ID 48

The formation of drops and sprays from complex liquids

 

Abstract:

The formation of drops and sprays by breakup of coherent liquid systems is influenced by deformation-induced stresses in the liquid. These stresses, depending on the rate of deformation, are given by the rheological constitutive equation of the liquid. Turning from Newtonian to Non-Newtonian liquid behavior, i.e. to complex liquids, jet or sheet breakup by Plateau-Rayleigh or Kelvin-Helmholtz instability may change dramatically, with impact on the size spectra of the drops formed. Experiments show that even small contents of a macromolecular polymeric substance in a liquid may change strongly the elongational viscosity of the liquid, while the shear viscosity may be left widely unaffected. Consequently, the influence of the elongational material behavior is predominantly seen in ligament-mediated sprays. The present talk discusses the material characterization relevant for complex liquid jet and sheet breakup. Linear jet and sheet stability analysis predicts the growth rate of unstable disturbances for linear viscoelastic liquid materials to be enhanced as compared to a corresponding Newtonian liquid with the same Ohnesorge number. The conflict with the experimental observation that macromolecular solutes in a liquid rather stabilize jets and sheets than destabilizing them is due to nonlinear macromolecular deformation. Experiments show that drop size spectra formed by breakup of jets of poly(ethylene-oxide) solutions in water-glycerol mixtures may be close to those predicted for Newtonian liquids. Mean drop sizes measured in complex liquid sprays from pre-filming atomizers are well predicted by functions of the liquid flow Weber number and an elongational Reynolds number, interpreted as a Deborah number.