Creep Ringing in Rheometry
or How to Deal with Oft-discarded Data in Step Stress Tests!
Randy Ewoldt and Gareth H. McKinley
Hatsopoulos Microfluids Laboratory, Department of Mechanical Engineering,
MIT, Cambridge, MA 02139
Inertial effects are ubiquitous and unavoidable in stress-controlled
readers of this Bulletin will have seen the kind of phenomena discussed in this
the fluid is sufficiently viscoelastic, inertio-elastic 'ringing' events (i.e.
oscillations) are observed in the angular displacement measured at the start of
test, as a result of the coupling of instrument inertia and sample
elasticity. Even if
the fluid is a simple viscous Newtonian fluid one may have noticed that the
strain response of any real creep test is always quadratic in time, rather than
simple linear response that is always taught in class. Although
theoretically, effectively dealing with the consequences of inertio-elastic
something with which practitioners of the coarse art of rheometry may not
comfortable. This note is intended to remind the reader of the sources of these
phenomena, and review some methods for extracting useful rheological information
data rather than simply discarding or deleting it.