Studies on Nanoparticle Collisions
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Nanosized clusters, termed nanoparticles, have attracted much attention recently because of physical properties unique to their size, 1-100 nm in diameter. They have mechanical and dynamical properties distinctively different from those of bulk counterparts due to their large surface-area-to-volume ratio and surface discreteness. Colliding nanoparticles often show intriguing and unexpected properties such as superelasticity, anomalous softening, and crystal phase transformation. In this dissertation, elastic and plastic properties of colliding nanoparticles are investigated by means of nonequilibrium classical molecular dynamics simulations. Approximately spherical nanoparticles made of a single face-centered cubic crystal are modeled for collisions. The radius of the nanoparticles ranges between 1.7-22 nm. Two identical nanoparticles are prepared and made to collide at a collision velocity varied approximately between 1-500 m/s. Purely repulsive nanoparticles, which represent coated nanoparticles, are studied in terms of elastic and plastic collisions. A superelastic collision of the repulsive nanoparticles shows that slowly colliding nanoparticles gain the translational kinetic energy from the internal degrees of freedom. Consequently, they rebound at a velocity faster than the incident velocity. It is shown that the time scale of the collision and the period of the surface vibrations are very close in such a peculiar collision. Unusual softening of the nanoparticles accompanying extensive plastic deformations is revealed in the rapidly falling coefficient of restitution. The softening is inconsistent with the continuum theory and experiments for colliding macroscopic bodies. Significant permanent deformations involving slip and twinning are found to occur in such single crystalline nanoparticles and presumably account for the softening. The smaller nanoparticles are found to be extremely hard. The yield velocity, a crossover between the elastic and plastic collisions, has a size dependence and becomes higher with reducing the nanoparticle size. This behavior is not explained by the conventional continuum based theory. We show that the size dependence of the yield velocity is attributed to its size-dependent dynamic yield strength. It is demonstrated that the surface roughness arising from facets and edges of the repulsive nanoparticles influences the inter-nanoparticle forces in the elastic collision regime. Two types of collision, the facet-facet collision and the edge-edge collision, are chosen for the force investigation. These collisions lead to distinctive contact surface evolutions as the compression progresses. Consequently, the edge-edge collision displays the Hertzian-like contact force as perfect elastic spheres experience, whereas the face-face collision displays the Hookean-like contact force. In adhesive nanoparticles, it is shown that the adhesiveness at both extremely low and high collision velocities gives rise to sticking of the nanoparticles. The adhesion effects on the coefficient of restitution and a critical velocity below which nanoparticles never bounce back are examined. The size dependence of the critical velocity found in our adhesive nanoparticles agrees with the prediction from the Johnson-Kendall-Roberts theory, a continuum theory for adhesive macroscopic spheres.