The origin of complex crater formation during high-speed impacts.

Complex crater formation is an incompletely understood phenomenon, referring to instances wherein a high-speed projectile impacts a surface and leaves a crater characterized by a central uplift. We elucidate the mechanism of complex crater formation by examining crater formation on different polymer substrates resulting from microparticle impacts with tunable microparticle diameter (1.8 to 6.1 micrometers) and impact velocities up to 840 meters per second. Central uplift is uniquely observed in craters on amorphous polymers, with the degree of complexity directly linked to the polymer thermal properties and homogeneity. We demonstrate that the complex crater volume scales with the ratio of the specific kinetic energy of the impacting object to the specific energy required to raise the impacted substrate to its glass transition temperature (the Eckert number). Our results also confirm that complex crater formation can occur not only for macroscopic celestial collisions but also during sufficiently high velocity collisions at the micrometer scale.