Controlling the elongational flow behavior of complex shear-thinning fluids without affecting shear viscosity

Complex flow fields including high elongational deformation occur in numerous industrial processes such as spraying, coating, fiber spinning, and screen or inkjet printing. Fully exploiting the potential of these technologies suffers from a lack of knowledge regarding how elongational flow properties of the processed fluids affect the results of these operations. Here, we present two strategies that allow for varying the elongational flow behavior independent of shear rheology. First, two acrylic thickener solutions that differ with respect to the fraction of hydrophobic co-monomers and hence with respect to their degree of inter- and intramolecular hydrophobic association were mixed to vary the elongational relaxation time, as determined using capillary breakup elongational rheometry (CaBER), by almost two orders of magnitude without affecting shear viscosity of these solutions in a wide, processing-relevant shear rate range. Second, a substantial increase in the elongational flow resistance was achieved by adding a small amount of plate-like particles without affecting the shear viscosity of these thickener solutions. A fourfold increase of the elongational relaxation time was observed upon the addition of 3.5 vol.% of glass flakes to such a highly shear-thinning system. A similar effect was also observed for an industrial waterborne automotive basecoat due to added aluminum flakes. This work may be useful for product development since the control of extensional viscosity can improve technological applications, and the introduced model systems may therefore be used for systematic, goal-oriented investigations of the relevance of elongational flow properties in the technological processes mentioned above.