How to extract the Newtonian viscosity from capillary breakup measurements in a filament rheometer

The liquid filament microrheometer originally described by Bazilevsky et al. (1990) provides a simple way of extracting material parameters for Newtonian and viscoelastic fluids from measurements of the capillary breakup of a thin fluid thread. However, there is an unresolved discrepancy in the value of the Newtonian viscosity obtained from the experimental data when using the existing theoretical analysis. We demonstrate how to correctly analyze measurements of the midpoint radius and present a simple formula that enables one to obtain quantitative values for the Newtonian viscosity for a range of viscous fluids. The validity of this correction is supported by numerical simulations and experiments with a number of viscous Newtonian fluids. In addition we analyze the role of gravitational body forces on modifying the dynamics of capillary thinning of a Newtonian liquid filament. Finally, we show how such capillary breakup devices may be used to make quantitative time-resolved measurements of changes in the viscosity of hygroscopic materials or fluids with a volatile solvent component that are exposed to an ambient atmosphere. (C) 2000 The Society of Rheology. [S0148-6055(00)01103-2].