Shear Induced Orientational Order of Rigid Rod Suspensions
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Shear Induced Orientational Order of Rigid Rod Suspensions H.D.
Ou-Yang, C. Wang, B.E. Vugmeister, D. Wong and A. Dawson Physics Department, Lehigh University
ABSTRACT Shear induced birefringence was measured for dilute and semidilute rigid PTFE rod suspension. Orientational order of the rods is deduced from the birefringence data and compared with theory by Doi and Edwards'. Although the theory was for dilute solution, the renormalized data agree qualitatively with the theory even far into the nonlinear regime. At high shear the order parameter approaches a saturated value, and at low shear it depends on the shear rate square. The major discrepancy between the theory and experiment comes from the expected slowing down of rod reorientation at higher concentrations. At high concentrations, the slowing down is more than what can be accounted for by the macroscopic viscosity along. INTRODUCTION Orientational order of highly concentrated rigid rod suspensions can occur spontaneously due to preferential packing 2-* The effort of using uniaxial rigid rod model to 4 explain the nematic liquid crystal transition has been relatively successful . More sophisticated theoretical models to include However, the nonequilibrium bi-axial symmetry are already underway-. the states for uniaxial symmetric rods, such as the behavior in the shear flow or dynamic relaxation to an oscillating field are much less understood except in very dilute case'. Since monodisperse rigid-rod suspensions have become more available, we are able to approach the problem experimentally. We are also interested in the cases when the external field is not small and nonlinear response has to be considered. In this paper we report a study on the orientational order of semidilute, rigid PTFE rods in aqueous suspensions under shear flow. The well below the theoretical concentration range in this study is still concentration predicted for nematic order. The study is made by measuring the induced birefringence along the shear gradient direction under shear flows. The induced birefringence provides a measure for the average particle orientation. For uniaxial symmetry the average particle orientational order can be represented by the second order Legendre polynomial Q=3/2 where 0 is the angle of the rod axis to a reference direction such as that of external field and the angular bracket stands for thermal average. Here Q is the order parameter. For dilute solution of rigid rods in a simple shear flow, the case has been solved by Doi and Edwards' and the order parameter Q is related to the shear rate y by 2 54 1+ Q 1-O 2 1+Q
(
2 =) 2
(1)
Dr
Here we have defined P., the Peclet number, to be a dimensionless shear rate and D, the dilute solution rotational diffusion coefficient for the rod with length L and width b is given by Dý= 3k21n (L/b)
(2)
where % is the solvent viscosity, k the 2Boltzmman constant and T the absolute temperature. For small P., Q=(1/54)P. and for large P., Q approaches
Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials Research Society
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