Molecular dynamics of rolling and twisting motion of amorphous nanoparticles

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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.

Section: Normal and Tangential Restitution Coefficientsmentioning

confidence: 99%

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

Nietiadi

Urbassek

2022

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“…The density for the liquid phase is used, since data seem to be more easily available for this phase and also because the structure of our amorphous NPs more closely resembles that of the liquid state. Note that the density of amorphous LJ material is 37 ; for crystalline LJ solids, it is somewhat higher, 38 . The triple-point and critical temperature of LJ material are and , respectively 39 – 41 ; the latter value depends strongly on the cut-off radius of the potential 42 .…”

Section: Methodsmentioning

confidence: 99%

“…The triple-point and critical temperature of LJ material are and , respectively 39 – 41 ; the latter value depends strongly on the cut-off radius of the potential 42 . For the convenience of the reader, we note that for the amorphous LJ solids used here, the specific surface energy amounts to ; the Young’s modulus is and the Poisson ratio 0.37 37 .…”

Section: Methodsmentioning

confidence: 99%

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

Nietiadi

Rosandi

Bringa

et al. 2022

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Molecular dynamics simulations are used to study collisions between amorphous ice nanoparticles consisting of CO, CO$$_2$$ 2 , Ar and H$$_2$$ 2 O. The collisions are always sticking for the nanoparticle size (radius of 20 nm) considered. At higher collision velocities, the merged clusters show strong plastic deformation and material mixing in the collision zone. Collision-induced heating influences the collision outcome. Partial melting of the merged cluster in the collision zone contributes to energy dissipation and deformation. Considerable differences exist—even at comparable collision conditions—between the ices studied here. The number of ejecta emitted during the collision follows the trend in triple-point temperatures and increases exponentially with the NP temperature.

An atomistic study of sticking, bouncing, and aggregate destruction in collisions of grains with small aggregates

Nietiadi,

Urbassek,

Rosandi

2024

Molecular dynamics simulations are used to study central collisions between spherical grains and between grains and small grain aggregates (up to 5 grains). For a model material (Lennard-Jones), grain–grain collisions are sticking when the relative velocity v is smaller than the so-called bouncing velocity and bouncing for higher velocities. We find a similar behavior for grain–aggregate collisions. The value of the bouncing velocity depends only negligibly on the aggregate size. However, it is by 35% larger than the separation velocity needed to break a contact; this is explained by energy dissipation processes during the collision. The separation velocity follows the predictions of the macroscopic Johnson–Kendall–Roberts theory of contacts. At even higher collision velocities, the aggregate is destroyed, first by the loss of a monomer grain and then by total disruption. In contrast to theoretical considerations, we do not find a proportionality of the collision energy needed for destruction and the number of bonds to be broken. Our study thus sheds novel light on the foundations of granular mechanics, namely the energy needed to separate two grains, the difference between grain–grain and grain–aggregate collisions, and the energy needed for aggregate destruction.