Everyday Science #1: The shower-curtain effect

Everyday Science #1: The shower-curtain effect

Sometimes, weird events happen in everyday life. One of them might happen every single day while you take a shower. It never occurred to you that the shower curtain seems to approach you when the water is on, like it is coming for you? Well, it does and there is a scientific explanation for that. Today’s post will answer to that question: why does the shower curtain stick to you skin?

Shower curtain

Curtain moving itself

Surprisingly, this question was left unanswered for a long time, with a lot of theories but nether proofs. We had to wait until 2001 for David Schmidt to finally give a valid proof. And he won a IgNobel, a prize given for research that “first make people laugh, and then make them think”, for his discover. [1]

Simulations showed that the droplets of water decrease their speed because they are transferring their energy to the air inside the shower. It results in the making of a tiny hurricane, a vortex, with a center at a lower pressure than the surroundings. In addition, faster is the stream of water, lower gets the pressure. The temperature doesn’t affect much this effect.

Air moving from high pression areas (anticyclone) to low pression areas (depression), the air outside the shower move towards the inside of the shower. Doing so, the shower curtain is pushed, through air movement, to whatever is inside the shower (usually, you).

Basically, you recreate a particular meteorological phenomenon every time you take a shower.

Sticky curtain

So now, the shower curtain got in contact with your skin. But it won’t go away on its own. This is due to another physical phenomenon called surface tension.

A medium, a fluid, always tends to stability, a state that minimise energy. To do so, it modifies its interface by reducing its area. The surface tension maintains this state. The energy on the interface between two fluids (for example between water and air) is higher than the one inside both fluid.

For our case, the water on the skin gets in contact with the shower curtain. The tension between the two increases causing them to attract each other, the same way water filled in a recipient is attracted to its edge, causing a meniscus. It will stay attracted until a force high enough is applied to separate them.

Surface tension has other useful applications. First, there is capillarity, when a liquid climb inside a narrow tube. The tension makes the liquid go up on the edges (like a meniscus), stabilises and does it again depending of the molecules cohesion forces of the liquid. It is how a straw works. Second, there is coalescence, where to droplets merge into one. It is how rain is formed.

It is amazing how we can find scientific principles behind everyday events. It is by observing and studying them that we can understand how the world works.

References

[1] The Ig Nobel Prize Winners, 2001, https://www.improbable.com/ig/miscellaneous/ig-2001-winners.html

 

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