SOME OLD AND NEW PUZZLES IN THE DYNAMICS OF FLUIDS

Ihor Mryglod (Personal webpage)

One of the basic concepts of modern physics with a long prehistory is a fluid inthe meaning of a substance that continually flows (or deforms) under an applied shearstress. In this sense fluids form a wide subset are the phases of matter and includeliquids, dense gases, plasmas, and to some extent even plastic solids. The fluidity isone of the main dynamical characteristics that depends strongly on materialproperties or details of local structure and parameters of many-particle interactions inthe terms of statistical physics. The last ones determines some relevant time thatdivide the fluid behavior in two main time regimes, characterizing material like as aviscous liquid over a long time period and as an elastic-like solid over a short timeperiod. Such a property is typical for a viscoelastic substance or in other words for afluid.

Another aspect to be important for understanding of the fluid dynamics isconnected with the structural ordering in the arrangement of atoms and molecules.Orderliness over distances comparable to interatomic distances is usually treated asshort-range order, whereas orderliness repeated over infinitely large distances iscalled long-range order. Both long-range and short-range order are absent in the idealgas, but liquids and amorphous solids exhibit short-range order. The physics ofphonon in crystalline solids with long-range order is well understood. In liquids,however, the atomic structure is changing with time and the concept of phononbecomes questionable for long time processes. How the phonon-like excitations aswell as other collective modes determine the fluid dynamics and are reflected in theresponse functions?

Theoretically the flow of a liquid is commonly described by continuumhydrodynamic theories. Less attention has been paid to the atomic level dynamicsbecause it has been believed that the liquids are so random that details of the atomicmotion are irrelevant to the physics of liquid flow. Is it true for fluid systems withshort-range order?

In this lecture we try to find answers for some old and new questions that makethe fluid dynamics still very attractive for the theoretical studies.