Solutions of polymer molecules and suspended solid particles are used in a range of products from paints and coatings to health care and foods. The polymer molecules and the suspended particles act cooperatively to create a range of complex fluid behaviors that impact their processing. While much is known about how particles and polymers act independently to alter fluid responses, little is known about their coupled effects. This project will develop theories to predict 1) the stretching and alignment of polymer molecules by the disturbance to the flow caused by the particles, 2) how elongated solid particles or fibers align in the presence of polymers, and 3) the flow response of mixtures of particles and polymers. The influence of flow rate, flow history, and flow geometry on the behavior of polymer-particle mixtures will be examined. As part of this research, a graduate student and several undergraduates will receive training in fluid dynamics and theoretical and numerical methods. Both the principal investigator and the graduate student will participate in mentoring an undergraduate student from a group that is underrepresented in STEM fields.
The interplay of particle and polymer stresses will be explored in steady shear, oscillatory shear, and extensional rheology of suspensions of spheres and fibers in polymer solutions. The hydrodynamic disturbance of the particles can dramatically alter the polymer conformation in the surrounding fluid while the polymer stress can lead to unexpected changes in the particle stresslet. The rheology of fiber suspensions is strongly influenced by the orientations of the fibers. While previous theoretical studies have suggested that polymer stresses induce fiber alignment with the vorticity axis of a steady simple shear flow, experiments reveal a greater diversity of orientations even at low particle concentrations. A search will be made for stable fixed orientations of fibers in simple shear flow and the dependence of the orientations on the rheological properties of the fluid. The rheology of dilute particle-filled polymeric liquids will be determined using an ensemble averaged equation approach. To facilitate quasi-analytic results, perturbation analyses based on small polymer concentration will be performed. In contrast to previous studies that considered second-order fluids, this approach allows an exploration of the strain rate (or Deborah number) dependence of the rheology even while considering weak polymeric stresses. Fully nonlinear computations using a finite volume method in a body-fitted coordinate system will complement the theoretical studies of fiber orientation in simple shear flow. A variety of rheological models that include and exclude shear thinning and finite extensibility will be used to understand the effect that different polymer rheological properties have on particle-polymer interactions. To broaden the diversity of future graduate students in the field, the PI and graduate student on the project will provide on-going mentorship throughout the undergraduate career of a prospective female or URM student from another university as well as offering undergraduate research projects on complex fluids.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
