Paper Number
IN1 

Session
Flow Induced Instabilities and Non-Newtonian Fluids

Title
Characterizing the extensional rheology of weakly elastic fluids using capillary breakup technique: An experimental and numerical study

Presentation Date and Time
October 21, 2019 (Monday) 9:50

Track / Room
Track 4 / Room 305B

Authors (Click on name to view author profile)

  1. Du, Jianyi (Massachusetts Institute of Technology, Department of Mechanical Engineering)
  2. Ohtani, Hiroko (Ford Motor Company, Vehicle Manufacturing and Additive Manufacturing-Metals)
  3. Ellwood, Kevin (Ford Motor Company, Vehicle Manufacturing and Additive Manufacturing-Metals)
  4. McKinley, Gareth H. (Massachusetts Institute of Technology, Department of Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Jianyi Du1, Hiroko Ohtani2, Kevin Ellwood2, and Gareth H. McKinley1
1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA; 2Vehicle Manufacturing and Additive Manufacturing-Metals, Ford Motor Company, Dearborn, MI 48121

Speaker / Presenter 
Du, Jianyi

Text of Abstract
Many commercial fluid materials appear close to Newtonian in shear rheometry but can be subtly distinguished by the weakly viscoelastic properties exhibited in strong extensional flows. Typical examples include automotive lubricants and commercial paints. This weak elasticity can profoundly alter the final stages of filament breakup processes due to the large accumulated strains and strain rates, hence it is of practical importance to understanding industrial processes such as jetting, painting and fragmentation. Rheological characterization of such weakly elastic fluids can be achieved using capillary breakup extensional rheometry (CaBER). The experimental results obtained with two commercial motor oils are shown to be broadly consistent with the predictions of the Second Order Fluid model, and the normal stress differences arising from both the viscous (first order) and elastic (second order) terms are found to be comparable in magnitude. Because of the additional elastic stress, the slender self-similar profile expected for a viscous Newtonian fluid filament is no longer valid. In order to understand the evolution of the measured filament profile R(z,t), we solve numerically the axisymmetric one-dimensional Cauchy momentum equation for the Second Order Fluid model using a fully implicit formulation. Very close to pinch-off, the mid-plane radius is found to asymptotically thin quadratically with the time to breakup, and the resulting filament profiles become slenderer than for a Newtonian fluid of the same viscosity. The midplane curvature ratios in the radial and axial directions exhibit new power-law relationships in time, suggesting self-similar dynamics. Careful analysis of the simulation results shows that modification is required in the numerical coefficients that enter the self-similar solutions used for analyzing the thinning profile, if accurate values for the viscosity and other material coefficients are to be obtained for weakly elastic fluid filaments undergoing thinning and breakup.