ACM Transactions on Graphics

Additive manufacturing has recently seen drastic improvements in resolution, making it now possible to fabricate features at scales of hundreds or even dozens of nanometers, which previously required very expensive lithographic methods. As a result, additive manufacturing now seems poised for optical applications, including those relevant to computer graphics, such as material design, as well as display and imaging applications. In this work, we explore the use of additive manufacturing for generating structural colors, where the structures are designed using a fabrication-aware optimization process. This requires a combination of full-wave simulation, a feasible parameterization of the design space, and a tailored optimization procedure. Many of these components should be re-usable for the design of other optical structures at this scale. We show initial results of material samples fabricated based on our designs. While these suffer from the prototype character of state-of-the-art fabrication hardware, we believe they clearly demonstrate the potential of additive nanofabrication for structural colors and other graphics applications

Colored fused filament fabrication

Fused filament fabrication is the method of choice for printing 3D models at low cost and is the de-facto standard for hobbyists, makers, and schools. Unfortunately, filament printers cannot truly reproduce colored objects. The best current techniques rely on a form of dithering exploiting occlusion, that was only demonstrated for shades of two base colors and that behaves differently depending on surface slope. We explore a novel approach for 3D printing colored objects, capable of creating controlled gradients of varying sharpness. Our technique exploits off-the-shelves nozzles that are designed to mix multiple filaments in a small melting chamber, obtaining intermediate colors once the mix is stabilized. We apply this property to produce color gradients. We divide each input layer into a set of strata, each having a different constant color. By locally changing the thickness of the stratum, we change the perceived color at a given location. By optimizing the choice of colors of each stratum, we further improve quality and allow the use of different numbers of input filaments. We demonstrate our results by building a functional color printer using low cost, off-the-shelves components. Using our tool a user can paint a 3D model and directly produce its physical counterpart, using any material and color available for fused filament fabrication

Računalna mehanika u znanosti i inženjerstvu – Quo vadis

Computational Mechanics has many applications in science and engineering. Its range of application has been enlarged widely in the recent decades. Hence, nowadays areas such as biomechanics and additive manufacturing are among the new research topics, in which computational mechanics helps solve complex problems and processes. In this contribution, these emerging areas will be discussed together with new discretization schemes, e. g. virtual element method and particle methods, whereby the latter need high performance computing facilities in order to solve problems such as mixing in an accurate way. Failure analysis of structures and components is another topic that is developing fast. Here, modern computational approaches rely on the phase field method that simplifies discretizations schemes. All these approaches and methods are discussed and evaluated by means of examples.Računalna mehanika ima široku primjenu u znanosti i inženjerstvu. Njeno područje primjene se znatno povećalo u zadnjim desetljećima. Danas polja kao biomehanika i aditivna proizvodnja nova su područja istraživanja u kojima računalna mehanika pomaže rješavati složene probleme i procese. U radu se razmatraju ova granična područja zajedno s novim diskretizacijskim postupcima kao što su metoda virtualnih elemenata i metoda čestica, gdje potonja zahtijeva moćnu računalnu opremu da bi se mogli točno riješiti problemi kao što je miješanje. Analiza oštećenja konstrukcija i njenih komponenata je drugo područje koje se brzo razvija, pa se ovdje moderni računalni postupci odnose na metodu faznih polja koja pojednostavljuje diskretizacijske sheme. Svi navedeni postupci i metode su razmatrani i vrednovani u numeričkim primjerima

Valorizing Sewage Sludge: Using Nature-Inspired Architecture to Overcome Intrinsic Weaknesses of Waste-Based Materials

Sewage sludge, a biosolid product of wastewater processing, is an often-overlooked source of rich organic waste. Hydrothermal processing (HTP), which uses heat and pressure to convert biomass into various solid, liquid, and gaseous products, has shown promise in converting sewage sludge into new materials with potential application in biofuels, asphalt binders, and bioplastics. In this study we focus on hydrochar, the carbonaceous HTP solid phase, and investigate its use as a bio-based filler in additive manufacturing technologies. We explore the impact of HTP and subsequent thermal activation on chemical and structural properties of sewage sludge and discuss the role of atypical metallic and metalloid dopants in organic material processing. In additive manufacturing composites, although the addition of hydrochar generally decreases mechanical performance, we show that toughness and strain can be recovered with hierarchical microstructures, much like biological materials that achieve outstanding properties by architecting relatively weak building blocks

Research Activities of JWRI

Abstracts of the Article

Understanding Homogeneous Nucleation in Solidification of Aluminum by Molecular Dynamics Simulations

Homogeneous nucleation from aluminum (Al) melt was investigated by million-atom molecular dynamics (MD) simulations utilizing the second nearest neighbor modified embedded atom method (MEAM) potentials. The natural spontaneous homogenous nucleation from the Al melt was produced without any influence of pressure, free surface effects and impurities. Initially isothermal crystal nucleation from undercooled melt was studied at different constant temperatures, and later superheated Al melt was quenched with different cooling rates. The crystal structure of nuclei, critical nucleus size, critical temperature for homogenous nucleation, induction time, and nucleation rate were determined. The quenching simulations clearly revealed three temperature regimes: sub-critical nucleation, super-critical nucleation, and solid-state grain growth regimes. The main crystalline phase was identified as face-centered cubic (fcc), but a hexagonal close-packed (hcp) and an amorphous solid phase were also detected. The hcp phase was created due to the formation of stacking faults during solidification of Al melt. By slowing down the cooling rate, the volume fraction of hcp and amorphous phases decreased. After the box was completely solid, grain growth was simulated and the grain growth exponent was determined for different annealing temperatures.Comment: 41 page

Multiphysics Modeling and Numerical Simulation in Computer-Aided Manufacturing Processes

The concept of Industry 4.0 is defined as a common term for technology and the concept of new digital tools to optimize the manufacturing process. Within this framework of modular smart factories, cyber-physical systems monitor physical processes creating a virtual copy of the physical world and making decentralized decisions. This article presents a review of the literature on virtual methods of computer-aided manufacturing processes. Numerical modeling is used to predict stress and temperature distribution, springback, material flow, and prediction of phase transformations, as well as for determining forming forces and the locations of potential wrinkling and cracking. The scope of the review has been limited to the last ten years, with an emphasis on the current state of knowledge. Intelligent production driven by the concept of Industry 4.0 and the demand for high-quality equipment in the aerospace and automotive industries forces the development of manufacturing techniques to progress towards intelligent manufacturing and ecological production. Multi-scale approaches that tend to move from macro- to micro- parameters become very important in numerical optimization programs. The software requirements for optimizing a fully coupled thermo-mechanical microstructure then increase rapidly. The highly advanced simulation programs based on our knowledge of physical and mechanical phenomena occurring in non-homogeneous materials allow a significant acceleration of the introduction of new products and the optimization of existing processes.publishedVersio

Study of dynamics and nanosegregation in ionic liquids and deep eutectic solvents by fluorescence correlation spectroscopy

Ionic liquids (ILs) and deep eutectics solvents (DESs) have emerged as a promising alternative to traditional organic solvents. This is due to their unique properties such as extremely low volatility, electroconductivity, unusual solvation and tuned miscibility, among other properties. Although, ILs and DESs are different types of solvents based on their molecular structures, they share many characteristics and properties that make them potentially attractive for a diversity of applications such as electrochemistry, synthesis, separation technologies, catalysis, materials science, and biochemistry. These unique properties have been interpreted as the result of their organization at the nanoscale level. Thus, the presence of nanosegregation in ILs and DESs is proposed to be important for many applications using these solvents, yet this nanoscale heterogeneity is poorly understood. In this dissertation, the translational diffusion dynamics of fluorophores in ILs and DESs films is reported as measured by fluorescence correlation spectroscopy. Theoretical studies have predicted a high degree of nanosegregation in tetraalkylphosphonium-based ILs. However, experimental studies that confirm these findings are scarce. To this end, fluorescence correlation spectroscopy was used to study molecular diffusion in a series of tetraalkylphosphonium ILs films. The primary motivation for this study was to understand how the nanostructural organization affects the diffusion behavior of fluorophores of different polarities, polar (Atto 590), and nonpolar fluorophore (DiD), when the cation and anion in tetraalkylphosphonium ILs are altered. From the results, it was concluded that spatial heterogeneity is present in these classes of ILs, given that the diffusion of the fluorescent probes deviates from the Brownian diffusion behavior. These deviations are attributed to the presence of structural heterogeneities in the tetraalkylphosphonium ILs. DESs have demonstrated increased potential for a diversity of applications, especially in separation technologies. Similar to ILs, nanostructural heterogeneity has been observed in DESs by theoretical and experimental studies. However, the fundamental understanding of DESs structure at the molecular level remains in a relatively early stage. To gain further insight into the presence of nanostructural heterogeneity in carboxylic-based DESs, fluorescence correlation spectroscopy experiments were performed for studying the translational diffusion properties of a hydrophilic (Atto 590) and a hydrophobic (DiI) fluorophore. Anomalous diffusion behavior was observed for the fluorescent molecules in all studied DESs. This anomalous diffusion behavior is characteristic of heterogeneous systems