Rheology

Rheology, as it is defined today, is a study of the deformation and flow of matter. A classic Newtonian fluid is described by Newton’s law, which relates the stress to the strain rate via a constant of proportionality, called viscosity. For Newtonian fluids, the viscosity is constant. Whenever the viscosity becomes a function of strain rate, we have what researchers refer to as a non-Newtonian fluid. Typical examples of non-Newtonian fluids are: polymer solutions, thermo\-plastics, drilling fluids, paints, fresh concrete, and biological fluids, to name a few. One of the most striking properties of non-Newtonian fluids is that they exhibit normal-stress differences. Non-Newtonian fluids display many effects that are not seen in Newtonian fluids, such as shear-thickening, shear-thinning, thixotropy, rheoplexy, extrudate swell, and normal-stress differences, among many others (Paul et al. 2021).

Characterization of normal stress differences for viscoelastic solids

In standard rheometry, these normal-stress differences are measured using a parallel plate rheometer in conjunction with a cone and plate rheometer. When used together, these rheometers allow for the accurate measurement of normal-stress differences. The cone and plate setup for a rheometer is especially advantageous, because it produces a constant state of stress at the cone surface.  The need to use both rheometers becomes problematic whenever one has a soft material that is more of a viscoelastic solid as opposed to a viscoelastic liquid.  This prohibits the use of a cone and plate setup, and thus, accurate measurement of the normal-stress differences is not feasible. In this paper, we introduce an alternative method for the calculation of both normal-stress differences by adopting a Gram-Schmidt or QR decomposition of the deformation gradient (Paul et al. 2021).

Thixotropic behavior of parafin/wax suspensions.

Three-dimensional printing (3DP) of functional materials is increasingly important for advanced applications requiring objects with complex or custom geometries or prints with gradients or zones with different properties. A common 3DP technique is direct ink writing (DIW), in which printable inks are comprised of a fluid matrix filled with solid particles, the latter of which can serve a dual purpose of rheology modifiers to enable extrusion and functional fillers for performance-related properties. Although the relationship between filler loading and viscosity has been described for many polymeric systems, a thorough description of the rheological properties of three-dimensional (3D) printable composites is needed to expedite the creation of new materials.

Energy dissipation in phase change salogels

Phase change salogels are physical networks designed to shape stabilize salt hydrate phase change materials (PCMs) and inhibit the leakage of molten PCM. In contrast to chemically crosslinked gels, salogels can restructure by breaking of prior and reforming of new bonds. Therefore, salogels are susceptible to dissipate energy during deformations due to the nature of physical bonds. Different concentrations of diethylenetriamine, as a physical crosslinker, were added to a constant concentration of polyvinyl alcohol (PVA) in a high-salinity environment of a fluid inorganic PCM (lithium nitrate trihydrate), and network viscoelasticity was studied using oscillatory rheology. The energy dissipated was calculated by computing the area inside Lissajous stress-strain curves performed at various strain amplitudes and a constant frequency of 1 rad/s.

Relevant Publications

1. Cipriani, Ciera E., Yalan Shu, Emily B. Pentzer, and Chandler C. Benjamin. “Viscoelastic and thixotropic characterization of paraffin/photopolymer composites for extrusion-based printing.” Physics of Fluids 34, no. 9 (2022): 093106.
2. Karimineghlani, Parvin, Abdelrahman A. Youssef, and Chandler C. Benjamin. “Energy dissipation in phase change salogels under shear stress.” Polymer (2022): 124977.
3. Cipriani, Ciera E., Taekwang Ha, Oliver B. Martinez Defilló, Manoj Myneni, Yifei Wang, Chandler C. Benjamin, Jyhwen Wang, Emily B. Pentzer, and Peiran Wei. “Structure–processing–property relationships of 3D printed porous polymeric materials.” ACS Materials Au 1, no. 1 (2021): 69-80.
4. Paul, Sandipan, Alan D. Freed, and Chandler C. Benjamin. “Application of the Gram–Schmidt factorization of the deformation gradient to a cone and plate rheometer.” Physics of Fluids 33.1 (2021): 017113.

Publications by Collaborators

1. Paul, Sandipan, and Alan D. Freed. “A simple and practical representation of compatibility condition derived using a QR decomposition of the deformation gradient.” Acta Mechanica 231 (2020): 3289-3304.
2. Paul, Sandipan, and Alan D. Freed. “Characterizing geometrically necessary dislocations using an elastic–plastic decomposition of Laplace stretch.” Zeitschrift für angewandte Mathematik und Physik 71.6 (2020): 1-22.