Exploring the significance of structural hierarchy in material systems—A review

Abstract

Structural hierarchy and heterogeneity are inherent features in biological materials, but their significance in affecting the system behaviors is yet to be fully understood. In Section 1, this article first identifies the major characteristics that manifest, or are resulted from, such hierarchy and heterogeneity in materials. Then in Section 2, it presents several typical natural material systems including wood, bone and others from animals to illustrate the proposed views. The paper also discusses a man-made smart material, textiles, to demonstrate that textiles are hierarchal, multifunctional, highly complex and arguably the engineered material closest on a par with biological materials in complexity, and more importantly we can still learn quite a few new things from them in development of novel materials.In Section 3, the paper summarizes several general approaches in developing a hierarchal material system at various scales, including structure thinning and splitting, laminating and layering, spatial and angular orientation, heterogenization and hybridization, and analyzes the advantages associated with them. It also stresses the adverse consequences once the existing structural hierarchy breaks down due to various mutations in biological systems. It discusses in particular the influences of moisture and air on material properties, given the near ubiquitousness of both air and water in materials.It next deals with in Section 4 some theoretical issues in material research including packing and ordering, the bi-modular mechanics, the behavior non-affinities due to disparity in hierarchal levels, the importance of system dimensionality in a hierarchal material system, and more philosophically, the issues of Nature's wisdom versus Intelligent Design. Section 5 then offers some concluding remarks, including a recap of the major issues covered in this article, and some general conclusions derived from the analyses and discussions. The main purpose of this paper is to make an effort to explore, identify, derive or theorize some generic principles based on the existing results, not to offer another comprehensive review of current research activities in the fields for that there already exist some excellent ones. This paper examines the related topics with several approaches to not only reveal the underlying geometrical and physical mechanisms, but also to emphasize the ways in which such mechanisms may be applied to developing engineered material systems with novel properties.

Fabrication and characterization of low-cost and green vacuum insulation panels with fumed silica/rice husk ash hybrid core material

“…Moreover, S gradually decreased from 193.9 m 2 /g to 87.5 m 2 /g with the increase of RHA content in the range of 0-46 wt.%. This was because the system volume (cubic of the length) grew faster with the component size than its S (square of the length) [34]. Thus, a greater S was found in the HCMs with a higher proportion of fine components (i.e., FS).…”

Measuring the Material Properties Related to Human Perceptions  

“…Even in the simple case of weight measurement, we use the graduation readings (the length) on the balance to find (represent) the weight -a twist from weight to length is needed. In consumer products, when our concern is the thermal comfort of a wool coat, it is the cloth density, not the thermal conductivity of the wool fiber that dominates the result 33 . To change the drape of a skirt, we have to check the tensile and shear resistances of the cloth 34,35 .…”

Nanophotonics-enabled smart windows, buildings and wearables

“…In other words, how far can we go to make city life feel more "natural"? A key aspect of "natural" nanophotonics is hierarchical nanostructures [12]. Such materials when reverse engineered into components, or imaged at several distinct magnifications, reveal multiple components each with a characteristic topology and scale, spatial arrangement and shape.…”