Brownian Motion & Viscosity Lab
Microscopy:
Students look at microscopy slides with polystyrene particles with diameters of 0.5
mm, 1.1 mm and 1.9
mm diffusing in water and water-glycerol mixtures.
Student web page:
Click here for web movies
of Brownian motion, which may be useful if you don't have
access to a microscope and/or particles with controlled sizes.
Falling ball experiments: Students perform falling ball
experiments. They measure the fall time of a glass sphere in
glycerol and in water. Note that while the liquid's density
does play a role (in changing the buoyant force on the sphere),
with glass spheres this secondary effect doesn't change the
results by much, as the density of glass = 2.6 g/cm^3 is much
larger than the density of all of the liquids. In a more
mathematically rigorous class, the velocity of the sphere can
be measured and compared to the drag force, buoyant force,
and weight of the sphere, to determine a value for viscosity.
The Stokes drag force for slowly moving spheres is F_drag =
6 pi eta R v, with eta the viscosity, R the radius of the
sphere, and v the velocity. For fast-moving spheres, this
equation is not correct, but nonetheless can distinguish between
the viscosities of these different liquids.
Drainage experiments: Students time how fast it takes
water, mineral oil, and glycerol to drain out of a plastic
beaker through a 1/8" hole drilled at the bottom of the beaker.
Viscosities of these liquids vary significantly whereas their
densities are similar. Density plays a role in determining the
hydrostatic pressure within the draining container, which causes
the fluid to flow out, but the main influence on draining time is
the viscosity.
Prepare for class:
- Brownian motion experiments
- slides with particles of different sizes
- microscope
- falling ball experiments experiments
- need two graduated cylinders: one filled with water, second one filled with glycerol
- Spheres of various sizes and densities -- can be
purchased from Small Parts
Inc for example, for ~$25.
- place a small piece of sponge at the bottom of the cylinders
if necessary to avoid the cylinder being broken by the falling spheres
- use watches or stop watches to measure the time of the fall of the sphere
- use a magnet to get the steel spheres out of the cylinder filled with glycerol
- three plastic beakers (or regular coffee cups) with a 1/8" hole in the bottom
of the each beaker
- preclass reading: reading 2 (MS
Word)
(HTML format)
- lab report: lab report 1 (MS Word)
(HTML format)
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