The flow – orientation coupling of rods

Christian Lang, Joachim Kohlbrecher, Lionel Porcar and Minne Paul Lettinga, "The Connection between Biaxial Orientation and Shear Thinning for Quasi-Ideal Rods", Polymers 2016

Like every common liquid, water possesses a certain objection to flow. This results from an inner friction of the molecules and their complicated interactions. Since the molecules are very small compared to, for example, a glass of water and also very quickly moving due to thermal motion, the objection of molecules to flow in the glass is always the same, no matter how fast we stir the glass.

Rods, with a length almost a million times the water molecule size and a width only 28 times the water molecule size, can be added to the water. This results in a much higher inner friction and a larger objection to flow. One finds such mixes almost everywhere in our daily consumables. Examples are food and health care products.

Interestingly enough, the resulting mix of water and rods is harder to move in a slow flow than it is in a faster flow. This means that the inner friction of the mix is dominated by the motion of rods and not by the flow of the water molecules between the rods.

In our research, we have, therefore, tried to observe the rods under different flow conditions in order to see the cause of this phenomenon. Since the rods are much too small to be seen, we have chosen to shoot neutrons at the mix and investigate the changes in the resulting scattering pattern of neutrons scattered from the flow aligning rods. The scattering pattern, thereby, serves as a fingerprint of the interaction of rods and, hence, gives us the necessary microscopic information.

In this study, we have observed that the rods are under thermally induced motion even if the mix is not brought to flow by us, resulting in a completely disordered state with a high objection to flow. As soon as we bring the whole mix to flow, the situation drastically changes. The rods align with the flow direction into a very ordered state and the friction between rods is strongly reduced, resulting in a friction very close to pure water. The thermal motion, thereby, is dominated by the flow alignment. 

On top of this orienting behavior, we were able to detect for the first time that the order is disparate along different directions in our sample. This phenomenon is usually called biaxiality and it is caused by the difference in momentum transfer under shear in different directions inside the sample. So far, this phenomenon had been only described by theory but not directly observed in an experiment.

With this knowledge, we are now enabled to review the microscopic theory of rod-dynamics under shear. We found an overall qualitatively correct description of the experiment by theory. However, certain very profound discrepancies between theory and measurement are present. In our experiment, we observe that the transport of rods is more hindered by the surrounding rods in a slow flow than it is in a faster flow, while the interaction of rods in a faster flow is much stronger compared to the slow flow case and also compared to theory. Additionally, the mass transport is direction dependent, or in mathematical terms, a tensorial quantity.

Based on these observations we can now aim on the development of an improved theory, taking the mentioned points into account. Also, we are enabled to understand the flow of food and health care products, containing rod-like objects, better than before.    

A full version of the article can be found here