Transient normal stress response in a concentrated suspension of spherical particles

The transient normal force response in a concentrated suspension of spherical particles upon startup of shear, following a period of rest, was found to depend on the direction in which shear was restarted. When shear was restarted in the same direction, the normal force signal rapidly grew to its positive steady-state value. However, when shear was restarted in the opposite direction, the normal force signal was initially negative and decreased, within the response time of the instrument, to a negative minimum, from which it gradually increased, passing through zero, to its positive steady-state value. This is believed to be the first experimental confirmation of this phenomenon, which had been suggested by the numerical simulations of previous investigators. (C) 2002 The Society of Rheology.