Titel: Rapid stretching of liquid bridges by an accelerated plate
Autoren/Herausgeber: Christina Maria Weickgenannt
Aus der Reihe: Forschungsberichte Strömungslehre und Aerodynamik
Ausgabe: 1. Auflage
Format: 21 x 14,8 cm
Gewicht: 209 g
The present study focus on stretching and breakup of liquid bridges of Newtonian and complex liquids. In particular fast stretching is examined, which has rarely been considered in literature until now. Comprehensive experimental and theoretical investigations have been performed and a fundamental understanding of the process physics and influencing factors were obtained. The results are of great importance for all industrial processes where liquids experience a strong extensional deformation, like in printing, spraying extrusion processes or fiber spinning.
An experimental setup is designed that realizes a wide range of adjustable constant stretching accelerations. For Newtonian liquids it is found that for sufficiently high stretching accelerations of a = 100 m/s² the bridge contour evolution is no longer influenced by an additional increase of the dynamics or by liquid properties, as it is for lower accelerations. The limit is reached when the stretching velocity exceeds the capillary wave velocity. A numerical modeling of the process reveals further that the region of maximal strain shifts with increasing acceleration towards the accelerated fluid and that the bridge deforms asymmetrical. Based on the experimental insights, a theoretical model is developed that allows to estimate the bridge breakup times.
The stretching and breakup of suspension liquid bridges is qualitatively similar to those of Newtonian liquids for a particle volume fractions up to Ø = 0.2. However, particles tend to prematurely initiate the breakup in comparison with Newtonian liquids of the same constant shear viscosity. It is assumed that the reason for this are local variations in particle density. For the case of higher particle volume fractions unexpected phenomena like jamming and dilatancy are observed, which depend among other things on the stretching acceleration. In order to investigate the observed phenomena in more detail, knowledge of the rheological behavior of the liquid under strain rates that are comparable to those in the experiments is required.
For this purpose an algorithm is proposed that allows material points to be tracked inside the stretched liquid bridge, based on shadow graphic images. In contrast to commonly existing rheometers, which measure at one fixed point, typically the geometrical middle of the bridge, it is shown that more precise measurements are possible. This includes for example the determination of elongational viscosity based on asymmetrical ligament deformation due to high elongation rates or based on irregular formed jets due to local particle density variations.