Instant yield stress measurement from falling drop size: The syringe test

We analyze different flow regimes of a filament formed by extrusion of a material through a cylindrical die. We deduce that the elongational yield stress of a simple yield stress fluid (i.e., with negligible thixotropy effects) can be determined from the mass of the droplet after filament breakage and an estimation of the critical radius at pinch-off at the solid-liquid regime transition. We demonstrate that such a simple characterization is relevant in a relatively wide range of extrusion velocities, i.e., this velocity slightly affects the drop mass in this range. For the simple yield stress fluids used, Carbopol gel, clay-water paste at different concentrations, and emulsion, covering a large range of yield stress values (50-1000 Pa), the elongational yield stress appears to be equal to the simple shear yield stress times a factor equal to about 1.5 root 3 . As a consequence, this simple test may be used to obtain, almost instantaneously and without sophisticated apparatus (a syringe and a balance are sufficient), a good estimate of the shear yield stress of simple yield stress fluids. In that case, the main source of uncertainty (up to about 20%) is the value of the critical radius at the solid-liquid transition. Finally, we review the operating conditions (material properties and extrusion characteristics) for which we can expect this approach to be valid. (c) 2023 The Society of Rheology.

Automated summary

We investigate different flow regimes of a filament generated through extrusion and propose a simple method to determine the elongational yield stress of simple yield stress fluids. We show that the drop mass after filament breakage and the estimation of the critical radius at the solid-liquid transition can be used to estimate the shear yield stress. Our findings suggest that this approach is valid in a wide range of extrusion velocities and can provide an instantaneous estimation of the shear yield stress without sophisticated equipment.