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A novel passive micromixer concept is presented. The working principle is to make a controlled 90 degree rotation of a flow cross section followed by a split into several channels; the flow in each of these channels is rotated a further 90 degrees before a recombination doubles the interfacial area between the two fluids. This process is repeated until achieving the desired degree of mixing. The rotation of the flow field is obtained by patterning the channel bed with grooves. The effect of the mixers has been studied using computational fluid mechanics and prototypes have been micromilled in poly(methyl methacrylate). Confocal microscopy has been used to study the mixing. Several micromixers working by the principle of lamination have been reported in recent years. However, they require three dimensional channel designs which can be complicated to manufacture. The main advantage with the present design is that it is relatively easy to produce using standard microfabrication techniques while at the same time obtaining good lamination between two fluids.

SYNOPSISThe tensile properties of polypropylene fibers, produced in a short-spin line, are correlated with the parameters of the three processing stages (spinning, drawing, and annealing), and with the molecular weight distribution. In general, tensile stiffness and strength increase with increasing molecular orientation, while the elongation at break decreases. The degree of orientation is determined by the deformation ratios and temperatures of the first two stages. Tensile modulus and strength also increase with increasing annealing stage shrinkage ratio. All the tensile properties, including the elongation at break, increase with increasing average molecular weight. The mechanisms of crystallization and deformation are related to the molecular weight distribution in different ways. Hence, the tensile modulus is highest for broad distributions when the draw ratio is low, and for narrow distributions when the draw ratio is high. The tensile strength increases and the elongation at break decreases as the width of the molecular weight distribution decreases, for all combinations of processing parameters. The distribution of tensile strength, for fibers with high draw ratios, broadens as the molecular weight distribution narrows. The total draw ratio of fibers, as experienced during processing and testing, and the true stress at break, are discussed in terms of deformation rates and relaxation times.

Effects of material parameters on the haze of blown films were analyzed. Four linearlow‐density polyethylenes (two metallocene grades and two Ziegler‐Natta grades) were studied in combination with three additives (two sorbitol‐based clarifying agents and a low‐molecular‐weight long‐chain branched polyethylene). One of the sorbitol‐based additives reduced the haze of both the metallocene materials in this study, but did not have any positive effect on the two Ziegler‐Natta materials. The variation in haze among the four base materials was directly related to the root‐mean‐square surface roughness (σ). When considering all 16 material/additive combinations, the link between haze and surface topography was not a simple σ‐haze relationship, but the haze was correlated with the average distance between adjacent surface profile peaks, the average slope, and the power spectral density at high lateral frequencies. Both of the mechanisms referred to in the literature, extrusion‐induced haze and crystallization‐induced haze, were probably active for the films in this study.

Approximate analytic expressions for haze (and gloss) of Gaussian randomly rough surfaces for various types of correlation functions are derived within phase-perturbation theory. The approximations depend on the angle of incidence, polarization of the incident light, the surface roughness, σ, and the average of the power spectrum taken over a small angular interval about the specular direction. In particular it is demonstrated that haze(gloss) increase(decrease) with σ/λ as exp(−A(σ/λ)2 ) and decreases(increase) with a/λ, where a is the correlation length of the surface roughness, in a way that depends on the specific form of the correlation function being considered. These approximations are compared to what can be obtained from a rigorous Monte Carlo simulation approach, and good agreement is found over large regions of parameter space. Some experimental results for the angular distribution of the transmitted light through polymer films, and their haze, are presented and compared to the analytic approximations derived in this paper. A satisfactory agreement is found. In the literature haze of blown polyethylene films has been related to surface roughness. Few authors have quantified the roughness and other have pointed to the difficulty in finding the correct roughness measure.

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Improving the success rate in additive manufacturing and designing highly optimized structures require proper understanding of material behaviour. This study proposes a novel experimental method by which anisotropic mechanical properties of additively manufactured materials can be assessed. The procedure is based on tensile testing of flat specimens, manufactured by laser powder bed fusion (LPBF) at different orientations relative to the build plate. In this study, the procedure was applied to the Inconel 718 alloy. Three identical specimen sets were built, each of which received complementary postprocessing treatments. The tensile tests were carried out on specimens with as-built surface finish. Digital image correlation was used to record the strain field evolution on two perpendicular surfaces of the tensile specimens under loading. An optimization algorithm is also proposed for determining the anisotropic elastic constants using only a few tensile test results. It was observed that both build orientation and postprocessing have strong influence on the anisotropic mechanical properties of the material. The effect of microstructure was also investigated and characterised. Consequently, three transversely isotropic compliance matrices were constructed, representing the effect of the different processing conditions.

Warpage of various semicrystalline polyethylenes (linear low density polyethylene [LLDPE]) has been investigatedunder typical rotational molding conditions, which means slow cooling from only one side. We have developed an experimental technique that is able to quickly rate different materials with respect to warpage under typical process conditions. We have also developed a numerical model simulating the experiments assuming a thermoelastic material including crystallization. As has been observed in practical rotational molding, it has been found in both experiments and simulations that materials with high crystallinity have in general higher warpage. The simulations also showed that the crystallization kinetics has implications on the warpage because the crystallinity gradient during solidification depends on the rate of crystallization. POLYM. ENG. SCI., 45: 945-952, 2005.