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,
Cambridge, MA 02139

Inertial effects are ubiquitous and unavoidable in stress-controlled rheometry.  Many
readers of this Bulletin will have seen the kind of phenomena discussed in this note.  If
the fluid is sufficiently viscoelastic, inertio-elastic 'ringing' events (i.e. damped
oscillations) are observed in the angular displacement measured at the start of a creep
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 initial
strain response of any real creep test is always quadratic in time, rather than the
simple linear response that is always taught in class.  Although well-understood
theoretically, effectively dealing with the consequences of inertio-elastic ringing is
something with which practitioners of the coarse art of rheometry may not always be
comfortable. This note is intended to remind the reader of the sources of these
phenomena, and review some methods for extracting useful rheological information from the
data rather than simply discarding or deleting it.