Geometries for Measuring Flow Birefringence

This section provides a brief discussion of the experimental geometries that have been used for flow birefringence measurements on polymer liquids. These techniques may be classified into three groups according to whether they determine birefringence in die x—y plane in shear, in the x—z plane in shear, or in elongation. The first category has been by far the most extensively employed. 

The discussion leading to eqs 9.4.5 and 9.4.6 was based on the assumption of a shear flow with the light beam propagating along the neutral z direction. There are three standard experimental realizations of this situation. 

The geometry used for flow birefringence measurements in an oscillatory shear (Miller and Schrag, 1975) consists of two flat, parallel surfaces, one of which is fixed the other is made to oscillate in the plane parallel to the fixed surface. The liquid is confined to the thin layer between the surfaces. The light beam propagates through the layer in a direction perpendicular to the flow direction and parallel to the surfaces. This experimental arrangement and its application are discussed in detail below. 

In the case of an x-y shear flow, it is also possible to direct the light beam along either the x or the y axis, thus enabling determination of A 23 or Artis, respectively. According to the SOR, optical measurements of T22 - T33 or n 1 - T33 are then possible. en either quantity is combined with the first normal stress difference obtained from the more common measurement of Arti2 and X, the second normal stress difference can be determined. Measurement of Art 13 in the x-y plane has been achieved by at least two geometries. 

In this apparatus—one of the first to be used for flow birefringence measurements of polymer liquids—one glass plane is translated parallel to another with the liquid contained between them (Dexter et al., 1961). Thus, the light beam can propagate directly through the apparatus along the y direction. 

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