SOME OLD AND NEW PUZZLES IN THE DYNAMICS OF FLUIDS
Institute for Condensed Matter Physics of National Academy of Sciences of UkraineOne of the basic concepts of modern physics with a long prehistory is a fluid in the meaning of a substance that continually flows (or deforms) under an applied shear stress. In this sense fluids form a wide subset are the phases of matter and include liquids, dense gases, plasmas, and to some extent even plastic solids. The fluidity is one of the main dynamical characteristics that depends strongly on material properties or details of local structure and parameters of many-particle interactions in the terms of statistical physics. The last ones determines some relevant time that divide the fluid behavior in two main time regimes, characterizing material like as a viscous liquid over a long time period and as an elastic-like solid over a short time period. Such a property is typical for a viscoelastic substance or in other words for a fluid. Another aspect to be important for understanding of the fluid dynamics is connected with the structural ordering in the arrangement of atoms and molecules. Orderliness over distances comparable to interatomic distances is usually treated as short-range order, whereas orderliness repeated over infinitely large distances is called long-range order. Both long-range and short-range order are absent in the ideal gas, but liquids and amorphous solids exhibit short-range order. The physics of phonon in crystalline solids with long-range order is well understood. In liquids, however, the atomic structure is changing with time and the concept of phonon becomes questionable for long time processes. How the phonon-like excitations as well as other collective modes determine the fluid dynamics and are reflected in the response functions? Theoretically the flow of a liquid is commonly described by continuum hydrodynamic theories. Less attention has been paid to the atomic level dynamics because it has been believed that the liquids are so random that details of the atomic motion are irrelevant to the physics of liquid flow. Is it true for fluid systems with short-range order? In this lecture we try to find answers for some old and new questions that make the fluid dynamics still very attractive for the theoretical studies.