Wondwosen Abebe Aregawi

Modeling of Coupled Water Transport and Large Deformation During Dehydration of Apple Tissue

Water loss of fruit during storage has a large impact on fruit quality and shelf life and is essential to fruit drying. Dehydration of fruit tissues is often accompanied by large deformations. One-dimensional water transport and large deformation of cylindrical samples of apple tissue during dehydration were modeled by coupled mass transfer and mechanics and validated by calibrated X-ray CT measurements. Uni-axial compression–relaxation tests were carried out to determine the nonlinear viscoelastic properties of apple tissue. The Mooney–Rivlin and Yeoh hyperelastic potentials with three parameters were effective to reproduce the nonlinear behavior during the loading region. Maxwell model was successful to quantify the viscoelastic behavior of the tissue during stress relaxation. The nonlinear models were superior to linear elastic and viscoelastic models to predict deformation and water loss. The sensitivity of different model parameters using the nonlinear viscoelastic model using Yeoh hyperelastic potentials was studied. The model predictions proved to be more sensitive to water transport parameters than to the mechanical parameters. The large effect of relative humidity and temperature on the deformation of apple tissue was confirmed by this study. The validated model can be employed to better understand postharvest storage and drying processes of apple fruit and thus improve product quality in the cold chain.

Novel Application of Neutron Radiography to Forced Convective Drying of Fruit Tissue

Neutron imaging is a promising technique to study drying processes in food engineering as it is a non-intrusive, non-destructive technique, which provides quasi-real-time quantitative information of the water loss during drying and of the internal water distribution, at a high spatial and dynamic resolution. Particularly, the high sensitivity to water is its main advantage for drying studies, despite the limited accessibility to reactor facilities, which produce neutrons. This technique was used to investigate forced convective drying of fruit tissue (pear and apple), placed in a small wind tunnel. Water loss, water distribution in the sample and sample shrinkage were evaluated as a function of time. The water loss, determined quantitatively from neutron radiographs, was underestimated slightly compared to gravimetrical measurements. The overall drying behaviour agreed well with control measurements performed in a climatic chamber and was very similar for both fruit tissues. The corresponding shrinkage behaviour of both tissues was also similar. The large shrinkage, which is characteristic for soft biological materials such as food products, however, hindered post-processing to some extent. From the internal water distribution, the water gradients within the sample, induced by drying, were visualised and were found to predominantly occur at the air–material interface, indicating that the water transport inside the tissue dominated the water loss, instead of the convective exchange with the air flow. Neutron imaging was shown to exhibit unique benefits for studying drying processes of food.

Quantitative neutron imaging of water distribution, venation network and sap flow in leaves

AbstractMain conclusionQuantitative neutron imaging is a promising technique to investigate leaf water flow and transpiration in real time and has perspectives towards studies of plant response to environmental conditions and plant water stress. The leaf hydraulic architecture is a key determinant of plant sap transport and plant–atmosphere exchange processes. Non-destructive imaging with neutrons shows large potential for unveiling the complex internal features of the venation network and the transport therein. However, it was only used for two-dimensional imaging without addressing flow dynamics and was still unsuccessful in accurate quantification of the amount of water. Quantitative neutron imaging was used to investigate, for the first time, the water distribution in veins and lamina, the three-dimensional venation architecture and sap flow dynamics in leaves. The latter was visualised using D2O as a contrast liquid. A high dynamic resolution was obtained by using cold neutrons and imaging relied on radiography (2D) as well as tomography (3D). The principle of the technique was shown for detached leaves, but can be applied to in vivo leaves as well. The venation network architecture and the water distribution in the veins and lamina unveiled clear differences between plant species. The leaf water content could be successfully quantified, though still included the contribution of the leaf dry matter. The flow measurements exposed the hierarchical structure of the water transport pathways, and an accurate quantification of the absolute amount of water uptake in the leaf was possible. Particular advantages of neutron imaging, as compared to X-ray imaging, were identified. Quantitative neutron imaging is a promising technique to investigate leaf water flow and transpiration in real time and has perspectives towards studies of plant response to environmental conditions and plant water stress.

1/(2 + Rc): A simple predictive formula for laboratory shrinkage-stress measurement

OBJECTIVE This paper presents and verifies a simple predictive formula for laboratory shrinkage-stress measurement in dental composites that can account for the combined effect of material properties, sample geometry and instrument compliance. METHODS A mathematical model for laboratory shrinkage-stress measurement that includes the composites elastic modulus, shrinkage strain, and their interaction with the samples dimensions and the instruments compliance has previously been developed. The model contains a dimensionless parameter, Rc, which represents the compliance of the instrument relative to that of the cured composite sample. A simplified formula, 1/(2+Rc), is proposed here for the normalized shrinkage stress to approximate the original model. The accuracy of the simplified formula is examined by comparing its shrinkage-stress predictions with those given by the exact formula for different cases. These include shrinkage stress measured using instruments with different compliances, samples with different thicknesses and composites with different filler fractions. RESULTS The simplified formula produces shrinkage-stress predictions that are very similar to those given by the full formula. In addition, it correctly predicts the decrease in shrinkage stress with an increasing configuration factor for compliant instruments. It also correctly predicts the value of the so-called flow factor of composites despite the fact that creep is not considered in the model. SIGNIFICANCE The new simple formula significantly simplifies the prediction of shrinkage stress for disc specimens used in laboratory experiments without much loss in precision. Its explicit analytical form shows clearly all the important parameters that control the level of shrinkage stress in such measurements. It also helps to resolve much of the confusion caused by the seemingly contradictory results reported in the literature. Further, the formula can be used as a guide for the design of dental composite materials or restorations to minimize their shrinkage stress.

Multiscale Modeling of Food Processes

Food unit operations involve complex transport phenomena. As experimental characterization of transport processes is not usually straightforward, mathematical models are often used to improve our understanding of them, and, more importantly, to design and optimize food processes. Contrary to typical engineering materials such as steel or brick, food tissues are intrinsically multiscale assemblies with different characteristics at each spatial scale. As a consequence, multiscale modeling is required. This article gives a systematic introduction to multiscale modeling in food processes, including imaging techniques for food microstructure, model formulation, and numerical solution techniques. Several applications in food process engineering will be presented. The article concludes with a discussion on future prospects for the use of multiscale modeling.

Can pulpal floor debonding be detected from occlusal surface displacement in composite restorations

OBJECTIVES Polymerization shrinkage of resin composite restorations can cause debonding at the tooth-restoration interface. Theory based on the mechanics of materials predicts that debonding at the pulpal floor would half the shrinkage displacement at the occlusal surface. The aim of this study is to test this theory and to examine the possibility of detecting subsurface resin composite restoration debonding by measuring the superficial shrinkage displacements. METHODS A commercial dental resin composite with linear shrinkage strain of 0.8% was used to restore 2 groups of 5 model Class-II cavities (8-mm long, 4-mm wide and 4-mm deep) in aluminum blocks (8-mm thick, 10-mm wide and 14-mm tall). Group I had the restorations bonded to all cavity surfaces, while Group II had the restorations not bonded to the cavity floor to simulate debonding. One of the proximal surfaces of each specimen was sprayed with fine carbon powder to allow surface displacement measurement by Digital Image Correlation. Images of the speckled surface were taken before and after cure for displacement calculation. The experiment was simulated using finite element analysis (FEA) for comparison. RESULTS Group I showed a maximum occlusal displacement of 34.7±6.7μm and a center of contraction (COC) near the pulpal floor. Group II had a COC coinciding with the geometric center and showed a maximum occlusal displacement of 17.4±3.8μm. The difference between the two groups was statistically significant (p-value=0.0007). Similar results were obtained by FEA. The theoretical shrinkage displacement was 44.6 and 22.3μm for Group I and II, respectively. The lower experimental displacements were probably caused by slumping of the resin composite before cure and deformation of the adhesive layer. SIGNIFICANCE The results confirmed that the occlusal shrinkage displacement of a resin composite restoration was reduced significantly by pulpal floor debonding. Recent in vitro studies seem to indicate that this reduction in shrinkage displacement could be detected by using the most accurate intraoral scanners currently available. Thus, subject to clinical validation, the occlusal displacement of a resin composite restoration may be used to assess its interfacial integrity.

The two sides of the C-factor

OBJECTIVE The aim of this paper is to investigate the effects on shrinkage strain/stress development of the lateral constraints at the bonded surfaces of resin composite specimens used in laboratory measurement. METHODS Using three-dimensional (3D) Hookes law, a recently developed shrinkage stress theory is extended to 3D to include the additional out-of-plane strain/stress induced by the lateral constraints at the bonded surfaces through the Poissons ratio effect. The model contains a parameter that defines the relative thickness of the boundary layers, adjacent to the bonded surfaces, that are under such multiaxial stresses. The resulting differential equation is solved for the shrinkage stress under different boundary conditions. The accuracy of the model is assessed by comparing the numerical solutions with a wide range of experimental data, which include those from both shrinkage strain and shrinkage stress measurements. RESULTS There is good agreement between theory and experiments. The model correctly predicts the different instrument-dependent effects that a specimens configuration factor (C-factor) has on shrinkage stress. That is, for noncompliant stress-measuring instruments, shrinkage stress increases with the C-factor of the cylindrical specimen; while the opposite is true for compliant instruments. The model also provides a correction factor, which is a function of the C-factor, Poissons ratio and boundary layer thickness of the specimen, for shrinkage strain measured using the bonded-disc method. For the resin composite examined, the boundary layers have a combined thickness that is ∼11.5% of the specimens diameter. SIGNIFICANCE The theory provides a physical and mechanical basis for the C-factor using principles of engineering mechanics. The correction factor it provides allows the linear shrinkage strain of a resin composite to be obtained more accurately from the bonded-disc method.