77th Annual Meeting of The Society of Rheology edition:77 location:Vancouver, Cananda date:16-20 October 2005
This paper discusses specific features of breakup of capillary jets of dilute polymer solutions associated with transition from dripping to jetting. Experiments monitored with a high speed camera reveal a new scenario of transition and breakup via periodic growth and detachment of large terminal drops. In this regime, thin short jet terminates with large terminal drop, much greater than the drops usually observed in the course of `normal' jet breakup. The jet is apparently steady-state; the terminal drop experiences a regularly repeating evolution. It first grows while slowly moving upstream; then eventually it turns back, accelerates, becomes much thicker than the jet, and detaches. The phenomenon is specific for macromolecular solutions. Due to capillary instability the primary jet starts to develop the beads-on-string pattern characteristic for polymeric jets before meeting the terminal drop. Therefore, the process appears as `gobbling' of the chain of tiny beads by a greedy terminal drop. Quantitative characteristics of `gobbling' are determined using computer-aided prosessing of video images. The underlying mechanism is discussed and a basic theory of breakup is presented. The dynamics of the terminal drop growth and motion prove to be governed primarily by mass and momentum balances involving capillary, gravity and inertial forces, and the drop detachment event is controlled by kinetics of thinning of interconnecting ligaments that form between droplets. The elasto-capillary thinning process is driven by surface tension and resisted by viscoelasticity of dissolved polymer molecules. Quantitative characteristics of oscillatory regimes are derived from the above qualitative description using mass and momentum balances for the jet and the terminal drop and the theory of polymeric fluid filaments thinning under combined action of the capillary pressure and axial tensile force.