Cover story in Communications Physics (Nature)
The smooth yet chaotic motion of atoms in a liquid leads to a rapid onset of friction below a characteristic frequency.
News from Jul 10, 2020
Friction describes how a body that is immersed in a fluid resists to dragging it. The friction experienced by the molecules of a liquid is a crucial parameter: it controls energy dissipation and sets the time scale of virtually all processes in liquid matter; examples range from viscous Stokes flow to macromolecular transition rates. On the other hand, the forces between individual molecules and atoms lead to motion that is free of dissipation. The origin of friction from such conservative forces remains as one of the grand challenges of the physics of fluids.
The bridge between these atomistic and hydrodynamic pictures of the same liquid was found by researchers of the MATH+ project EF4-4 and the Collaborative Research Center SFB 1114 at Freie Universität Berlin. The team of theoretical
physicists combined high-performance simulations with mathematics and reports in the open access journal Communications Physics (Nature Publishing) on precise dissipation spectra, which reveal that friction in liquids emerges abruptly below a certain frequency. Above this frequency, viscous liquids appear as non-dissipative, elastic solids. The rapid onset occurs in the terahertz regime and implies that the effective Brownian forces on the molecules are pronouncedly non-Markovian. The researchers plan to employ their approach to other problems where memory effects are relevant, for example cell migration and the intriguing mechanical properties of soft materials.
A. V. Straube, B. G. Kowalik, R. R. Netz, and F. Höfling,
Rapid onset of molecular friction in liquids bridging between the atomistic and hydrodynamic pictures,
Commun. Phys. 3, 126 (2020) – cover story in the month of October.