Viscosity: Molecular friction

Chemical Viscosity

Today’s lesson was all about viscosity. Viscosity is fluid friction, which we can describe as the interactions between molecules of a fluid that affect the rate at which that fluid is able to flow.  Viscosity is not related to density or stickiness, but is related to temperature.  Before we experimented with fluids, however, we refreshed our memory of how friction works between solids.  We rubbed our hands together, and noticed that the friction between our hands caused some of the kinetic energy of the motion to be converted into heat energy, warming up our fingers!

For our activity, we had a viscosity race between eight common fluids including some you might find in the bathroom, kitchen cupboard, or even in your family’s car!  Students set up a test of the viscosities of the different liquids.  They made hypotheses about which fluids were more viscous and less viscous.  Using a cookie sheet they placed drops of their liquids on the sheet and raised one end and then measured the distance each drop traveled.  The students then compared their findings to their hypotheses.  The students should all have a much clearer understanding of the expression “slower than molasses in January” now!

This is an activity that is easy to try at home, with as many different fluids as you can find (detergent, lotion, shampoo, syrups, pudding, yogurt, etc).  You can also look at the effect of temperature on viscosity (how warm does Vaseline need to be before it will flow?)….  Happy experimenting!

Author

Dr. Maureen Griffin

Maureen earned a Ph.D. in Chemical and Biomolecular Engineering from the University of Pennsylvania in 2003. While at Penn, she developed a novel micro-mechanical technique called micropipette peeling to investigate the role of muscle cell adhesion in normal and diseased skeletal muscle cells. After graduating, Maureen worked full time as a post-doctoral researcher and then a staff scientist a SelectX Pharmaceuticals. She joined the teaching staff in 2008 and was excited to be made an executive staff member in 2009. Maureen also continued to consult part time for SelectX until her daughter's birth in 2009; now she is focused on Science from Scientists and, of course, her children. Maureen uses her spare time to read, blog, cook, and renovate her house.

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