Effect of viscoelastic relaxation on fluid and species transport in biopolymeric materials

Pawan Preet Singh, Purdue University

Abstract

Hybrid mixture theory was used to develop a two-scale and a three-scale thermomechanical theory for swelling biopolymeric media. At the microscale, the solid polymeric matrix interacted with the solvent through surface contact. The relaxation processes within the polymeric matrix were incorporated by modeling the solid phase as viscoelastic and the solvent phase as viscous. Novel equations for the total stress tensor, chemical potential of the solid phase, heat flux and the generalized Darcy's law were obtained at the meso and macro scales. Thermoviscoelastic and thermoviscous effects were incorporated by coupling the thermal gradients with strain-rate tensors for the solid phase and the deformation-rate tensors for the liquid phases. The developed three-scale Darcy's law was coupled with the mass balance equations of Bennethum and Cushman (1996) to obtain a fluid transport model. The model includes a novel integral term, which is related to the viscoelastic properties of the biopolymeric matrix. The model suggests that fluid transport is Fickian in the rubbery and glassy states when the biopolymers are sufficiently far from the glass transition region. In the proximity of glass transition the flow of fluids is anomalous or non-Fickian. These predictions are in agreement with the experimental observations of Kim et al. (1996). The model was solved using the finite element method for predicting moisture transport and viscoelastic stresses during sorption and drying of soybeans. Predicted drying curves were validated using experimental data of Misra and Young (1980) (average absolute difference 6–13%). At drying temperatures used in the soybean processing industry (70–93°C), smooth moisture profiles were obtained, which indicated Fickian transport. As the drying temperature approached the glass transition temperature (25°C at 10% moisture content), the moisture profiles became sharper, which indicated non-Fickian transport. The viscoelastic stress profiles clearly exhibited the difference in interaction of the force terms during sorption and drying. The increase in stress due to reduction in moisture content below 10% was not compensated by an increase in drying temperature. Drying of soybeans below 10% moisture content should be avoided in the industry because this will lead to thicker flakes that reduce the amount of oil recovery.

Degree

Ph.D.

Advisors

Cushman, Purdue University.

Subject Area

Agricultural engineering

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