September 9, 2022

Soupy Physics

When tomato soup is heated in a saucepan, we can often see the tops of convection cells.  The darker areas of the soup surface consist of the warmer soup that has risen from the bottom near the heat source.  The fluid spreads from the center of a rising column or, cell, and as the surface cools it falls back toward the bottom of the pan.  Where it falls, it tends to leave the small air bubbles that continue to float on top.  When you watch the soup - before it starts to boil - you can see the soup rising in the center of the cells and propagate towards the cell walls.

 

 

 


The picture at right shows a schematic diagram of the flow of the soup in the pan that is heated by a hot plate from below.  The convection cells are referred to as Bénard cells named after the French physicist Henri Bénard who studied them extensively in 1900.  The analytical fluid mechanics of the process is very complicated.  The convection flow in an unevenly heated (or cooling) liquid tends to form in a mesh of Bénard cells due to the buoyancy differences with temperature and also heavily dependent on the surface tension.  The liquid that reaches the top has to spread out in all directions and eventually sink.  The whole volume is more stable if the convection is broken up in the individual cells.  

Bénard cells are also noticed in some cloud formations when there is a rather thin layer of fairly small puffy clouds.  Those will be shown in a future Physics Photo of the Week post when an opportunity becomes available. 

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

Observers are invited to submit digital photos to:

 

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