Volume 24, Issue 1, February 1980
Index of content:
24(1980); http://dx.doi.org/10.1122/1.549554View Description Hide Description
The phenomenon of droplet breakup in nonuniform shear flow (e.g., in a cylindrical tube) is of practical importance in many polymer processing operations. A theoretical and experimental study was carried out on the stability of a long threadlike viscoalastic droplet suspended in a viscoelastic medium flowing through a cylindrical tube. The stability analysis was carried out using linearized dynamic equations with the Jeffreys model. A perturbation method was used for the case of small wave numbers, and the case of small Reynolds numbers was also considered. A theoretical criterion for droplet breakup was obtained in terms of the rheological properties of the fluids concerned, and the flow conditions. The results reveal that a long threadlike droplet cannot be stable under all disturbances, and that both the elastic and viscous properties affect the stability of extending droplets. Furthermore, it was found that the interfacial tension tends to destabilize the extending threadline. For the experimental study, the same apparatus and fluid systems as those described in part I of this series [H. B. Chin and C. D. Han, J. Rheol., 23(5), 557–590 (1979)] were used. The breakup patterns of droplets were recorded on both movie and still films. It was observed that the droplets were elongated greatly at the entrance region of the tube and then, under certain flow conditions, broke up into smaller droplets in the cylindrical tube section. The breakup conditions were correlated to the rheological properties of the droplet phase and the suspending medium.
24(1980); http://dx.doi.org/10.1122/1.549555View Description Hide Description
A simple model of a polymer melt has been offered to explain the normal stress effect and the Barus effect in case of a steady shear flow. Assuming that a polymer melt consists of numerous flowing units which in turn are aggregates of radial Voigt elements with balls at their outer ends, the primary normal stress difference and swelling ratio were calculated theoretically. The experiments were carried out to test the theory. The exit pressure and swelling ratio were measured in cases of two high‐density polyethylenes, a polypropylene, and a low‐density polyethylene. The primary normal stress difference was calculated from the measured exit pressure. Insofar as our experiments are concerned, the theoretical results show reasonable agreement with the experimental results in both cases of the normal stress effect and the Barus effect.
24(1980); http://dx.doi.org/10.1122/1.549556View Description Hide Description
Measurements were taken of both flow patterns and stress birefringence patterns of viscoelasticpolymeric melts in a slit channel having various sizes of transverse slots. Materials used for the experiment were polystyrene, high‐ and low‐density polyethylenes, and polycarbonate. The slit channel had two glass windows (optical grade quartz), which permitted us to take photographs of the stress birefringence patterns of the molten polymers with the aid of a polariscope. With the polariscope removed, we were able to take photographs of the flow patterns. It was observed that secondary flows occur inside the pressure holes, and that neither the isochromatic fringe patterns nor the flow streamlines are symmetric about the centerline of the pressure hole. Also, measurements were taken of flow patterns of both Newtonian liquid and viscoelasticpolymer solutions in the flow channel having various sizes of circular holes. Two additional flow channels, one with a circular cross section (a cylindrical tube) and the other with a rectangular cross section (a slit flow channel), were constructed of Plexiglas. The transparency of the Plexiglas permitted us to take photographs of the flow patterns of test solutions in the flow channel as well as inside the pressure hole. Aqueous solutions of polyacrylamide at various concentrations were used as viscoelastic test fluids and glycerine was used as the Newtonian one. It was found that the fluid streamlines are asymmetric about the centerline of the pressure hole, secondary flows start inside the pressure holes at Reynolds numbers much lower than unity and the severity of secondary flow increases with wall shear stress (or shear rate). In view of the fact that when the asymmetry of the stress and flow patterns about the centerline of the pressure hole prevails the existing theories may not be useful, doubt is cast on the validity of the experimental studies of Higashitani and Lodge and of Baird, claiming that their hole pressure measurements may be used for determining the normal stress differences of viscoelastic fluids. In other words, the Higashitani‐Prichard theory was not tested critically by the experimental studies of Higashitani and Lodge and of Baird.
24(1980); http://dx.doi.org/10.1122/1.549557View Description Hide Description
The rheological properties of a 500 ppm aqueous solution of the polyacrylamide Separan MG200 obtained using a capillary tube viscometer are presented. The flow behavior index for the solution is shear‐rate dependent and above shear rates of 600 near‐Newtonian behavior is observed.
24(1980); http://dx.doi.org/10.1122/1.549589View Description Hide Description
The storage shear modulus and loss tangent have been measured for gelatin gels in water and glycerol‐water mixtures containing 60, 73, and 91% glycerol by weight. The concentration of gelatin (molecular weight 35,000) was 15.0% and the temperatures were 5, 12, and 20°C. Measurements were made from 0.08 to 12.5 Hz in the Plazek torsion pendulum and (for one gel) from 0.05 to 500 Hz in the modified Birnboim transducer apparatus. Single composite curves extending over five logarithmic decades of frequency were obtained for most of the data by plotting logarithmically and against where is (approximately) at the lowest frequency of measurement, ω the radian frequency, the solventviscosity, and the average molecular weight of a network strand (estimated from by the statistical theory of Pearson and Graessley); the subscript 0 refers to a reference state of water at 12.0°C. The relaxation spectrum calculated from and by standard approximation methods, followed the Rouse‐Mooney proportionality to over several logarithmic decades, and the monomeric friction coefficient calculated from it was reasonable in magnitude. It may be concluded that the network strands behave as flexible chains whose mobility is determined by the frictional resistance of the surrounding medium (though this is not necessarily true for other protein gels).
24(1980); http://dx.doi.org/10.1122/1.549558View Description Hide Description
Elastic properties in capillary and rotational viscosimetry have been examined for a series of styrene/acrylic terpolymers. Swelling ratio data confirm earlier contentions that melt flow behavior of these polymers is dominated by entanglement effects above an apparent critical temperature of about 180°C, and by a combination of specific interaction and entanglement factors at lower process temperatures. Maximum postextrusion swelling correlates well with zero‐shear melt viscosity, while a die‐swell reduction parameter has been introduced which appears to vary linearly with the recoverable shear component in capillary extrusion, and which also generates useful master curves over appreciable ranges of wall stress. It has been shown that some solid‐state properties of the polymers, including elastic modulus and bond strength of joints sealed by the polymers, vary with processing conditions, thereby also depending on the flow mechanisms operative during processing.