Ranajay Ghosh

Hierarchical honeycomb auxetic metamaterials

Most conventional materials expand in transverse directions when they are compressed uniaxially resulting in the familiar positive Poisson’s ratio. Here we develop a new class of two dimensional (2D) metamaterials with negative Poisson’s ratio that contract in transverse directions under uniaxial compressive loads leading to auxeticity. This is achieved through mechanical instabilities (i.e., buckling) introduced by structural hierarchy and retained over a wide range of applied compression. This unusual behavior is demonstrated experimentally and analyzed computationally. The work provides new insights into the role of structural organization and hierarchy in designing 2D auxetic metamaterials, and new opportunities for developing energy absorbing materials, tunable membrane filters, and acoustic dampeners.

Origami-based cellular metamaterial with auxetic, bistable, and self-locking properties

We present a novel cellular metamaterial constructed from Origami building blocks based on Miura-ori fold. The proposed cellular metamaterial exhibits unusual properties some of which stemming from the inherent properties of its Origami building blocks, and others manifesting due to its unique geometrical construction and architecture. These properties include foldability with two fully-folded configurations, auxeticity (i.e., negative Poisson’s ratio), bistability, and self-locking of Origami building blocks to construct load-bearing cellular metamaterials. The kinematics and force response of the cellular metamaterial during folding were studied to investigate the underlying mechanisms resulting in its unique properties using analytical modeling and experiments.

Contact kinematics of biomimetic scales

Dermal scales, prevalent across biological groups, considerably boost survival by providing multifunctional advantages. Here, we investigate the nonlinear mechanical effects of biomimetic scale like attachments on the behavior of an elastic substrate brought about by the contact interaction of scales in pure bending using qualitative experiments, analytical models, and detailed finite element (FE) analysis. Our results reveal the existence of three distinct kinematic phases of operation spanning linear, nonlinear, and rigid behavior driven by kinematic interactions of scales. The response of the modified elastic beam strongly depends on the size and spatial overlap of rigid scales. The nonlinearity is perceptible even in relatively small strain regime and without invoking material level complexities of either the scales or the substrate.

Nonlinear deformation and localized failure of bacterial streamers in creeping flows

We investigate the failure of bacterial floc mediated streamers in a microfluidic device in a creeping flow regime using both experimental observations and analytical modeling. The quantification of streamer deformation and failure behavior is possible due to the use of 200 nm fluorescent polystyrene beads which firmly embed in the extracellular polymeric substance (EPS) and act as tracers. The streamers, which form soon after the commencement of flow begin to deviate from an apparently quiescent fully formed state in spite of steady background flow and limited mass accretion indicating significant mechanical nonlinearity. This nonlinear behavior shows distinct phases of deformation with mutually different characteristic times and comes to an end with a distinct localized failure of the streamer far from the walls. We investigate this deformation and failure behavior for two separate bacterial strains and develop a simplified but nonlinear analytical model describing the experimentally observed instability phenomena assuming a necking route to instability. Our model leads to a power law relation between the critical strain at failure and the fluid velocity scale exhibiting excellent qualitative and quantitative agreeing with the experimental rupture behavior.

Type-IV Pilus Deformation Can Explain Retraction Behavior

Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100 pN during their retraction before stalling, powering surface translocation(twitching). Single TFP level experiments have shown remarkable nonlinearity in the retraction behavior influenced by the external load as well as levels of PilT molecular motor protein. This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly. In order to explain this behavior, we analyze the coupling of TFP elasticity and interfacial behavior with PilT kinetics. We model retraction as reaction controlled and elongation as transport controlled process. The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load. Elongation is controlled by monomer transport which suffer entrapment due to excess PilT in the cell periplasm. Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.

Numerical Analysis of the Response of Biomimetic Cellular Materials Under Static and Dynamic Loadings

Biological structures distinguish themselves in preserving material heterogeneity for better mechanical behavior in contrast to man-made structures where they are deliberately removed through design. In this book chapter, we explore the distinct advantages in mechanical behavior brought about by regular, irregular, and functionally graded heterogeneities inherent in these cellular materials under quasi-static and dynamic crushing as well as impact loading. In particular we investigate energy dissipation and deformation modes during such loadings. We also investigate the effect of increasing self-similar structural hierarchy and cell wall material plasticity on the mechanical properties and failure behavior of these structures. Specifically, we discuss how mechanical response can be tuned using structural variations alone in particular, the tuning of strength to stiffness envelopes. Therefore, we show that material hierarchy can potentially reverse the conventional practice of design of structures using a variety of materials rather than vice versa. We also show how variation of structure can fundamentally change the deformation and damage patterns of these structures under dynamic crushing and impact loading. Finally we discuss some recent advances in fabrication and characterization of novel ultralight structures which exploit these essential self-similar properties of biological structures to create materials with remarkable stress–strain behavior strikingly different from the very materials originally used to make them.

Non-ideal effects in bending response of soft substrates covered with biomimetic scales

Biomimetic scales are known to substantially alter the mechanics response of the underlying substrate engendering complex nonlinearities that can manifest even in small deformations due to scales interaction. This interaction is typically modeled using a-priori homogenization with an enforced periodicity of engagement. Such a framework is fairly useful especially when dealing with the structural length scale which is at least one order of magnitude greater than the scales themselves since individual tracking of a large number of scales become insurmountable. On the other hand, this scheme makes several assumptions whose validity has not yet been investigated including infinite length of the substrate and rigidity of the scales. The validity of these assumptions and the accuracy and limitations of associated analytical models are investigated. Finite element based numerical studies were carried out to identify the critical role of edge effects and other non-ideal behavior such as violation of periodicity and nonlinear constitutive response on scale rotation. Our investigation shows that several important quantities show a strong saturation characteristic which justify many of the simplifying assumptions whereas others need much greater care.

Frictional effects in biomimetic scales engagement

Scales engagement can contribute significantly to nonlinear bending behavior of elastic substrates with rigid biomimetic scales. In this letter, we investigate the role of friction in modulating the nonlinearity that arises due to self-contact of scales through an analytical investigation. We model the friction as dry Coulomb type friction between rigid links and the substrate is taken to be linear elastic. Our results reveal that frictional effects give rise to two possible locking mechanisms, namely static friction lock and kinetic friction lock. These locks arise due to a combination of interfacial behavior and geometry. In addition to these extremes, the frictional behavior is found to increase stiffness of the structure. This dual nature of friction which influences both system operation and its terminal limit results in the maximum relative frictional work to lie at intermediate friction coefficients and not at the extremes of frictional limits.

Thermal conductivity of biomimetic leaf composite

The venous morphology of a typical plant leaf affects its mechanical and thermal properties. Such a material could be considered as a fiber reinforced composite structure where the veins and the rest of the leaf are considered as two materials having highly contrast mechanical and thermal properties. The variegated venations found in nature is idealized into three principal fibers—the central mid-fiber corresponding to the mid-rib, straight parallel secondary fibers attached to the mid-fiber representing the secondary veins, and then another set of parallel fibers emanating from the secondary fibers mimicking the tertiary veins of a typical leaf. This paper addresses the in-plane thermal conductivity of such a composite by considering such a venous fiber morphology embedded in a matrix material. We have considered two cases, fibers having either higher or lower conductivity respect to the matrix. The tertiary fibers do not interconnect the secondary fibers in our present study. We carry out finite element based computational investigation of the thermal conductivity of these composites under uniaxial thermal gradients and study the effect of different fiber architectures. To this end, we use two broad types of architectures both having similar central main fiber but differing in either having only secondary fibers or additional tertiary fibers. The fiber and matrix volume fractions are kept constant and a comparative parametric study is carried out by varying the inclination of the secondary fibers. We find the heat conductivity in the direction of the main fiber (Y direction) increases significantly as the fiber angle of the secondary increases. Furthermore, for composite with metal fibers, the conductivity in the Y direction is further enhanced when composite is manufactured by having secondary fibers forming a closed cell structure. However, for composite with ceramic fibers, the conductivity of the composite in the Y direction is little affected by having secondary fibers closed. An opposite behavior is observed when considering conductivity of the composite in the X direction. The conductivity of the composite in the X direction is reduced with increase in the angle of the secondary fibers. Higher conductivity in the X direction is achieved for composite with no closed cells for composites with metal fibers. The results also indicate that for composites with the constant fiber volume fraction, morphology of tertiary fibers may not significantly alter material conductivities. In conclusion, introducing a leaf-mimicking topology in fiber architecture can provide significant additional degrees of tunability in design of these composite structures.

Near wall void growth leads to disintegration of colloidal bacterial streamer

We investigated the failure of thick bacterial floc-mediated streamers in a microfluidic device with micropillars. It was found that streamers could fail due to the growth of voids in the biomass that originate near the pillar walls. The quantification of void growth was made possible by the use of 200 nm fluorescent polystyrene beads. The beads get trapped in the extracellular matrix of the streamer biomass and acted as tracers. Void growth time-scales could be characterized into short-time scales and long time-scales and the crack/void propagation showed several instances of fracture-arrest ultimately leading to a catastrophic failure of the entire streamer structure. This mode of fracture stands in strong contrast to necking-type instability observed before in streamers.