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Abstract
The Capillary Breakup Extensional Rheometer (CaBER) is the only commercially available elongational rheometer. The CaBER can measure the diameter of a low viscous sample under stretching as a function of time. The analysis of this setup is consequently limited to the time dependent diameter of the capillaries D (t). Some samples do not develop a proper capillary flow, because of, for example, a lack of a substantial surface tension or a cylindrical fluid filament. As a consequence the interpretation of the results based on D (t) is problematic. On the other hand there is a huge fundamental industrial interest in measuring properties like strand formation, stickiness, spreadability, and so on. These properties are related to the extensional properties, specifically the elongational viscosity of a sample. Therefore, additional information beside D (t) is needed. In this publication a newly developed extension of the CaBER is presented and first applications are shown. The viscous sample is placed between two plates and afterward extended quickly by moving the upper plate. The normal force acting on the lower plate is measured as a function of time. To measure these small forces (<0.05 mN) with very short time resolution (e.g., dwell‐times down to 0.2 ms) a piezo transducer is used in combination with modern data oversampling techniques to further increase the sensitivity of the transducer. In this article the CaBER modifications are presented in detail to simultaneously measure the diameter and the force down to 5*10−5 N, with 10−3 s time resolution.
ACKNOWLEDGMENTS
We thank H. Buggisch from the Department of Applied Mechanics, Karlsruhe University for the support in developing the theory, and together with B. Hochstein for discussions and suggestions. We also acknowledge the workshop from the Institute of Technical Chemistry and Polymer Chemistry of the University of Karlsruhe for manufacturing the mechanical parts. We thank F. Hetzel from Beiersdorf AG Hamburg for their support and helpful discussions. Furthermore, we thank B. Schröder from Thermo (Durlach, Germany) for making this apparatus commercially available.