- The first involves a rather surprising result from a very simple set of experiments. For bubble of gas rising through a non-Newtonian fluid, a plot of terminal velocity vs. bubble size shows a large discontinuity:
- The next problem is much more of a modelling issue in my mind than an intellectual curiosity. The picture below shows the behavior of a polyacrylamide solution flowing downward and leaving through a small opening.
One small bone to pick in this section of the paper was the introductory sentence:"Contraction flows occur in many polymer manufacturing processes, as molten polymer is extruded to form (ideally) a smooth, uniform thread." (Emphasis added). I would say that contraction flows occur in ALL or MOST polymer manufacturing processes. Any process that initially requires melting the polymer and then shaping it, such as extrusion (sheet, film, fiber...), or molding (blow, injection,...) will have contraction flows and the associated extensional components. Rotomolding (filling a hollow mold with polymer pellets, then tumbling the mold so that the molten polymer uniformly coats the walls of the mold) would be the only process that is on the edge of this definition. You still are melting the polymer in order to shape it, but the shaping is done only by gravity, not by any externally supplied pressure, a very unique situation for polymer processing.
- Lastly, there is the asymmetric flow patterns that occur in cross-slot flows. In the diagram below, the fluid is pumped into the die in opposing directions (from the left and right in this case) and exit the die through the top and the bottom.
This is not strictly an academic problem as cross-slot flows are used for extensional flow viscometry. Being able to better understand the origin of the breakdown in the flow field could lead to more robust instrumentation.
Wednesday, March 14, 2012
Open Problems in Non-Newtonian Fluids - The Details
As promised yesterday, here's a more detailed look at some open problems in non-Newtonian fluids.