Oil Drop
Experiment
Updated September 2011
How can we find the
height of a molecule
without fancy equipment?
Updated September 2011
Ben Franklin’s Oil Drop Experiment
Ben noticed that oil
would spread out
very thin…
“I fetched out a cruet of oil and dropped a little
of it on the water. I saw it spread itself with
surprising swiftness upon the surface…
Though not more than a teaspoonful, produced
an instant calm over a space several yards
square which spread amazingly and extended
itself gradually till it reached the lee side,
making all that quarter of the pond, perhaps
half an acre, as smooth as a looking glass.”
Portrait of Benjamin Franklin by
Joseph Siffred Duplessis
Circa 1785
Updated September 2011
Let’s use these BBs to
understand how we could
find the height of a molecule.
Imagine that each BB is one
molecule.
How can we find out how tall
each one is, assuming
we could not actually
measure it?
Updated September 2011
Image by HighPoint Learning
We know the volume of
the BBs.
Now, let’s pour them
into a single layer.
Image by HighPoint Learning
Updated September 2011
Measure the diameter of
the single layer.
If the shape were not a
circle, how could you get a
diameter?
Given the diameter,
calculate the Area.
Area=π(Diameter/2)2
Updated September 2011
Image by HighPoint Learning
Now, let’s determine the height of this (1 layer thick)
Cylinder.
Use Height = Volume/Area.
Updated September 2011
Measure the height of a single BB to see
How close our calculation is.
?
Updated September 2011
Now, you will use this same method to calculate the
height of a single oil molecule.
Image by Ben Mills
Updated September 2011
1) Prepare the oil and alcohol.
• Mix 9 ml of alcohol with 1 ml of mineral oil.
• Stir until they have combined well.
• Each ml of this mixture will now be 1/10 ml
oil.
Updated September 2011
2) Determine what fraction of a ml
one drop is.
• Fill the pipette to a ml mark and count how
many drops it takes for the level to go down to
the next ml mark.
• How many drops are in a ml (e.g. 10 drops)?
• What fraction of a ml is one drop(e.g. 1/10
ml)?
Updated September 2011
3) Calculate the volume of oil in the
drop.
• The volume of oil will be the 1/10 oil times
the fraction of a ml that the oil drop (e.g. 1/10
x 1/10 = 1/100 ml of oil).
• What is the volume of oil in one drop of the
oil alcohol mix?
Updated September 2011
4) Prepare the pan of water.
• Fill the pan with water.
• Beat two chalkboard erasers together over the
pan leaving a very thin film of dust on the
surface.
Updated September 2011
5) Drop the oil drop.
• Place the pipette of the oil alcohol mix close
to the surface of the water in the middle of
the pan.
• Press one drop out onto the surface. It should
push the chalk making a circle of oil that is
easy to see.
Updated September 2011
6) Measure the diameter of the oil
film.
• Take several measurements of the diameter of
the oil film and average them.
• What is the average diameter of the oil film?
Updated September 2011
7) Calculate the area of the oil film
• Area = π x (Diamerer/2)2
• What is the area of the oil film?
Updated September 2011
8) Calculate the height of the oil
film
• Height of the oil molecule = Volume of oil in
the drop(from #3) / Area(from #7)
• What is the height of the oil molecule?
• What is the height of the oil molecule in nm?
Updated September 2011
This module is one of a series designed to introduce faculty and high school
students to the basic concepts of nanotechnology. Each module includes a
PowerPoint presentation, discussion questions, and hands-on activities, when
applicable.
The series was funded in part by:
The National Science Foundation
Grant DUE-0702976
and the
Oklahoma Nanotechnology Education Initiative
Any opinions, findings and conclusions or recommendations expressed in the
material are those of the author and do not necessarily reflect the views of the
National Science Foundation or the Oklahoma Nanotechnology Education Initiative.
Updated September 2011
Image Credits
Aumier, Guillaume (Designer) Carbon Nanotubes.svg [Digital Image]. France. Wikimedia
Commons (commons.wikimedia.org)
Brandrodungswanderfuldhackbau (Photographer) 787-flight-check.jpg.[Digital Image].
Germany. Wikimedia Commons (commons. Wikimedia.org)
Duplessis, Joseph Siffred (Artist). Portrait of Benjamin Franklin. [Oil on Canvas]. United States.
National Portrait
Gallery, Smithsonian Institution
Highpoint Learning Solutions. (Photographers/designers) [digital images] United States.
Fokkix (Photographer). Elephants.jpg. [digital Image]. United States. Wikimedia Commons
(commons.wikimedia.org)
Updated September 2011
Image Credits
MilborneOne (Artists B787-2155a.jpg [Digital Image]. United Kingdom. Wikimedia
Commons (commons.wikimedia.org)
Mills, Ben. (Designer). Ball-and-stick model of the Oil Red O molecule. [Digital Image].
Wikimedia Commons. (commons.wikimedia.org)
Molhave, Kristian (Professor. Fe-cat CVD MWCNT SEM overview mats with scalebar.jpg.
[Scanning Electron Microscope Image]. United Kingdom. WIkimedi Commons
(commons.wikimedia.org)
Updated September 2011
References
Estimating the size of a molecule using an oil film (2004). Practical Physics. Retrieved from
http://www.practicalphysics.org/go/Experiment_633.html
Franklin, Benjamin. The Autobiography of Benjamin Franklin. [Kindle Edition]. Retrieved from http://www.amazon.com
Updated September 2011