Drop Size and Material

name         Gerri
status       educator
Grade        6-8
location     TX

Question -   I have a new lab kit and some of the answers
are not explained in depth.  Why do the larger intermolecular 
attractions make the drop size larger for water than ethanol?  I 
know that water has a much greater surface tension.  My students 
seemed to interpret that a larger surface tension makes drop size 
smaller with the data that they collected.  Thanks for your help.
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Without having the "answers" at hand it is difficult to assess 
the accuracy of the explanation. A good way to think about surface 
tension is the following: How much energy it takes to increase the 
surface area of a fluid per unit area. If it takes a lot of energy 
to increase the surface area then that fluid has a large surface 
tension. This point of view eliminates the need to take into 
account the nature of the surrounding fluid, the temperature, and 
other variables that can cause confusion.
   If a fluid has a strong attraction to itself, compared to its 
surroundings, it will have a large surface tension. Water is an 
example of such a fluid. Even more so are liquid metals, e.g. mercury.
   If there is a component in the fluid that weakens the attraction 
of the fluid in the region near the surface -- for example a 
soap-like component --
the surface tension of the solution will decrease, because such 
components tend to accumulate at the surface, and they do not 
attract one another very strongly.
   Your students are on the right track, associating higher surface 
tension (greater attraction for molecules for one another) with 
smaller drop size being more stable. It is perhaps more clearly 
seen by considering the inverse process. In order to make a film of 
a fluid easier to stretch, it is necessary to add a component to 
the solution that weakens the attraction of the fluid for its 
neighbors. That is what happens when you add soap to water. The 
surface film of soap weakens the attraction of water molecules for 
one another, allowing it to be easier to stretch, thus forming 
bubbles (or foam).

Vince Calder
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Gerri,

If I understand you correctly, you are asking how to explain to your 
students why higher surface tension equals larger, not smaller, drop 
sizes, correct?

Rubber balloons are often used as an example of surface tension, but 
I believe to explain your dilemma, plastic shopping bags might work better.
The thicker and stronger the bags, (ie: higher surface tension) the 
more material they can hold, thus a larger drop size.

Ryan Belscamper
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