Synergy of Fiber Surface Chemistry and Flow: Multi-Phase Transcrystallization in Fiber-Reinforced Thermoplastics.

Fiber-reinforced polymer composites are largely employed for their improved strength with respect to unfilled matrices. Considering semi-crystalline materials under relevant processing conditions, the applied pressure and flow induce shear stresses at the fiber-polymer interface. These stresses may strongly enhance the nucleation ability of the fiber surface with respect to the quiescent case. It is thus possible to assume that the fiber features are no longer of importance and that crystallization is dominated by the effect of flow. However, by making use of an advanced experimental technique, i.e., polarization-modulated synchrotron infrared microspectroscopy (PM-SIRMS), we are able to show that the opposite is true for the industrially relevant case of isotactic polypropylene (iPP). With PM-SIRMS, the local chain orientation is measured with micron-size spatial resolution. This orientation can be related to the polymer nucleation density along the fiber surface. For various combinations of an iPP matrix and fiber, the degree of orientation in the cylindrical layer that develops during flow correlates well with the differences in nucleation density found in quiescent conditions. This result shows that the morphological development during processing of polymer composites is not solely determined by the flow field, nor by the nucleating ability of the fiber surface alone, but rather by a synergistic combination of the two. In addition, using finite element modeling, it is demonstrated that, under the experimentally applied flow conditions, the interphase structure formation is mostly dominated by the rheological characteristics of the material rather than perturbations in experimental conditions, such as shear rate, layer thickness, and temperature. This once again highlights the importance of matrix-filler interplay during flow and, thus, of material selection in the design of hybrid and lightweight composite technologies.

Torsional fracture of viscoelastic liquid bridges.

Short liquid bridges are stable under the action of surface tension. In applications like electronic packaging, food engineering, and additive manufacturing, this poses challenges to the clean and fast dispensing of viscoelastic fluids. Here, we investigate how viscoelastic liquid bridges can be destabilized by torsion. By combining high-speed imaging and numerical simulation, we show that concave surfaces of liquid bridges can localize shear, in turn localizing normal stresses and making the surface more concave. Such positive feedback creates an indent, which propagates toward the center and leads to breakup of the liquid bridge. The indent formation mechanism closely resembles edge fracture, an often undesired viscoelastic flow instability characterized by the sudden indentation of the fluid's free surface when the fluid is subjected to shear. By applying torsion, even short, capillary stable liquid bridges can be broken in the order of 1 s. This may lead to the development of dispensing protocols that reduce substrate contamination by the satellite droplets and long capillary tails formed by capillary retraction, which is the current mainstream industrial method for destabilizing viscoelastic liquid bridges.

A filament stretching rheometer for in situ X-ray experiments: Combining rheology and crystalline morphology characterization.

We present a rheometer that combines the possibility to perform in situ X-ray experiments with a precise and locally controlled uniaxial extensional flow. It thus allows us to study the crystallization kinetics and morphology evolution combined with the rheological response to the applied flow field. A constant uniaxial deformation rate is ensured, thanks to a fast control scheme that drives the simultaneous movement of the top and bottom plates during a pulling experiment. A laser micrometer measures the time evolution of the smallest diameter, where the highest stress is concentrated. The rheometer has a copper temperature-controlled oven with the ability to reach 250 °C and a N2 connection to create an inert atmosphere during the experiments. The innovation of our rheometer is the fixed location of the midfilament position, which is possible because of the simultaneous controlled movement of the two end plates. The copper oven has been constructed with four ad hoc windows: two glass windows for laser access and two Kapton windows for X-ray access. The key feature is the ability to perfectly align the midfilament of the sample to the laser micrometer and to the incoming X-ray beam in a synchrotron radiation facility, making it possible to investigate the structure and morphologies developed during extensional flow. The rheological response measured with our rheometer for low-density polyethylene (LDPE) is in agreement with the linear viscoelastic envelope and with the results obtained from the existing extensional rheometers. To demonstrate the capability of the instrument, we have performed in situ-resolved X-ray experiments on LDPE samples exhibiting extensional flow-induced crystallization.

Behavior of viscoelastic models with thermal fluctuations.

Fluctuating viscoelasticity for conformation-tensor-based models is studied at equilibrium, in simple-shear deformation, and in uniaxial extension. The models studied are the upper-convected Maxwell model, the FENE-P model with finite chain-extensibility, and the Giesekus model with anisotropic drag. Using numerical simulations, the models are compared in detail both with each other and with analytical predictions for the Maxwell model. At equilibrium, the models differ only marginally, both in terms of static and dynamic characteristics. When deformed, the average mechanical response of the Maxwell model is unaffected by the strength of thermal fluctuations, while the mechanical response of the FENE-P and Giesekus models show a slight decrease the stronger the fluctuations in simple shear, whereas the decrease in uniaxial extension is marginal. For all models, the standard deviation of the mechanical response increases with increasing strength of fluctuations, and the magnitude of the standard deviation relative to the average for given fluctuation strength generally decreases the stronger the deformation, this effect being stronger for uniaxial extension than for simple-shear deformation.

Microfluidic Magnetic Mixing at Low Reynolds Numbers and in Stagnant Fluids.

Microfluidic mixing becomes a necessity when thorough sample homogenization is required in small volumes of fluid, such as in lab-on-a-chip devices. For example, efficient mixing is extraordinarily challenging in capillary-filling microfluidic devices and in microchambers with stagnant fluids. To address this issue, specifically designed geometrical features can enhance the effect of diffusion and provide efficient mixing by inducing chaotic fluid flow. This scheme is known as "passive" mixing. In addition, when rapid and global mixing is essential, "active" mixing can be applied by exploiting an external source. In particular, magnetic mixing (where a magnetic field acts to stimulate mixing) shows great potential for high mixing efficiency. This method generally involves magnetic beads and external (or integrated) magnets for the creation of chaotic motion in the device. However, there is still plenty of room for exploiting the potential of magnetic beads for mixing applications. Therefore, this review article focuses on the advantages of magnetic bead mixing along with recommendations on improving mixing in low Reynolds number flows (Re ≤ 1) and in stagnant fluids.

A novel experimental setup for in situ optical and X-ray imaging of laser sintering of polymer particles.

We present a unique laser sintering setup that allows real time studies of the structural evolution during laser sintering of polymer particles. The device incorporates the main features of classical selective laser sintering machines for 3D printing of polymers and at the same time allows in situ visualization of the sintering dynamics with optical microscopy as well as X-ray scattering. A main feature of the setup is the fact that it provides local access to one particle-particle bridge during sintering. In addition, due to the small scale of the device and the specific laser arrangement process, parameters such as the temperature, laser energy, laser pulse duration, and spot size can be precisely controlled. The sample chamber provides heating up to 360 °C, which allows for sintering of commodity as well as high performance polymers. The latter parameters are controlled by the use of a visible light laser combined with an acousto-optic modulator for pulsing, which allows small and precise spot sizes and pulse times and pulse energies as low as 500 µs and 17 µJ. The macrostructural evolution of the particle bridge during sintering is followed via optical imaging at high speed and resolution. Placing the setup in high flux synchrotron radiation with a fast detector simultaneously allows in situ time-resolved X-ray characterizations. To demonstrate the capabilities of the device, we studied the laser sintering of two spherical PA12 particles. The setup provides crucial real-time information concerning the sintering dynamics as well as crystallization kinetics, which was not accessible up to now.

Physical Ageing of Polystyrene: Does Tacticity Play a Role?

The ageing kinetics of amorphous atactic (a-PS), isotactic (i-PS), and syndiotactic (s-PS) polystyrene were studied by means of flash-differential scanning calorimetry. The specimens were aged for up to 2 h at six different ageing temperatures: the optimum ageing temperature, that is, the temperature at which the enthalpy overshoot at the glass transition is maximal for the given elapsed time, and five ageing temperatures ranging from 20 to 80 K below the optimum ageing temperature. A logarithmic increase of the enthalpy overshoot with ageing time is observed for specimens at their optimum ageing temperatures. For temperatures significantly lower than the optimum, there is a range where the enthalpy overshoot is constant, but for higher temperatures (still below the optimum), a logarithmic increase is also observed. Moreover, the ageing kinetics appear to depend on tacticity, with s-PS and i-PS exhibiting the slowest and fastest ageing kinetics, respectively, and a-PS exhibiting ageing kinetics between these two extremes.

Laser sintering of polymer particle pairs studied by in situ visualization.

Merging of particle pairs during selective laser sintering (SLS) of polymers is vital in defining the final part properties. Depending on the sintering conditions, polymers can undergo full or partial sintering whereby incomplete sintering results in poor mechanical properties. At present, the underlying mechanisms and related conditions leading to various consolidation phenomena of polymer particles are not well understood. In the present work, a novel in-house developed experimental setup is used to perform laser sintering experiments on polystyrene (PS) particle doublets while performing in situ visualization of the sintering dynamics. From the recorded images, the evolution of the growth of the neck radius formed between both particles is analyzed as a function of time. Sintering conditions such as heating chamber temperature, laser pulse energy and duration, laser spot size and particle size are precisely controlled and systematically varied. A non-isothermal viscous sintering model is developed that allows qualitative prediction of the observed effects of the various parameters. It is shown that the sintering kinetics is determined by a complex interplay between the transient rheology caused by the finite relaxation times of the polymer and the time-dependent temperature profile which also affects the polymer viscosity. The combination of a full material characterization with sintering experiments under well-defined conditions has resulted in a general understanding of the effects of material and process parameters on laser sintering. Thereby a strong foundation is laid for the route towards rational design of laser sintering.

Shear-Induced Migration of Rigid Particles near an Interface between a Newtonian and a Viscoelastic Fluid.

Simulations of rigid particles suspended in two-phase shear flow are presented, where one of the suspending fluids is viscoelastic, whereas the other is Newtonian. The Cahn-Hilliard diffuse-interface model is employed for the fluid-fluid interface, which can naturally describe the interactions between the particle and the interface (e.g., particle adsorption). It is shown that particles can migrate across streamlines of the base flow, which is due to the surface tension of the fluid-fluid interface and a difference in normal stresses between the two fluids. The particle is initially located in the viscoelastic fluid, and its migration is investigated in terms of the Weissenberg number Wi (shear rate times relaxation time) and capillary number Ca (viscous stress over capillary stress). Four regimes of particle migration are observed, which can roughly be described by migration away from the interface for Wi = 0, halted migration toward the interface for low Wi and low Ca, particle adsorption at the interface for high Wi and low Ca, and penetration into the Newtonian fluid for high Wi and high Ca. It is found that the angular velocity of the particle plays a large role in determining the final location of the particle, especially for high Wi. From morphology plots, it is deduced that the different dynamics can be described well by considering a balance in the first-normal stress difference and Laplace pressure. However, it is shown that other parameters, such as the equilibrium contact angle and diffusion of the fluid, are also important in determining the final location of the particle.

Effect of particle-size dynamics on properties of dense spongy-particle systems: Approach towards equilibrium.

Open-porous deformable particles, often envisaged as sponges, are ubiquitous in biological and industrial systems (e.g., casein micelles in dairy products and microgels in cosmetics). The rich behavior of these suspensions is owing to the elasticity of the supporting network of the particle, and the viscosity of permeating solvent. Therefore, the rate-dependent size change of these particles depends on their structure, i.e., the permeability. This work aims at investigating the effect of the particle-size dynamics and the underlying particle structure, i.e., the particle permeability, on the transient and long-time behavior of suspensions of spongy particles in the absence of applied deformation, using the dynamic two-scale model developed by Hütter et al. [Farad. Discuss. 158, 407 (2012)1359-664010.1039/c2fd20025b]. In the high-density limit, the transient behavior is found to be accelerated by the particle-size dynamics, even at average size changes as small as 1%. The accelerated dynamics is evidenced by (i) the higher short-time diffusion coefficient as compared to elastic-particle systems and (ii) the accelerated formation of the stable fcc crystal structure. Furthermore, after long times, the particle-size dynamics of spongy particles is shown to result in lower stationary values of the energy and normal stresses as compared to elastic-particle systems. This dependence of the long-time behavior of these systems on the permeability, that essentially is a transport coefficient and hence must not affect the equilibrium properties, confirms that full equilibration has not been reached.

Magnetic interaction of Janus magnetic particles suspended in a viscous fluid.

We studied the magnetic interaction between circular Janus magnetic particles suspended in a Newtonian fluid under the influence of an externally applied uniform magnetic field. The particles are equally compartmentalized into paramagnetic and nonmagnetic sides. A direct numerical scheme is employed to solve the magnetic particulate flow in the Stokes flow regime. Upon applying the magnetic field, contrary to isotropic paramagnetic particles, a single Janus particle can rotate due to the magnetic torque created by the magnetic anisotropy of the particle. In a two-particle problem, the orientation of each particle is found to be an additional factor that affects the critical angle separating the nature of magnetic interaction. Using multiparticle problems, we show that the orientation of the particles has a significant influence on the dynamics of the particles, the fluid flow induced by the actuated particles, and the final conformation of the particles. Straight and staggered chain structures observed experimentally can be reproduced numerically in a multiple particle problem.

Monocytic cells become less compressible but more deformable upon activation.

AIMS: Monocytes play a significant role in the development of atherosclerosis. During the process of inflammation, circulating monocytes become activated in the blood stream. The consequent interactions of the activated monocytes with the blood flow and endothelial cells result in reorganization of cytoskeletal proteins, in particular of the microfilament structure, and concomitant changes in cell shape and mechanical behavior. Here we investigate the full elastic behavior of activated monocytes in relation to their cytoskeletal structure to obtain a better understanding of cell behavior during the progression of inflammatory diseases such as atherosclerosis. METHODS AND RESULTS: The recently developed Capillary Micromechanics technique, based on exposing a cell to a pressure difference in a tapered glass microcapillary, was used to measure the deformation of activated and non-activated monocytic cells. Monitoring the elastic response of individual cells up to large deformations allowed us to obtain both the compressive and the shear modulus of a cell from a single experiment. Activation by inflammatory chemokines affected the cytoskeletal organization and increased the elastic compressive modulus of monocytes with 73-340%, while their resistance to shape deformation decreased, as indicated by a 25-88% drop in the cell's shear modulus. This decrease in deformability is particularly pronounced at high strains, such as those that occur during diapedesis through the vascular wall. CONCLUSION: Overall, monocytic cells become less compressible but more deformable upon activation. This change in mechanical response under different modes of deformation could be important in understanding the interplay between the mechanics and function of these cells. In addition, our data are of direct relevance for computational modeling and analysis of the distinct monocytic behavior in the circulation and the extravascular space. Lastly, an understanding of the changes of monocyte mechanical properties will be important in the development of diagnostic tools and therapies concentrating on circulating cells.

A ternary model for double-emulsion formation in a capillary microfluidic device.

To predict double-emulsion formation in a capillary microfluidic device, a ternary diffuse-interface model is presented. The formation of double emulsions involves complex interfacial phenomena of a three-phase fluid system, where each component can have different physical properties. We use the Navier-Stokes/Cahn-Hilliard model for a general ternary system, where the hydrodynamics is coupled with the thermodynamics of the phase field variables. Our model predicts important features of the double-emulsion formation which was observed experimentally by Utada et al. [Utada et al., Science, 2005, 308, 537]. In particular, our model predicts both the dripping and jetting regimes as well as the transition between those two regimes by changing the flow rate conditions. We also demonstrate that a double emulsion having multiple inner drops can be formed when the outer interface is more stable than the inner interface.

Valinomycin biosynthetic gene cluster in Streptomyces: conservation, ecology and evolution.

Many Streptomyces strains are known to produce valinomycin (VLM) antibiotic and the VLM biosynthetic gene cluster (vlm) has been characterized in two independent isolates. Here we report the phylogenetic relationships of these strains using both parsimony and likelihood methods, and discuss whether the vlm gene cluster shows evidence of horizontal transmission common in natural product biosynthetic genes. Eight Streptomyces strains from around the world were obtained and sequenced for three regions of the two large nonribosomal peptide synthetase genes (vlm1 and vlm2) involved in VLM biosynthesis. The DNA sequences representing the vlm gene cluster are highly conserved among all eight environmental strains. The geographic distribution pattern of these strains and the strict congruence between the trees of the two vlm genes and the housekeeping genes, 16S rDNA and trpB, suggest vertical transmission of the vlm gene cluster in Streptomyces with no evidence of horizontal gene transfer. We also explored the relationship of the sequence of vlm genes to that of the cereulide biosynthetic genes (ces) found in Bacillus cereus and found them highly divergent from each other at DNA level (genetic distance values >or= 95.6%). It is possible that the vlm gene cluster and the ces gene cluster may share a relatively distant common ancestor but these two gene clusters have since evolved independently.

Understanding and Optimizing the SMX Static Mixer.

Using the Mapping Method different designs of SMX motionless mixers are analyzed and optimized. The three design parameters that constitute a specific SMX design are: The number of cross-bars over the width of channel, N(x) , the number of parallel cross-bars per element, N(p) , and the angle between opposite cross-bars θ. Optimizing N(x) , somewhat surprisingly reveals that in the standard design with N(p) = 3, N(x) = 6 is the optimum using both energy efficiency as well as compactness as criteria. Increasing N(x) results in under-stretching and decreasing N(x) leads to over-stretching of the interface. Increasing N(p) makes interfacial stretching more effective by co-operating vortices. Comparing realized to optimal stretching, we find the optimum series for all possible SMX(n) designs to obey the universal design rule N(p) = (2/3) N(x) -1, for N(x) = 3, 6, 9, 12, ….

Chaotic mixing induced by a magnetic chain in a rotating magnetic field.

Chaotic mixing, induced by breakup and reformation of a magnetic chain under the influence of a rotating magnetic field, is studied. A direct simulation method combining the Maxwell stress tensor and a fictitious domain method is employed to solve flows with suspended magnetic particles. The motion of the chain is significantly dependent on the Mason number (Ma), the ratio of viscous force to magnetic force. The degree of chaos is characterized by the maximum Lyapunov exponents. We also track the interface of two fluids in time and calculate the rate of stretching as it is affected by the Mason number. The progress of mixing is visualized via a tracer particle-tracking method and is characterized by the discrete intensity of segregation. Within a limited range of Mason number, a magnetic chain rotates and breaks into smaller chains, and the detached chains connect again to form a single chain. The repeating topological changes of the chain lead to the most efficient way of chaotic mixing by stretching at chain breakup and folding due to rotational flows.

Numerical investigation of the effect of insoluble surfactants on drop deformation and breakup in simple shear flow.

The effect of insoluble surfactants on drop deformation and breakup in simple shear flow is studied using a combination of a three-dimensional boundary-integral method and a finite-volume method to solve the coupled fluid dynamics and surfactant transport problem over the evolving interface. The interfacial tension depends nonlinearly on the surfactant concentration, and is described by the equation of state for the Langmuir isotherm. Results are presented over the entire range of the viscosity's ratio lambda and the surface coverage x, as well as the capillary number Ca that spans from that for small deformation to values that are beyond the critical one Ca(cr). The values of the elasticity number E, which reflects the sensitivity of the interfacial tension to the maximum surfactant concentration, are chosen in the interval 0.1 < or = E < or = 0.4 and a convection dominated regime of surfactant transport, where the influence of the surfactant on drop deformation is the most significant, is considered. For a better understanding of the processes involved, the effect of surfactants on the drop dynamics is decoupled into three surfactant related mechanisms (dilution, Marangoni stress and stretching) and their influence is separately investigated. The dependence of the critical capillary number Ca(cr)(lambda) on the surface coverage is obtained and the boundaries between different modes of breakup (tip-streaming and drop fragmentation) in the (lambda; x) plane are searched for. The numerical results indicate that at low capillary number, even with a trace amount of surfactant, the interface is immobilized, which has also been observed by previous studies. In addition, it is shown that for large Péclet numbers the use of the small deformation theory to measure the interfacial tension in the case where surfactants are present can introduce a significant error.