60th Annual Report on Research 2015

Progress on work funded by this grant in 2014-15 included:

1. Work continued on a paper "Hysteresis and lubrication in shear thickening of cornstarch suspensions," arXiv:1405.7233, by C. E. Chu (undergraduate), J. A. Groman (researcher supported by this grant), H. L. Sieber (undergraduate), J. G. Miller (co-P.I.), R. J. Okamoto (WU researcher) and J. I. Katz (P.I.), the original version of which was submitted to Physical Review E in 2013-14. We began a program to determine the origin of the critical strain rate at which discontinuous shear thickening sets in. The initial version of this paper hypothesized this was the strain rate at which the stress in the unstiffened state became sufficient to move the three phase (solid confining surface, suspension, air) contact line to admit shear dilation. Experiments at a range of plate separations demonstrated that this was not the case; the critical strain rate is nearly independent of plate separation.

2. Further experiments were begun to investigate the explanation of the hysteresis discussed in the paper cited in paragraph 1. The hypothesis is that the jammed state is metastable if it is not in contact with unjammed fluid, in analogy with the behavior of bismuth oxychloride. In ongoing rheometer experiments in parallel plate geometry, the central region of the starch suspension, where the strain rate is low and the suspension is expected to remain unstiffened, is replaced with a soft solid such as grease or Crisco.

3. A paper "Rheology in the teaching lab: properties of starch suspensions," arXiv: 1410.5371, by J. A. Groman (researcher supported by this grant), J. G. Miller (co-P.I.) and J. I. Katz (P.I.) was submitted to the American Journal of Physics. This describes a very simple rheometer, involving the descent of a metal rod into a vertical cylinder filled with suspension, that is suitable for construction and use in the teaching lab, but that produces research-quality results within its domain of application.