Cluster Impact
1) Cratering
Atomistic simulation data on crater formation due to hypervelocity impact of nanoprojectiles of up to 55 nm diameter and with targets containing up to 1.1 · 1010 atoms, are compared to available experimental data on μm-, mm-, and cm-sized projectiles. We show that previous scaling laws do not hold in the nano-regime and outline the reasons: within our simulations we observe that the cratering mechanism changes, going from the smallest to the largest simulated scales, from an evaporative regime to a regime where melt and plastic flow dominate, as it is expected in larger micro-scale experiments. The importance of strain-rate dependence of strength and of dislocation production and motion are discussed.
2) Cratering in Thin Films
We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km/s on copper films ranging in thickness from from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimetre scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallization. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/dp < 1.5, where f is the film thickness and dp is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account.
3) Experimental method to generate hypervelocity micro dust
To model the size distribution and composition of interstellar and interplanetary dust grains, and their effect on a wide range of phenomena, it is vital to understand the mechanism of dust-shock interaction. We demonstrate a new laser experiment that subjects dust grains to pressure spikes similar to those of colliding astrophysical dust, and that accelerates the grains to astrophysical velocities. This new method generates much larger data sets than earlier methods; we show how large quantities (thousands) of grains are accelerated at once, rather than accelerating individual grains, as is the case of earlier methods using electric fields. We also measure the in-flight velocity (~4.5km/s) of hundreds of grains simultaneously by use of a particle image velocimetry (PIV) technique.
4) Cluster Stopping
Using molecular-dynamics simulations, we study the processes underlying the stopping of energetic clusters upon impact in matter. We investigate self-bombardment of both a metallic (Cu) and a van-der-Waals bonded (frozen Ar) target. Clusters with sizes up to N = 104 atoms and with energies per atom of E/N = 0.1 − 1600 eV/atom were studied. We find that the stopping force exerted on a cluster follows a N2/3- dependence with cluster size N; thus large clusters experience less stopping than equivelocity atoms. In the course of being stopped, the cluster is strongly deformed and attains a roughly pancake shape. Due to the cluster inertia, maximum deformation occurs later than the maximum stopping force. The time scale of projectile stopping is set by t0, the time the cluster needs to cover its own diameter before impacting the target; it thus depends on both cluster size and velocity. The time when the cluster experiences its maximum stopping force is around (0.7 − 0.8)t0. We find that the cluster is deformed with huge strain rates of around (1/2) t0; this amounts to 10^11−10^13/s for the cases studied here.
5) Carbon grain collision
In this study we analyze, using molecular dynamics, the collision dynamics of two clusters of diamond of radius 1nm, with different velocities and impact parameters. The research includes the analysis of the structural and thermodynamic changes produced by the collision. When the impact speed is high, the configuration of the atoms changes, i.e. the sp3 hybridization of diamond changes to sp2 (graphitic) hybridization. This transition occurs after a few picoseconds and modifies greatly the properties of nanograins.
Collision scheme. X is the impact paramer, d the diameter and v/2 the individual velocity.
6) Silicate grain collisions
The collision of granular clusters can result in a number of complex outcomes, from sticking, to partial or full destruction of the clusters. This outcome will contribute to the size distribution of dust aggregates, changing their optical properties and their capability to contribute to solid-state astrochemistry. We study the collision of two clusters of equal size, formed by approximately 7000 sub-micron grains each, with a mass and velocity range which is difficult to sample in experiments. We obtain the outcome of the collision: compaction, fragmentation and size distribution of ejecta, and type of outcome, as a function of velocity and impact parameter. We compare our results to other models and simulations, at both atomistic and continuum scales, and find some agreement together with some discrepancies.
Final snapshots for different collision events showing the regimes of agglomeration, partial, and total fragmentation. (a) v = 2 m/s (v = 12vfrag ). (b) v = 5 m/s (v = 59vfrag ). (c) v= 10 m/s (v = 176vfrag ). In all cases the impact parameter was selected as b = 0.6R.
Publications:
- "Collisions of porous clusters: a granular-mechanics study of compaction and fragmentation". Christian Ringl, Eduardo M. Bringa, Dalía S. Bertoldi and Herbert M. Urbassek. Astrophysics Journal (2011)
- "Penetration scaling in atomistic simulations of hypervelocity impact", Andrew Higginbotham, E.M. Bringa, Emma A. Taylor, Giles Graham, International Journal of Impact Engineering 38 (2011) 247-251.
- "A new method to generate dust with astrophysical properties", J.F. Hansen, W. van Breugel, E.M. Bringa, B. Eberly, G.A. Graham, B.A. Remington, E.A. Taylor, and A.G.G.M. Tielens, J. Inst. 6 (2011) 5010.
- "Digging a crater: Why nano-projectiles work differently than macro-impactors", Christian Anders, Eduardo M. Bringa, Gerolf Ziegenhain, Giles A. Graham, J. Freddy Hansen, Nigel Park, Nick E. Teslich, and Herbert M. Urbassek, Phys. Rev. Lett. (2012).
- "Stopping of hypervelocity clusters in solids", Christian Anders, Eduardo M. Bringa, Gerolf Ziegenhain and Herbert M. Urbassek, New J. Phys.(2011).
- "Collisions of porous clusters: a granular-mechanics study of compaction and fragmentation" Christian Ringl, Eduardo M. Bringa, Dalía S. Bertoldi, and Herbert M. Urbassek1. (Dated: August 30, 2011)
Integrantes
Collaborators:
H.M. Urbassek and his group (TU Kaiserslautern)
A. Higginbotham (University of Oxford, UK)
N. Park (AWE, UK)
G. Graham (National History Museum, UK)