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

The liquid filament microrheometer originally described by Bazilevsky et al . (1990) provides a simple way of extracting material parameters for Newton ian and viscoelastic fluids from measurements of the capillary breakup of a thin fluid thread. However, there is an unresolved discrepancy in the valu e 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 enab les one to obtain quantitative values for the Newtonian viscosity for a ran ge of viscous fluids. The validity of this correction is supported by numer ical 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 quantitativ e time-resolved measurements of changes in the viscosity of hygroscopic mat erials or fluids with a volatile solvent component that are exposed to an a mbient atmosphere. (C) 2000 The Society of Rheology. [S0148-6055(00)01103-2 ].