Diffusion of spherical and rod-shaped Janus nanoparticles

Ali Kharazmi and Nikolai V. Priezjev

Understanding the processes of self- and directed-assembly of nanoparticles is important for design of various nanostructured materials with advantageous mechanical and optical properties. Recent progress in synthesis of Janus particles, that can form a variety of predictable superstructures, makes their production feasible at an industrial scale. The dynamics of assembly, however, depends on the shape of the particle, distribution of wettability and charges, and boundary conditions at the particle surface in a solvent. 

This figure shows a snapshot of the Janus nanoparticle that consists of 72 atoms rigidly fixed on vertices of a convex polyhedron. The surface of the particle is composed of wetting (blue atoms) and nonwetting (green atoms) hemispheres. The diameter of the particle is about 4s in Lennard-Jones units (about 2nm) and the particle is 50 times heavier than a solvent molecule. 

In this animation, one can see a typical diffusive trajectory of the Janus particle during a time interval of about 200t. The total number of solvent molecules is 46531 (not shown). 

In our recent study, we investigated the translational and rotational diffusion of a single Janus nanoparticle in a quiescent solvent using molecular dynamics simulations. The Lennard-Jones interaction energy between particle atoms and solvent molecules is fixed at the wetting hemisphere, while the wettability of the other side is a variable parameter. These parameters correspond to partial slip boundary conditions at both hemispheres. The diffusion process of such particle was compared against two limiting cases of uniformly wetting and nonwetting particles.

We found that the diffusion coefficient of a Janus particle is bounded between the two limiting cases and it increases with decreasing surface energy at the nonwetting hemisphere. The analysis of the translational and angular velocity autocorrelation functions showed that the exponents of the long-time power-law decay are the same for Janus and homogeneous particles. Interestingly, the diffusive motion of Janus particles involves a subtle correlation between translational and angular velocities; i.e., a non-zero velocity component parallel to the particle symmetry plane causes a finite rotation of the nonwetting side towards the displacement vector to reduce drag. 

In a separate study, the combined effect of particle shape anisotropy and wettability contrast on diffusion of a rod-shaped Janus particle in the limit of infinite dilution was investigated using molecular dynamics simulations. In particular, it was shown that diffusion coefficients for displacement of the center of mass in the direction perpendicular and parallel to the major axis agree well with theoretical predictions in the case of a uniformly wetting rod. The analysis of the particle trajectories revealed that the effective center of rotation of Janus particles is displaced along the major axis toward the wetting end. Another unusual feature of the diffusive motion of Janus particles is that the nonwetting end of the Janus particle is rotated on average along the displacement vector of the center of mass in order to reduce the friction force from the surrounding fluid. 

The support of the National Science Foundation (grant CBET-1033662) is gratefully acknowledged. Molecular dynamics simulations were performed at Michigan State University's High Performance Computing Facility and the Ohio Supercomputer Center using the LAMMPS numerical code. 

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