Granular materials flow like complex fluids

• Published in: Nature (London) Vol. 551; no. 7680; pp. 360 - 363
• Main Authors: Kou, Binquan, Cao, Yixin, Li, Jindong, Xia, Chengjie, Li, Zhifeng, Dong, Haipeng, Zhang, Ang, Zhang, Jie, Kob, Walter, Wang, Yujie
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.11.2017; Nature Publishing Group
• Subjects: 639/301/1023/303; 639/301/923/218; 639/766/119/1002; 639/766/530; Bulk solids flow; Complex fluids; Condensed Matter; Disordered Systems and Neural Networks; Displacement; Ellipsoids; Fluid dynamics; Fluid flow; Foams; Friction; Geotechnical engineering; Glass formation; Granular materials; Humanities and Social Sciences; letter; Measurement; Mechanical properties; multidisciplinary; Normal distribution; Physics; Power law; Science; Shape memory; Shear strain; Statistical Mechanics
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature24062

The relaxation dynamics of granular materials is more like that of complex fluids than that of thermal glass-forming systems, owing to the absence of the ‘cage effect’. Against the grain We can all claim familiarity with granular materials (think of sand, for example), so it might be surprising to find that there is still much to learn about their properties. When left undisturbed, a granular system will eventually settle into a stable structure; but perturb the system and the grains will move about, in a manner thought to resemble that of atoms in a slowly flowing glassy system. New results by Binquan Kou et al . show that this glassy analogy doesn't hold. The team used X-ray tomography to follow the three-dimensional motion of grains in a perturbed granular medium, and see dynamic behaviour that is distinct from that encountered in a glassy system. They trace this deviant behaviour to the presence of friction between the grains. Granular materials such as sand, powders and foams are ubiquitous in daily life and in industrial and geotechnical applications 1 , 2 , 3 , 4 . These disordered systems form stable structures when unperturbed, but in the presence of external influences such as tapping or shear they ‘relax’, becoming fluid in nature. It is often assumed that the relaxation dynamics of granular systems is similar to that of thermal glass-forming systems 3 , 5 . However, so far it has not been possible to determine experimentally the dynamic properties of three-dimensional granular systems at the particle level. This lack of experimental data, combined with the fact that the motion of granular particles involves friction (whereas the motion of particles in thermal glass-forming systems does not), means that an accurate description of the relaxation dynamics of granular materials is lacking. Here we use X-ray tomography to determine the microscale relaxation dynamics of hard granular ellipsoids subject to an oscillatory shear. We find that the distribution of the displacements of the ellipsoids is well described by a Gumbel law 6 (which is similar to a Gaussian distribution for small displacements but has a heavier tail for larger displacements), with a shape parameter that is independent of the amplitude of the shear strain and of the time. Despite this universality, the mean squared displacement of an individual ellipsoid follows a power law as a function of time, with an exponent that does depend on the strain amplitude and time. We argue that these results are related to microscale relaxation mechanisms that involve friction and memory effects (whereby the motion of an ellipsoid at a given point in time depends on its previous motion). Our observations demonstrate that, at the particle level, the dynamic behaviour of granular systems is qualitatively different from that of thermal glass-forming systems, and is instead more similar to that of complex fluids. We conclude that granular materials can relax even when the driving strain is weak.