Cooperative clusters in a simulated cooling liquid start shrinking below a certain temperature

The clusters Laudicina measured are defined by how the particles move. Some particles in a supercooled liquid barely move at all, while others form groups that travel together, and his simulations tie the large changes in viscosity to that unevenness ^[7][8]. He described the crowding this way: "A bit like at a festival, where, in the crowded areas, everyone has to move a little if you want to get a beer at the back of the field." ^[18]

In the phys.org interview Laudicina said that around the glass transition "the viscosity increases extremely rapidly" while "the material's internal structure barely changes" ^[3]. A glass, he said, "behaves like a solid, while its internal structure is more like that of a liquid" ^[2]. The static arrangement is the part that stays put; the simulations follow how the motion sorts itself into groups ^[7].

The simplification was deliberate. Theoretical physicists are primarily "striving for simplicity", Laudicina said, and here that meant reducing real liquids to a system of perfectly spherical particles interacting in relatively simple ways ^[17][5]. Time scales are the hard part of the measurement. He pointed to the University of Queensland's pitch drop livestream: pitch is rock-hard at room temperature and still an extremely viscous liquid, dripping on average once every ten years ^[13].

What he set out to measure was how the clusters change as the liquid cools, and how that relates to the gradual slowing of the liquid through the glass transition ^[19]. The method was to measure the different clusters precisely and track them over longer periods of time ^[20]. The simulated particles were spheres interacting simply, so the turnover he reports cannot say whether a liquid of real molecules shows the same maximum ^[5][9].