Sedimentation of nanoparticles is common in science and technology, yet the dynamics of sedimentation may sometimes be unexpected. We demonstrate spontaneous ordering of sedimenting nanoparticles into well defined layers of constant number density; the corresponding density profiles thus adopt a staircase-like appearance (see Figure).
Similar effects have been observed in micron-sized colloids back in 1884. However, the physical mechanism of these phenomena remained controversial, due to their notorious sensitivity to tiny temperature gradients, such as induced by the radiated body heat of the experimenter. The layering effect in nanoparticles appears to be much more robust, which allowed the critical conditions for the onset of layering to be measured, elucidating the physics of this spectacular phenomenon[1].
In particular, we demonstrate that the layering, breaking the translational symmetry within the sediments, is induced by particle polydispersity and transverse temperature gradients. These archetypal sources of noise, usually maximizing the disorder, are responsible in our case for the emergence of order; strikingly, similar order-inducing role is believed to be played by noise in formation of zebra stripes and in the morphogenesis. In our suspensions, this order-inducing noise competes with thermal diffusion, giving rise to the observed criticality[1]. The understanding of layering thus achieved opens a wide perspective for development of nanopattern formation technologies and new methods for nanoparticle characterization, as also for deeper understanding of the emergence of order in complex noisy systems.
[1] A. V. Butenko, P. M. Nanikashvili, D. Zitoun, and E. Sloutskin, Phys. Rev. Lett. 112, 188301 (2014)
eli.sloutskin@biu.ac.il
