Swans on Tea

This month’s Physical Review Focus: Nanoparticles Stick a Perfect Landing

They found that for speeds less than 1.2 kilometers per second, the nanoparticle bounces off the surface like a basketball. But at higher speeds, some of the nanoparticle undergoes a phase transition to a compressed state called β-tin, where each atom bonds to six neighbors. This transition is surprising, Dumitrică says, because the collision energy is not high enough to induce a phase transition in a macroscopic object. However, the impact force is applied over a few square nanometers, so the pressure inside the nanoparticle is extremely large–around 200,000 atmospheres, which is more than enough to cause the phase transition.

The β-tin state only lasts a few picoseconds, though. As the nanoparticle begins to bounce back, there is a second phase transition to an amorphous, or disordered, state. The combination of the two phase transitions, plus some heat generation, takes up all of the kinetic energy, and the particle remains on the surface. After all of this action, “the recoil is too weak to beat the adhesion forces between the nanoparticle and the substrate,” Dumitrică says.

However

A silicon nanoparticle flying at 8 times the speed of sound can slam into a surface and stick, but it bounces off if colliding at half that speed.

The speed of sound in what, pray tell? I wish journalists would remember (learn?) that the speed of sound is not a constant of nature.