MadSci Network: Physics

Re: Why are Cheerios drawn to each when they're floating in milk?

Date: Mon Apr 5 08:12:25 1999
Posted By: Dave Koppenhaver, Staff, Forensic Chemist, Indiana State Police Laboratory
Area of science: Physics
ID: 920087586.Ph

Eva, you have asked a very good question.  Iím sorry I have not gotten 
back to you sooner.  This strange phenomenon has plagued Cheerios eaters 
for centuries.  The first thing we have to consider is the milk is not 
completely still.  The scientific reasons for the Cheerios finding each 
other and sticking together are the small currents in the milk and 
adhesive forces.  Surface tension plays a part in this too.

Surface tension is a property which enable a needle to float on water and 
a water strider (we called them water skimmers) to float on the water.  
Surface tension also comes into play when washing clothes.  The soap 
decreases the surface tension, which lets the soap between the fibers and 
remove the dirt.  I have read where a duck canít float on soapy water 
because the soap has decreased the surface tension.  I have not verified 
this because my wife wonít let me put a duck in her washing machine, 
honestly Iíve tried.

What does this have to do with my Cheerios you ask?  Well surface tension 
is one reason they float.  Now, adhesive forces what are these?  In a 
laboratory, you can measure liquids in a glass graduated cylinder.  A 
graduated cylinder is a glass tube maybe 10Ē tall and a quarter inch in 
diameter (there are different sizes).  If you put milk into the graduated 
cylinder and look at the level you should see the milk curve upward near 
the walls of the glass and is lower in the middle.  This is adhesive 
forces which means the milk stick to the side of the bowl to a small 
extent, and to the Cheerios.  The surface tension and attractions between 
other milk molecules pull it down in the middle.

Ok so the real reason your Cheerios behave so strangely is because:  1.  
The currents in the milk make them move around, 2. Surface tension makes 
them float along with density, 3. The milk sticks to the sides of the bowl 
(adhesive forces) and to the Cheerios, which makes the other Cheerios 
stick to each other, and also stick to the side of the bowl.

Eva keep up the great thinking.  Now all of this has made me hungry, Iím 
going to get some Cheerios.

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