Maxwell Maxwell Effect (flow birefringence) Maxwell Legacy Concepts
In a nutshell

Unlike many of the concepts attributed to Maxwell, the 'Maxwell effect' derives more from experiments he conducted than theoretical analysis. The effect is the appearance of birefringence in many flowing liquids, birefringence that disappears when the liquid come to rest. Birefringence is a characteristic of anistropy, showing that light passing through the material is divided into two beams of different polarisation. Anisotropy can be produced by internal strain. The surprising aspect of the phenomenon is that liquids were generally considered as being unable to support internal strain, for as soon as you try to 'bend' a liquid it simply flows.

In 1866 Maxwell had retired from King's College, London to return to his country estate in Kirkubrightshire, partly to give himself more time to research and write. He did, though, keep on his London flat for a few years. He had clearly been thinking about viscous flow (see the accompanying piece on viscoelastic materials) and had come to the conclusion that viscosity in a flowing fluid could be thought of as an extreme case of solid rigidity under shear stress that broke down in some short time and then reformed, only to break down again. He built this concept into his treatment of viscosity in fluids but must have wondered if it could be demonstrated.

In the early 1850s Maxwell had investigated in detail how strained solids produced birefringence that could be seen by placing a sample between cross-polarisers. In 1866 he used this technique to investigate viscous liquids, procuring himself a cylindrical glass-bottomed box that he placed between crossed polarisers. The box was part filled with liquid and a central cylinder rotated to spin the liquid around. The effect he had been expecting was seen with some liquids, the case of Canada balsam being singled out as particularly noticeable. The birefringence disappeared soon after the agitation was stopped.

The effect is sometimes known as flow birefringence. Many liquids show the effect, including olive oil, gelatin solution and many polymeric solutions. The underlying cause is either anisotropy of the molecules of the fluid or deformation of the molecules when flowing. Maxwell published a brief account of his experiments in 1873. He credited Poisson with the initial concept that a liquid was like a solid whose elasticity conntinually boke down.

Reference: James Clerk Maxwell, On double refraction in a viscous fluid in motion, Proc. Roy. Soc. Lond., XXII, pp 46-47, 1874

JSR 2016

Flow birefringenceStills from a YouTube video showing flow birefringence. The first image shows the stationary liquid and plunger disc (black) at the end of a red rod. In the image on the right the plunger is in motion, stirring the liquid which shows birefringence lines between crossed-polarisers. See