(353d) Effects of Geometry and Fluid Elasticity during Polymeric Droplet Pinch-off in Microfluidic Environments
We investigate the effects of fluid elasticity and channel geometry on polymeric droplet pinch-off by performing systematic experiments using viscoelastic polymer solutions which possess practically shear rate-independent viscosity (Boger fluids). Three different molecular weight poly(ethylene oxide) solutions and a Newtonian fluid with comparable viscosities are used to investigate the role of elasticity on pinch-off dynamics. Four different geometric sizes (width and depth are scaled up proportionally at the ratio of 0.5, 1, 2, 20) are used to study the effect of the length scale, which in turn influences the ratio of elastic to viscous forces as well as the Rayleigh time scale associated with the interfacial instability of a cylindrical column of liquid. We observe a power law relationship between the dimensionless (scaled with respect to the Rayleigh time scale) capillary pinch-off time, T, and the elasticity number, E, defined as the ratio of the fluid relaxation time to the time scale of viscous diffusion. In general, T increases dramatically with increasing E and for E = 10, T is greater than 100. Furthermore, coil-to-stretch transition effects can be inferred as the Weissenberg number, defined as the ratio of the fluid relaxation time to the inverse of the characteristic extension rate, approaches 0.5. The inhibition of ?bead-on-a-string? formation is observed for flows with effective Deborah number, De, defined as the ratio of the fluid relaxation time to the Rayleigh time scale becomes greater than 10. It is shown that models that neglect the enhanced extensional viscosity of the polymer solutions become increasingly more inaccurate as the De increases. Results indicate that for sufficiently large values of De, the Rayleigh instability may be modified substantially by fluid elasticity.