报告人：《Physics of Fluid》主编GIACOMIN（杰克明）教授
报告题目：The New Physics of Fluids
Professor Giacomin has led Physics of Fluids as its Editor-in-Chief since 2016. This lecture is designed to interest authors, new and old. It tells the story of the journal’s surging success, with special focus on growth from China.
报告题目：Exploiting Large-Amplitude Oscillatory Shear Flow
Large-amplitude oscillatory shear flow (LAOS) is the most popular laboratory experiment for investigating the nonlinear rheological behavior of polymeric liquids (see Fig. 1). This nonlinear experiment brings out distortion in the stress response. Attempts to analyze this distortion using continuum approaches have lead us to quantitative responses but these teach us little about the molecular origins of the distortions, namely molecular structure and orientation. Here, we investigate stress responses from LAOS from continuum and molecular approaches. We also explore traverse the bridge between these two approaches, which we callmacromolecular continua.
In the continuum framework, the Oldroyd 8-constant framework2is a versatile set of popular fluid models3. Moreover, when generalized to multimode fluid using the Spriggs relations, this fluid model can successfully predict nonlinear behaviour of polymeric liquids. Finally, the Oldroyd 8-constant framework yields exact solutions, for both shear stress and normal stress difference responses. Whereas these exact solutions give us general idea on how material reacts to nonlinear input, using them can be cumbersome. We can exploit these exact solutions to get simpler forms called Padé approximants. Under some circumstances, the best of these, for either the shear stress or the normal stress differences, gives the same level of accuracy as the exact expression. Lastly, we can also exploit these exact solutions for the thermodynamic analysis of flow stability to depict when LAOS is unstable (see Fig. 2).
Molecular structure and orientation are the primary causes of nonlinearity in viscoelastic behaviors of polymeric liquids. To explain the molecular origins of polymer fluid elasticity, we find the rigid dumbbell suspension to be the simplest relevant. By relevant we at least mean, predicting distortion in the stress responses. The stress expressions for molecular models take the approximate form of truncated power series. We can thus exploit the best Padé approximants to perfect these truncated expansions. Since arriving at these truncated expansions involves difficulty, perfecting these so easily is useful.
Figure. 1.Rigid dumbbells in LAOS in Cartesian coordinates.
Figure 2.Ziegler instability corotational Maxwell fluid in LAOS and over Pipkin space.
1.T-T. Tee, and J.M. Dealy,Trans. Soc. Rheol.,19, 595 (1975);T.-T. Tee,Large amplitude oscillatory shearing of polymer melts, Ph.D. Thesis, Chem. Eng. Dept., McGill University, Montreal, Canada, 1974
2. J.G. Oldroyd,Proc. Royal Soc. A,245, 278 (1958).
3. C. Saengow, A.J. Giacomin, and C. Kolitawong,Physics of Fluids,29(4), 043101 (2017).