title: Probing the vapor-liquid phase behaviors of near-critical and supercritical fluids using a shear mode piezoelectric sensor.
abstract: With the rapidly expanding industrial and research applications of near-critical and supercritical technology there is a pressing need for a simple and inexpensive sensor that may be used to determine the phase coexistence regions of fluid mixtures and to establish whether a fluid system is below, at, or above, a critical point. Mechanically vibrating AT-cut quartz plates may be used to determine the product of the fluid density and viscosity of a fluid in which it is immersed, through measurement of the impedance minimum of the electrical equivalent circuit or of the corresponding frequency. The density-viscosity product changes abruptly between fluid phases and rapidly along the isotherm corresponding to the critical temperature, enabling such a plate to act as a sensor of these fluid features. We consider the limitations and linearity of such a sensor and its behavior when a liquid-gas meniscus crosses its surface. We demonstrate for the first time the effective use of an AT-cut quartz sensor in mapping the phase behavior of fluids, using measurements made on carbon dioxide and ethane for calibration and then investigating an ethane-carbon dioxide mixture. The advantages of this experimental approach are that (i) piezoelectric sensors are available for operation up to 1,000 degrees C and at extremely high pressures and (ii) the measurement of the density-viscosity product of supercritical fluids is inherently simpler than traditional techniques for determining phase behavior.
authors.affiliation: School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Oag
authors.forename: Peter J
authors.initials: PJ
authors.lastname: King
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Mellor
authors.forename: Michael W
authors.initials: MW
authors.lastname: George
authors.forename: Jie
authors.initials: J
authors.lastname: Ke
authors.forename: Martyn
authors.initials: M
authors.lastname: Poliakoff