Ermittlung der wahren Dehnviskosität niedrigviskoser Fluide durch zusätzliche Kraftmessung bei der Capillary Breakup Extensional Rheometry
Final Report Abstract
We have introduced a new way of performing CaBER experiments named tilted CaBER method. This method comprises a horizontal stretching of fluid filaments and allows for a determination of the axial force in the liquid bridge from the gravity-driven bending of the filament applying chain bending theory. This method provides reliable values for the axial force in a liquid filament in a range of N = 0.1 - 1,000 µN and allows for a determination of the true elongational viscosity without additional assumptions or specific constitutive equations. For Newtonian fluids, the existence of a negative axial normal stress was proved experimentally, and the solution of Papageorgiou (Papageorgiou1995) was confirmed for the linear thinning of Newtonian liquids by using two different sets of samples covering a viscosity range of η = 0.9 - 60 Pas. The tilted CaBER method has been applied to non-Newtonian PEO solutions with concentrations c > c* and c > ce. The assumption of vanishing axial normal stress for evaluating CaBER experiments was confirmed for PEO solutions with c < ce, and a positive axial normal stress was detected for concentrations above the entanglement concentration. However, for the PS and CPyCl/NaSal/NaCl solutions an intermediate Newtonian thinning regime was confirmed based on the measured force ratio X = 0.713 as already suggested by Clasen. Such an intermediate regime can be related to the low elastocapillary number Ec0 < 1. Deviations from X = 1 directly cause a difference between the apparent and true elongational viscosity. Therefore, apparent values calculated from the diameter decay have to be corrected by the time dependent factor 2X-1. For X < 1 the apparent elongational viscosity ηe,app is greater than the true elongational viscosity ηe (Newtonian controlled thinning) and ηe,app < ηe is found for X > 1 (PEO solutions with c > ce). However, in all cases the elongational viscosity exhibits the well-known elongational hardening behavior. A direct measurement of the axial force using commercial silicon based force transducers seems to be unsuitable due to serious problems during handling. To avoid these problems, a customized force transducer has been designed where the deflection of a spring is measured with a laser interferometer.
Publications
- (2012) Determination of axial forces during the capillary breakup of liquid filaments - the tilted CaBER method. Rheologica Acta 51:909–923
Sachsenheimer D, Hochstein B, Buggisch H, Willenbacher N
(See online at https://doi.org/10.1007/s00397-012-0649-3) - Experimental study on the capillary thinning of entangled polymer solutions. Rheologica Acta, Sept 2014, Vol 53, Issue 9, pp 725–739
Sachsenheimer D, Hochstein B, Willenbacher N
(See online at https://doi.org/10.1007/s00397-014-0789-8)
