Multiphysics modelling of the mechanical properties in polymers obtained via photo-induced polymerization

Photopolymerization is an advanced technology to trigger free radical polymerization in a liquid monomer solution through light-induced curing, during which mechanical properties of the material are significantly transformed. Widely used in additive manufacturing, parts fabricated with this technique display precisions up to the nanoscale; however, the performance of final components is not only affected by the raw material but also by the specific setup employed during the printing process. In this paper, we develop a multiphysics model to predict the mechanical properties of the photo-cured components, by taking into account the process parameters involved in the considered additive manufacturing technology. In the approach proposed, the main chemical, physical, and mechanical aspects of photopolymerization are modelled and implemented in a finite element framework. Specifically, the kinetics of light diffusion from a moving source and chain formation in the liquid monomer is coupled to a statistical approach to describe the mechanical properties as a function of the degree of cure. Several parametric examples are provided, in order to quantify the effects of the printing settings on the spatial distribution of the final properties in the component. The proposed approach provides a tool to predict the mechanical features of additively manufactured parts, which designers can adopt to optimize the desired characteristics of the products.

Active thermal cloaking and mimicking

We present an active cloaking method for the parabolic heat (and mass or light diffusion) equation that can hide both objects and sources. By active, we mean that it relies on designing monopole and dipole heat source distributions on the boundary of the region to be cloaked. The same technique can be used to make a source or an object look like a different one to an observer outside the cloaked region, from the perspective of thermal measurements. Our results assume a homogeneous isotropic bulk medium and require knowledge of the source to cloak or mimic, but are in most cases independent of the object to cloak.

Bats of the Prespa Basin in northern Greece: acoustic survey, new records and relationships to light pollution (Chiroptera)

The bat survey in the Greek part of the Prespa basin in 2018–2020 confirmed, despite the limited methods used, its extraordinary importance for bat diversity. The study documented the presence of 22 bat forms (20 species) by acoustic surveys and a favourable status of some roosts including several nursery colonies. Three of the recorded species are here reported for the first time, Eptesicus nilssonii (the first record for Greece), Nyctalus lasiopterus, and Barbastella barbastellus, however, confirmation by a capture or direct observation of an individual is still needed. The collected data showed that bats do not avoid the urbanized parts of the study area and underlined the importance of old houses, churches, and ruins providing suitable roosting and foraging opportunities. The measurements of light pollution in villages revealed a high amount of light diffusion caused by the used type of shieldless luminaires. Heat maps of light intensity revealed a slightly higher bat activity in almost all villages in 2020, upon the situation in 2018. Bat passes were recorded not only in the peripheries of the highest illumination zones but also inside them, although some of the bat passes may belong to bats flying enough in the dark above the light levels of the installed lamps. The study provides application of mitigation actions for the Municipality of Prespa with the main aim to reduce the current level of the ALAN (Artificial Light At Night).

Estimation of errors in determining corrosion grain sizes by analysis of diffuse light reflection signal

The approaches to estimation of lower boundary of the inverse problem solution error concerning the sizing the corrosion micro defects inside the submillimeter corrosion spots has been proposed. It is assumed that pointed error depends on random location of the corrosion spots. The method based on comparison of two estimations of light diffusion reflectance sensor’s signal discrepancy. The first estimation is based on the standard deviation for the discrepancy caused by randomly located corrosion spots. The second one is based on corrosion grains’ size deviation. Also, it is found that the discrepancy based on deviations of signal peaks positions provides more stable solution for the corrosion micro defects sizes.

Lighting Simulation in 3D Printing

3D printing for rapid prototyping and production of unique objects is being actively developed. Consumer-grade printers are now commonly available for a range of purposes, while increasingly advanced techniques allow us to fabricate novel shapes, mechanical properties, and appearances. The printers’ capabilities have improved dramatically from printing single-material objects to producing detailed structures with pervoxel material variation. Since the 2010s, it is possible to fabricate full-colour 3D objects with resolutions of hundreds of DPI (voxels’ dimensions are in the order of 10 μm). Such capabilities are most prominent in printers based on the photo-polymer jetting process. Ideally, it should be possible now to produce photorealistic appearances or visually indistinguishable objects copies for, e.g., cultural-heritage applications.However, the resins used as print materials in commercial devices are inherently translucent, i.e., exhibit significant sub-surface scattering. This serves effective colour mixing in full-colour print processes, thus commercial printer drivers offer high-quality colour reproduction. At the same time, the resulting light diffusion leads to over-blurring and potential colour bleeding when printingspatially-varying colour textures. This translucent ‘crosstalk’ between surface points also strongly depends on the internal structure of the volume surrounding each surface point.Previously existing scattering-aware methods used simplified models for light diffusion and accepted the visual blur as an immutable property of the print medium.In this talk, we present the series of works conducted by a consortium of several institutes (Max-Planck Institute for Informatics, Germany; Charles University in Prague, Czech Republic; Institute of Science and Technology, Austria; University Col-lege London, United Kingdom; Universita della Svizzera Italiana, Switzerland; The Keldysh Institute of Applied Mathematics RAS, Russia). Our work counteracts heterogeneous scattering to obtain the impression of a crisp albedo texture on top of the 3D print, by optimizing for a fully volumetric material distribution that preserves the target appearance.We build our iterative method on top of a general Monte-Carlo simulation of heterogeneous scattering. We find out that a certain arrangement of materials expands the gamut of achievable appearances and makes it possible to produce sharp textures. This knowledge built-in into the volume-update step enables convergence justafter 10–15 iterations. We verify these findings using an established stochastic gradient-descent optimization for small canonical objects where it is feasible computationally.Expansion of our method to fabrication of arbitrary 3D objects with the translucent resins opens a set of problems of achievable colour combinations on the two sides of thin shapes, in the extreme convex and concave shapes. Physically correct lighting simulation enables exploration of these extreme cases where no ideal solution is possible. It turns out that a re-formulation of established gamut-mapping methods is needed for the medium with the inherent cross-talk properties such as the scattering resins of the modern full-colour 3D printers.Elaborating further ideas from we also propose a fast forward predictor of the object's surface appearance based on a neural network to replace the Monte-Carlo simulation in order to speed up the preparation of the model by 300 times. The achieved acceleration allows to reduce simulation time to minutes for a single, GPU-equipped workstation.Thismakesthe print preparation timings practical.

Numerical Technique for Study of Noise Grating Dynamics in Holographic Photopolymers

Although the angular distribution of noise gratings in holographic photopolymer is understood to arise from Bragg matching, the details of scatter strength and dynamics are not fully understood. This confounds development of materials and recording techniques that minimize haze. Here, the kinetics are studied using a multi-physics numerical approach coupling diffraction of light from the dynamic material including scatter centers, reactions of chemical species initiated by this light, diffusion and swelling of these constituents, and the formation of the refractive index from the resulting composition. The approach is validated in the case of two-beam transmission holography by comparison to traditional harmonic series and rigorous coupled-mode approaches. Two beam holography in the presence of scatter is then used to study haze development. This reveals that haze due to weak noise gratings grows significantly above initial scatter only in reaction-limited materials, consistent with proposed Bragg-matched amplification mechanisms. Amplified haze is found to be proportional to initial scatter, quantifying the impact of clean sample fabrication. Conversely, haze is found to grow super-linearly with sample thickness, illustrating the significant challenge for applications requiring low haze in large thickness.

Transition from light diffusion to localization in three-dimensional amorphous dielectric networks near the band edge

Localization of light is the photon analog of electron localization in disordered lattices, for whose discovery Anderson received the Nobel prize in 1977. The question about its existence in open three-dimensional materials has eluded an experimental and full theoretical verification for decades. Here we study numerically electromagnetic vector wave transmittance through realistic digital representations of hyperuniform dielectric networks, a new class of highly correlated but disordered photonic band gap materials. We identify the evanescent decay of the transmitted power in the gap and diffusive transport far from the gap. Near the gap, we find that transport sets off diffusive but, with increasing slab thickness, crosses over gradually to a faster decay, signaling localization. We show that we can describe the transition to localization at the mobility edge using the self-consistent theory of localization based on the concept of a position-dependent diffusion coefficient.