Bouncing and spinning of amorphous Lennard-Jones nanoparticles under oblique collisions

Abstract

Collisions of Lennard-Jones nanoparticles (NPs) may be used to study the generic collision behavior of NPs. We study the collision dynamics of amorphous NPs for oblique collisions using molecular dynamics simulation as a function of collision velocity and impact parameter. In order to allow for NP bouncing, the attraction between atoms originating from differing NPs is reduced. For near-central collisions, a finite region of velocities – a ‘bouncing window’ – exists where the 2 NPs bounce from each other. At smaller velocities, energy dissipation and – at larger velocities – also NP deformation do not allow the NPs to surpass the attractive forces such that they stick to each other. Oblique collisions of non-rotating NPs convert angular momentum into NP spin. For low velocities, the NP spin is well described by assuming the NPs to come momentarily to a complete stop at the contact point (‘grip’), such that orbital and spin angular momentum share the pre-collision angular momentum in a ratio of 5:2. The normal coefficient of restitution increases with impact parameter for small velocities, but changes sign for larger velocities where the 2 NPs do not repel but their motion direction persists. The tangential coefficient of restitution is fixed in the ‘grip’ regime to a value of 5/7, but increases towards 1 for high-velocity collisions at not too small impact parameters, where the 2 NPs slide along each other.

Collisions between CO, CO$$_2$$, H$$_2$$O and Ar ice nanoparticles compared by molecular dynamics simulation

“…Only central collisions were considered. We found that for all velocities in the range of 150–100,000 m/s, collisions between CO NPs were always sticking; this finding is in agreement with earlier simulations on amorphous LJ NPs 52 and our previous results on water-ice NPs 28 . We therefore present here only the results of a central collision with a relative velocity of LJ units; this corresponds to velocities of 276 (298, 424, 742) m/s for Ar (CO, CO , H O) according to Table 2 .…”

“… Finally, our present study focused on central collisions, while in experiment and in nature large impact parameters will occur. Large impact parameters decrease the tendency for sticking as only parts of the colliding NPs come into close contact and experience their mutual attraction 52 , 59 . Recently, it was shown that energy dissipation in granular aggregates of water ice leads to different aggregation and fragmentation outcomes 60 .…”

“…Finally, our present study focused on central collisions, while in experiment and in nature large impact parameters will occur. Large impact parameters decrease the tendency for sticking as only parts of the colliding NPs come into close contact and experience their mutual attraction 52 , 59 .…”

The role of porosity in collisions of granular aggregates: A simulation study of fusion, sliding, and fragmentation collisions

“…Such a behavior has not been observed in the present study nor, to our knowledge, in other studies of aggregate collisions, except for unrealistically large filling factors (Wada et al 2011) or in aggregates composed of grains of very different masses such as dust-covered chondrules (Umstätter et al 2019). We note, however, that such a bouncing behavior is well-known for solid or liquid clusters (Kalweit & Drikakis 2006;Sommerfeld & Kuschel 2016;Nietiadi & Urbassek 2022).…”

“…We note that collisions of liquid droplets -of interest in technological applications such as spraying -have been classified similarly; however, the fragmentation regime is denoted as "stretching separation," where a multitude of small droplets are produced after the colliding droplets separate again (Ashgriz & Poo 1990;Estrade et al 1999;Sommerfeld & Kuschel 2016). Also collisions of solid clusters, analyzed with the help of molecular dynamics simulations, lead to a similar classification (Kalweit & Drikakis 2006;Nietiadi & Urbassek 2022).…”