Large Growth Deformations of Thin Tissue using Solid-Shells

Simulating large scale expansion of thin structures, such as in growing leaves, is challenging. Sold-shells have a number of potential advantages over conventional thin-shell methods, but have thus far only been investigated for small plastic deformation cases. In response, we present a new general-purpose FEM growth framework for simulating large plastic deformations using a new solid-shell growth approach while supporting morphogen diffusion and collision handling. Large plastic deformations are handled by augmenting solid-shell elements with \textit{plastic embedding} and strain-aware adaptive remeshing. Plastic embedding is an approach to model large plastic deformations by modifying the rest configuration in response to displacement strain. We exploit the solid-shell's ability of describing both stretching and bending in terms of displacement strain to implement both plastic stretching and bending using the same plasticity model. The large deformations are adaptively remeshed using a strain-aware criteria to anticipate buckling and eliminate low-quality elements. We perform qualitative investigations on the capabilities of the new solid-shell growth approach in reproducing buckling, rippling, rolling, and collision deformations, relevant towards animating growing leaves, flowers, and other thin structures. The qualitative experiments demonstrates that solid-shells are a viable alternative to thin-shells for simulating large and intricate growth deformations

Mechanical properties of mesoscopic objects

This thesis describes measurements of the mechanical properties on the nanoscale. Three different mesoscopic tubular objects were studied: MoS2 nanotubes, carbon nanotubes and microtubules. The main goal was to investigate the interplay between the fine structure of these objects and their mechanical properties. Measurements were performed by elastically deforming tubes deposited on porous substrates with the tip of an atomic force microscope. The first experimental part describes the mechanical characterization of MoS2 nanotube bundles. Elastic deformation of MoS2 nanotube bundles can be modelled, in analogy with carbon nanotube bundles, using two elastic moduli: the Young's modulus and the shear modulus describing the weak intertube coupling. The measured Young's modulus of 120GPa has later been confirmed by theoretical modelling. It is in the range of commonly used engineering materials. The shear modulus corresponding to intertube sliding is an order of magnitude lower than in the case of carbon nanotube bundles. MoS2 nanotubes could therefore prove an interesting model for studying 1D and weakly coupled systems. They could also be interesting as AFM tips, especially for biological applications thanks to their sulphur-based chemistry. In the case of carbon nanotubes, the weak intertube coupling is a serious problem that has to be solved before they could be used as reinforcing fibers or building blocks of macroscopic objects. This problem was addressed in the second part of this thesis. Stable crosslinks were introduced into carbon nanotube bundles by irradiating them with electrons inside a TEM. AFM measurements performed in parallel with TEM observations show that the irradiation process is composed of two competing mechanisms: crosslinking, which is dominant at low exposures, and degradation of the crystalline structure followed by amorphization in the later stages of irradiation. Theoretical modelling shows that the crosslinks are most probably formed by interstitial carbon atoms. The third part of this thesis describes measurements of the mechanical properties of microtubules performed in the liquid environment. The bending modulus shows a pronounced temperature dependence, in good agreement with previously published data on the dynamic instability of microtubules. The shear and the Young's moduli were simultaneously measured, on two different temperatures, using a substrate prepared by electron beam lithography. These measurements have demonstrated that microtubules behave as strongly anisotropic cylinders. This is due to their structure, with large gaps separating neighboring protofilaments. The observed stiffening of microtubules on low temperatures (<15°C) is due to increasing interaction between the protofilaments. This manifests itself as a decrease of disassembly velocity, showing that the dynamic behavior of microtubules is reflected in their mechanical properties

Physics of Microswimmers - Single Particle Motion and Collective Behavior

Locomotion and transport of microorganisms in fluids is an essential aspect of life. Search for food, orientation toward light, spreading of off-spring, and the formation of colonies are only possible due to locomotion. Swimming at the microscale occurs at low Reynolds numbers, where fluid friction and viscosity dominates over inertia. Here, evolution achieved propulsion mechanisms, which overcome and even exploit drag. Prominent propulsion mechanisms are rotating helical flagella, exploited by many bacteria, and snake-like or whip-like motion of eukaryotic flagella, utilized by sperm and algae. For artificial microswimmers, alternative concepts to convert chemical energy or heat into directed motion can be employed, which are potentially more efficient. The dynamics of microswimmers comprises many facets, which are all required to achieve locomotion. In this article, we review the physics of locomotion of biological and synthetic microswimmers, and the collective behavior of their assemblies. Starting from individual microswimmers, we describe the various propulsion mechanism of biological and synthetic systems and address the hydrodynamic aspects of swimming. This comprises synchronization and the concerted beating of flagella and cilia. In addition, the swimming behavior next to surfaces is examined. Finally, collective and cooperate phenomena of various types of isotropic and anisotropic swimmers with and without hydrodynamic interactions are discussed.Comment: 54 pages, 59 figures, review article, Reports of Progress in Physics (to appear

From Micro- to Nano-porous Cellular Materials with Layered 2D Microstructure

A large body of work has been committed to studying the unique properties of 2D materials such as graphene, with advancements in both the material quality and scale of mechanical exfoliation and chemical vapour deposition (CVD) methods. These emergent 2D materials have recently been engineered as the cell walls in three-dimensional structures, but their superb material properties are yet to be fully realized in this new form. This thesis investigates the CVD processing of a range of catalytic templates to open new routes towards the controlled fabrication of graphitic foams and lattices. As part of a full feedback loop, mechanical characterization of these unique cellular materials was undertaken in order to examine their deformation and failure mechanisms, including capturing their behaviour in a new hierarchical model framework. These novel structures have the potential to combine the properties of structured porous materials, i.e. low density, high geometric surface area, permeability and mechanical stability, with the intrinsic properties of 2D materials such as enhanced electrical and thermal conductivity, high mechanical strength and stiffness as well as resistance to damage from extreme temperatures and chemical attack. Such high quality 2D-material based cellular structures have manifold potential applications in electrochemistry, catalysis and filtration. Herein, freestanding graphitic foams are fabricated across a range of relative densities, and their uniaxial compressive responses are measured to investigate the operative deformation and failure mechanisms that govern the mechanical response of such foams. For this purpose, a hierarchical micromechanical model is developed, which traces the deformation of the hollow cell struts to the axial stretching of the cell walls. The waviness of the multilayered graphitic wall increases the axial compliance of each cell wall, and it is established that axial straining within the cell wall occurs by interlayer shearing. Crucially, this mechanism demonstrates that the continuum properties of such foams are dictated by the weak out-of-plane shear properties of the layered cell wall material, leading to a large knockdown in the macroscopic mechanical properties of the foam. Ordered graphene gyroid lattices possessing nanoscale unit cell sizes are then fabricated and characterized through a combination of nanoindentation and a multi-scale finite element analysis (FEA) study. These structured nanolattices were found to be highly conductive and possessed a high degree of elastic recovery and strength owing to the structural efficiency afforded by the stretching-dominated cellular architecture. However, the nanoscale interlayer shearing deformation mechanism was again found to be active in the cell walls of these structures, attenuating the continuum response of the lattice. The hierarchical micromechanical model developed herein rationalizes why CVD-grown multilayer graphitic foams and lattices possess diminished continuum elastic moduli and yield strengths in comparison to the exemplary in-plane mechanical properties of 2D materials, presenting a first step towards the understanding of porous materials whose cell walls are comprised of emergent 2D materials. In addition, the direct shrinkage of commercial polymer foams and 3D printed templates is used herein to offer a very simple and low-cost method for reaching identically-shaped structures with sub-200 μm unit cell sizes. The conformal addition of different thicknesses of alumina is shown to control the level of isotropic shrinkage, reducing the shrinkage ratio from 125x to 4x after addition of 25 nm of alumina, while inducing a surface stress mismatch that drastically increases the surface roughness of the material. Furthermore, efficient graphitization was demonstrated through the use of an electrolessly deposited Nickel film, resulting in the formation of a conductive multilayer graphenic coating at temperatures below 1100°C. These processes present the flexible production of multifunctional cellular materials with sub-mm unit cells, tuneable size, roughness and conductivity. A final study investigates the preparation of a nascent 2D material, WS$_2$, through the use of a deconstructed metal organic chemical vapour deposition (MOCVD) process which allowed insight into the role of each process step. The catalytic effect of an Au substrate is unambiguously demonstrated, which allowed for a reduction in the precursor partial pressures required to nucleate and grow WS$_2$ by over an order of magnitude in comparison to competing methods. This enabled the efficient low-pressure growth of WS$_2$ films with low levels of carbon contamination. Furthermore, the reaction process developed herein exhibited a self-limiting monolayer growth behaviour with exposure cycles lasting just 10 minutes, a significant improvement over prior MOCVD processes requiring growth times in excess of 1 hour. These insights foster our understanding of the key underlying mechanisms of WS$_2$ growth for future integrated manufacturing of transition metal dichalcogenides (TMDCs) and other 2D materials.Funded by the EPSRC (EP/G037221/1) - Cambridge NanoScience through Engineering to Application Doctoral Training Centre: Assembly of Functional NanoMaterials and NanoDevices, EPSRC (EP/K016636/1) - CVD enabled Graphene Technology and Devices (GRAPHTED), ERC (279342) - In-situ metrology for the controlled growth and interfacing of nanomaterials and ERC (206409) - Multi-phase lattice materials

Patterning by cell-to-cell communication

This thesis addresses the question of how patterning may arise through cell-to-cell communication. It combines quantitative data analysis with computational techniques to understand biological patterning processes. The fi�rst section describes an investigation into the robustness of an evolved arti�ficial patterning system. Cellular automata rules were implemented sequentially according to the instructions in a simple `genome'. In this way, a set of target patterns could be evolved using a genetic algorithm. The patterning systems were tested for robustness by perturbing cell states during their development. This exposed how certain types of patterning rule had very di�fferent levels of robustness to perturbations. Rules that generated patterns with complex divergent patterns were more likely to amplify the e�ffect of a perturbation. When smaller genomes, comprising less individual rules, were evolved to match certain target patterns, these were shown to be more likely to select complex patterning rules. As a result, the developmental systems based on smaller genomes were less robust than those with larger genome sizes. Section two provides an analysis of the patterning of microchaetes in the epithelial layer of the notum of Drosophila flies. It is shown that the pattern spacing is not sufficiently described by a model of lateral inhibition through Delta-Notch signalling between adjacent cells. A computational model is used to demonstrate the viability of long range signalling through a dynamic network of �filopodia, observed in the basal layer of the epithelium. In-vivo experiments con�rm that when fi�lopodia lengths are effected by mutations the pattern spacing reduces in accordance with the model. In the fi�nal section the behaviour of simple asynchronous cellular automata are analysed. It is shown how these diff�er to the synchronous cellular automata used in the fi�rst section. A set of rules are identifi�ed whose emergent behaviour is similar to the lateral inhibition patterning process established by the Delta-Notch signalling system. Among these rules a particular subset are found to produce patterns that adjust their spacing, over the course of their development, towards a more ordered and densely packed state. A re-examination of the Delta-Notch signalling model reveals that this type of packing optimisation could take place with either dynamic �filopodial signalling, or as an alternative, transient Delta signalling at each cell. Under certain parameter regimes the patterns become more densely packed over time, whilst maintaining a minimum zone of inhibition around each Delta expressing cell. The asynchronous CA are also used to demonstrate how stripes can be formed by cell-to-cell signalling and optimised, under certain conditions, so that they align in a single direction. This is presented as a possible novel alternative to the reaction-di�ffusion mechanism that is commonly used to model the patterning of spots and stripes

Fabricate 2020

Fabricate 2020 is the fourth title in the FABRICATE series on the theme of digital fabrication and published in conjunction with a triennial conference (London, April 2020). The book features cutting-edge built projects and work-in-progress from both academia and practice. It brings together pioneers in design and making from across the fields of architecture, construction, engineering, manufacturing, materials technology and computation. Fabricate 2020 includes 32 illustrated articles punctuated by four conversations between world-leading experts from design to engineering, discussing themes such as drawing-to-production, behavioural composites, robotic assembly, and digital craft

Studies in discrete and continuum mechanics

We have used a combination of theory and computation to investigate collective aspects of discrete mechanical systems. The analysis involves considerations from geometry, elasticity and hydrodynamics. We have developed continuum theories to describe these systems, in the spirit of compressing information by mathematical abstraction from the discrete description.Engineering and Applied Science

Plasma Nanoscience: from Nano-Solids in Plasmas to Nano-Plasmas in Solids

The unique plasma-specific features and physical phenomena in the organization of nanoscale solid-state systems in a broad range of elemental composition, structure, and dimensionality are critically reviewed. These effects lead to the possibility to localize and control energy and matter at nanoscales and to produce self-organized nano-solids with highly unusual and superior properties. A unifying conceptual framework based on the control of production, transport, and self-organization of precursor species is introduced and a variety of plasma-specific non-equilibrium and kinetics-driven phenomena across the many temporal and spatial scales is explained. When the plasma is localized to micrometer and nanometer dimensions, new emergent phenomena arise. The examples range from semiconducting quantum dots and nanowires, chirality control of single-walled carbon nanotubes, ultra-fine manipulation of graphenes, nano-diamond, and organic matter, to nano-plasma effects and nano-plasmas of different states of matter.Comment: This is an essential interdisciplinary reference which can be used by both advanced and early career researchers as well as in undergraduate teaching and postgraduate research trainin

Design exploration through bidirectional modeling of constraints

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Architecture, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 315-324).Today digital models for design exploration are not used to their full potential. The research efforts in the past decades have placed geometric design representations firmly at the center of digital design environments. In this thesis it is argued that models for design exploration that bridge different representation aid in the discovery of novel designs. Replacing commonly used analytical, uni-directional models for linking representations, with bidirectional ones, further supports design exploration. The key benefit of bidirectional models is the ability to swap the role of driver and driven in the exploration. The thesis developed around a set of design experiments that tested the integration of bidirectional computational models in domain specific designs. From the experiments three main exploration types emerged. They are: branching explorations for establishing constraints for an undefined design problem; illustrated in the design of a concept car. Circular explorations for the refinement of constraint relationships; illustrated in the design of a chair. Parallel explorations for exercising well-understood constraints; illustrated in a form finding model in architecture. A key contribution of the thesis is the novel use of constraint diagrams developed to construct design explorers for the experiments. The diagrams show the importance of translations between design representations in establishing design drivers from the set of constraints. The incomplete mapping of design features across different representations requires the redescription of the design for each translation.(cont.) This redescription is a key aspect of exploration and supports design innovation. Finally, this thesis argues that the development of design specific design explorers favors a shift in software design away from monolithic, integrated software environments and towards open software platforms that support user development.by Axel Kilian.Ph.D