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Department of Chemical Engineering and Biotechnology

PNAS article: Making complex fluids behave themselves

a strategy for tuning the flowability of shear thickening materials

Researchers from the Universities of Cambridge, Edinburgh and Cornell, including new department member Dr Christopher Ness, have devised a strategy for tuning the flowability of shear thickening materials. The development provides a foundational step toward employing such materials in advanced technological fields, such as 3D printing and robotics, and may also help solve industrial paste processing challenges. The findings are published in the Proceedings of the National Academy of Sciences of the United States of America.

Shear thickening, an increase of liquid viscosity during flow, is a widespread phenomenon closely related to many engineering applications. It can be easily seen in a 'kitchen experiment' by playing with a mixture of cornstarch and water. The mixture viscosity grows dramatically as the stirring speed increases. Thickening fluids have important technological applications including as liquid body armor for athletes and soldiers. However, they also cause problems in industry during paste processing and extrusion. Finding a way to tune the flowability on-the-fly will help us to better harness this fascinating flow behavior and solve many processing problems in industry.

In this article, we describe a method to tune the flowability of a thickening suspension on demand. Recent work showed that thickening is a consequence of particle contact clusters and their alignment: if we can design a method to break the particle clusters, we can make a thickened suspension flow smoothly again. To do this, we apply a rapid oscillatory flow across the main shearing direction to interfere with the particle contact alignment.  This interfering flow tilts clusters and extends them just enough so that the particles are pulled out of contact. The simple disturbance can effectively lower the viscosity of a thickening suspension by about a factor of a hundred. We find that to maximize the flowability, the perturbation needs to be imposed delicately: it is crucial that the interfering flow has a high frequency and small amplitude. The high frequency guarantees that the disturbance has its full effect before the clusters rearrange into new ones; the small amplitude ensures that the disturbance does not create new clusters of its own.

The tuning capability thus provides a foundational step toward employing dense suspensions in many advanced technological fields, such as 3D printing and robotics. While our method is reminiscent of the traditional fluidization method whereby dry grains are shaken violently to make them flow, the mechanism is intrinsically distinct. In dry-grain fluidization, the vibration has wildly large amplitudes that would actually induce shear thickening in our suspensions. Instead, we exploit the fragility of the clusters using low amplitude vibrations. This capability opens the door to using dense suspensions in many advanced technological fields. For instance, we will be able to 3D-print very concentrated suspensions that are otherwise extremely challenging to flow smoothly (cement, conducting suspensions comprising metal particles, or even chocolate). We can also imagine building robotic arms with tunable rigidities in their joints and grippers.

Full article:

“Tunable Shear Thickening in Suspensions”
Proceedings of the National Academy of Sciences USA
Neil Y. C. Lin1, Christopher Ness 2, Michael E. Cates3, Jin Sun2 and Itai Cohen1
1Physics Department, Cornell University, Ithaca, NY 14853, USA; 2School of Engineering, University of Edinburgh, Edinburgh EH9 3JL, United Kingdom; 3 DAMTP, University of Cambridge, Cambridge CB3 0WA, United Kingdom

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