A New Progress in the Study of Plasticity Mechanisms in Disordered Nanoparticle Systems
Recently, Prof. Xiaohui Liu from the Institute of Materials Modification and Modeling in School of Materials Science and Engineering of Shanghai Jiao Tong University (SJTU), in collaboration with Prof. Ju Li from Massachusetts Institute of Technology, Prof. Carpick from University of Pennsylvania, and Prof. Jie Zhang from the Institute of Natural Sciences and the Department of Physics and Astronomy of SJTU, published an online paper entitled "Friction and Adhesion Govern Yielding of Disordered Nanoparticle Packings: A Multiscale Adhesive Discrete Element Method Study" in the prestigious international journal Nano Letters.
Amorphous systems are widely found in nature, industrial applications, and daily life, such as soils, sand piles, colloids, glasses, etc. The moduli of these amorphous systems and the characteristic length scales of their constituents span several orders of magnitude, and the interactions between the constituents are drastically different. Despite these differences, recent studies have found striking commonalities in the yield strain, the equivalent size and the spatial correlation of the constituent rearrangements over a wide range of amorphous systems, reflecting the universal laws governing the mechanical behavior of amorphous systems generally. Nevertheless, the underlying mechanisms of plasticity and the effect of the interaction between constituents on the overall mechanical behavior of amorphous systems are still not fully understood.
To address these issues, the researchers chose to study disordered nanoparticle systems with significant frictional and adhesive interactions between the constituents, in order to compare with other amorphous systems such as atomic glasses and macroscopic granular matter, and thus explore the commonalities and differences in the mechanical behavior of various amorphous systems. On the other hand, although nanoparticle systems have been revealed with many excellent functionality, their poor mechanical properties largely limit their further application prospects. Therefore, it is of both scientific and engineering significance to reveal the microscopic mechanisms of plasticity and rheology of nanoparticle systems.
To capture the characteristics of nanoparticle systems, Prof. Xiaohui Liu developed a complicated multiscale adhesive discrete element method (MADEM). This method is able to obtain the normal contact forces between the particles in polydisperse systems under large deformation, and describe the friction (due to sliding, twisting, and rolling) and adhesion interactions between the particles. The particles’ trajectories (including the translational and rotational ones) as well as the evolution of the force chains, a network of forces connected by the interaction between the particles, can be obtained by numerically solving the kinetic equations, which is extremely challenging or even impossible to accomplish experimentally.
Figure 1 Typical load curve and affine von Mises strain distribution for disordered silica nanoparticle systems under nanoindentation.
Based on the developed MADEM, the researchers carried out numerical simulations on the previous experiments of nanoindentation on disordered silica nanoparticle systems. The simulation results are in good agreement with the experiments in terms of key characteristics of indentation load curves, energy dissipation, and plastic strain of the system, verifying the effectiveness of the MADEM method. On this basis, by tracing the trajectory of the particles and analyzing the evolution of the force chains, they revealed the microscopic mechanisms of the yielding and hardening phenomena of the packing under nanoindentation observed in the experiments. It was found that the yielding and hardening are the result of localized interparticle debonding and bonding, respectively, rather than the shear transformation zone (STZ) rearrangement and jamming mechanisms speculated in previous experiments. Unlike the commonly accepted STZ plasticity mechanism in amorphous systems, this interparticle debonding is not accompanied by significant particle rearrangements. This is a new paradigm for considering the nature of the unit processes involved in the plasticity of disordered systems, with yielding being sensitive to the interparticle friction and adhesion interactions, similar to many geological systems.
Figure 2 Evolution of the normal contact force chains and sliding frictional force chains in disordered silica nanoparticle systems under nanoindentation
By tuning two key parameters, the friction coefficient and the work of adhesion, the researchers also found a strong synergy between interparticle friction and adhesion in stabilizing and toughening the DNPs, which is not achieved in frictionless or nearly adhesionless packings. This is because for weakly adhesive systems, the mechanical strength is limited by the structural instabilities resulting in pile-up on the packings’ surface due to the shear dilatancy effect induced by the indentation. On the other hand, the adhesive but frictionless packings do gain enhanced stiffness in the case of stronger adhesion; however, generally, higher work of adhesion results in a larger pull-off force separating two particles, which will break the force balance of the global normal contact force chains to a greater extent and may lead to a more significant structural rearrangement, thus limiting the attainable mechanical strength of the packings. In contrast, for packing systems with both significant friction and adhesion, the strong adhesion considerably inhibits the shear dilatancy, while the tangential dissipative frictional force effectively retards the particle debonding and mitigates the particle rearrangements, thus significantly improving the mechanical stability and toughness of the system.
The interplay between friction and adhesion between particles revealed in this work suggests possibly establishing a unified jamming phase diagram that incorporates factors with both friction and adhesion, and may furnish possible guidance for toughening disordered nanoparticle systems. The MADEM developed here is expected to be employed to investigate other amorphous systems by varying the size, mass, and interaction of the constituents, providing a means to further explore the commonalities, differences, and key factors in the mechanical behavior of various amorphous systems.
This work was supported by the China Scholarship Council and the US National Science Foundation. Prof. Xiaohui Liu is the first author of the paper, with J. Lefever from University of Pennsylvania as the co-first author, and Prof. Ju Li and Prof. R. Carpick being the corresponding authors. Prof. Jie Zhang and Prof. D. Lee from University of Pennsylvania co-author this paper.
Contributed by: Institute of Materials Modification and Modeling