SoftComp Scientific Meeting, Softcomp Workshop: Micro (or nano-)scopic engineering of rheological properties location:Leuven, Belgium date:31 January - 1 February 2007
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 periodic and quasiperiodic drop dynamics that arise in dripping and jetting of polymeric solutions expelled from nozzles of different diameters..
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. An axial momentum balance incorporating capillarity, gravity and fluid elasticity captures many of the essential features of this “gobbling” phenomenon.