ISSN:
1089-7666
Source:
AIP Digital Archive
Topics:
Physics
Notes:
A time-resolved dynamic light scattering technique is presented for measuring the velocity gradient in transient but repeatable flows. The experimental technique is verified via measurements for a Newtonian fluid undergoing a known time-dependent flow. The method is then applied to the creeping flow of a Newtonian fluid in a corotating two-roll mill. It is demonstrated that the flow near the stagnation point can be accurately described by an analytical creeping flow solution for a two-roll mill in an unbounded fluid. The time dependence of the velocity gradient for a concentrated polymer solution in the startup of the two-roll mill has also been measured, it is believed, for the first time. The measurement provides direct evidence of the modification of the flow for the viscoelastic polymer liquid, and will ultimately lead to significant insights into the polymer dynamics for concentrated solutions in strong, extension-like flows. A second significant feature of the dynamic light scattering experiment is that the initial magnitude of the correlation function is related to the degree of optical anisotropy of the polymer molecules, i.e., to the geometric configuration of the polymer chains. Thus, it yields information on the time-dependent degree of polymer orientation and stretch that is equivalent to birefringence, but is obtained at the "point'' occupied by the scattering volume rather than as a two-dimensional average across the whole fluid as in birefringence. This measurement of polymer configuration is compared with birefringence data for the exact same flow.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.868411
