Proceedings of the XIVth International Congress on Rheology pages:RE33-1-3
XIVth International Congress on Rheology edition:XIV location:Seoul, Korea date:22-27 August 2004
The ability to visualize and quantify fugacious phenomena using high-speed digital video-imaging has increased tremendously in recent years due to increasing digital data-transfer, -storage and processing possibilities and decreasing costs for storage media. Unsteady jet and film flows of low-viscosity complex liquids such as polymeric, surfactant and micellar solutions are examples of flows that can now easily be visualized and analyzed. The slender geometry of pinching threads and films allows one to compare experimental observations with theoretical predictions, and also to analyze the dynamical processes in order to extract characteristic rheological properties of the complex fluids. In this paper we will demonstrate the capabilities of video-rheology for analyzing uniaxial elongation in:
- elastocapillary filament break-up and propagation of recoil waves in threads of wormlike micellar solutions and
- Periodic and quasiperiodic drop dynamics that arise in dripping and jetting of micellar and polymeric solutions expelled from nozzles of different diameters..
In the latter example, analysis of the jet dynamics in the flow transition region between dripping and jetting reveals the existence of quasiperiodic oscillations in the number and size of the drops formed. This is in sharp contrast to the regular periodic pinching behavior that is observed in simple Newtonian fluids. The ultimate pendant drop remains attached to the primary fluid jet in a ‘beads-on-a-string’ configuration and ‘feeds’ on the growing secondary droplets until it exceeds a critical size, at which point it detaches and the process repeats. A simple axial momentum balance incorporating capillarity, gravity and fluid elasticity captures many of the essential features of this “gobbling” phenomenon.