Dissipation and adhesion in collisions between amorphous FeO nanoparticles

How dust grains aggregate into planetesimals is still an open question. It is experimentally observed that binary collisions between micron-sized dust grains result in adhesion for collisions up to similar to 1 ms(-1). However, aggregates at scales similar to 1 mm and above have been shown to exhibit fragmentation or bouncing at relevant collision velocities, resulting in a barrier preventing the formation of larger aggregates. One key factor is the weak adhesion observed between dust particles in aggregates leading to ruptures even in low-velocity collisions. To better understand the structure and strength of the adhered interface resulting from collisions, we performed molecular-dynamics simulations of head-on collisions between amorphous FeO nanoparticles. The results demonstrate several important phenomena which indicate how bonding between aggregates might become stronger than is often observed in experiments. For nanograins, it is shown that strong attractive interactions result in a minimum relative collision speed upsilon(c) determined just before impact. The values of upsilon(c) for particle radii 7 nm and below are computed to be in the range 40-100 ms(-1). This high collision speed is shown to result in strong bond reordering at the interface. Moreover, increasing the incident collision speed upsilon(rel) is shown to increase the work of adhesion W-adh, which is correlated to substantial bond rearrangement. Specifically, for values of upsilon(rel) in the range between 10-90 ms(-1), the interface between adhered grains is structured very closely to perfectly coordinated FeO amorphous solid. The reported results suggest stronger bonding results from collisions between particles with unpassivated surfaces, especially when particles are small, have amorphous surface structures, or when collisions occur at higher relative speeds. These factors should generate aggregates with stronger bonds that are less easily broken in subsequent collisions, and hence might be responsible for aggregates less likely to fragment at larger length scales.

Automated summary

Experimental observations show that micron-sized dust grains exhibit adhesion in collisions, but larger aggregates exhibit fragmentation or bouncing. Weak adhesion between dust particles in aggregates is a key factor leading to this phenomenon.