Double refraction streaming

Jerrand, H. Theories of streaming double refraction. Chem. Rev. 59,345 (1959). 

Kuhn (129) considered gelatin molecules in warm aqueous solutions to be randomly contracted to near spherical shape, and to yield weak streaming double refraction because of stretching under the shearing... 

A clear, not too concentrated soap solution shows no Tyndall effect, no streaming double refraction, while no particles can be observed with the ultramicroscope. Nevertheless almost all the physical properties of the soap solutions change at a particular concentration and these changes point to aggregates of molecules or ions. 

Hartley s ideas can be applied very well to dilute soap solutions but in concentrated solutions other additional factors must be taken into account. Hartley s spherical micelle will not be able to explain the streaming double refraction of concentrated soap solutions. Thiessen and Triebel investigated the streaming double refraction of Na-oleate at various concentrations and temperatures. This double refraction depends greatly on the concentration and on the temperature (see Fig. 9). At 50 the double refraction of a 15.3% solution of Na oleate has almost completel] disappeared while that of an 18% solution is still fairly high. Taken altogether it follows that anisotropic particles must be present in high concentrations of soaps. 

Fig. 9. Streaming double refraction of Na-oleate solutions in dependence on tJae temperature and concentration (Thiessen and Triebel, 1931).

Disordered aggregates, with a much higher viscosity, steep rj rerc curve, dependent on the temperature, considerable structure viscosity and streaming double refraction but a poor X-ray picture. 

Some liquid colloid systems show a double refraction when they are set in laminar flow. The classical example is the V2O5 sol which (at least after ageing) consists of needle-shaped particles with a crystalline character the sol itself with its particles randomly oriented by Brownian motion is isotropic if now one makes it flow through a tube or between two coaxial cylinders, one of which is rotating, it becomes double refracting The same is the case with many macromolecular sols This phenomenon, streaming double refraction of sols, is also reckoned as accidental double refraction, because it only occurs through a constraint exerted on the system ... 

It will be clear that we shall also have to include under orientation double refirac-tion the streaming double refraction previously mentioned, which can be forced on some sols by passing them through a narrow tube or in some other way setting up a flow gradient ... 

Meanwhile it also happens that sols, which have isotropic spherical particles, exhibit streaming double refraction the cause then lies in the pressure which the particle experiences perpendicular to the direction of flow as a result of the liquid friction. 

With the phenomena of double refraction arc associated those of dichroism the ordinary and the extraordinary rays are often differently absorbed . Extensive information on this point is to be found in the papers quoted above on streaming double refraction ... 

The measurement of the streaming double refraction has been extensively applied to the investigation of macromolecular solutions As an example of this we shall take the investigation of Buchheim and Philippoff of a nitrocellulose solution in cyclohexanone for which the dependence of A and orientation angle x on the velocity drop q is reproduced in Fig IL Reference may also be made to Voi II, Ch V, 7, p, 142 ... 

It can be understood that it is a great simplification for the development of the theory that the axes of the intrinsic anisotropy as a rule coincide with those of the shape anisotropy. The experimental set-up is simpler than with streaming double refraction since the orientation is produced in this case by bringing the solution into an electric or a magnetic field. 

Fig. 1 and eq. (4) show that, apart from the direction of flow, also the directions making angles of 45 and 135 with the flow, are preferred directions. It is in these last two directions that the principal axes of streaming double refraction are situated (see chapter III 3 b, p. 107, this chapter 2 b, p. 345). 

The problem has been taken up by several authors. Boeder and KDhn both determined the mean orientation of the particles in the presence of BROWNian motion. At low shear there is a slight preference for orientation of rod-like particles at 45° with respect to the direction of flow. At higher rates of shear the angle of orientation diminishes until in the limit of an infinite rate of shear the particles are practically all the time parallel to the direction of flow. Boeder tested this by means of determinations of streaming double refraction. KuHisf used the partition function found to calculate the viscosity at different rates of shear. 

When a slight amount of salt is present, it forms a colloidal solution, containing rod-shaped particles. The solution shows streaming double refraction. Concentrated solutions may gel and a paste of the dye with water is typically plastic. 

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