Drop Size and Material
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.
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).
If I understand you correctly, you are asking how to explain to your
students why higher surface tension equals larger, not smaller, drop
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.
Update: June 2012