Many everyday fluids such as face creams, shampoos and sauces flow in unusual ways.

Because of their special properties they are called “complex liquids”. Their

viscosity may drop drastically when stirred or shaken, a phenomenon called ‘shear-thinning’ or thixotropy. For example, tip a bottle of ketchup upside down and the

sauce hardly moves; shake it and the liquid pours out. Cosmetics such as liquid foundation rely on thixotropy to give an even spread on the skin – the fingers supply

the shear force. Neutrons reveal for the first time the microscopic structural changes during shear-thinning of such thick complex liquids.

Such unusual flow properties derive from the behaviour of long chain-like molecules in the material. They obstruct movement of the liquid because they form networks, either as a result of becoming entangled or of being held together by weak attractive interactions. In household products, the chain structures are mostly polymers or surfactant micelles. However, solutions of carbon nanotubes (minute cigar rolls of graphitic carbon) or various nanofibres used in high-performance materials also behave in this way.

Understanding their flow properties during processing and manufacture is of considerable technological interest. Surprisingly, until recently, the shear-thinning behaviour of these materials was little understood. It seemed likely that the forced flow induces changes in the orientation of the chain-like molecules and the network structure, but no decisive experiments had been performed.

Measuring shear-induced structures

We addressed this question with a new device recently developed for the instrument D11 at the ILL. It consists of a high-precision rheometer that can be set up and aligned in the neutron beam. It allows us to probe changes in molecular orientation and network structure using SANS while simultaneously shearing the fluid and measuring the shear viscosity. We could therefore relate the viscosity – a bulk property – to the structure and orientation of the network strands – a molecular property. We employed ‘model’ solutions of worm-like micelles of varying thickness and stability. Depending on their

chemical structure, they have diameters between 4 and 40 nanometres. The image above taken with an atomic force microscope shows a typical micellar network.

Solutions with concentrations between 1 and 10 per cent show pronounced shear-thinning. We found that when such a solution was sheared above a certain rate it started to shear-thin, and the neutron scattering pattern became less symmetrical, stretching in one direction (as shown below). This is what we would expect to see as the micelles begin to orient in the direction of the flow. As the shear rate is slowly increased, the viscosity gradually drops and the stretching pattern becomes even more stretched out,

thus confirming that shear-thinning is directly related to the orientation of the chain molecules in the flow direction. Furthermore, to see shear-thinning in liquids

containing thin micelles, they had to be sheared much faster than those containing thicker micelles. The quality of the data allowed us for the first time to establish a clear relation between shear viscosity and the orientational order of the chain molecules. The scattering patterns could be analysed quantitatively using theoretical predictions for the distribution of orientations of the worm-like micelles. We were surprised to find a simple exponential relationship between the decrease in shear viscosity and the degree of orientational order – irrespective of micellar thickness and concentration.

These results considerably deepen our understanding of thixotropic fluids, allowing us to

predict and tailor their flow properties for particular applications.

Reference:

S. Förster, M. Konrad, P. Lindner

“Shear thinning and orientational ordering of wormlike micelles”

Phys. Rev. Lett. (2005) 94, 017803