Abstract
The research is motivated by the search for materials with negative viscosity to exploit the effect of negative mass. We introduce media (gaseous and liquid) that demonstrate negative viscosity. Consider the vibrated plate, which is vertically pulled through the ideal gas and built from the core-shell "meta-molecules". Vibrating the vertical plate supplies an excess vertical momentum to the core-shell meta-molecules. If the frequency of vibrations is larger than the resonant frequency, the excess moment is oriented against the direction of the vertical motion; thus, the effect of negative viscosity becomes possible. The effective viscosity becomes negative when the frequency of the plate vibrations approaches the resonant frequency from above. Thus, a novel physical mechanism resulting in negative viscosity is introduced. No violation of energy conservation is observed; the energy is supplied to the system by the external source vibrating the plate. The effect of the negative viscosity is also possible in liquids. Frequency dependence of the viscosity is addressed. Asymptotic expressions are derived for the frequency-dependent viscosity. Introduced meta-materials may be exploited for the development of media with prescribed rheological properties. Possible realizations of the negative-viscosity media are discussed.