New trends in viscometers

Abstract

To fulfil the need of research minded audience, modern viscometers using various physical phenomena are described. The viscometers discussed are based on tuning fork, ultrasonic, plate waves, Love waves, cantilevers and use of optical fibres. A tuning fork generates sound using the phenomenon of resonance at a constant frequency. Similarly a tuning fork-type viscometer resonate its sensor plates at a natural frequency and measures viscosity from the driving force (electromagnetic force) required to maintain constant amplitude. Measurement of acoustic and shear impedances of ultrasonic longitudinal waves are functions of viscosity of the liquid. A viscometer based on aforesaid principle has been described. Martin et al. showed that the attenuation of the plate waves propagating on thin silicon-nitride membranes in contact with viscous liquids depends upon the viscosity of the liquid while the decrease in frequency of maximum transmission depends upon the density of the liquid. Thus an ultrasonic plate wave viscometer along with its basic theory is briefly described. Love waves-plain polarised shear waves using micro devices have been profitably used for viscosity measurement. The Change in frequency of a PZT crystal is a function of the product of viscosity and density of the liquid around it. Piezoelectric resonator has a good potential to measure viscosity and density of the liquids. Such a device is also given. Similarly the micro-cantilevers used for measurement of very small forces can be used to determine density and viscosity of liquids. The change in amplitude and velocity of a vibrating optical fibres partially immersed in a liquid depends upon the viscosity of the liquid. The change in intensity of the diffraction pattern is proportional to the small changes in amplitude of the vibrating optical fibre. This phenomenon has been used to determine viscosity of liquids available in very small amount.