Polyatomic degrees of freedom and their temporal evolution extracted from the damping of micro-oscillators

Abstract

This paper investigates for the first time the influence of molecular degrees of freedom (DOF) on the damping behavior of micro-oscillators in the molecular flow regime. None of the existing theories of damping does take this aspect into account. The damping of piezoelectrically driven bending oscillators is determined by measuring the pressure dependent quality factor Q in an atmosphere consisting of noble gases or polyatomic gases. A neighboring plate with an adjustable gap width ranging from 150 to 3500 μm was mounted above the oscillator, limiting the time of flight of the gas molecules in between the delimitation to several microseconds. Taking the behavior of the noble gases with three translational DOF as a reference, the development of additional rotational and vibrational DOF of polyatomic gas molecules is determined as a function of the gap width, which can be correlated to the time of flight. The determined relaxation times and collision numbers of the DOF are compared to existing theories and to experimental results explored by other methods. Based on the experimental evidence, additions to the existing molecular damping theory are derived by including the higher DOF and adapting an existing thermodynamic excitation model.