(283i) Investigating Droplet-Breakup Dynamics for Characterizing Low-Viscosity Elasticity of Dilute Polymer Solutions

The addition of macromolecules to solvent, even in dilute quantities, can significantly alter a fluidâ??s response in an extensional, or elongational, flow. When high molecular weight polymers are extended in solution, extensional thickening can result, yielding viscosities that are orders of magnitude larger than values found in shear. Examples of applications where weakly viscoelastic fluids are used include inkjet printing, turbulent drag flow reduction, and the atomization of fertilizers and pesticides. Thus, understanding the response of such fluids, particularly in extension, is important for making relevant processing and design decisions. The present research was conducted in an effort to make improvements to and to provide an in-depth assessment of the techniques available for characterizing low-viscosity elasticity. Outcomes of this work include the creation of a new device for extensional characterization and the completion of a study comparing a number of previously described extensional techniques (e.g., ODES-DOS) in an effort to investigate discrepancies and identify limitations in device usage.

Similar to passive breakup of a droplet at a T-junction, a microfluidic device is presented that uses a cross-slot geometry to observe a two-phase droplet breakup process. For viscoelastic fluids, we report that a cylindrical filament forms between droplet segments, which decays exponentially in time. In optically tracking this decay, both transient extensional viscosity and relaxation times were evaluated. For validating and optimizing the device, a range of polyethylene oxide (PEO) and Newtonian solutions were tested. Comparisons of the evolution profiles due to the presence of elasticity were made. The resulting Trouton ratios versus Hencky strains were recorded and compared with the results from other emerging extensional techniques. Experiments were performed to observe the growth of instability, the dynamics of filament thinning, and the presence of any bead formation in the droplet breakup event. These results reveal a complex interplay between viscous, elastic, capillary, and inertial contributions in the extensional devices used.