The modified imbedded disc retraction method for measurement of interfacial tension in polymer melts

The imbedded disc retraction method is used to estimate interfacial tension in LLDPE/PS system with PS as the imbedded disc. Shape evolution of a disc of one material (PS) imbedded into the matrix of another material is observed (LLDPE). Three to five repetitions at three different temperature levels are observed. The Newtonian model of Rundqvist et al. (1996) for the imbedded disc retraction is modified to include elastic effects. The modified model is derived assuming uniaxial extension, starting with the lower convected Maxwell model. Both the original model and modified imbedded disc retraction model are used in data analysis. The mean values of interfacial tension at 190 °C, 200 °C, and 210 °C are 6.8 ± 0.7 mN/m, 3.9 ± 0.3 mN/m, and 3.7 ± 0.2 mN/m, respectively. A method of estimating whether elastic effects will significantly affect the estimated interfacial tension value during retraction for the given polymer pair is provided. Received: 6 August 1999 Accepted: 2 January 2001     

The imbedded disk retraction method for measurement of interfacial tension between polymer melts

A method for measuring interfacial tension of high viscosity polymer melts at elevated temperatures is described. The method involves the tracking of the shape evolution of a disk of one material imbedded in a second one. This makes it possible to determine the interfacial tension over a relatively short time period. The technique of preparing the samples makes it possible to measure on practically any combination of polymer melts without restrictions on viscosities and melting temperature, as long as one of the materials is transparent in the molten state.The retraction of the disk is observed by using a microscope with a high resolution video camera. The camera is connected to a video frame grabber in a personal computer which is programmed to collect images with preselected time intervals. Data of the retraction is acquired by using an image analysis software, measuring the average radius of the disk.The driving force for the shape evolution is interfacial tension and it is balanced by viscous forces. The analysis of the retraction process is done analytically with a simplified one-dimensional model. The model has been compared to experiments with the system PS/PMMA at 210 °C, covering viscosity ratios over a range of six decades and five different molecular weight values of PS. It is shown that interfacial tension can be determined over the whole range and a value of 1.1±0.2 mN/m was obtained for all samples.     

The Development of a High Rate Tensile Testing System for Micro Scaled Single Crystal Silicon Specimens

Structures have been built at micro scales with unique failure mechanisms that are not yet understood, in particular, under high-rate loading conditions. Consequently, microelectromechanical systems (MEMS) devices can suffer from inconsistent performance and insufficient reliability. This research aims to understand the failure mechanisms in micro-scaled specimens deforming at high rates. Single-crystal silicon (SCS) micro specimens that are 4 μm thick are subjected to tensile loading at an average strain rate of 92 s^?1 using a miniature Hopkinson tension bar. A capacitance displacement system and piezoelectric load cell are incorporated to directly measure the strain and stress of the silicon micro specimens. The average dynamic elastic modulus of the silicon micro specimens is measured to be 226.8?±?18.50 GPa and the average dynamic tensile strength of the silicon is measured to be 1.26?±?0.310 GPa. High-speed images show that extensive fragmentation of the specimens occurs during tensile failure.     

Scaling law behaviour of the retraction of a Newtonian droplet after a strain jump in a Newtonian matrix

We performed 3D visualisation of a Newtonian droplet embedded in an immiscible Newtonian liquid after a strain jump with a home-built counter-rotating shear device. The use of different linear polyurethanes for the droplet liquid allowed us to cover almost three decades of viscosity ratios (K) and to obtain a distinct interface with PDMS matrices with the same interfacial tension for all droplet/matrix pairs. During the droplet retraction, the major axis (L) showed universal time dependence. The apparent Hencky strain of L decayed linearly at large deformations and exponentially at small deformations. After large strain steps, the droplet axis along the vorticity direction (W) deflated and then inflated and the time dependence could be well described by a log normal function. The full width at half maximum was proportional to the droplet relaxation time for all K. The amplitude and the position of the minimum of W were proportional to the affine deformation. The results revealed interesting scaling law behaviour of the droplet retraction after large strain jumps.     

Rheological behaviour of vitreous humour

The vitreous humour (VH) is a complex biofluid that occupies a large portion of the eyeball between the lens and the retina, and exhibits non-Newtonian rheological properties that are key for its function in the eye. It is often possible to distinguish two different phases in VH, known as liquid and gel phases (Sebag J Eye 1: 254–262, 1987). In this work, we present a detailed rheological characterisation of the two phases of the VH under shear and extensional flow conditions. Healthy New Zealand rabbit eyes were used to measure the surface tension and the shear and extensional rheological properties of VH in different phase conformations and at different times after dissection. The results show that VH liquid phase exhibits a surface tension of 47.8 mN/m, a shear thinning behaviour reaching a viscosity plateau around 10^?3 Pa s for shear rates above ~1000 s^?1, and an average relaxation time of 9.7 ms in extensional flow. Interestingly, both VH phases present higher storage modulus than loss modulus, and the measurements performed with VH gel phase 4?±?1 h after dissection exhibit the highest moduli values. The compliance measurements for the gel phase show a viscoelastic gel behaviour and that compliance values decrease substantially with time after dissection. Our results show that the two VH phases exhibit viscoelastic behaviour, but with distinct rheological characteristics, consistent with a gel phase mostly composed of collagen entangled by hyaluronan and a second phase mainly composed of hyaluronan in aqueous solution.     

Experimental study to obtain the viscosity of CuO-loaded nanofluid: effects of nanoparticles’ mass fraction,temperature and basefluid’s types to develop a correlation

In this study, the impacts of temperature, nanoparticles mass fraction, and basefluid types were investigated on the dynamic viscosity of CuO-loaded nanofluids. The nanoparticles were dispersed in deionized water, ethanol, and ethylene glycol as basefluids separately and the measurements were performed on samples with nanoparticles loads ranging from 0.005 to 5 wt%, and the temperature range of 25 to 70 °C. TEM analysis were performed on dried nanoparticles and the results showed the average mean diameter of CuO nanoparticles ranged from 10 to 50 nm. The results of DLS analysis confirmed the results of nanoparticles size obtained by TEM analysis in mentioned basefluids and Zeta-Potential tests exhibited the high stability of the nanoparticles in the basefluids environment. The results indicate that by adding tiny amount of CuO nanoparticles to basefluids, relative viscosity of nanofluid increases. By the increase in nanoparticles load higher than 0.1 wt% the effect of both nanoparticles mass fraction and temperature would be more tangible, while for nanoparticles mass fraction lower than 0.1 wt% no significant change in viscosity was observed. In addition, the results declare that viscosity of nanofluid remains constant at various applied shear rates indicating Newtonian behavior of nanofluid at various nanoparticles load and temperature. According to experimental data, it is also evident that with the increase in temperature, the value of relative dynamic viscosity decreases significantly. Also it is concluded that for CuO/ethanol nanofluid, more interfacial interaction is resulted that causes higher relative dynamic viscosity while for CuO/water lower interfacial interaction between nanoparticles surface and water molecules are resulted which leads to the lower values for this parameter. The results of this study implied that with increase the temperature from 25 to 70 °C at the condition where nanoparticles mass fraction was chosen to be 5 wt%, the value of dynamic viscosity of CuO/ethanol, CuO/deionized water, CuO/ethylene glycol declined 69%, 66%, and 65% respectively. Finally, a correlation was proposed for the relative dynamic viscosity of nanofluid based on the CuO nanoparticles mass fraction and temperature of the basefluid and nanoparticles.     

Synchronous Full-Field Strain and Temperature Measurement in Tensile Tests at Low,Intermediate and High Strain Rates

Tensile tests with simultaneous full-field strain and temperature measurements at the nominal strain rates of 0.01, 0.1, 1, 200 and 3000 s^?1 are presented. Three different testing methods with specimens of the same thin and flat gage-section geometry are utilized. The full-field deformation is measured on one side of the specimen, using the DIC technique with low and high speed visible cameras, and the full-field temperature is measured on the opposite side using an IR camera. Austenitic stainless steel is used as the test material. The results show that a similar deformation pattern evolves at all strain rates with an initial uniform deformation up to the strain of 0.25–0.35, followed by necking with localized deformation with a maximum strain of 0.7–0.95. The strain rate in the necking regions can exceed three times the nominal strain rate. The duration of the tests vary from 57 s at the lowest strain rate to 197 μs at the highest strain rate. The results show temperature rise at all strain rates. The temperature rise increases with strain rate as the test duration shortens and there is less time for the heat to dissipate. At a strain rate of 0.01 s^?1 the temperature rise is small (up to 48 °C) but noticeable. At a strain rate of 0.1 the temperature rises up to 140 °C and at a strain rate of 1 s^?1 up to 260 °C. The temperature increase in the tests at strain rates of 200 s^?1 and 3000 s^?1 is nearly the same with the maximum temperature reaching 375 °C.     

Retraction and Adhesion of a Single Droplet Normal Impact on the Solid Surface

The impact behavior of individual biomass oil droplets was investigated on solid surfaces having different structures (flat, cylindrical, and spherical) using the high-speed video technique. This makes it possible to compare the evolution of the droplet impact on various surface structures. The impact behaviors of retraction–oscillation and adhesion are analyzed for different hydrophobic surfaces. The influence of the Weber number (We), the surface structure, and the surface curvature is further examined by focusing on the retraction and stable adhesion (thickness, adhesion, and contact angle) for different biomass oil droplets. The results show that the retraction factor gradually increases as We increases to some critical value, beyond which the increase rate slows down or the retraction factor begins to decrease. The largest retraction factor is observed on the flat surface and the smallest one appears on the spherical surface. The adhesion thickness of the liquid film oscillates periodically over time, and its oscillation amplitude gradually decreases with a constant frequency, which is smaller for the more hydrophobic surfaces. The curvatures of the cylinder and sphere have little influence on the stable adhesion behavior. For the different droplet types, the adhesion diameter on the flat surface gradually increases as We rises, whereas the adhesion thickness gradually decreases with increase in We. These results are helpful for understanding the impact behaviors of biomass oil droplets with high viscosity and small surface tension on solid surfaces.

     

Understanding droplet coalescence and its use to estimate interfacial tension

The study of coalescence of polymer droplets is presented in the viscosity ratio range (p) going from 0.1 to 10. It is shown that the determination of the characteristic time of coalescence is a good way to estimate the interfacial tension. Polydimethylsiloxane (PDMS) is mixed with polyisobutylene (PIB) and the temperature change provides a way to modify the interfacial tension of the PDMS/PIB system significantly, as measured using a pendant drop apparatus. We obtain a dependence of the reduced coalescence time as a function of p^-1/2 which gives access to the interfacial tension. This technique can be an interesting choice for estimating interfacial tension without requiring sophisticated techniques. In a further attempt to correlate these observations with a theoretical model (Verdier C (2001) Polymer 42), the flow field inside and outside the droplets is investigated. PIV measurements are carried out where the evidence of elongational regimes is demonstrated. Such experiments are also interesting for future comparisons with numerical results.     

Effect of temperature and molecular weight on the interfacial tension of PS/PDMS blends

The imbedded-fiber retraction (IFR) method was used to study the effect of temperature and PDMS molecular weight on the interfacial tension of PS/PDMS blends. The interfacial tension decreased with increasing temperature and analysis of the temperature dependence using a simple linear fit gave –dγ/dT value of 0.058±0.010 dyn/cm-deg. Reported –dγ/dT values of PS/PDMS blends are highly dependent on the molecular weights of the polymers and can have values that are <0, 0, or >0. Our interfacial tension values were independent of the molecular weight of PDMS and this was attributed to the molecular weights studied here being well above the entanglement values of both polymers. However, analysis of interfacial tension data from this work and the literature showed the following empirical relationship between apparent blend molecular weight, M_b, and interfacial tension of PS/PDMS blends with a correlation of 0.94: γ₁₂=γ₀+k₂M_b ^(–2/3), where γ₀=7.3±0.3 dyn/cm; k₂=–517±41 (dyn/cm)(g/mol)^2/3.     

Assessment of volume of fluid and immersed boundary methods for droplet computations

The volume of fluid (VOF) and immersed boundary (IB) methods are two popular computational techniques for multi‐fluid dynamics. To help shed light on the performance of both techniques, we present accuracy assessment, which includes interfacial geometry, detailed and global fluid flow characteristics, and computational robustness. The investigation includes the simulations of a droplet under static equilibrium as a limiting test case and a droplet rising due to gravity for Re?1000. Surface tension force models are key issues in both VOF and IB and alternative treatments are examined resulting in improved solution accuracy. A refined curvature model for VOF is also presented. With the newly developed interfacial treatments incorporated, both IB and VOF perform comparably well for the droplet dynamics under different flow parameters and fluid properties. Copyright © 2004 John Wiley & Sons, Ltd.     

Role of droplet bridging on the stability of particle-containing immiscible polymer blends

The effect of micron-sized hydrophobic calcium carbonate particles on the stabilization of polydimethylsiloxane (PDMS)/polyisobutylene (PIB) immiscible model blends is investigated in this study. The analytical splitting of bulk and liquid–liquid interface contributions from the droplet bridging one is successfully performed due to the negligible contribution of hydrophobic microparticles to the bulk rheology of phases. The presence of particles at the fluid–fluid interface is supported by wetting parameter calculation and verified by optical microscopy observations. Moreover, direct visualizations shows that particles are able to form clusters of droplets by simultaneously adsorbing at two fluid–fluid interfaces and glue-dispersed droplets together, probably due to the patchy interactions induced by heterogeneous distribution of particles along the interface. Rheological studies show that the flow-induced coalescence is slowed down upon addition of particles and almost suppressed with the addition of 4 wt% particles. The linear viscoelastic response is modeled to estimate interfacial tension by considering the contribution of particle-induced droplet aggregation in addition to bulk and droplet deformation ones. From linear and nonlinear viscoelastic responses, the improved stability of filled polymer blends is attributed to the interfacial rheology and/or the bridged structure of droplets, even though the interfacial area is not fully covered by particles. Furthermore, Doi–Ohta scaling relations are investigated by employing stress growth response upon step-up of shear flow.     

Load-Inversion Device for the High Strain Rate Tensile Testing of Sheet Materials with Hopkinson Pressure Bars

A high strain rate tensile testing technique for sheet materials is presented which makes use of a split Hopkinson pressure bar system in conjunction with a load inversion device. With compressive loads applied to its boundaries, the load inversion device introduces tension into a sheet specimen. Two output bars are used to minimize the effect of bending waves on the output force measurement. A Digital Image Correlation (DIC) algorithm is used to determine the strain history in the specimen gage section based on high speed video imaging. Detailed finite element analysis of the experimental set-up is performed to validate the design of the load inversion device. It is shown that under the assumption of perfect alignment and slip-free attachment of the specimen, the measured stress–strain curve is free from spurious oscillations at a strain rate of 1,000 s^?1. Validation experiments are carried out using tensile specimens extracted from 1.4 thick TRIP780 steel sheets. The experimental results for uniaxial tension at strain rates ranging from 200 s^?1 to 1,000 s^?1 confirm the oscillation-free numerical results in an approximate manner. Dynamic tension experiments are also performed on notched specimens to illustrate the validity of the proposed experimental technique for characterizing the effect of strain rate on the onset of ductile fracture in sheet materials.     

Interfacial localization of nanoclay particles in oil-in-water emulsions and its reflection in interfacial moduli

The localization of nanoclay particles dispersed in the oil phase of a model oil-in-water emulsion depends on the wetting property of layered nanoparticles. Investigation at a single droplet interface shows that nanoclay is located at different interfacial regions depending on the hydrophilic property of the nanoclay surface. Hydrophobic nanoclays do not present Pickering phenomena at the interface and hardly form an interfacial layer. Hydrophilic nanoclay particles quickly move to the interface and form a Pickering interface with a high interfacial shear modulus. With surfactant, poor hydrophilic nanoclays can be located at the interface due to improvement of the wetting behavior caused by the surfactants dissolved in the aqueous continuous phase. With ionic molecules changing the wetting behavior of particles, the interfacial localization of nanoclays can be controlled and improve the mechanical property of emulsion.     

High-Resolution Deformation Mapping Across Large Fields of View Using Scanning Electron Microscopy and Digital Image Correlation

This paper details the creation of experimental and computational frameworks to capture high-resolution, microscale deformation mechanisms and their relation to microstructure over large (mm-scale) fields of view. Scanning electron microscopy with custom automation and external beam control was used to capture 209 low-distortion micrographs of 360 μm?×?360 μm each, that were individually correlated using digital image correlation to obtain displacement/strain fields with a spatial resolution of 0.44 μm. Displacement and strain fields, as well as secondary electron images, were subsequently stitched to create a 5.7 mm × 3.4 mm field of view containing 100 million (7678?×?13,004) data points. This approach was demonstrated on Mg WE43 under uniaxial compression, where effective strain was shown to be relatively constant with respect to distance from the grain boundary, and a noticeable increase in the effective strain was found with an increase in the basal Schmid factor. The ability to obtain high-resolution deformations over statistically relevant fields of view enables large data analytics to examine interactions between microstructure, microscale strain localizations, and macroscopic properties.     

Structural and Mechanical Properties of Gastropod Connective and Smooth Muscle Tissue

The pedal integument of terrestrial gastropod Arion rufus is composed mainly of smooth muscle cells (SMCs, 45 %), haemocoelic cavities (36 %), and collagen connective tissue. Using stereological methods, SMC two-dimensional length density (0.12 μm^?1), numerical density (426,000 mm^?3), and mean distance (31 μm) in the cluster were assessed. The average SMC could be approximated by an ellipsoid 72 μm in length with semi-axes of 3 μm. Three-dimensional reconstructions of SMCs and haemocoelic cavities of gastropod tissue were created using serial thick and semi-thin sections. These reconstructions showed the spatial arrangement of individual SMCs within the tissue: longitudinally, perpendicularly, and obliquely oriented to the main axis of the gastropod body. Using uniaxial mechanical loading with linearly increasing load or elongation at various loading rates (2, 10, and 20 mN/min; 2 and 3 mm/min) in transverse and longitudinal orientations to the main gastropod body axis, the Young’s modulus of elasticity for small (23–27 kPa) and large deformations (49–132 kPa) as well as ultimate stress (105–250 kPa) and strain (300–400 %) were determined. There was a trend toward stiffer integument tissue in the longitudinal direction compared to the transversal direction and toward increasing stiffness with loading velocity.     

Temperature dependence of the interfacial tension of PS/PMMA, PS/PE, and PMMA/PE blends

The effect of temperature on the interfacial tension for PS/PMMA, PS/PE, and PMMA/PE was measured using the imbedded fiber retraction method. Interfacial tensions for PS/PMMA, PS/PE, and PMMA/PE were measured over temperature ranges of 160–250 °C, 140–220 °C, and 140–220 °C, respectively. The interfacial tension was found to follow a dependence of 3.6–0.013 T dyn/cm, 7.6–0.051 T dyn/cm and 11.8–0.017 T dyn/cm for PS/PMMA, PS/PE, and PMMA/PE, respectively. Comparison of the data with the mean field theory of Helfand and Sapse were made; however, a simple linear fit to the data described the temperature dependence in the experimental window as well as the predictions of the mean field theory. Received: 6 July 1999 Accepted: 23 March 2000     

Fibrin network adaptation to cell-generated forces

Fibrin promotes wound healing by serving as provisional extracellular matrix for fibroblasts that realign and degrade fibrin fibers, and sense and respond to surrounding substrate in a mechanical-feedback loop. We aimed to study mechanical adaptation of fibrin networks due to cell-generated forces at the micron-scale. Fibroblasts were elongated-shaped in networks with ≤?2 mg/ml fibrinogen, or cobblestone-shaped with 3 mg/ml fibrinogen at 24 h. At frequencies f?<?10² Hz, G′ of fibroblast-seeded fibrin networks with ≥?1 mg/ml fibrinogen increased compared to that of fibrin networks. At frequencies f?>?10³ Hz, G″ of fibrin networks decreased with increasing concentration following the power-law in frequency with exponents ranging from 0.75?±?0.03 to 0.43?±?0.03 at 3 h, and of fibroblast-seeded fibrin networks with exponents ranging from 0.56?±?0.08 to 0.28?±?0.06. In conclusion, fibroblasts actively contributed to a change in viscoelastic properties of fibrin networks at the micron-scale, suggesting that the cells and fibrin network mechanically interact. This provides better understanding of, e.g., cellular migration in wound healing.

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Graphical abstract

     

Measurements of the stabilisation of liquid film flow by the soluble surfactant sodium dodecyl sulfate (SDS)

Gravity-driven film flow of aqueous solutions of SDS is studied experimentally and the evolution of small-amplitude, regular inlet disturbances is investigated. With the addition of SDS, strong attenuation of non-linear growth is observed, with traveling waves remaining relatively small in height and near-sinusoidal over an impressive parametric range. The critical Reynolds number of the primary instability rises by an order of magnitude. Maximum stabilization is observed at small surfactant loadings (characterized by surface tension 60–65 mN/m) and the critical Reynolds number gradually decreases with further addition of surfactant. Observations are interpreted by the competing effects of surface elasticity -which increases with the adsorbed SDS and intensifies Marangoni stresses- and surfactant mass transfer between bulk and interface -which also increases with the amount of SDS and mitigates interfacial gradients and Marangoni stresses.     

Direct numerical simulation of surfactant-stabilized emulsions

A 3D lattice Boltzmann model for two-phase flow with amphiphilic surfactant was used to investigate the evolution of emulsion morphology and shear stress in starting shear flow. The interfacial contributions were analyzed for low and high volume fractions and varying surfactant activity. A transient viscoelastic contribution to the emulsion rheology under constant strain rate conditions was attributed to the interfacial stress. For droplet volume fractions below 0.3 and an average capillary number of about 0.25, highly elliptical droplets formed. Consistent with affine deformation models, gradual elongation of the droplets increased the shear stress at early times and reduced it at later times. Lower interfacial tension with increased surfactant activity counterbalanced the effect of increased interfacial area, and the net shear stress did not change significantly. For higher volume fractions, co-continuous phases with a complex topology were formed. The surfactant decreased the interfacial shear stress due mainly to advection of surfactant to higher curvature areas. Our results are in qualitative agreement with experimental data for polymer blends in terms of transient interfacial stresses and limited enhancement of the emulsion viscosity at larger volume fractions where the phases are co-continuous.