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We analyzed bubble rupture and hole opening dynamics in a non-Newtonian fluid by investigating the retraction process of thin films after inflation at different blowing rates. The experiments were modeled through a dimensional analysis, with the aim of establishing a general approach on the bubble rupture dynamics and discerning the role of viscous, elastic, surface, and inertial forces on the opening velocity, according to the nature of the specific fluid. A new mathematical model, which includes all possible contributions to the hole opening dynamics, was proposed, to the best of our knowledge for the first time. The experimental evidence on the opening velocity as a function of the inflation rate was found to be in good agreement with the prediction of the model. The sensitivity of our modeling was tested by comparing our results with the existing models of retracting velocity.

The 2019 pandemic locked lab doors to students and guest colleagues, with an impoverishment for all. Inspired by the 2015 work on "cloud chemistry" at the University of Nottingham, we designed an experimental facility to go beyond doors and boundaries (Skilton et al., 2015). On December 16th, 2020, an entire class of grad students virtually entered the foamlab of the University of Naples Federico II and remotely performed a plastic foaming experiment. The autoclave design, the plastic granules-handling robot, the process control and the vision acquisition system were all engineered to allow easy control by inexperienced operators and to guarantee safety to the local instructor. The digitalization of the foaming process is an exhaustive and modern pedagogical activity whereby chemical engineering and materials science and engineering students test their knowledge of transport phenomena, thermodynamics and process control. Students' response was enthusiastic: each of them felt at the centre of the active learning process.

Insulating polymeric foams have an important role to play in increasing energy efficiency and therefore contributing to combating climate change. Their development in recent years has been driven towards the reduction of thermal conductivity and achievement of the required mechanical properties as main targets towards sustainability. This perception of sustainability has overseen the choice of raw materials, which are often toxic, or has placed research efforts on optimizing one constituent while the other necessary reactants remain hazardous. The transition to the circular economy requires a holistic understanding of sustainability and a shift in design methodology and the resulting research focus. This paper identifies research needs and possible strategies for polymeric foam development compatible with Circular Product Design and Green Engineering, based on an extensive literature review. Identified research needs include material characterization of a broader spectrum of polymer melt–gas solutions, ageing behavior, tailoring of the polymer chains, detailed understanding and modeling of the effects of shear on cell nucleation, and the upscaling of processing tools allowing for high and defined pressure drop rates.

In carbon dioxide-blown polymer foams, the solubility of carbon dioxide (CO

The micropatterning of thin films represents a challenging task, even for additive manufacturing techniques. In this work, we introduce the use of inkjet-printing technology coupled with a gas-foaming process, to produce patterned porosities on polymeric thin films, to develop a bubble-writing method. Inkjet printing of an aqueous solution of poly (vinyl alcohol) (PVA), a well-known gas-barrier polymer, allows the selective coating of a thin poly (lactic acid) (PLA) film, which is, successively, exposed to a gas-foaming process. The foaming of the thin PLA film is effective, only when PVA is printed on top, since the PVA barrier hinders the premature loss of the gas, thus allowing the formation of cavities (bubbles) in the covered areas; then, removing the PVA coating by water washing forms a bubble pattern. As a proof of concept, the surface-morphology features of the patterned porous PLA films have been proven effective at driving endothelial cell growth. A new technological platform is, hence, introduced in the field of tissue engineering and, in general, in fields involving thin films, where a patterned porous structure may add value.

The incorporation of graphene-related materials as nanofiller can produce multifunctional foams with enhanced specific properties and density reduction. Herein we report on the preparation of microcellular thermoplastic polyurethane/graphene foams by batch foaming. Solution blending was first adopted to disperse graphene nanoplatelets (GNP) in the elastomeric matrix. Then, a foaming process based on the use of supercritical CO2 was adopted to produce the microcellular TPU/GNP composite foams with graphene content up to 1 wt%. The EMI shielding behaviour of the TPU/GNP foams has been assessed in the THz range, and has revealed their potential in comparison with other graphene-filled foams presented in the literature, that exhibit similar specific shielding effectiveness but at much higher content of graphene-related materials (10-30 wt%).

In this study, we investigated the influence of mixing conditions on the foaming process of water blown polyurethane (PU) foams obtained at different mixing speeds (50, 500, 1000 and 2000 rpm). In particular, the morphological evolution during the foaming process, in terms of the bubble size and bubble density, was studied via optical observations, while the effects on the reaction kinetics were monitored using in situ FTIR spectroscopy. At the slow mixing speed (50 rpm), no air bubbles were included and the early foaming process was characterized by the formation of new bubbles (CO2 nucleation), provided by the blowing reaction. Later on, it was observed that the coalescence affected the overall foaming process, caused by the gelling reaction, which was inhibited by the indigent mixing conditions and could not withstand the bubbles expansion. As a result, a PU foam with a coarse cellular structure and an average bubble size of 173 µm was obtained. In this case, the bubbles degeneration rate, dN/dt, was −3095 bubble·cm−3·s−1. On the contrary, at 500 rpm, air bubbles were included into the PU reaction system (aeration) and no formation of new bubbles was observed during the foaming process. After this, the air bubbles underwent growth caused by diffusion of the CO2 provided by the blowing reaction. As the gelling reaction was not strongly depleted as in the case at 50 rpm, the coalescence less affected the bubble growth (dN/dt = −2654 bubble·cm−3·s−1), leading to a PU foam with an average bubble size of 94 µm. For the foams obtained at 1000 and 2000 rpm, the bubble degeneration was first affected by coalescence and then by Ostwald ripening, and a finer cellular structure was observed (with average bubble sizes of 62 µm and 63 µm for 1000 rpm and 2000 rpm, respectively). During the first foaming stage, the coalescence was less predominant in the bubble growth (with dN/dt values of −1838 bubble·cm−3·s−1 and −1601 bubble·cm−3·s−1, respectively) compared to 50 rpm and 500 rpm. This occurrence was ascribed to the more balanced process between the bubble expansion and the PU polymerization caused by the more suitable mixing conditions. During the late foaming stage, the Ostwald ripening was only responsible for the further bubble degeneration (with dN/dt values of −89 bubble·cm−3·s−1 and −69 bubble·cm−3·s−1, respectively).

A visual observation experiment mimicking the pressure history in foam injection molding was utilized to infer the bubbles formation in core-back operations. The different stages of bubble nucleation, dissolution, and re-nucle- ation, corresponding to injection, packing, and core-back opening, respectively, were studied. A reduction of the number density of re-nucleated bubbles with respect to first nucleation was observed. In order to understand this observa- tion, the evolution of the blowing agent concentration around the dissolved bubbles was analyzed. A characteristic time was introduced, required to achieve blowing agent uniformity or, equivalently, required to cancel the mass transport-mediated memory of the system. A numerical model, assuming Fickian behavior, was derived and validated with the experimental results. The study allowed for the development of a predictive tool to design core-back operations in foam injection molding.

The influence of mild CO2 treatment on thermal properties of poly(L-lactic acid) (PLLA) is discussed in this article. A slowly crystallizing PLLA with 4% D-isomer was treated with CO2 at room temperature and moderate pressures (up to 6.0 MPa) for short times, not longer than 20 min. These mild treatments are sufficient to develop ε-form (PLLA/CO2 complex), which evolves to α"-mesophase after desorption of CO2. The amount of mesophase depends on the pressure of CO2 and the sorption times, and significantly affects the thermal properties of PLLA. Although the mesophase melts immediately above the glass transition, it affects subsequent cold crystallization. The influence of CO2 sorption time and pressure on the cold crystallization kinetics of PLLA is quantified here, and it is found that only a few minutes of treatment with low-pressure CO2 is sufficient to enhance the crys- tallization rate. After longer exposure to CO2, α-crystals also develop, especially under high CO2 pressure, hence proper adjustment of these parameters can allow tuning the structure and thermal properties of PLLA. The results demonstrate the possibility of producing CO2-induced mesophase at room temperature using very short sorption times, thus introducing a fast, green and cost-effective way to tailor the crystallization kinetics of PLLA.

This study investigates the effect of liquid-type organofluorine additives (OFAs) on the morphology, thermal conductivity and mechanical properties of rigid polyurethane (PU) and polyisocyanurate (PIR) foams. Foams were characterized in terms of their morphology (density, average cell size, anisotropy ratio, open cell content), thermal conductivity and compressive as well as flexural properties. Based on the results, we observed that OFAs efficiently reduced the average cell size of both PU and PIR foams, leading to improved thermal insulating and mechanical properties.

In this study, we investigated the preservation mechanisms induced by the use of organofluorine additives (OFAs), at different contents on the foaming process of water−pentane blown rigid polyurethane and polyisocyanurate foams (PUFs and PIRFs). The morphological evolution was observed via optical observation and characterized in terms of average bubble size and bubble density. For both PUFs and PIRFs, the foaming process was formed by a two-stage mechanism, consistent with the bubble growth and degeneration. The first foaming stage was affected by combined bubble coalescence and Ostwald ripening (OR) in the case of PUFs, while coalescence only in the case of PIRFs. The second foaming stage was affected by only OR for all the PU and PIR formulations. Interestingly, it was revealed that OFAs can act with two possible mechanisms: (i) enhanced air bubble inclusion during the mixing stage and (ii) OR inhibition effect on the bubble degeneration. The dual function was related to the unique chemical structures of the OFAs, conferring a high dissolution capacity for the gas phase such as air bubbles included in the polymeric phase, in which they are insoluble. In particular, the OR inhibition was regulated by the addition of OFA that possibly induced two effects: (i) a reduction in system surface tension that mitigated the Laplace pressure and (ii) the incompatibility of OFA with the polymeric phase that remains confined within the air bubbles, leading the partial pressure of OFA to contribute together with the partial pressure exerted by the other solutes so that the pressure of the internal bubble counterbalances the pressures of Laplace and the reacting mixture. In addition, morphology features such as foam density, average bubble size, anisotropy ratio, and open cell content were also measured. Based on the results, we observed that OFAs efficiently reduced the average bubble sizes of both PUFs and PIRFs, with negligible effects on the foam density, weakening of the bubble walls, and orientation of the bubble distribution.

The production of thermoplastic polyurethane-based porous heterostructures combining physical foaming with fused deposition modeling is detailed in this contribution. The choice of combining these two techniques lies in the possibility of creating objects endowed with a dual-scale structure at millimeter scale by fused deposition modeling and at microscopic scale by gas foaming. Thermal stability and rheological properties of the neat polymer were studied prior to foaming to design a suitable processing protocol and three different combined techniques are proposed: pressure quench, temperature rise and direct 3D foam printing. Foam morphologies were evaluated by SEM and foamed samples were characterized by thermal and mechanical analyses to highlight the differences among the combined processing techniques. Samples foamed via pressure quench exhibit the highest degree of crystallinity and a uniform cell morphology, also resulting in the largest stiffness. The results presented in this contribution open up the possibility of producing objects with complex geometry and porosity architecture at the dual scale.

We report the design of a microcapillary rheometer (μCR) that allows to perform experiments rapidly and in a broad range of shear rates (i.e., from 0.1 to 1000 s−1), using small amounts of material (i.e., just few milligrams). Additionally, multiple measurement parallelization makes it suitable for High-Throughput Rheological Experimentation of polymer melts (HT-Rheo-E). The novel rheometer consists of a set of three cylindrical microcapillaries in which the fluid flows driven by a controlled pressure. A camera, placed at the capillary exit, records the fluid motion to measure its flow rate, from which the fluid viscosity can be determined. The optimization of the setup allowed for reliable and fast viscosity measurements using ca. 10 mg of material. The current work reports the design of the rheometer and validation measurements on several model fluids. The microfabricated μCR is of potential interest for applications in quality control and research where rapid and repeated measurements using limited milligrams of polymer are required, as well as for High-Throughput-Experimentation of complex fluids (e.g., biological systems).

This work aims at obtaining a systematic description and prediction of the multicomponent gas separation behavior of membranes, using the Non-Equilibrium Lattice Fluid (NELF) model for gas sorption and the Standard Transport (ST) model for gas permeation. The scheme is applied to a comprehensive analysis of CO2/CH4 separation with cellulose acetate membranes. A dedicated experimental campaign (pressure-volume-temperature (pVT), differential scanning calorimetry (DSC) and pure gas sorption tests) was performed to obtain reliable model parameters, accounting also for crystallinity. The approach was validated against a complete set of literature data, including mixed gas sorption and permeation.The parameters obtained were used to perform predictive simulations of mixed CO2/CH4 sorption and permeation in a wide mixture composition range. The NELF model accurately predicts the effect of temperature on sorption, as well as the strong competitive exclusion of CH4 when CO2 is present, that enhances the solubility-selectivity. The ST model correctly estimates the experimentally observed lower-than-ideal CO2/CH4 perm-selectivity. The model shows that low perm-selectivity is due to mixed gas diffusivity-selectivity (which is not available experimentally): even though solubility-selectivity increases in the mixed gas case, diffusivity-selectivity suffers a larger departure from pure gas conditions, ultimately leading to a lower perm-selectivity under multicomponent conditions.

Optimization of post polymerization processes of polyolefin elastomers (POE) involving solvents is of considerable industrial interest. To this aim, experimental determination and theoretical interpretation of the thermodynamics and mass transport properties of POE-solvent mixtures is relevant. Sorption behavior of n-hexane vapor in a commercial propylene-ethylene elastomer (V8880 VistamaxxTM from ExxonMobil, Machelen, Belgium) is addressed here, determining experimentally the sorption isotherms at temperatures ranging from 115 to 140 °C and pressure values of n-hexane vapor up to 1 atm. Sorption isotherms have been interpreted using a Non Random Lattice Fluid (NRLF) Equation of State model retrieving, from data fitting, the value of the binary interaction parameter for the n-hexane/V8880 system. Both the cases of temperature-independent and of temperature-dependent binary interaction parameter have been considered. Sorption kinetics was also investigated at different pressures and has been interpreted using a Fick’s model determining values of the mutual diffusivity as a function of temperature and of n-hexane/V8880 mixture composition. From these values, n-hexane intra-diffusion coefficient has been calculated interpreting its dependence on mixture concentration and temperature by a semi-empiric model based on free volume arguments.

Polymeric foams are low-density key components for many lightweight structures used in construction and means of transportation, and for applications where porosity determines specific functionalities for insulation, packaging, cushioning, safety, and biomedical uses. The aims of this chapter are (i) to illustrate the evolution of the foam concept and its functions from the beginning to the current applications, (ii) to provide an overview on the main properties for conventional and more advanced materials, and (iii) to introduce some of the most important current developments.

Hypothesis Thin liquid films are important in many scientific fields. In particular, films with both the surface layers exposed to a different fluid phase, known as freestanding films, are relevant in the ambit of foams and emulsions. Hence, there is a great interest in developing novel techniques allowing to form large and stable freestanding liquid films and to follow their dynamics. Experiments We develop a novel opto-mechanical tool allowing to perform and study the preparation and the capillary leveling flow of axisymmetric bare freestanding liquid films. The tool is composed by a customized motorized iris diaphragm and by an innovative joint imaging setup combining digital holography and white light color interferometry that enables real-time measurement of film thickness over a large field of view. The dynamics of films made of a model Newtonian fluid, i.e., high-viscosity silicone oil, is studied. Direct numerical simulations and a hydrodynamic model based on the lubrication theory are used to support the experimental results. Findings Iris opening induces the formation of large circular freestanding films with a stepped profile. Once iris opening is stopped, the films undergo a capillary leveling flow tending to flatten their profile. The leveling flow follows the theoretical scaling given by Ilton et al. [1]. We prove through numerical simulations that an equi-biaxial extensional flow occurs at the film center. Furthermore, we observe the formation and dynamics of dimples in bare freestanding films for the first time.

Knowledge of the thermodynamics of sorption of low molecular weight compounds in rubbery and glassy polymers is of great relevance to understanding the physics and dynamics of polymer foaming and for the technological implementation of foaming processes. This chapter is devoted to the illustration of the recent advances in statistical thermodynamics approaches toward the elaboration of a theoretical framework aimed at the description of sorption thermodynamics of small molecules in polymers, addressing also systems that involve specific intra- and intermolecular interactions. In particular, the attention is focused on sorption models rooted on Equation of State theories. Notably, the models discussed here are structured to specifically account for nonrandom distribution of molecular species and for dealing with several kinds of specific interactions, addressing both for the case of rubbery polymers and of out-of-equilibrium glassy polymers. Moreover, a description of the experimental approaches devoted to the measurement of sorption isotherms is provided. Several kinds of experimental methodologies aimed at the description of sorption in polymers are described with attention also to the molecular-level characterization of the polymer-penetrant systems provided by vibrational spectroscopy (FTIR, Raman), which allows for the development of in situ, time-resolved measurements. Sorption thermodynamics in polymers is examined here considering the contact of polymer with a penetrant in a state of a subcritical vapor, of a supercritical fluid, or of a gas.

This study investigates the influence of air bubbles inclusion on the foaming of water blown and water-pentane blown rigid polyurethane and polyisocyanurate foams. It was observed that, when a large number of air bubbles were included in the reacting system during the mixing stage (fast mixing), no further bubble nucleation from water reaction or pentane evaporation occurred. In this case, foam morphology was solely dictated by the mixing stage and by later bubble coarsening. Instead, when no air bubbles were included during mixing (slow mixing), nucleation of new bubbles from CO2 (from water-isocyanate reaction) and pentane was observed. Furthermore, both bubble coalescence and Ostwald ripening were observed as mechanisms responsible for the foam morphology coarsening, the latter being less effective in the case of polyisocyanurate foams.

The intensive exploitation of resources on a global level has led to a progressive depletion of mineral reserves, which were proved to be insufficient to meet the high demand for high-technological devices. On the other hand, the continuous production of Waste from Electrical and Electronic Equipment (WEEE) is causing serious environmental problems, due to the complex composition of WEEE, which makes the recycling and reuse particularly challenging. The average metal content of WEEE is estimated to be around 30% and varies depending on the manufacturing period and brand of production. It contains base metals and precious metals, such as gold and palladium. The remaining 70% of WEEEs is composed of plastics, resins, and glassy materials. The recovery of metals from WEEEs is characterized by two main processes well represented by the literature: Pyrometallurgy and hydrometallurgy. Both of them require the pre-treatment of WEEEs, such as dismantling and magnetic separation of plastics. In this work, the selective adsorption of precious metals has been attempted, using copper, gold, and palladium aqueous solutions and mixtures of them. A screening on different adsorbent materials such as granular activated carbons and polymers, either as pellets or foams, has been performed. Among these, PolyEther Block Amide (PEBA) was elected as the most performing adsorbent in terms of gold selectivity over copper. Spent PEBA has been then characterized using scanning electron microscope, coupled with energy dispersive spectroscopy, demonstrating the predominant presence of gold in most analyzed sites, either in the pellet or foam form.

Although a significant amount of research has already been conducted on the various aspects of foaming of a large variety of polymeric materials, much work still needs to be done in certain areas. The main scientific open questions and technological challenges, together with the current research trends and developments, are analyzed in this chapter with the aim of examining the perspectives for further advances in the field.

The first wave of the COVID-19 pandemic brought about a broader use of masks by both professionals and the general population. This resulted in a severe worldwide shortage of devices and the need to increase import and activate production of safe and effective surgical masks at the national level. In order to support the demand for testing surgical masks in the Italian context, Universities provided their contribution by setting up laboratories for testing mask performance before releasing products into the national market. This paper reports the effort of seven Italian university laboratories who set up facilities for testing face masks during the emergency period of the COVID-19 pandemic. Measurement set-ups were built, adapting the methods specified in the EN 14683:2019+AC. Data on differential pressure (DP) and bacterial filtration efficiency (BFE) of 120 masks, including different materials and designs, were collected over three months. More than 60% of the masks satisfied requirements for DP and BFE set by the standard. Masks made of nonwoven polypropylene with at least three layers (spunbonded–meltblown–spunbonded) showed the best results, ensuring both good breathability and high filtration efficiency. The majority of the masks created with alternative materials and designs did not comply with both standard requirements, resulting in suitability only as community masks. The effective partnering between universities and industries to meet a public need in an emergency context represented a fruitful example of the so-called university “third-mission”.

Significance The bursting bubbles are central in many natural and engineering processes because they influence the heat, mass, and momentum transfer; for example, fragmentation and cell death are influenced by the mechanical stresses produced by the hydrodynamic flows caused by the fast and frequent bubble bursting on viscoelastic surfaces in bioreactors. We investigate the bursting bubbles with viscoelastic interfaces and demonstrate that the interfacial viscoelasticity changes the bubble rupture mode. We present the characteristics of the bubble rupture mode that produces flower-shape bubble bursting and a validated modeling that can predict the number of petals formed during the rupture. This work presents insights for understanding and controlling the bursting bubbles with viscoelastic interfaces with both fundamental and practical implications., The lifetime of bubbles, from formation to rupture, attracts attention because bubbles are often present in natural and industrial processes, and their geometry, drainage, coarsening, and rupture strongly affect those operations. Bubble rupture happens rapidly, and it may generate a cascade of small droplets or bubbles. Once a hole is nucleated within a bubble, it opens up with a variety of shapes and velocities depending on the liquid properties. A range of bubble rupture modes are reported in literature in which the reduction of a surface energy drives the rupture against inertial and viscous forces. The role of surface viscoelasticity of the liquid film in this colorful scenario is, however, still unknown. We found that the presence of interfacial viscoelasticity has a profound effect in the bubble bursting dynamics. Indeed, we observed different bubble bursting mechanisms upon the transition from viscous-controlled to surface viscoelasticity-controlled rupture. When this transition occurs, a bursting bubble resembling the blooming of a flower is observed. A simple modeling argument is proposed, leading to the prediction of the characteristic length scales and the number and shape of the bubble flower petals, thus paving the way for the control of liquid formulations with surface viscoelasticity as a key ingredient. These findings can have important implications in the study of bubble dynamics, with consequences for the numerous processes involving bubble rupture. Bubble flowering can indeed impact phenomena such as the spreading of nutrients in nature or the life of cells in bioreactors.

Quantitative modeling for the design of polymer foaming processes requires the knowledge of thermal properties of the mixtures of polymer and blowing agents. In fact, the presence of a penetrant within a polymer significantly affects the glass transition and crystallization/melting temperature of the starting neat polymer. Moreover, also energy transport parameters are affected, with particular reference to thermal conductivity. In this chapter, the phenomenology of glass transition in polymer-penetrant systems is first discussed, illustrating several theoretical approaches aimed at its interpretation. The effect of the presence of penetrant on the crystallization/melting behavior of the mixtures is then presented, introducing the recent advancements in their theoretical interpretation. Further discussion of these issues, in the perspective of the foaming process, is provided in Chapter 13 . Finally, experimental and modeling results for thermal conductivity of polymer-penetrant mixtures are discussed in some detail. All these aspects have been considered including the case of contact of a polymer with a physical blowing agent in the supercritical fluid state.

The gas foaming, which makes use of a physical blowing agent (e.g., carbon dioxide, nitrogen, hydrocarbons, etc.) to form bubbles in a softened polymer, is the most used method for the making of polymeric foams, mainly because of its high productivity. In this chapter, an overview of the state of the art of the gas foaming method and the proposed modeling approaches will be provided.