Interfacial dynamics in processing of materials with normal stress differences

Huntington, Benjamin Ashley
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Processing of elastic non-Newtonian fluids is of critical importance to many industrial manufacturing processes. Three different processes are analyzed in this work: co-extrusion of polymer melts, inclined plane flow of soft particle pastes, and roll-to-roll processing of soft particle pastes. These three processes are examined using stability theory and finite element simulation as tools and, when possible, experimental results obtained by collaborators are used to verify and test findings. The polymer co-extrusion process analyzed in this work is an experimental device at Case Western Reserve University (CWRU) that creates many layered polymer films with individual layer thicknesses on the order of microns to 100s of nanometers. Due to forces acting between the layers during the co-extrusion process, the layered structure can be damaged or destroyed. Two key components of this process are analyzed using the finite element method: the feedblock for the co-extruder and the layer multiplier dies. The study of the feedblock identifies two critical improvements for the process that help mitigate the destruction of the layered structure. The finite element analysis of the multiplier dies identify a way to reduce the high pressure drop through the multiplier die, and a design that helps preserve the layered structure. These results are confirmed experimentally by collaborators at CWRU. In the second part of this work, flow of a soft particle paste down an inclined plane is analyzed using a linear stability theory. This problem is tackled a preliminary study to the roll-to-roll processing of the same material. Stability of inclined plane flow has been studied in the literature for a variety of different materials. The destabilizing second normal stress differences exhibited by the soft particle paste are found to compete with the stabilizing force of surface tension. Stable and unstable wavenumber ranges are determined for this problem, as well as the fastest growing mode. This is then used to compute the expected wave lengths seen for varying yield stress. Lastly, the stability of flow of a soft particle paste in a forward roll coating process is analyzed. Forward roll coating of soft particle pastes is a common industrial process, particularly in the area of paint application. The analysis examines the impact of material properties on the so –called ribbing instability that is known to occur in many roll-to-roll processes. A method for analyzing the stability of Newtonian fluids in forward roll coating is expanded to power law fluids. The results show that stability strongly depends on the capillary number and the power law index.