TIME-RESOLVED VELOCITY-GRADIENT AND OPTICAL ANISOTROPY IN LINEAR FLOWBY PHOTON-CORRELATION SPECTROSCOPY

A time-resolved dynamic light scattering technique is presented for me asuring the velocity gradient in transient but repeatable flows. The e xperimental technique is verified via measurements for a Newtonian flu id undergoing a known time-dependent flow. The method is then applied to the creeping flow of a Newtonian fluid in a corotating two-roll mil l. It is demonstrated that the flow near the stagnation point can be a ccurately described by an analytical creeping flow solution for a two- roll mill in an unbounded fluid. The time dependence of the velocity g radient 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 fl ow for the viscoelastic polymer liquid, and will ultimately lead to si gnificant insights into the polymer dynamics for concentrated solution s 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 anisotrop y of the polymer molecules, i.e., to the geometric configuration of th e polymer chains. Thus, it yields information on the time-dependent de gree of polymer orientation and stretch that is equivalent to birefrin gence, but is obtained at the ''point'' occupied by the scattering vol ume rather than as a two-dimensional average across the whole fluid as in birefringence. This measurement of polymer configuration is compar ed with birefringence data for the exact same flow