Phase Boundary, Immiscible Liquids

Name: Stephano
Status: student
Grade: other
Location: LA

Question: What exactly are we seeing in a phase boundary between
immiscible liquids?
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Stephano,

Picture a common example of a two phase system of immiscible liquids -- oil 
and vinegar salad dressing.  When the bottle has been just sitting for at least 
several minutes you see the characteristic boundary.  The vinegar (a dilute, 
aqueous solution of acetic acid) is on the bottom.  The oil (a hydrocarbon, 
extremely water-hating -- hydrophobic) on the top.  Light in the room passes 
through each liquid (how else would you see the boundary?).  The light does not 
pass through each liquid at the same rate, and it does not exit at the same 
angle that it entered. The boundary between the layers results from the difference 
of the angles of the light exiting the liquids.  See Snell's Law in a physics text 
for a deeper discussion of light in transparent materials.

Warren Young
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At the most fundamental level, we see the difference in the speed of light between 
liquid (1) and liquid (2). This difference forms a meniscus (in the case of a 
transparent tube) or a "bent" line in the case of an optical path between the 
two transparent phases, for example a fishing line. This is the most basic 
observation, however, depending upon the experimental arrangement it may not 
appear to be "exactly" what we are seeing.

Vince Calder
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