Following last month’s blog by Mahmoud on the role of structure in the design of materials capable of controlling the propagation of mechanical waves, I would like to delve into the characteristics of the structure itself. The propagation of mechanical waves can be controlled via scattering induced by a material’s structure.The propagation of waves for example is significantly disturbed when their wavelength and direction become correlated with the medium’s structure. This is known as Bragg scattering and can be achieved simply by embedding voids or inclusions (of different physical properties) in a material [1].

Engineering structures are made out of materials and as such there is a natural hierarchy in which a material’s intrinsic properties contribute to shaping up the structure’s response. It is possible however to reverse this hierarchy and engineer materials that are made out of structures. In this case, the intrinsic properties of a material are shaped by the structural response. Such a configuration can only be realized with a repeated structure, forming an array of identical unit cells.

Porous materials can be created by a variety of methods and exhibit properties that are advantageous in certain applications, e.g. insulation, energy absorption, and core materials in sandwich panels. As the length scale of the pores/ligaments is reduced below one micron, size effects arise and cause changes in the deformation mechanisms that operate in the ligament material. The mechanical properties can change dramatically, especially for so-called “nanoporous metals”, which have pores and ligaments as small as a few nanometers.

Hierarchical Mechanics of Diatom Algae: From Atoms to Organism and Weakness to Strength

This month’s iMechanica Journal Club theme is the hierarchical structure and mechanics of diatom algae, silicified organisms that use silica (“sand”) – abundantly available in the ocean – to construct strong, tough and stiff structures [1-10]. The interest in this area has been revived recently given recent advances in the combined measurement, modeling and synthesis of these materials, leading to exciting research being conducted at the interface of mechanics and biology.

Introduction:

The October 2011 journal club theme is "AFM in Nano-biomechanics". Nano-biomechanics is an emerging field that aims at exploring fundamental science and engineering related to biological materials at the nanoscale (http://www.technologyreview.com/biomedicine/16475/ and http://en.wikipedia.org/wiki/Nanobiomechanics). Atomic force microscope (AFM) has been one of the instrumental tools in this field by providing pN force sensitivity, and better than nanometer spatial resolution.

Energy harvesting is the process of converting energy that will otherwise be dissipated into the ambient environment, into useful energy to do work.  I shall focus this discussion on motion-based energy harvesting.  Motion-based energy harvesting is the process of converting dissipated mechanical energy into electrical energy.  Sources of mechanical energy include the ocean waves, wind, human motion, vehicular traffic, and vibrations in buildings and bridges.  This source of energy is ubiquitous and pervasive, and yet, it is one of the least developed energy harvesting technology.

From venation of leaves to the blood and lymph vessels and tracheae of insects, 3D filamentous branching networks are a common pattern in all higher organisms.  These busy “highways” supply the tissue with nutrition and oxygen, expel waste and heat, as well as conduct immune reactions and other signal pathways. These microvascular networks are also essential for effective response of external stimuli in some sensitive plants, such as Venus flytrap and Mimosa pudica.

The response of structural materials to external mechanical load may strongly depend on the rate at which the load is imposed. For example, a specimen may exhibit ductile fracture if loaded at quasi-static rate (strain rate below 1.0/s), but may show brittle fracture under impact (high-rate) loading. According to the classic monograph of Professor Marc Meyers, if the strain rate is above 100/s, it can be put into the high-strain rate regime. The mechanical behavior of structural materials under such loading conditions is dubbed dynamic.

Investigations into the dynamic behaviors of materials dates back to the 19th century. It was shown that stress wave propagation becomes predominant.

Coupling between electrical and mechanical phenomena is ubiquitous in nature and underpins the functionality of materials and systems as diversified as ferroelectrics and multiferroics, electroactive molecules, and biological systems. In ferroelectrics, electromechanical behavior is directly linked to polarization order parameter and hence can be used to study complex phenomena including polarization reversal, domain wall pinning, multiferroic interaction, and electron-lattice coupling. The very basis of functionalities of biological systems is electromechanics - from nerve-controlled muscle contraction on macroscale to cardiac activity and hearing on microscale and to energy storage in mitochondria, voltage-controlled ion channels and electromotor proteins on nanoscale. More broadly, electromechanical coupling is a key component of virtually all electrochemical transformations, and is a nearly universal part of energy conversion and transport processes. It forms a basis for many device applications, and is directly relevant to virtually all existing and emerging aspects of materials science and nanobiotechnology.

• RSS feed for the posts:   http://imechanica.org/taxonomy/term/85/0/feed
• RSS feed for comments:   http://imechanica.org/crss/term/85
• Full list of PDF files of papers from the Suo Group
• Slides of research presentations

You will be alerted whenever someone posts to the Job Channel if you subscribe to RSS feeds.

The RSS feed for this Channel:
http://imechanica.org/taxonomy/term/73/0/feed

Learn how to subscribe to RSS feeds. They are easy, effective, and free.