Return Home

Research

The Axisymmetric Abrupt Contraction-Expansion

The image to the left is a pseudo streak imageof the flow of a highly elastic solution through an axisymmetric 4:1:4 contraction-expansion. The fluid is a 0.025wt% high molecular weight monodispersepolystyrene dissolved in an oligomeric polystyrene. To view a movie showingthe growth of the enhanced upstream vortex structure and the onset of anelastic instability click on the avi file below.

4_1_4.avi (7.1MB)
Notes to Aid Numerical Simulations 
We have generated a series of notes entitled ViscoelasticFlows in Abrubt Contraction-Expansions which should provide the necessary information for those interested in numerically simulating this complexflow.  These notes include a complete description of the shear andtransient extensional rheology of our test fluid (Fluid_Rheology.pdf), a discussion of how we chose the proper relaxation time (Relaxation_Time.pdf), the specifications for our test geometry (Geometry.pdf) and the definition of the dimensionless pressure drop we report (Pressure_Drop.pdf). Please also see our article entitled Extensional Flow of a PolystyreneBoger Fluid Through a 4:1:4 Axisymmetric Contraction/Expansion recently published in the Journal of Non-Newtonian Fluid Mechanics (GHM38.pdf).

Stretching and Breakup of Polymeric Liquids in a Microfilament Rheometer

A microfilament rheometer (MFR) that can be used to readily differentiate between the response of different fluid formulations.The device relies on a detailed observation of the rate of extensionalthinning of a Newtonian or a viscoelastic fluid filament and provides adirect measurement of the ultimate time to break-up of the fluid filament.  Measurements are performed in a controlled temperature and environmentalconditions. We consider four different classes of entangled polymer liquids that are of importance commercially including (i) pressure sensitive adhesives,(ii) branched and linear polymer melts, (iii) concentrated polymer solutionsand (iv) aqueous solutions of associating polymers such as HEUR (Hydrophobically modified urethane-ethoxylate). We observed that the breakup dynamics in these liquids depend on the extensional viscosity, on molecular parameterssuch as the chain-length, entanglement density or degree of chain branching,and on external factors such as solvent volatility. Varying these factors changes the dominant time scales in the extensional flow.

For example, pressure sensitive adhesives are elastically stabilized against breakupand are 'tacky' if the solvent evaporation rate l-1evap= h/R0 (where h is the mass transfer coefficient for the solvent and R0 is the initial radiusof the filament) is significantly larger than the stress relaxation ratel-1whereas they are 'non-tacky' and undergo capillary breakup if the evaporation rate is much slower than the capillary necking rate l-1neck= s /h 0R0)( where s is the surface tension and h0is the zero shear viscosity). In the case of associative polymers, the breakup mechanism also depends on the concentration of micelles and theionic surfactant strength.

Elasto-Capillary Thining in a Microfilament Rheometer,Test Fluid: Glycerin
entov_glycerin.avi(1.6 MB)

Elasto-Capillary Thining in a Microfilament Rheometer, Test Fluid: HEUR (Associative Polymer)
entov_heur.avi(1.6 MB)

Elasto-Capillary Thining in a MicrofilamentRheometer, Test Fluid: Pressure Sensitive Adhesive (PSA)
entov_psa.avi(2.3 MB)

The Collapse of Highly Viscous Bubbles In this video a high molecular weight (high viscosity) Poly(dimethyl siloxane) polymer has a gas bubble insterted into it.  The free bubble at the surface is then rupturedfrom above using a pin.  Due to the high viscosity of the silicone oil film drainage is not possible and hence the bubble "skin" must collapse under gravity.  The stresses introduced result in the clearly visible buckling of the skin.  Note the bubble is viewed from above. bubble.avi (1.4 MB)

Purely Elastic Instability Between Two Rotating Coaxial Parallel Plates
(text to come)
pplate.avi (2.2 MB)

Purely Elastic Instability Between a Rotating Cone-and-Plate
(text to come)
coneplate.avi (4.3 MB)
Flow Past a Cylinder
(text to come)
cylinder.avi (4.3 MB)