In this work, our attention is dedicated to a beetle in nature, called bombardier beetle, which ejects 100 °C hot and corrosive spray as a defensive mechanism. The spray is the product of an explosive biochemical reaction taking place in the reaction chambers (RCs). Knowledge of the thermal properties, such as thermal conductivity, of the RCs will help to shed light on why bombardier beetles can withstand the explosive reaction taking place in their bodies. In this paper, we developed a method to measure the thermal conductivity of RCs, which is challenging for the traditional technologies because of the submillimeter dimension of RC. The reliability of the method was proved, and the applicability can be extended to other materials especially polymeric ones. The results in this paper not only help to understand the superior thermal insulation of the RC wall but also offer a facile approach to measuring the thermal conductivity of unknown materials in small scale.

Reference:

• Z. Guo, W. Sha, H. Yao^*, 2019. Measuring thermal conductivity of ultra-small materials exampled by the reaction chambers of bombardier beetles, Int. J. Heat and Mass Transfer 134 , 1318-1322.

Biofouling refers to the unfavorable attachment and accumulation of marine sessile organisms (e.g., barnacles, mussels, and tubeworms) on the solid surfaces immerged in ocean. The enormous economic loss caused by biofouling in combination with the severe environmental impacts induced by the current antifouling approaches entails the development of novel antifouling strategies with least environmental impact. Inspired by the superior antifouling performance of the leaves of mangrove tree Sonneratia apetala, here we propose to combat biofouling by using surface with microscopic ridge-like morphology. Settlement tests with tubeworm larvae on polymeric replicas of S. apetala leaves confirm that the microscopic ridge-like surface morphology can effectively prevent biofouling. A contact mechanics-based model is then established to quantify the dependence of tubeworm settlement on the structural features of the microscopic ridge-like morphology, giving rise to theoretical guidelines to optimize the morphology for better antifouling performance. Under the direction of the obtained guidelines, a synthetic surface with microscopic ridge-like morphology is developed, exhibiting antifouling performance comparable to that of the S. apetala replica. Our results not only reveal the underlying mechanism accounting for the superior antifouling property of the S. apetala leaves, but also provide applicable guidance for the development of synthetic antifouling surfaces.

Reference:

• J. Fu, H. Zhang, Z. Guo, D-qing Feng, V. Thiyagarajan, H. Yao^*, 2018. Combat biofouling with microscopic ridge-like surface morphology: a bioinspired study. J. R. Soc. Interface 20170823, doi: 10.1098/rsif.2017.0823.

Helical structures are ubiquitous in nature at length scales of a wide range. In this paper, we studied a helical architecture called microscopic screw dislocation (μ-SD), which is prevalently present in biological laminated composites such as shells of mollusks P. placenta and nacre of abalone. Mechanical characterization indicated that μ-SDs can greatly enhance resistance to scratching. To shed light on the underlying reinforcing mechanisms, we systematically investigated the mechanical behaviors of μ-SD using theoretical modelling in combination with finite element simulation. Our analysis on an individual μ-SD showed that the failure of a μ-SD under tension involves the delamination of the prolonged spiral interface, giving rise to much higher toughness compared to those of the planar counterpart. The corporation of multiple μ-SDs was further investigated by analyzing the effect of areal density on the mechanical reinforcement. It was found that higher areal density of μ-SD would lead to more improvement in toughness. However, the operation of such reinforcing mechanism of μ-SD requires proclivity of cracking along the spiral interface, which is not spontaneous but conditional. Fracture mechanics-based modelling indicated that the proclivity of crack propagation along the spiral interface can be ensured if the fracture toughness of the interface is less than 60% of that of the lamina material. These findings not only uncover the reinforcing mechanisms of μ-SDs in biological materials but imply a great promise of applying μ-SDs in reinforcing synthetic laminated composites.

Reference:

• Y. Gao, Z. Guo, Z. Song, H. Yao*, 2017. Spiral interface: A reinforcing mechanism for laminated composite materials learned from nature, J. Mech. Phys. Solids 109, 252-263.

The excellent mechanical properties of natural biomaterials have attracted intense attention from researchers with focus on the strengthening and toughening mechanisms. Nevertheless, no material is unconquerable under sufficiently high load. If fracture is unavoidable, constraining the damage scope turns to be a practical way to preserve the integrity of the whole structure. Recent studies on biomaterials have revealed that many structural biomaterials tend to be fractured, under sufficiently high indentation load, through ring cracking which is more localized and hence less destructive compared to the radial one. Inspired by this observation, here we explore the factors affecting the fracture mode of structural biomaterials idealized as laminated materials. Our results suggest that fracture mode of laminated materials depends on the coating/substrate modulus mismatch and the indenter size. A map of fracture mode is developed, showing a critical modulus mismatch (CMM), below which ring cracking dominates irrespective of the indenter size. Many structural biomaterials in nature are found to have modulus mismatch close to the CMM. Our results not only shed light on the mechanics of inclination to ring cracking exhibited by structural biomaterials but are of great value to the design of laminated structures with better persistence of structural integrity.

References:

• H. Yao*, Z. Xie, C. He, M. Dao, 2015. Fracture mode control: a bio-inspired strategy to combat catastrophic damage, Sci. Rep. 5, 8011.

• C. He, Z. Xie, Z. Guo, H. Yao^*, 2015. Fracture-mode map of brittle coatings: theoretical development and experimental verification, J. Mech. Phys. Solids 83, 19-35.

Biological armors such as mollusk shells have long been recognized and studied for their values in inspiring novel designs of engineering materials with higher toughness and strength. However, no material is invincible and biological armors also have their rivals. In this paper, our attention is focused on the teeth of black carp (Mylopharyngodon piceus) which is a predator of shelled mollusks like snails and mussels. Nanoscratching test on the enameloid, the outermost layer of the teeth, indicates that the natural occlusal surface (OS) has much higher wear resistance compared to the other sections. Subsequent X-ray diffraction analysis reveals that the hydroxyapatite (HAp) crystallites in the vicinity of OS possess c-axis preferential orientation. The superior wear resistance of black carp teeth is attributed to the c-axis preferential orientation of HAp near the OS since the (001) surface of HAp crystal, which is perpendicular to the c-axis, exhibits much better wear resistance compared to the other surfaces as demonstrated by the molecular dynamics simulation. Our results not only shed light on the origin of the good wear resistance exhibited by the black carp teeth but are of great value to the design of engineering materials with better abrasion resistance.

Reference:

• J. Fu, C. He, B. Xia, Y. Li, Q. Feng, Q. Yin, X. Shi, X. Feng, H. Wang, H. Yao^*, 2016. c-axis preferential orientation of hydroxyapatite accounts for the high wear resistance of the teeth of black carp (Mylopharyngodon piceus), Scientific Reports 6, 23509.

In this work, we systematically investigate the factors that affect the edge orientation of the mechanically exfoliated 2D materials. Our study manifests that the preferred fractured direction during mechanical exfoliation is determined by both tearing direction and anisotropy of fracture energy. Our theory, in combination with the knowledge of crystallographic structure of a specific 2D material, allows us to predict the possible edge orientations of mechanically exfoliated 2D materials as well as their probabilities. Our theoretical predication is indirectly verified by examining the inter-edge angles of the mechanically exfoliated 2D materials including graphene, MoS₂, PtS₂, and black phosphorous. This work not only sheds light on the mechanics of exfoliation of 2D materials, but also opens a new area of deriving information of edge orientation of mechanically exfoliated 2D materials by data mining of their macroscopic geometric features.

Reference:

• J. Yang, Y. Wang, Y. Li, H. Gao, Y. Chai^*, H. Yao^*, 2018. Edge orientations of mechanically exfoliated anisotropic two-dimensional materials, J. Mech. Phys. Solids 112, 157-168.