Who ordered these nanoparticles?
Sedimentation of nanoparticles is commonly used in science and technology, yet this process may also lead to completely unexpected phenomena. We demonstrated spontaneous ordering of sedimenting nanoparticles into well defined layers of constant number density, so that the density decreases with the height above the bottom of the cell in a staircase-like manner.
Video: While the density profile in silica microparticles is monotonic in these conditions,
the experimental (Cu:Ag) and the simulated nanoparticles exhibit distinct density layers
during their sedimentation in a centrifuge.
Similar effects have been observed back in 1884 in microparticle suspensions. However, their physical mechanism remained controversial, with all quantitative measurements of these effects precluded by their notorious sensitivity to tiny temperature gradients: even the irradiated body heat of the experimenter is sufficient to eliminate the layering. We demonstrated that the layering is much more robust in systems of nanoparticles. This robustness allowed the critical conditions for the onset of layering to be measured, elucidating the physics of this spectacular phenomenon.
Figure. Kinetics of layer formation. The layers (shown in cyan) are separated by
local maxima in horizontal convection velocity (red).
We have demonstrated that the layering, breaking the translational symmetry within the sediments, is induced by particle polydispersity and transverse temperature gradients. These archetypal sources of noise, which usually maximize the disorder, are responsible in our case for the emergence of order. In this sense, the mechanism of layering is reminiscent of other noise-driven ordering phenomena, believed to play an important role in formation of zebra stripes and in morphogenesis, where stripe-like protein concentration profiles emerge during embryonic development of drosophila. In our experiments, this order-inducing noise competes with order-inhibiting thermal diffusion, giving rise to the observed criticality. The understanding of layering thus achieved opens a wide perspective for development of analytical methods for nanoparticle characterization and nanopatterning technologies, as also for deeper understanding of the emergence of order in complex noisy systems.
References for additional information:
A. V. Butenko, P. M. Nanikashvili, D. Zitoun, and E. Sloutskin,
Phys. Rev. Lett. 112, 188301 (2014). 2. Layering in sedimenting nanoparticle suspensions: the order-inducing role of randomness.
P. M. Nanikashvili, Y. Kapilov Buchman, L. L. Israel, J.-P. Lellouche, D. Zitoun, A. V. Butenko, and E. Sloutskin,
Colloids Surf. A 483, 248 (2015).