A design protocol for failure resilient architected metamaterials

There has been a recent explosion in the development of light and strong mechanical metamaterials reporting extreme effective and functional properties. As additive manufacturing progresses to proliferate these metamaterials, their application as structural materials is ultimately limited by their tolerance to damage and defects. While significant advances have been made in reporting their stiffness and strength, material properties that enable us to define the tolerance to defects as yet remain unclear. All work to-date has a-priori assumed that a material property known as fracture toughness exists for these materials akin to usual continuum solids, without a-posteriori experimental validation. In fact, all existing experimental measurements are based on metamaterial specimens comprising only dozens to at most a few hundred unit cells where the so called “K-field” required to define an effective toughness is not established. Thus, an understanding of defect sensitivity in these metamaterials has remained unknown. In this work, we perform a series of fracture toughness measurements (uniaxial to multiaxial loadings) coupled with in-situ X-ray CT visualization on a range of octet-truss specimens comprising up to millions of unit cells. This was combined with large-scale numerical calculations and a theoretical analysis to decipher the elusive fracture behaviour of 3D metamaterials. It is demonstrated that (i) stress intensity factor K_I is insufficient to characterize fracture and (ii) standard fracture testing protocols, established over the last 50 years, are inappropriate for such materials. We uncover the significance of T-stress (T) effects in elastic-brittle fracture of open-cell architected metamaterials. Using asymptotic analysis we extend the findings to construct fracture mechanism maps (with K_I \& T) that can characterize the failure of slender-beam (relative densities less than 20%) periodic truss 3D metamaterials under arbitrary loadings. These findings led to a revision of elastic fracture mechanics and thereby the development of a general design methodology and testing protocol for mechanical metamaterials. The framework is envisioned to form the basis for fracture characterization in other discrete elastic-brittle solids where the notion of fracture toughness is known to breakdown.The author acknowledge funding from the Office of Naval Research (N00014-18-1-2658 and N00014-20-1-2504:P00001).The author is supported by Cambridge-India Ramanujan scholarship from the Cambridge Trust and the SERB (Govt. of India)

Exploration of truss metamaterials with graph based generative modeling

In the expanding landscape of metamaterial design, Zheng and colleagues introduces a framework that bridges design and properties, using machine learning to enhance truss metamaterials. A neural network creates an interpretable, low-dimensional space, empowering designers to tailor mechanical properties

Exploration of truss metamaterials with graph based generative modeling

In the expanding landscape of metamaterial design, Zheng and colleagues introduces a framework that bridges design and properties, using machine learning to enhance truss metamaterials. A neural network creates an interpretable, low-dimensional space, empowering designers to tailor mechanical properties

Evaporating sessile droplet pair: Insights into contact line motion, flow transitions and emergence of universal vaporisation pattern

We have deciphered that the vaporization rate of a pair of sessile droplets placed in a close vicinity of each other not only gets suppressed but also approached a universal pattern in the long time asymptotic limit, irrespective of substrate hydrophobicity. In a short time, these droplets exhibit a series of naturally evolving characteristics such as alteration of evaporation modes, flow transitions, asymmetric deformation, and motion of the contact line. Such dynamics are uniquely determined by the degree of pinning. In addition, we show that the enhanced hydrophobicity does not always lead to lower evaporation rate in droplets. Published by AIP Publishing

Insight into the Evaporation Dynamics of a Pair of Sessile Droplets on a Hydrophobic Substrate

In this work, we have demonstrated three unique regimes in the evaporation lifecycle of a pair of sessile droplets placed in variable proximity on a hydrophobic substrate. For small separation distance, the droplets undergo asymmetric spatiotemporal,evaporation leading to contact angle hysteresis and suppressed vaporization. The reduced evaporation has been attributed quantitatively to the existence of a constrained vapor-rich dome between the two droplets. However, a dynamic decrease in the droplet radius due to solvent removal marks a return to symmetry in terms of evaporation and contact angle. We have described the variation in evaporation flux using a universal correction factor. We have also demonstrated the existence of a critical separation distance beyond which the droplets in the, droplet pair do not affect each other. The results are crucial to a plethora of applications ranging from surface patterning to lab-on-a-chip devices

Research Data supporting "3D observations provide striking findings in rubber elasticity"

There are five .rar files, corresponding to Figure 1 to Figure 5. Please unzip to access the dataset. In each figure folder, there are subfolders (named as “A”, “B”, “C”, …) that contain data of each subfigure, namely Figure 1A, Figure 1B, etc... There are four types of data format, namely vtk, xlsx, mat, and opju. Here is a detailed guidance on how to utilize/access these four different types of data format. (1) For vtk files, use the Paraview software (https://www.paraview.org/). Paraview is an open source software free to download. A simple tutorial is also available on the website. With the paraview installed and opened, simply drag vtk file to the paraview, and the software will display the 3D information stored in the vtk file. (2) For xlsx file, use the Microsoft Excel software (https://www.microsoft.com/en-gb/microsoft-365/excel). The software is also free to use. Once open, the names of the columns and rows in the xlsx file clearly indicates the information as they correspond to the figure caption. The files contain 2D data for all the line plots and point plots in the figures. (3) For mat file, use the Matlab software (https://ww2.mathworks.cn/products/matlab.html). Matlab is widely available from different sources. The mat files are accessible for Matlab version after 2016. Similar to xlsx files, the names of the columns and rows in the mat file clearly indicates the information as they correspond to the figure caption. The files contain 2D data for all the line plots and point plots in the figures. (4) For opju file, use Origin software (https://www.originlab.com/). The software is widely available from different sources. Any version after 2018 will work. Similar to xlsx and mat files, the names of the columns and rows in the opju file clearly indicates the information as they correspond to the figure caption. The files contain 2D data for all the line plots and point plots in the figures. Additionally, the opju file contains the 2D plots generated from the data, same as those appeared in the figures. Readers are recommended to read the paper thoroughly before looking into this dataset

Gravity enables self‐assembly

Abstract: Crystallization of granular assemblies has broad implications for rapid and scalable creation of architected materials with applications ranging from structural materials to microarchitected battery electrodes. While significant advances have been made in understanding colloidal self‐assembly at nano to micro scale, the governing mechanisms for organization of dry assemblies of hard spheres remain unclear. Here, we investigate crystallization of mono‐size hard spheres with and without imposed vibration. Using X‐ray computed tomographic analysis coupled with discrete‐element simulations, we unravel the roles of gravity and imposed vibration on the three‐dimensional self‐assembly of the dry spheres. We use these insights to introduce gravity‐mediated epitaxial crystal growth with slow pouring of balls on seeding templates. Contrary to vibration‐induced crystallization, this method can form large single crystals with both close‐packed and rather surprisingly, nonclose‐packed metastable particle arrangements. Our results provide insight for the scalable manufacture of defect‐free granular assemblies that can be used as space‐holding templates to manufacture cellular materials, such as inverse opals and other related topologies. Key points: Self‐assembly of hard spheres is a critical step for the scalable manufacture of micro‐architected solids. Via a combination of vibration experiments, 3D X‐ray tomographic observations, and simulations, we elucidate the critical role of gravity in the self‐assembly of hard spheres. We design seeding templates that can not only induce the self‐assembly into stable close‐packed crystal structures but also rather counterintuitively into metastable single crystal structures

Sessile nanofluid droplet can act like a crane

Interactive droplet systems form the backbone for emerging avenues in droplet based technologies like cell sorting, inkjet printing and digital microfluidics, to name a few. These and their associated fields have gained significant importance in the recent times. Here, we report one such phenomenon wherein a naturally evaporating nanocolloidal sessile droplet interacts with a porous silica gel bead to mimic a macro scale mechanical crane assembly. Precisely, we show a sequence of events displayed by the particle laden aqueous droplet (nanoparticles of silica at different loading rates placed on a hydrophobic substrate) when brought in contact with a meso-porous silica gel bead. First, preferential self-assembly along droplet-bead interface is followed by formation of an adhesive bond. The phenomenon continues until the evaporating droplet naturally lifts the bead. The kinematics of the lift mechanism can be represented by a simple four bar linkage. This work provides insights into interactions between droplets and freely placed porous objects across multiple spatio-temporal scales. Present study should not just motivate researchers to design interactive droplet based systems but also use the same to perform engineering tasks like the crane action. (C) 2017 Elsevier Inc. All rights reserved

3D observations provide striking findings in rubber elasticity.

The mechanical response of rubbers has been ubiquitously assumed to be only a function of the imposed strain. Using innovative X-ray measurements capturing the three-dimensional spatial volumetric strain fields, we demonstrate that rubbers and indeed many common engineering polymers undergo significant local volume changes. But remarkably, the overall specimen volume remains constant regardless of the imposed loading. This strange behavior which also leads to apparent negative local bulk moduli is due to the presence of a mobile phase within these materials. Combining X-ray tomographic observations with high-speed radiography to track the motion of the mobile phase, we have revised classical thermodynamic frameworks of rubber elasticity. The work opens broad avenues to understand not only the mechanical behavior of rubbers but a large class of widely used engineering polymers