Foam as a geometer

Foams are found everywhere: in nature, in technology, in our home. They are examples of cellular materials: assemblies or clusters of cells (from Latin cella: a small compartment or enclosed region) packed together so that they fill space without gaps. Foams come in different kinds. Ordinary liquid foam is an experimental system that solves some difficult geometry problems

Simulation of defects in bubble clusters

Topological defects in foam, either isolated (disclinations and dislocations) or in pairs, affect the energy and stress, and play an important role in foam deformation. Surface Evolver simulations were performed on large finite clusters of bubbles. These allow us to evaluate the effect of the topology of the defects, and the distance between defects, on the energy and pressure of foam clusters of different sizes. The energy of such defects follows trends similar to known analytical results for a continuous medium

Built-In Self-Test Quality Assessment Using Hardware Fault Emulation in FPGAs

This paper addresses the problem of test quality assessment, namely of BIST solutions, implemented in FPGA and/or in ASIC, through Hardware Fault Emulation (HFE). A novel HFE methodology and tool is proposed, that, using partial reconfiguration, efficiently measures the quality of the BIST solution. The proposed HFE methodology uses Look-Up Tables (LUTs) fault models and is performed using local partial reconfiguration for fault injection on Xilinx(TM) Virtex and/or Spartan FPGA components, with small binary files. For ASIC cores, HFE is used to validate test vector selection to achieve high fault coverage on the physical structure. The methodology is fully automated. Results on ISCAS benchmarks and on an ARM core show that HFE can be orders of magnitude faster than software fault simulation or fully reconfigurable hardware fault emulation

In-line rheo-optical investigation of the dispersion of organoclay in a polymer matrix during twin-screw compounding

The dispersion mechanisms in a clay-based polymer nanocomposite (CPNC) during twin-screw extrusion are studied by in-situ rheo-optical techniques, which relate the CPNC morphology with its viscosity. This methodology avoids the problems associated with post extrusion structural rearrangement. The polydimethylsiloxane (PDMS) matrix, which can be processed at ambient and low temperatures, is used to bypass any issues associated with thermal degradation. Local heating in the first part of the extruder allows testing of the usefulness of low matrix viscosity to enhance polymer intercalation before applying larger stresses for clay dispersion. The comparison of clay particle sizes measured in line with models for the kinetics of particle dispersion indicates that larger screw speeds promote the break-up of clay particles, whereas smaller screw speeds favor the erosion of the clay tactoids. Thus, different levels of clay dispersion are generated, which do not simply relate to a progressively better PDMS intercalation and higher clay exfoliation as screw speed is increased. Reducing the PDMS viscosity in the first mixing zone of the screw facilitates dispersion at lower screw speeds, but a complex interplay between stresses and residence times at larger screw speeds is observed. More importantly, the results underline that the use of larger stresses is inefficient per se in dispersing clay if sufficient time is not given for PDMS to intercalate the clay galleries and thus facilitate tactoid disruption or erosion.The authors thank the FCT (Fundação para a Ciência e Tecnologia) for financial support under the framework of Strategic Funding grant UID/CTM/50025/2020. L.H. acknowledges funding from the FCT Investigator Programme through grant IF/00606/2014

What is the shape of an air bubble on a liquid surface?

We have calculated the equilibrium shape of the axially symmetric meniscus along which a spherical bubble contacts a flat liquid surface, by analytically integrating the Young-Laplace equation in the presence of gravity, in the limit of large Bond numbers. This method has the advantage that it provides semi-analytical expressions for key geometrical properties of the bubble in terms of the Bond number. Results are in good overall agreement with experimental data and are consistent with fully numerical (Surface Evolver) calculations. In particular, we are able to describe how the bubble shape changes from hemispherical, with a shallow flat bottom, to lenticular, with a deeper, curved bottom, as the Bond number is decreased

Ordering of Oblate Hard Particles between Hybrid Penetrable Walls.

We report a Monte Carlo (MC) simulation study of a model discotic liquid crystal (DLC) confined between hybrid walls with controllable penetrability. The model consists of oblate hard Gaussian overlap (HGO) particles. Particle-substrate interactions are modeled as follows: each substrate sees a particle as a disc of zero thickness and diameter D less than or equal to that of the actual particle, σ0, embedded inside the particle and located halfway along, and perpendicular to, its minor axis. This allows us to control the anchoring properties of the substrates, from planar (edge-on) for D ≈ 0 to homeotropic (face-on) for D ≈ σ0, which can be done independently at either substrate. Depending on the values of Ds ≡ D/σ0 at the top (D s t ) and bottom (D s b ) substrates, we find domains in (D s b , D s t ) space in which particle alignment is uniform planar (UP), is uniform homeotropic (UH), or varies linearly from planar at one substrate to homeotropic at the other (Lin). These domains are separated by regions of bistability (P-Lin and H-Lin), which appear to be wider than for prolate HGOs, and there may be also a small tristable (P-H-Lin) region. Results are compared with the predictions of density functional theory, implemented at the level of Onsager's second-virial approximation with Parsons-Lee rescaling. As in the case of symmetric confinement studied previously, the agreement between theory and simulation is substantially less good than for prolate HGOs: in particular, for the investigated substrate separation L = 6σ0, the Lin configuration is never predicted. These discrepancies are likely a consequence of the fact that Onsager's theory is less accurate for discs than for rods

Re-entrant phase behaviour of network fluids: A patchy particle model with temperature-dependent valence

We study a model consisting of particles with dissimilar bonding sites ("patches"), which exhibits self-assembly into chains connected by Y-junctions, and investigate its phase behaviour by both simulations and theory. We show that, as the energy cost epsilon(j) of forming Y-junctions increases, the extent of the liquid-vapour coexistence region at lower temperatures and densities is reduced. The phase diagram thus acquires a characteristic "pinched" shape in which the liquid branch density decreases as the temperature is lowered. To our knowledge, this is the first model in which the predicted topological phase transition between a fluid composed of short chains and a fluid rich in Y-junctions is actually observed. Above a certain threshold for epsilon(j), condensation ceases to exist because the entropy gain of forming Y-junctions can no longer offset their energy cost. We also show that the properties of these phase diagrams can be understood in terms of a temperature-dependent effective valence of the patchy particles. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3605703

Análise numérica da propagação de ondas sobre a plataforma continental interna e antepraia da costa sul do Brasil: de Torres a Mostardas

The shoreface and the inner shelf along the northern and the central coast of Rio Grande do Sul (RS) state, Brazil, between Torres and Mostardas, although displaying a homogeneous bottom relief show variations in slope and width. In the northern sector, between Torres and Pinhal, the inner shelf is wide, while the shoreface is narrow. Towards the south, these patterns are just the opposite. In this study, a model based on the parabolic approximation of the mild slope equation, REF/DIF.1, is used to simulate the wave propagation in this region. Wave refraction diagrams based on the predominant wave climate show a dominant and invariable divergent pattern for wave incidence from 90° to 135°. Numerical analyses of changes in wave energy, regarding differences in width and slope using a bottom friction coefficient fw = 0.01 in the different coastal sectors, show little influence of the bottom friction over the wave transformation processes. In general, a friction coefficient fw = 0.2 shows that wave energy reduction due to bottom friction over the wider shoreface south of Pinhal by comparison with the sector to the north is compensated by its shorter inner shelf width. The analyses show that lateral gradients of wave energy along the 10 m isobaths regarding the morphological differences between the two sectors are negligible.A plataforma continental interna e a antepraia ao longo do litoral norte e médio do estado do Rio Grande do Sul (RS), Brasil, entre Torres e Mostardas, têm o relevo do fundo homogêneo, mas apresentam variações de inclinação e largura. No setor norte, entre Torres e Pinhal, a plataforma interna é mais larga, enquanto a largura de antepraia é mais estreita. Por outro lado, o oposto ocorre ao Sul de Pinhal. Este trabalho analisa numericamente a propagação de ondas nesta região através do REF/DIF 1, modelo que adota a aproximação parabólica da equação de declive suave. Os diagramas de refração de ondas, baseados em um clima de onda predominante, indicam que o padrão dominante é o divergente para ondas incidentes de 90° a 135°. Análises numéricas das variações de energia de ondas, investigando a influência das variações da largura da plataforma interna e da largura de antepraia, e da fricção com o fundo considerando um coeficiente de atrito fw = 0,01, revelam a pouca influência do atrito com o fundo sobre a transformação das ondas. Adotando-se um coeficiente de atrito fw = 0,2, constata-se, de forma geral, que a influência do aumento da largura da antepraia do sul de Pinhal para o norte no atrito com o fundo é compensada pela influência da diminuição da largura da plataforma continental interna. As análises mostram que os gradientes laterais da energia da onda ao longo da isobatimétrica de 10 m, mesmo considerando as diferenças morfológicas entre os dois setores, são desprezíveis

Film blowing of PHB-based systems for home compostable food packaging

One of the routes to minimize the environmental impact of plastics waste is the use of bio-sourced and biodegradable alternatives, particularly for packaging applications. Although Polyhydroxyalkanoates (PHA) are attractive candidates for food packaging, they have poor processability, particularly for extrusion film blowing. Thus, one relatively successful alternative has been blending PHA with a biodegradable polymer. This work proposes film blowing of a co-extruded Poly (hydroxybutyrate) (PHB) layer with a poly butylene adipate-co-terephtalate (PBAT) layer to enhance bubble stability, mechanical and barrier properties. Co-extrusion is detailed, together with the different strategies followed to improve adhesion between film layers and the PHB content in the films. Films with thicknesses below 50 micron and elongation at break beyond 500 % were consistently produced.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 774088. LH also acknowledges funding from the Portuguese Foundation for Science and Technology Investigator Programme through grant IF/00606/2014

Idade, origem e protólitos de granitos variscos de três áreas portuguesas

Publicação digital - Online em 5 Fev. 201