The equation describes the flow of a liquid inside a capillary of a given radius. I need to do a bit more of reading, but it relates the penetration of the liquid to time, viscosity , surface tension , radius, and contact angle .
The original equation is from 1906, so I assume there should be a lot of work and characterizations done for multiple materials.
This relationship can be of great help to characterize the liquid inside the hollow optical fiber setup of Dispertech : if we can determine the viscosity of the medium independently (i.e. without asking the user to input it), we can have a truly self-calibrating method of determining size.
See: Wikipedia as starting point.