Frej Mighri

Influence of elastic properties on drop deformation and breakup in shear flow

In this article we report experimental results on the deformation and the critical breakup conditions of a single drop in a medium under simple shear flow. The role played by both drop and matrix elasticities is quantified by using constant viscosity elastic (Boger) fluids. The experiments were conducted using two transparent parallel disks mounted on a R-18 Weissenberg rheogoniometer. The critical shear rate was determined by imposing successive small changes in shear rate from lower to higher values until the drop breakup was observed. The results show remarkable differences in the mode of deformation and breakup for Newtonian and elastic fluid systems. It is also found that the drop resistance to deformation and breakup increases with increasing elasticity ratio. The contribution of the drop and matrix elasticities is quantified by using an empirical relation established between the drop deformation and the capillary number, Ca. The critical breakup conditions, such as a dimensionless breakup time, tb*...

Influence of elastic properties on drop deformation in elongational flow

We report experimental results on the deformation of a single drop suspended in a medium under uniaxial elongational flow along the central axis of a converging conical channel made of Plexiglas. Both the drop and the continuous phases consist of constant viscosity elastic fluids, so-called Boger fluids. This study reveals several interesting features about the role played by both the drop and matrix elasticities on the drop deformability. In a given matrix fluid, the drop deformation decreases as its elasticity increases. For a given drop fluid, the matrix elasticity has the opposite effect: the drop deformation increases with increasing matrix elasticity. An empirical relation between the drop and matrix deformations is established as a function of the drop and matrix characteristic elastic times.

Dispersion visualization of model fluids in a transparent Couette flow cell

Dispersion mechanisms in model fluid systems of different viscosity ratios were studied in a transparent Couette flow cell. The counter-rotating concentric cylinders were driven by two independent dc motors. Drops of the minor phase were then maintained at a constant position by fixing the inner and outer cylinders’ rotational speeds. The advantage of this new setup is that visualization can be made at high shear rates without any secondary flow effects, usually observed with cone-plate or parallel plates geometry. Constant viscosity viscoelastic drops (Boger fluid) and Newtonian drops [high viscosity polydimethylsiloxane, (PDMS)], deformed at low shear rates in a Newtonian matrix (low viscosity PDMS), oriented along the flow field and drop deformation increased with shear rate, as expected. However, when a critical shear rate (characteristic of the fluid system used) was reached, the deformed drops began to contract in the flow direction. When increasing the shear rate over this critical value, drop cont...

Relationship between rheological and electrical percolation in a polymer nanocomposite with semiconductor inclusions

Microstructure, electrical conductivity, and rheological properties of nanocomposites based on isotactic polypropylene (iPP) containing semiconductor nanoparticles of TiO2 were studied. Compatibilized and uncompatibilized nanocomposites containing a wide range of TiO2 concentrations (up to 15 vol%) were prepared by melt compounding in a twin-screw extruder via a masterbatch method. An anhydride-modified PP (AMPP) was used as the compatibilizer. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and image analysis techniques were utilized to study the morphology evolution in the samples. Analyzing the results of direct current (DC) electrical conductivity measurements showed a lower percolation threshold for the uncompatibilized samples, compared to the compatibilized ones. In order to estimate the percolation threshold, linear and nonlinear melt-state viscoelastic properties of the samples were studied. Liquid-solid transition and nonterminal behavior of the uncompatibilized samples were observed at relatively lower range of TiO2 loading, compared to the compatibilized samples. It was an indication of lower rheological percolation threshold in the uncompatibilized nanocomposites which was in agreement with the electrical percolation threshold. Scaling analysis of strain sweep tests above the percolation thresholds of the nanocomposites resulted in a lower fractal dimension for the uncompatibilized samples.

Surface orientation of hydrophilic groups in sulfonated poly(ether ether ketone) membranes.

Sulfonated poly(ether ether ketone) copolymers (SPEEK) with a range of sulfonate contents (SC 77-51%) were synthesized via nucleophilic substitution polycondensation from hydroquinone and sulfonated hydroquinone. Membranes obtained by solvent casting from dimethylacetamide onto glass surfaces were analyzed for surface behavior. The surfaces of a membrane were hydrophobic in air, but hydrophilic in water. This surface behavior was corroborated by water contact angle vs. time, using sessile drop measurements. Hydrophilic sulfonic group aggregates on SPEEK chain and various media contacting with the top or bottom surfaces of the membrane during the fabrication process caused differences in surface behavior. Angle-dependent XPS showed that there was a higher atomic S/C ratio of the bottom surface than on the corresponding top surface. The hydrophilic sulfonic groups were in higher concentration within the membrane, with the concentration gradually decreasing towards the surface for SPEEK-HQ-80 and SPEEK-HQ-70 membranes. Acidification with strong acid and higher temperature induced a more hydrophilic surface on a membrane than a milder acidification process. The depth profile at the membrane surface was examined by a combination of contact angle, XPS and ATR-FTIR.

Morphology development of polypropylene cellular films for piezoelectric applications

In the present work, the dielectric nature of polypropylene and the softness of the cellular structure in the thickness direction of polypropylene cellular films were combined together to create a low cost and easily processable piezoelectric material. The effects of processing parameters, polymer crystallinity, filler type and concentration on the final structure of the cellular films were investigated. Three grades of calcium carbonate (CaCO3) filler having an average particle size of 0.7, 3 and 12 µm were used. An optimized cellular film was developed by biaxial stretching and inflating a film made from three hot-pressed polypropylene sheets filled with 20 wt% of calcium carbonate particles with an average size of 12 µm. The cells have an average length of 35 µm and height of 4 µm with a density ratio of 0.8. Its particle size distribution compares favorably with those available in open literature.

Current Issues and Challenges in Polypropylene Foaming: A Review

Thermoplastic foams have several advantages in comparison with unfoamed polymers such as lightweight, high strength to weight ratio, excellent insulation property, high thermal stability, high impact strength and toughness, as well as high fatigue life. These outstanding properties lead cellular plastics to various industrial applications in packaging, automotive parts, absorbents, and sporting equipment. Nowadays, polypropylene (PP), because of its outstanding characteristics such as low material cost, high service temperature, high melting point, high tensile modulus, low density, and excellent chemical resistance, is a major resin in the foaming industry. However, foaming of conventional PP is limited by its low melt strength leading to poor cell morphology, cell rupture/coalescence and limited density reduction. To improve PP melt strength, several strategies including particle addition as nucleating agent, introduction of long chain branching, blending with high melt strength polymers and crosslinking have been proposed. In this review, these issues are discussed and analyzed in terms of mechanical, thermal, and rheological characterizations.

Development of Porous Electrode Gas Diffusion Layers for Proton Exchange Membrane Fuel Cells

The aim of this work was to develop a porous film structure for an electrode gas diffusion layer (GDL) used for proton exchange membrane fuel cells (PEMFCs). This film was made from a matrix composed of two immiscible polymers filled with a mixture of electrically conductive materials fabricated via a twin-screw extrusion process followed by selective extraction of one of the two polymers. The matrix consisted of low-viscosity polypropylene and polystyrene (PS) and the conductive additives were composed of high specific surface area carbon black and synthetic flake graphite. The conductive blends were first compounded in a corotating twin-screw extruder and subsequently extruded through a flexible film die to obtain a GDL film of around 500 μm having high electronic conductivity. The PS phase was then extracted with tetrahydrofuran (THF) solvent and a film of controlled porosity was generated. The morphology of the GDL porous structure was then analyzed by scanning electron microscopy. GDL porosity characterization was done by both Brunauer-Emmett-Teller (BET) and mercury-intrusion porosimeter. The effects of PS concentration and extraction time with THF on GDL porosity were also studied. Pore-size distribution obtained by BET and mercury-intrusion porosimetry revealed that the GDL structure is composed by both mesopores and macropores. Mesopores represent more than 60% of the total pore volume inside the GDL film.

Chitosan-Coated Collagen Membranes Promote Chondrocyte Adhesion, Growth, and Interleukin-6 Secretion

Designing scaffolds made from natural polymers may be highly attractive for tissue engineering strategies. We sought to produce and characterize chitosan-coated collagen membranes and to assess their efficacy in promoting chondrocyte adhesion, growth, and cytokine secretion. Porous collagen membranes were placed in chitosan solutions then crosslinked with glutaraldehyde vapor. Fourier transform infrared (FTIR) analyses showed elevated absorption at 1655 cm−1 of the carbon–nitrogen (N=C) bonds formed by the reaction between the (NH2) of the chitosan and the (C=O) of the glutaraldehyde. A significant peak in the amide II region revealed a significant deacetylation of the chitosan. Scanning electron microscopy (SEM) images of the chitosan-coated membranes exhibited surface variations, with pore size ranging from 20 to 50 μm. X-ray photoelectron spectroscopy (XPS) revealed a decreased C–C groups and an increased C–N/C–O groups due to the reaction between the carbon from the collagen and the NH2 from the chitosan. Increased rigidity of these membranes was also observed when comparing the chitosan-coated and uncoated membranes at dried conditions. However, under wet conditions, the chitosan coated collagen membranes showed lower rigidity as compared to dried conditions. Of great interest, the glutaraldehyde-crosslinked chitosan-coated collagen membranes promoted chondrocyte adhesion, growth, and interleukin (IL)-6 secretion. Overall results confirm the feasibility of using designed chitosan-coated collagen membranes in future applications, such as cartilage repair.

Effect of Processing Conditions on the Cellular Morphology of Polypropylene Foamed Films for Piezoelectric Applications

In this work, continuous extrusion-calendering was used to produce polypropylene (PP) foam films for piezoelectric applications. The setup is based on physical foaming using supercritical nitrogen (SC-N2) and calcium carbonate (CaCO3) as nucleating agent. In particular, the extrusion parameters (screw design, temperature profile, blowing agent and nucleating agent content) and post-extrusion conditions (calendaring temperature and speed) were optimized to achieve a specific stretched eye-like cellular structure with uniform cell size distribution. The morphology in both machine and transverse directions, as well as tensile properties were characterized. The results show that a cellular structure with a higher cell aspect ratio has a lower Youngs modulus, which is appropriate for piezoelectric cellular films. Generally, the developed foam morphology presents high potential for the production of ferroelectret PP films used in different piezoelectric applications.