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•Bubble coalescence has been observed at the very early stages of foam formation.•Extensive change in the cell densities have been observed as a consequence of different foaming ...procedures.•Talc in polystyrene has been observed to hinder early bubble coalescence, instead of nucleating more cells.•Pressure drop rate was observed to increase bubble nucleation rather linearly in a log-log plot.
In the underlying literature, bubble coalescence has been so far observed at the latest foaming stages, where bubble growing causes large extensional loads on the polymer film between two neighboring bubbles and eventually leads to the film rupture and to the merging of the two bubbles in a single one. This phenomenon is responsible for the coarsening of the foam morphology and, in turn, the worsening of the final foam properties. Here, the unanticipated, indirect, observation of a bubble coalescence phenomenon occurring at the very early stage of foaming, when most of the blowing agent is still solubilized in the polymer, is reported. Likewise, this phenomenon is responsible for a significant coarsening of the foam morphology. With the aim of investigating this phenomenon, a novel pressure vessel for foaming was utilized, capable of imposing two different cooling histories on two polymer samples, foamed at the same time. Results are presented on polystyrene foamed with carbon dioxide.
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.
Polyphenyl sulfone (PPSU) is a highly stable and rigid polymer with good chemical and mechanical resistance used in automotive and aerospace industries in high‐temperature applications during ...long‐operating cycles. PPSU foams are applied as core materials in lightweight composites ensuring high strength, fire resistance, thermal and acoustic insulation. In this paper, PPSU slabs are foamed using CO2, N2, and He as blowing agents (BAs). An experimental campaign is carried out to estimate the BAs' diffusivity in the polymer. In details, CO2 diffusivity ranges from 8E‐11 to 1E‐09 m2/s at temperatures from 220 to 260°C; diffusivity of N2 ranges from 2E‐10 to 5E‐09 m2/s (220–260°C) and it is 4E‐09 m2/s for He at 220°C. Foaming tests reveal an expansion ratio as high as 400% for CO2, 130% for He, and 150% for N2.Scanning electron microscopy analysis is also performed on produced samples, obtaining a cell number density of 1.3E07, 2.6E05, and 1.8E06 cells/cm3 when saturating, respectively, with CO2 at 220°C and 100 bar, N2 at 220°C and 100 bar, and He at 230°C and 100 bar.
Study of apparent diffusivity for different blowing agents, finalized at PPSU foam saturation.
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.
Graphene wetting by polymers is a critical issue to both the success of polymer-aided transfer of large size sheets onto specific substrates and to the development of well performing nanocomposites. ...Here we show for the first time that high temperature contact angle measurements can be performed to investigate the wettability of CVD graphene by molten polymers. In particular, poly(methyl methacrylate), a widely used polymer support for CVD graphene transfer, has been adopted herein for this proof-of-concept study and the values of contact angle and work of adhesion have been provided in the temperature range 170–200 °C.
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•We show that high temperature contact angle measurements can be performed to investigate the wettability of CVD graphene by molten polymers.•As proof-of-concept, the system molten PMMA on CVD graphene has been investigated in the temperature range 170 – 200 °C.•Kinetics of wetting were studied and the duration of the transient is consistent with the characteristic time for interfacial-tension-driven motions.•The work of adhesion between graphene and PMMA has been calculated in the investigated temperature range.
Elasticity in Bubble Rupture Tammaro, Daniele; Pasquino, Rossana; Villone, Massimiliano Maria ...
Langmuir,
05/2018, Letnik:
34, Številka:
19
Journal Article
Recenzirano
When a Newtonian bubble ruptures, the film retraction dynamics is controlled by the interplay of surface, inertial, and viscous forces. In case a viscoelastic liquid is considered, the scenario is ...enriched by the appearance of a new significant contribution, namely, the elastic force. In this paper, we investigate experimentally the retraction of viscoelastic bubbles inflated at different blowing rates, showing that the amount of elastic energy stored by the liquid film enclosing the bubble depends on the inflation history and in turn affects the velocity of film retraction when the bubble is punctured. Several viscoelastic liquids are considered. We also perform direct numerical simulations to support the experimental findings. Finally, we develop a simple heuristic model able to interpret the physical mechanism underlying the process.
Designing the foam structures in terms of density and morphology gives the chance to tune their mechanical and functional properties to the specific application. Nowadays, this design has been ...leveled up by the introduction of graded foams which are characterized by spatially nonuniform densities and/or morphologies. Graded foams have proved superior compared to uniform one in numerous examples and loading conditions but in pure compressive loading, where properties such as the Young's modulus of graded foams are always inferior to uniform ones. When using sintered beads foams, macroscopic pure compression may induce local bending on the single bead, which can be exploited to induce stiffening. This thesis was investigated on both polystyrene and thermoplastic polyurethane samples made by foamed beads sintered in a cylindrical mold. The response to compressive tests of graded foams made by sintering beads having nonuniform cells structures was compared to the uniform counterparts. The results evidenced that, at same average density, foams had up to a 30% ca. of increase of the Young's modulus when the beads are characterized by a denser outer layer.
This work represents an overview on types of wounds according to their definition, classification and dressing treatments. Natural and synthetic polymeric wound dressings types have been analyzed, ...providing a historical overview, from ancient to modern times. Currently, there is a wide choice of materials for the treatment of wounds, such as hydrocolloids, polyurethane and alginate patches, wafers, hydrogels and semi-permeable film dressings. These systems are often loaded with drugs such as antibiotics for the simultaneous delivery of drugs to prevent or cure infections caused by the exposition of blood vessel to open air. Among the presented techniques, a focus on foams has been provided, describing the most diffused branded products and their chemical, physical, biological and mechanical properties. Conventional and high-pressure methods for the production of foams for wound dressing are also analyzed in this work, with a proposed comparison in terms of process steps, efficiency and removal of solvent residue. Case studies, in vivo tests and models have been reported to identify the real applications of the produced foams.
This communication reports the design and fabrication of porous scaffolds of poly(ε‐caprolactone) (PCL) and PCL loaded with hydroxyapatite (HA) nanoparticles with bimodal pore size distributions by a ...two step depressurization solid‐state supercritical CO2 (scCO2) foaming process. Results show that the pore structure features of the scaffolds are strongly affected by the thermal history of the starting polymeric materials and by the depressurization profile. In particular, PCL and PCL‐HA nanocomposite scaffolds with bimodal and uniform pore size distributions are fabricated by quenching molten samples in liquid N2, solubilizing the scCO2 at 37 °C and 20 MPa, and further releasing the blowing agent in two steps: (1) from 20 to 10 MPa at a slow depressurization rate, and (2) from 10 MPa to the ambient pressure at a fast depressurization rate. The biocompatibility of the bimodal scaffolds is finally evaluated by the in vitro culture of human mesenchymal stem cells (MSCs), in order to assess their potential for tissue engineering applications.
The successful fabrication of bimodal porous scaffolds made of polycaprolactone (PCL) and PCL loaded with hydroxyapatite nanoparticles via the two step depressurization of solid‐state supercritical CO2 foaming is reported. The entire process was performed at 37°C, so opening new routes to the design of porous bioactive scaffolds for tissue engineering.
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