Generalized behavior of the breakup of viscous drops in confinements

The breakup of confined drops in shear flow between parallel plates is investigated as a function of viscosity ratio and confinement ratio. Using a boundary-integral method for numerical simulations and a counter-rotating experimental device, critical capillary numbers in shear flow are obtained. It is observed that different viscosity ratios yield different trends with increasing confinement ratio: a low viscosity ratio drop shows an increase in critical capillary number, at a viscosity ratio of unity no major trend is seen, and the critical capillary number for a high viscosity ratio drop decreases significantly. A generalized explanation for all viscosity ratios is that confinement affects the orientation of the drop with respect to the direction of the local strain field. At moderate confinement ratios, the drop orients more towards the strain direction, where it experiences a stronger flow and hence, the critical capillary number is decreased. As the drop gets more confined, it aligns more in the flow direction. Hence, the drop experiences a weaker flow and thus, additionally stabilized by wall effects, it breaks at a higher critical capillary number. In principle, this behavior is the same for all viscosity ratios, but transitions occur at different confinement ratios. Most of the breakup is of a binary nature, but ternary breakup can occur if the drop length is larger than 6 undeformed drop radii, consistent with arguments based on the Rayleigh-Plateau instability

Deformation and relaxation of PMMA/PS and PMMA/PSOX blends

In this work we study the role of the interfaces in the rheological behavior of blends of PMMA with PS and PS functionalized with oxazoline (PSOX). The blends here studied are concentrated (20 w% of dispersed phase) and have viscosity ratios higher than 1. Linear and non-linear (both in shear and extension) experiments were performed. In terms of relaxation, the cessation of flow after steady shear experiments reveal the appearance of second very slow relaxation mechanism in the PMMA/PSOX blend as do those upon cessation of a step uniaxial extension. Small Angle Light Scattering (SALS), performed on 1 w% dispersed phase blends, was used during step shear to infer about the relaxation of the droplets and/or interface and the patterns show that this behavior is not due to a higher deformation of the PSOX droplets, but should be attributed to a relaxation of the interfaces. The reason for the very high elasticity of the interface is not a priori obvious, but is probably related with the existence of extra physical entanglements at the interface in the PMMA/PSOX blends since the chemical analysis does not indicate any type of chemical interaction between the oxazoline groups and the PMMA.The authors would like to thank the Foundation for Science and Technology-FCT for awarding one of them (Jorge Silva) aPhD Fellowship (ref BD/12833/2003)

The influence of swelling on elastic properties of polyacrylamide hydrogels

Polyacrylamide (PAM) hydrogels are commonly used as substrates for cell mechanical and mechanobiological studies because of their tunable stiffness and ease of handling. The dependence of bulk rheological and local elastic properties (assessed by Atomic Force Microscopy, or AFM) of PAM hydrogels on its composition and polymerization temperature has been extensively studied. PAM hydrogels swell when immersed in media, but the influence of swelling on local elastic properties is poorly characterized. Direct measurements of the effect of swelling on PAM elastic properties are scarce. We report here, for the first time, the direct measurements of volumetric swelling and local elastic properties of PAM gels throughout the post-polymerization swelling process until equilibrium. First, local and global elastic properties (measured by rheology), were obtained during polymerization in the absence of swelling, and showed good agreement with each other. Four PAM hydrogel compositions were characterized thus, with corresponding storage shear moduli (as measured immediately after polymerization) of 4,530 Pa (termed stiffest), 2,900 Pa (stiff), 538 Pa (soft), and 260 Pa (softest). Next, all compositions were subjected to swelling in phosphate buffered saline. Swelling ratios and local elastic moduli were measured at 0, 3, 6, 9, 12, and 24 h post-polymerization for the soft and softest compositions, and once daily till 6 days post-polymerization for all four compositions. For the stiffest and stiff gels, swelling ratio, and local elastic modulus changed negligibly with time, while for the soft and softest gels, substantial changes between Day 0 and Day 1 were found for both swelling ratio (increased by 21.6 and 133%, respectively), and local elastic modulus decreased (by 33.7 and 33.3%, respectively), substantially. Experimental data were analyzed by a model that combined ideal elastomer mechanics and poroelastic swelling kinetics model. Model predictions confirmed the validity of present measurements with respect to past studies where swelling and elastic properties were not measured simultaneously. The present study underlines the important effect swelling can have on PAM elastic properties and provides detailed quantitative data to guide the duration taken to reach equilibrium—a useful information for cell mechanics experiments. In addition, the simultaneous measurements of swelling and local elastic moduli provide novel data for the validation of theoretical models

Extensional rheology, cellular structure, mechanical behavior relationships in HMS PP/montmorillonite foams with similar densities

Producción CientíficaThe main goal of this work is to analyze the relationships between the extensional rheological behavior of solid nanocomposites based on high melt strength polypropylene (HMS PP) and montmorillonites (MMT) and the cellular structure and mechanical properties of foams produced from these materials. For this purpose two systems have been analyzed. The first one incorporates organomodified MMT and a compatibilizer and the second system contains natural clays and is produced without the compatibilizer. Results indicate that the extensional rheological behavior of both materials is completely different. The strain hardening of the polymer containing organomodified clays decreases as the clay content increases. As a consequence, the open cell content of this material increaseswith the clay content and hence, themechanical properties get worse. However, in the materials produced with natural clays this relationship is not so clear. While no changes are detected in the extensional rheological behavior by adding these particles, the nano-filled materials show an open cell structure, opposite to the closed cell structure of the pure polymer, which is caused by the fact of having particle agglomerates with a size larger than the thickness of the cell walls and a poor compatibility between the clays and the polymer.Financial support from PIRTU contract of E. Laguna-Gutierrez by Junta of Castile and Leon (EDU/289/2011) and cofinanced by the European Social Fund is gratefully acknowledged. Cristina Saiz-Arroyo would like to acknowledge Spanish Ministry of Economy and Competitiveness (MINECO) via Torres Quevedo Program (PTQ-12-05504). Finally, financial assistance from MINECO and FEDER program (MAT 2012 – 34901) MINECO, FEDER, UE (MAT2015-69234-R) and the Junta de Castile and Leon (VA035U13) are gratefully acknowledged

Enhancing the rheological performance of wheat flour dough with glucose oxidase, transglutaminase or supplementary gluten

The enzymes glucose oxidase and transglutaminase are frequently used to improve the breadmaking performance of wheat flours, as they have the ability to considerably alter the viscoelastic nature of the gluten network. To evaluate a flour’s breadmaking performance, rheological tests offer an attractive framework. In this study, the rheological impact of adding glucose oxidase or transglutaminase to wheat flour dough is investigated by means of linear oscillatory shear tests, creep-recovery shear tests and startup extensional tests. The former tests reveal that the enzymes render the dough stiffer and enhance its elastic character, until saturation is reached. In the breadmaking process, the use of excessive amounts of enzyme is known to be counterproductive. The strain-hardening index clearly reveals this overcross-linking effect. Besides enzymes, the gluten network can also be reinforced by adding supplementary gluten, which was indeed found to enhance the extent of strain-hardening.</p