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Helpful Information

Capillary Action

Capillary action happens when the molecules of a liquid are more attracted to a surface than to each other.  In paper towels, the molecules move along tiny fibers.  In plants (like celery), they move upward through narrow tubes called capillaries.   Capillary action occurs on Earth, but can be difficult to observe- except in small capillaries- because of gravity.  While experiments fall in a drop tower, where there seems to be almost no gravity, capillary effects are easy to see and study.

For more basic information about capillary action, see

Drop Tower

In PSU’s Dryden Drop Tower, your experiment will be dropped down a tall shaft, and it will behave as if there is nearly no gravity while it is falling.  Gravity will still be present, but our sensation of gravity and weight comes from a resistance to its pull, for example because of the floor holding us up.  But while freely falling, we feel weightless and that is the basis for many amusement park rides.  This works because all objects fall at the same acceleration in a gravitational field unless acted upon by another force.  As one result, the astronauts and the ISS fall together (around the Earth) such that the astronauts float within the space station… even though they are so close to the Earth that gravity in low-Earth orbit is only about 10% less than that on the planet’s surface.  While this is space science, the concept of apparent weightlessness through free fall (microgravity) was first put to practical use in the late eighteenth century when shot towers were built to produce superior shot for hunting.  In these towers, droplets of liquid lead became spherical because of the surface tension resulting from the liquid’s attraction to itself.

You can learn more about microgravity at:


Professor Mark Weislogel of Portland State University is a world leader in the study of capillary action. With the support of NASA, he has had such experiments conducted in drop towers, on the space shuttles, on the Russian Mir space station, and now on the International Space Station (ISS), where the latter includes:

Capillary Channel Flow (CCF)
Capillary Flow Experiment (CFE)
CELERE enables students to participate in research related to space station science and learn about computer technology (e.g., CAD), both of which can inspire the pursuit of STEM careers – where STEM stands for Science, Technology, Engineering, and Mathematics.


Draftsight Download

PSU’s Dryden Drop Tower

Videos from the Dryden Drop Tower

NASA: What is Microgravity?

NASA Education: Microgravity

NASA Teaching Materials