4th Annual European Rheology Conference (AERC 2007) edition:4 location:Naples date:12-14 April 2007
The ability to visualize and quantify flow 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.
This paper discusses specific features of the breakup of capillary jets of dilute polymer solutions and the dynamics associated with the transition from dripping to jetting in a complex fluid. High speed digital video imaging reveals a new scenario of transition and breakup via periodic growth and detachment of large terminal drops. The underlying mechanism is discussed and a basic theory for the mechanism of breakup is also presented. The dynamics of the terminal drop growth and trajectory prove to be governed primarily by mass and momentum balances involving capillary, gravity and inertial forces, whilst the drop detachment event is controlled by the kinetics of thinning of the interconnecting viscoelastic ligaments that form between droplets. This elasto-capillary thinning process is driven by surface tension and resisted by the viscoelasticity of dissolved polymer molecules. Rheological characteristics of the polymer solutions inferred from these experiments are found in good agreement with conventional measurements, which provide a benchmark for the novel video-rheological techniques.