Anna Park
Supervisor: Gareth McKinley
Fluid Mechanics Lab
Non-Newtonian Fluid Dynamics Research Group

Extensional Viscosity of Complex Fluids and the Effects of Pre-Shear

        It is important to understand the fluid mechanics of complex fluids since they are processed and used in many aspects of everyday life.  Foods such as yogurt, ketchup, and bread dough are examples of such fluids.  Paint and other suspensions are also complex fluids.  In order to optimize material properties, the fluids are studied and characterized.  Extensional viscosity is an important parameter since fluids get extended when they are spread, mixed, or extruded.  The extensional viscosity of a fluid can affect the process modeling and design.

       The main experiments are conducted using a Capillary Breakup Extensional Rheometer (CaBER).  The CaBER has three major components: a laser, two cylinders, and a solenoid.  The two cylinders are held vertically one on top of the other with a small gap in between them.  The fluid to be tested is injected in the gap.  Using the solenoid, a step strain is applied to the sample.  The laser aligned to the middle of the stretched fluid measures the midpoint radius of the filament as it thins.  The combination of viscous, elastic, and capillary forces affect the way in which the fluid drains towards the two cylinders.  A theoretical fit is performed on the data to determine the midpoint radius as a function of time.  The equation governing the evolution takes into account the viscous stress as the fluid drains, tensile force from the stretching, elastic stress of the material, and the capillary pressure due to surface tension.  Using the result, extensional viscosity and various other material parameters such as elastic modulus and relaxation time are computed and analyzed.  The sagging of the fluid due to gravity is evaluated and examined using the Bond number.

      Yogurt, paint, nanoparticles suspended in polystyrene, and several other fluids are tested using the CaBER.  The difference in fluid property of regular and non-fat yogurt is evaluated to understand the effect of milk fat suspended in yogurt.  Mainly, the extensional viscosity and its dependence on strain are computed for each type of yogurt and the results are compared.  Repeated tests are performed on paint samples to obtain the change in extensional viscosity of paint as it dries.  Experiments are conducted on nanoparticles (specifically clay) in polystyrene to understand how the small suspensions change the fluid properties of the mixture.  Various samples of different particle concentrations are tested.

       A modification is then made to the CaBER so that fluid samples can be pre-sheared by specified amounts.  A motor with an optical encoder is mounted under the base plate and attached to the bottom cylinder.  The motor is incorporated into the CaBER software so that it can be controlled using a computer.  Using this new mount, fluids are sheared differing amounts before being stretched.  The effect of adding shear can be studied by comparing the data from these experiments to those from the previous experiments.  Changes in extensional viscosity are expected since many complex fluids show changes in material properties after undergoing strain.