Structure of Non-Crystalline Sediments
Sedimentation of granular materials is abundant. The structure of granular sediments plays an important role in various industrial processes; for example, it determines the density of randomly packed materials, such as flour or sugar, if these are poured to fill a container. The physics of sedimentation determines the mechanical strength of various construction materials; in astrophysics, it is believed to play an important role in formation of planets by ballistic deposition processes. Slow sedimentation processes form rocks and minerals; faster sedimentation forms snowdrifts, disastrous for the traffic flow in the major cities of the world. Last, but not the least, the physics of randomly-packed granular matter is believed to be deeply related to the physics of glass formation. Both in glasses and in the granular matter, the emergence of significant mechanical rigidity is accompanied by only very small structural changes on the microscopic scale, remaining mysterious after decades of an intensive scientific research.
Colloids are unique, bridging between thermodynamics and granular matter physics. While successively mimicking thermodynamical phases of atoms and molecules when freely moving in a solvent, colloids rapidly sediment subject to an effective gravity in a table-top centrifuge if their mass density is even slightly mismatched with that of the surrounding solvent. The ratio between a typical sedimentation rate of a colloidal particle and its rate of diffusion, known as the Péclet number (Pe), can be tuned to any value between 1e-5 to 1e3. When the Péclet number is significantly smaller than unity, the particles explore their phase space by Brownian motion; when the Péclet number is much larger than unity, Brownian motion is negligible, and the particles sediment rapidly by gravity akin to macroscopic granular objects (at low Reynolds' numbers). Thus, colloids are unique in that they are exactly on the verge between thermodynamics and granular matter physics. This potentially allows the long-sought hypothetical thermodynamics of granular matter to be based in future on direct experiments.
References for additional information:
1. Dense colloidal fluids form denser amorphous sediments. S. R. Liber, S. Borohovich, A. V. Butenko, A. B. Schofield, and E. Sloutskin, PNAS 110, 5769 (2013). Supporting video 2. Locating particles accurately in microscope images requires image-processing kernels to be rotationally-symmetric. P. J. Lu, M. Shutman, E. Sloutskin, and A. V. Butenko, Opt. Express 21, 30755 (2013).
A. P. Cohen, S. Dorosz, A. B. Schofield, T. Schilling, and E. Sloutskin,
Phys. Rev. Lett. 116, 098001 (2016).
P. M. Nanikashvili, A. V. Butenko, S. R. Liber, D. Zitoun, and E. Sloutskin,
Soft Matter 10, 4913 (2014). Supporting video1 Supporting video2
