Research progress in the anomalous confined diffusion
Diffusion is ubiquitous in nature and is a common way of nanoparticle transportation. Different from the Brownian diffusion in simple liquids, the motion of nanoparticles in complex fluids is influenced by the heterogeneous fluid structures nearby, and thus exhibits anomalous behavior of confined diffusion (for instance, the nonlinear relation between mean square displacement MSD and elapsed time). Recently, an anomalous behavior called “non-Gaussian yet Brownian” was unveiled, immediately attracting broad attention. It was observed that the non-Gaussianity emerges clearly while the linear relation between MSD and time still existed in some time scales. However, the source and the underlying mechanisms have not been well explained yet.
The researchers from the group of micro-/nanofluidics at the Institute of Mechanics, CAS, use particle tracking technique to experimentally measure the diffusive motion of nanoparticles (size d=40, 100 and 200 nm) in different polyethylene oxide solutions (PEO, molecular weight 600k, 2M, 4M and 8M, concentration 0.05-1.5%). By systematically measuring the non-Gaussian parameter, the displacement probability distribution, and the displacement auto-correlation, they analyze the source of the non-Gaussianity in the anomalous confined diffusion. It is proposed that the hopping diffusion of some nanoparticles, which demonstrates the large displacement when nanoparticle move across the polymer networks, should be the main reason for the “non-Gaussian yet Brownian” phenomena. They also observe that the non-Gaussianity varies with time and nanoparticle size. When the size of nanoparticle is smaller than the typical size of polymer networks, the non-Gaussianity will become more significant and exist in a longer time. The previous studies that only focused on the short-time nanoparticle-polymer interaction failed to explain the non-Gaussianity in a long-time stage. Thus the mechanism of hopping diffusion proposed in the current study presents new physical insights in the anomalous confined diffusion. Furthermore, hopping diffusion could offer explanations for the rare events observed in the catalytic reactions and signaling and metabolite shuttling in biological systems.
This research has appeared in the Journal of Physical Chemistry Letters (http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.5b02624). The corresponding author is Dr. Xu Zheng, and the first author is Chundong Xue, along with Kaikai Chen, Prof. Yu Tian and Prof. Guoqing Hu as the co-authors. This work is supported by the Ministry of Science and Technology and the National Natural Science Foundation of China.
Figure 1. The “fat tail” exhibited in the displacement probability distribution (DPD) is a clear signal of hopping diffusion, which is also the main source of the non-Gaussianity of the nanoparticles’ anomalous diffusive behaviors.