Measurement of fluid viscosity at microliter volumes using quartz impedance analysis

Abstract

The purpose of this work was to measure viscosity of fluids at low microliter volumes by means of quartz crystal impedance analysis. To achieve this, a novel setup was designed that allowed for measurement of viscosity at volumes of 8 to 10 μL. The technique was based on the principle of electromechanical coupling of piezoelectric quartz crystals. The arrangement was simple with measurement times ranging from 2 to 3 minutes. The crystal setup assembly did not impose any unwanted initial stress on the unloaded quartz crystal. Quartz crystals of 5- and 10-MHz fundamental frequency were calibrated with glycerol-water mixtures of known density and viscosity prior to viscosity measurements. True frequency shifts, for the purpose of this work, were determined followed by viscosity measurement of aqueous solutions of sucrose, urea, PEG-400, glucose, and ethylene glycol at 25°C ± 0.5°C. The measured viscosities were found to be reproducible and consistent with the values reported in the literature. Minor inconsistencies in the measured resistance and frequency shifts did not affect the results significantly, and were found to be experimental in origin rather than due to electrode surface roughness. Besides, as expected for a viscoelastic fluid, PEG 8000 solutions, the calculated viscosities were found to be less than the reported values due to frequency dependence of storage and loss modulus components of complex viscosity. From the results, it can be concluded that the present setup can provide accurate assessment of viscosity of Newtonian fluids and also shows potential for analyzing non-Newtonian fluids at low microliter volumes.