be attracted towards the soap bit? Rather it seems that soap and water repel each other. Could you please explain why this happens? You could also repeat this experiment in your house.

EDIT $1:$ I repeated the experiment with the tiles in my drawing room which are very smooth and nothing of that sort happened. I think only the ("little") rough tiles (which are in my bathroom) can be used to see "soap repel water". Moreover the tiles in my drawing room also don't spread water when a bit of water is thrown on one.

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  • $\begingroup$ What's your shower floor made of? Ceramic tiles? Do small amounts of water spread out on it, or does it form into beads (droplets)? Consider what soap does to the surface tension. $\endgroup$
    – PM 2Ring
    Mar 30, 2019 at 10:28
  • $\begingroup$ @PM 2Ring water spreads and doesn't form into droplets $\endgroup$
    – user8718165
    Mar 30, 2019 at 10:55

1 Answer 1

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tl;dr- When the soap touched the water, part of it dissolved, reducing the local surface tension. Since the higher surface tension further away from the soap pulled more strongly than the weaker surface tension near the soap, the water moved in that direction. This video shows a similar effect.

I'd guess that this is the Marangoni effect:

Since a liquid with a high surface tension pulls more strongly on the surrounding liquid than one with a low surface tension, the presence of a gradient in surface tension will naturally cause the liquid to flow away from regions of low surface tension.

In this case, the water is moving away from the soap because that's the direction of increasing surface tension.

Why? Because a little bit of the soap dissolved upon contact, and soap, as a surfactant, reduces surface tension.

Here's a neat demo of it:

And a corresponding YouTube clip:

In this demo, a little paper boat is sitting in a bit of water. The water near the boat is touched with a bit of soap, reducing the local surface tension, which drives the boat in the opposite direction.

You're probably observing much the same thing, just without the little boat.

When the Marangoni effect causes a fluid (like the water) to move along a surface (like the floor) due to a concentration gradient (like the dissolved soap's), it's called "diffusioosmosis":

Diffusioosmosis, also referred to as capillary osmosis, is flow of a solution relative to a fixed wall or pore surface, where the flow is driven by a concentration gradient in the solution.

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  • $\begingroup$ thanks for the answer but is the effect tile dependent? I couldn't reproduce the results on wood or the smooth tiles which I have got. Moreover unlike my bathroom tiles, those tiles also don't spread water when it falls on them. $\endgroup$
    – user8718165
    Mar 30, 2019 at 13:54
  • $\begingroup$ @user8718165 You're inducing a dynamic effect, where there's a temporary force due to a temporary difference in surface tension. I'd expect it to be most observable on a very hydrophobic surface, on which the water is beaded up, since the water's own forces would dominate there, maximizing the effect. On a very hydrophillic surface, it'd seem like the water would tend to interact less with itself, and be more attached to the surface itself, such that this effect would probably be less observable. $\endgroup$
    – Nat
    Mar 30, 2019 at 14:01
  • $\begingroup$ Is the tile where water spreads out rather than getting beaded up a hydrophobic surface and the tile where water doesn't spread out at all a hydrophilic surface? $\endgroup$
    – user8718165
    Mar 30, 2019 at 14:04
  • $\begingroup$ @user8718165 If the water spreads out rather than beading, that'd sound more hydrophillic. Admittedly I'm half-asleep right now, but it sounds a bit weird to me that your bathroom tiles would be more hydrophillic than other tiles in your house. I mean, your bathroom is like the one place where it's most important for surfaces to be water-repelling. They even sell bathroom products specifically for this purpose (random example on Amazon.com). $\endgroup$
    – Nat
    Mar 30, 2019 at 14:07
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    $\begingroup$ I remember doing a magic trick as a kid, sprinkling black pepper on a bowl of water and touching it with a tiny amount of soap at the fingertip. The black pepper immediately repels. Now I know why! $\endgroup$
    – Marc.2377
    Mar 30, 2019 at 15:53

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