Trees and other woody plants are hierarchically structured to achieve a wide range of mechanical properties-necessary for their survival under various and varying loading conditions-with a narrow range of chemical composition. The hierarchical level understood to be directly responsible for modulating the longitudinal elastic modulus and yielding behavior is that of the composition of the cell walls, which consists of crystalline cellulose fibrils embedded in an amorphous matrix of hemicellulose and lignin. Here we report a simple coarse-grained model of the cellulose fibrils and hemicellulose chains to provide a bottom-up description of the mechanics. We identify the mechanical behaviors for varying fibril angles, plot force-strain relationships and compare the data with experimental results and theoretical predictions, providing insight into fundamental structure-property relations of wood. Our model shows that hemicellulose deforms in a stick-slip motion as the fibrils shear, resulting in permanent deformation without causing material damage, and leading to the characteristic softening of wood as deformation is increased. The main contribution of this work is the development of a model that can be fine-tuned and adapted to other applications. While it presents simple formulation, it can successfully describe several key phenomena, specifically the three regimes of mechanical behavior: elastic, plastic, and high-strain stiffening, as well as the dependence of modulus on the microfibril angle, and as such provides a bottom-up mechanistic approach to wood mechanics.

In this topical review we discuss recent advances in the use of physical insight into the way biological materials function, to design novel engineered materials 'from scratch', or from the level of fundamental building blocks upwards and by using computational multiscale methods that link chemistry to material function. We present studies that connect advances in multiscale hierarchical material structuring with material synthesis and testing, review case studies of wood and other biological materials, and illustrate how engineered fiber composites and bulk materials are designed, modeled, and then synthesized and tested experimentally. The integration of experiment and simulation in multiscale design opens new avenues to explore the physics of materials from a fundamental perspective, and using complementary strengths from models and empirical techniques. Recent developments in this field illustrate a new paradigm by which complex material functionality is achieved through hierarchical structuring in spite of simple material constituents.

The settlement of spores and larvae on hard substrates has been shown to be influenced by many factors, but few studies have evaluated how underlying bedrock may influence recruitment. The characteristics of coastal rock types such as color, heat capacity, mineral size, and free energy have all been implicated in settlement success. We examined the influence of naturally occurring rock types on the initial attachment of zygotes of the brown alga Fucus spiralis Linnaeus 1753. We also assessed the dislodgment of zygotes on four bedrock types after initial attachment in laboratory experiments using wave tanks. Settling plates were prepared from limestone, basalt, schist, and granite, found in the region of Orrs Island, Maine, USA. The plate surfaces tested were either naturally rough or smooth-cut surfaces. We measured the density of attached zygotes after 1.5 h of settlement and subsequently after a wave treatment, in both winter and summer. The pattern of initial attachment was the same on natural and smooth surfaces regardless of season: highest on limestone (range 7.0–13.4 zygotes/cm2), intermediate on schist (1.8–8.5 zygotes/cm2) and basalt (3.5–14.0 zygotes/cm2), and lowest on granite (0.8–7.8 zygotes/cm2). Patterns of survivorship following the wave treatment were similar to those of initial settlement with the mean survivorship varying from 60.1% (SE = 3.8) (limestone, smooth substrate) to 31.8% (SE = 0.59) (granite, natural substrate), and with the highest mean survival on limestone, basalt, and schist, and the lowest on granite. Our results suggest that rock type has a significant effect on zygote attachment and persistence. Patterns of attachment were the same on smooth and rough surfaces, indicating that surface roughness is not the predominant factor controlling the difference in successful attachment among rock types. Other properties of bedrock, possibly grain size, surface free energy, or chemical interaction with the adhesives used by the zygotes, directly affect the attachment of these algal propagules. These results suggest that patterns of benthic community structure could be determined in part by the distribution of bedrock types.

No abstract