Bottom-up model for understanding the effects of wheat endosperm microstructure on its mechanical strength

Investigation of food microstructure and texture using atomic force microscopy: A review

We review recent applications of atomic force microscopy (AFM) to characterize microstructural and textural properties of food materials. Based on interaction between probe and sample, AFM can image in three dimensions with nanoscale resolution especially in the vertical orientation. When the scanning probe is used as an indenter, mechanical features such as stiffness and elasticity can be analyzed. The linkage between structure and texture can thus be elucidated, providing the basis for many further future applications of AFM. Microstructure of simple systems such as polysaccharides, proteins, or lipids separately, as characterized by AFM, is discussed. Interaction of component mixtures gives rise to novel properties in complex food systems due to development of structure. AFM has been used to explore the morphological characteristics of such complexes and to investigate the effect of such characteristics on properties. Based on insights from such investigations, development of food products and manufacturing can be facilitated. Mechanical analysis is often carried out to evaluate the suitability of natural or artificial materials in food formulations. The textural properties of cellular tissues, food colloids, and biodegradable films can all be explored at nanometer scale, leading to the potential to connect texture to this fine structural level. More profound understanding of natural food materials will enable new classes of fabricated food products to be developed.

Tensile strength of granular aggregates: Stress chains across particle phase versus stress concentration by pores

We use the bond-based peridynamics approach to analyze the strength and fracture of dense granular aggregates with variable amount of a solid binding matrix, distributed according to a simple protocol in the interstitial space between particles. We show the versatility of the peridynamics approach in application to crack propagation and its scaling behavior in a homogeneous medium (in the absence of particles and pores). Then we apply this method to simulate the deformation and failure of aggregates as a function of the amount of the binding matrix under tensile loading. We find that the tensile strength is a strongly nonlinear function of the matrix volume fraction. It first increases slowly and levels off as the gap space in-between touching particles is gradually filled by the binding matrix, up to nearly 90% of the total pore volume, and then a rapid increase occurs to the maximum strength as the remaining interstitial space, composed of isolated pores between four or more particles, is filled. By analyzing the probability density functions of stresses in the particle and matrix phases, we show that the adhesion of the matrix to the particles and the thickening of stress chains (i.e., stresses distributed over larger cross sections) control the strength in the first case whereas the homogenizing effect of the matrix by filling the pores (hence reducing stress concentration) is at the origin of further increase of the strength in the second case. Interestingly, these two mechanisms contribute almost equally to the total strength.

Stress transmission in cemented bidisperse granular materials

We analyze stress distributions in a two-dimensional bidisperse cemented granular packing for a broad range of the values of particle-size ratio, the volumes of large and small particles, and the amount of cementing matrix. In such textured porous materials, the stress concentration, which controls the fracture and fragmentation of the material under tensile loading or in grinding processes, reflects not only the porosity but also the contact network of the particle phase and the resulting stress chains. By means of peridynamic simulations under tensile loading, we show how both the texture and stress distribution depend on size ratio, volume ratio, and the amount of the cementing matrix. In particular, the volume fraction of the class of small particles plays a key role in homogenizing stresses across the system by reducing porosity. Interestingly, the texture controls not only the porosity but also the distribution of pores inside the system with its statistical variability, found to be strongly correlated with the homogeneity of stresses inside the large particles. The most homogeneous stress distribution occurs for the largest size ratio and largest volume fraction of small particles, corresponding to the lowest pore size dispersion and the cushioning effect of small particles and its similar role to the binding matrix for stress redistribution across the packing. At higher porosity, the tensile stresses above the mean stress fall off exponentially in all phases with an exponent that strongly depends on the texture. The exponential part broadens with decreasing matrix volume fraction and particle-size ratio. These correlations reveal the strong interplay between size polydispersity and the cohesive action of the binding matrix for stress distribution, which is significant for the behavior of textured materials in grinding operations.