Force transmission during repose of flexible granular chains.

We study the mechanics of standing columns formed during the repose of flexible granular chains. It is one of the many intriguing behaviours exhibited by granular materials when links capable of transmitting tension exist between particles. We develop and calibrate a discrete element method contact model to simulate the mechanics of the macroscopic flexible granular chains and conduct simulations of the angle of repose experiments of these chains by extracting a chain-filled cylinder and allowing the material to flow out under gravity and repose. We evaluate various micro-mechanical, topological and macroscopic parameters to elucidate the mechanics of the repose behaviour of chain ensembles. It is the ability of the links connecting the individual particles to transmit tensile forces along the chain backbone that provides lateral stability to the column, enabling them to stand. In particular, the contact force rearrangement inside the columns generates a self-confining radial stress near the base of the columns, which provides an important stabilizing stress.

Column to pile transition in quasi-static deposition of granular chains.

The repose angle is a key geometric property that characterises the inter-particle friction and thereby granular flows. One of the common methods to measure this property is to deposit a pile by extracting a pre-filled cylinder and letting the material flow out. While the repose angle of spherical beads is insensitive to the aspect ratio of this pre-filled column, we find that long flexible granular chains show a remarkable transition from stable vertical columns to conical piles depending on the aspect ratio. Below a critical aspect ratio, the cessation of flow of granular chains due to inter-chain entanglement stabilises the columns, while above the critical aspect ratio the conical piles of long granular chains arise not out of shear flow but instead through a series of column collapse instabilities during the deposition process. We also identify the critical chain length below which the granular chains flow and behave similar to spherical particles.

Fabrication and Mechanical Characterization of Hydrogel Infused Network Silk Scaffolds.

Development and characterization of porous scaffolds for tissue engineering and regenerative medicine is of great importance. In recent times, silk scaffolds were developed and successfully tested in tissue engineering and drug release applications. We developed a novel composite scaffold by mechanical infusion of silk hydrogel matrix into a highly porous network silk scaffold. The mechanical behaviour of these scaffolds was thoroughly examined for their possible use in load bearing applications. Firstly, unconfined compression experiments show that the denser composite scaffolds displayed significant enhancement in the elastic modulus as compared to either of the components. This effect was examined and further explained with the help of foam mechanics principles. Secondly, results from confined compression experiments that resemble loading of cartilage in confinement, showed nonlinear material responses for all scaffolds. Finally, the confined creep experiments were performed to calculate the hydraulic permeability of the scaffolds using soil mechanics principles. Our results show that composite scaffolds with some modifications can be a potential candidate for use of cartilage like applications. We hope such approaches help in developing novel scaffolds for tissue engineering by providing an understanding of the mechanics and can further be used to develop graded scaffolds by targeted infusion in specific regions.

Micromechanics of contact-bound cohesive granular materials in confined compression.

The mechanical behavior of granular materials results from interparticle interactions, which are predominantly frictional. With the presence of even very small amounts of cohesion this frictional interparticle behavior significantly changes. In this study, we introduce trace amounts of cohesive binder between the intergranular contacts in a sample of quartz particles and apply one-dimensional (1D) compression loading. X-ray computed tomography is performed in situ during 1D compression. We make observations at three different length scales. At the macroscopic or ensemble scale, we track the evolution of the porosity, particle size and the stress-strain response during this compression. At the microstructure or interparticle scale, we compute the directional distribution of contacts and the particles. We also track the evolution of the fabric chains with continued compression. We also evaluate particle rotations, displacements, contact twist, rotation, and sliding. We show through our experiments that even a small amount of cohesion (as low as 1% by weight) significantly changes the response at multiple length scales. This interparticle cohesion suppresses the fragmentation of grains, alters force transmission and changes the structure of the ensemble.

Moisture alone is sufficient to impart strength but not weathering resistance to termite mound soil.

Soil is used for the construction of structures by many animals, at times admixed with endogenous secretions. These additives, along with soil components, are suggested to have a role in biocementation. However, the relative contribution of endogenous and exogenous materials to soil strength has not been adequately established. Termite mounds are earthen structures with exceptional strength and durability including weathering resistance to wind and rain. With in situ and laboratory-based experiments, we demonstrate that the fungus-farming termite Odontotermes obesus which builds soil nest mounds, when given a choice, prefers soil close to its liquid limit for construction. At this moisture content, the soil-water mixture alone even in the absence of termite handling undergoes self-weight consolidation and upon drying attains a monolithic, densely packed structure with compressive strength comparable to the in situ strength of the mound soil; however, the soil-water mixture alone has lower resistance to water erosion than the in situ mound samples, suggesting that termite secretions impart weathering resistance and thereby long-term stability to the mound. Therefore, weathering resistance and compressive strength are conferred by different aspects of termite soil manipulation. Our work provides novel insights into termite mound construction and strength correlates for earthen structures built by animals.

Bi-layered architecture facilitates high strength and ventilation in nest mounds of fungus-farming termites.

Mass-energy transfer across the boundaries of living systems is crucial for the maintenance of homeostasis; however, it is scarcely known how structural strength and integrity is maintained in extended phenotypes while also achieving optimum heat-mass exchange. Here we present data on strength, stability, porosity and permeability of termite mounds of a fungus-farming species, Odontotermes obesus. We demonstrate that the termite mound is a bi-layered structure with a dense, strong core and a porous shell that is constantly remodelled. Its safety factor is extraordinarily high and is orders of magnitude higher than those of human constructions. The porous peripheries are analogous to the mulch layer used in agriculture and help in moisture retention crucial for the survival of fungus gardens, while also allowing adequate wind-induced ventilation of the mounds. We suggest that the architectural solutions offered by these termites have wider implications for natural and industrial building technologies.

Building mud castles: a perspective from brick-laying termites.

Animal constructions such as termite mounds have received scrutiny by architects, structural engineers, soil scientists and behavioural ecologists but their basic building blocks remain uncharacterized and the criteria used for material selection unexplored. By conducting controlled experiments on Odontotermes obesus termites, we characterize the building blocks of termite mounds and determine the key elements defining material choice and usage by these accomplished engineers. Using biocement and a self-organized process, termites fabricate, transport and assemble spherical unitary structures called boluses that have a bimodal size distribution, achieving an optimal packing solution for mound construction. Granular, hydrophilic, osmotically inactive, non-hygroscopic materials with surface roughness, rigidity and containing organic matter are the easiest to handle and are crucial determinants of mass transfer during mound construction. We suggest that these properties, along with optimal moisture availability, are important predictors of the global geographic distribution of termites.

Deformation field heterogeneity in punch indentation.

Plastic heterogeneity in indentation is fundamental for understanding mechanics of hardness testing and impression-based deformation processing methods. The heterogeneous deformation underlying plane-strain indentation was investigated in plastic loading of copper by a flat punch. Deformation parameters were measured, in situ, by tracking the motion of asperities in high-speed optical imaging. These measurements were coupled with multi-scale analyses of strength, microstructure and crystallographic texture in the vicinity of the indentation. Self-consistency is demonstrated in description of the deformation field using the in situ mechanics-based measurements and post-mortem materials characterization. Salient features of the punch indentation process elucidated include, among others, the presence of a dead-metal zone underneath the indenter, regions of intense strain rate (e.g. slip lines) and extent of the plastic flow field. Perhaps more intriguing are the transitions between shear-type and compression-type deformation modes over the indentation region that were quantified by the high-resolution crystallographic texture measurements. The evolution of the field concomitant to the progress of indentation is discussed and primary differences between the mechanics of indentation for a rigid perfectly plastic material and a strain-hardening material are described.

Deformation field in indentation of a granular ensemble.

An experimental study has been made of the flow field in indentation of a model granular material. A granular ensemble composed of spherical sand particles with average size of 0.4 mm is indented with a flat ended punch under plane-strain conditions. The region around the indenter is imaged in situ using a high-speed charge-coupled device (CCD) imaging system. By applying a hybrid image analysis technique to image sequences of the indentation, flow parameters such as velocity, velocity gradient, and strain rate are measured at high resolution. The measurements have enabled characterization of the main features of the flow such as dead material zones, velocity jumps, localization of deformation, and regions of highly rotational flow resembling vortices. Implications for validation of theoretical analyses and applications are discussed.