Rheol Acta (2012) 51:909–923 DOI 10.1007/s00397-012-0649-3 ORIGINAL CONTRIBUTION Determination of axial forces during the capillary breakup of liquid filaments – the tilted CaBER method Dirk Sachsenheimer · Bernhard Hochstein · Hans Buggisch · Norbert Willenbacher Received: 24 February 2012 / Revised: 11 July 2012 / Accepted: 26 July 2012 / Published online: 28 August 2012 © Springer-Verlag 2012 Abstract The capillary breakup extensional rheometry (CaBER) is a versatile method to characterize the elon- gational behavior of low-viscosity fluids. Commonly, data evaluation is based on the assumption of zero normal stress in axial direction (σ zz = 0). In this pa- per, we present a simple method to determine the axial force using a CaBER device rotated by 90 ◦ and analyzing the deflection of the filament due to grav- ity. Forces in the range of 0.1–1,000 μN could be as- sessed. Our study includes experimental investigations of Newtonian fructose solutions and silicon oil mixtures (viscosity range, 0.9–60 Pa s) and weakly viscoelastic polyethylene oxide (PEO, M w = 10 6 g/mol) solutions covering a concentration range from c ≈ c∗ (critical overlap concentration) up to c > c e (entanglement con- centration). Papageorgiou’s solution for the stress ratio σ zz /σ rr in Newtonian fluids during capillary thinning is experimentally confirmed, but the widely accepted assumption of vanishing axial stress in weakly viscoelas- tic fluids is not fulfilled for PEO solutions, if c e is exceeded. Keywords CaBER · Elongational viscosity · Uniaxial extension · Force measurement D. Sachsenheimer (B ) · B. Hochstein · H. Buggisch · N. Willenbacher Karlsruhe Institute of Technology (KIT), Institute for Mechanical Process Engineering and Mechanics, Group Applied Mechanics (AME), Gotthard-Franz-Straße 3, 76131 Karlsruhe, Germany e-mail: sachsenheimer@kit.edu Introduction General remarks Many industrial applications and processes such as coating (Fernando et al. 1989, 2000), spraying (Dexter 1996; Prud’homme et al. 2005) (including mist for- mation (James et al. 2003) and its prevention (Chao et al. 1984)), inkjet printing (Agarwal and Gupta 2002; Han et al. 2004; Vadillo et al. 2010) or fiber spinning (McKay et al. 1978) include flow kinematics with large elements of elongational flow. Due to this high technical relevance, the correlation between rhe- ological properties against elongational deformation and fluid behavior in processes is a major subject of research. Technically relevant liquids are often complex multi- component systems with special flow properties ad- justed by adding small amounts of rheological modifier or thickener. A great number of these additional materials are commercially available, e.g., biopoly- mers (polysaccharides) like xanthan gum, starch, car- rageenan, or especially cellulose derivatives, inorganic substances like silica or water-swellable clay, or simply synthetic polymers like polyacrylates, polyvinylpyrili- done, or polyethylene oxide (PEO) (Braun and Rosen 2000). Therefore, understanding the elongational flow properties of such viscoelastic polymer solutions is of fundamental importance in process optimization and product development. Unfortunately, measuring the elongational viscosity of low viscosity fluids is still a very challenging task, resulting in only a few investiga- tions which correlate the elongational behavior of com- plex fluids with their application properties (Meadows et al. 1995; Kennedy et al. 1995; Ng et al. 1996; Solomon