Pradipto K. Bhattacharjee

Extensional stress growth and stress relaxation in entangled polymer solutions

We report an evaluation of the double constraint release model with chain stretch (DCR-CS) suggested by Ianniruberto and Marrucci [J. Rheol. 45, 1305–1318 (2001)], in predicting the transient stress growth and stress relaxation behavior of two well-characterized entangled polymer solutions undergoing homogeneous uniaxial extensional flow. The experiments are conducted using a filament stretching rheometer. The DCR-CS model belongs to a family of simplified single-segment models that incorporate constraint release, double reptation, and segmental stretching into the basic reptation mechanism proposed in the original Doi–Edwards theory and seeks to extend the predictive capacity of the theory to more complex flow fields. We show that the single-mode DCR-CS differential model performs well in predicting the transient extensional stress growth and steady-state extensional viscosity over a range of stretch rates. The model also predicts the observed stress relaxation following cessation of stretching satisfactorily. We further show that the model predicts shear thickening even in steady shear flow.

Extensional flow of low-viscosity fluids in capillary bridges formed by pulsed surface acoustic wave jetting

Forming capillary bridges of low-viscosity (10 mPa s) fluids is difficult, making the study of their capillary-thinning behavior and the measurement of the fluids extensional viscosity difficult as well. Current techniques require some time to form a liquid bridge from the stretching of a droplet. Rapidly stretching a liquid bridge using these methods can cause its breakup if the viscosity is too low. Stretching more slowly allows the bridge to thin and break up before a suitable bridge geometry can be established to provide reliable and accurate rheological data. Using a pulsed surface acoustic wave to eject a jet from a sessile droplet, a capillary bridge may be formed in about 7.5 ms, about seven times quicker than current methods. With this approach, capillary bridges may be formed from Newtonian and non-Newtonian fluids having much lower viscosities—water, 0.04% by weight solution of high-molecular-weight (7 MDa) polystyrene in dioctyl phthalate and 0.25% fibrinogen solution in demineralized water, for example. Details of the relatively simple system used to achieve these results are provided, as are experimental results indicating deviations from a Newtonian response by the low-viscosity non-Newtonian fluids used in our study.

Electrospinning and polymer nanofibers: Process fundamentals

Electrospinning is a popular technique for manufacturing nanofibers using an electrified jet of a polymeric fluid. The fibrous, nonwoven mat that typically results from electrospinning finds applications in a wide range of industries. While setting up an electrospinning experiment is relatively simple, controlling the attributes of the end-products, such as the diameter and morphology of the fiber formed, the orientation of the deposited fiber, the porosity, and the surface properties of the resultant mat, etc., is considerably more challenging. Significant progress has been made in recent years in understanding the physical mechanisms that govern the process of fiber formation through electrospinning. This chapter reviews these advances and provides an overview of the present status of the technology. The process of electrospinning is outlined and the mathematical framework for studying the mechanics involved is discussed. The relevant electrical and the rheological aspects of polymeric fluids are also reviewed. An account of the advances in controlling the fiber diameter, morphology, and surface properties, as well as in controlling the properties of the mat, is provided. This chapter also discusses the limitations of the current technology and provides a perspective for future developments.

Response of an entangled polymer solution to uniaxial and planar deformation

Experimental data on homogeneous planar extension on well-characterized polymeric fluids are rare. As a result, the relationship between planar and uniaxial extensional viscosity is also unclear. We present measurements of the steady–state planar extensional viscosity of a well-characterized polymer solution with about 30 entanglements per chain. The measurements are made by modifying the filament stretching rheometer to enable the stretching of the sample in the shape of a cylindrical tube. When a constant strain rate is imposed on the sample, a well-defined steady-state in the planar extensional viscosity is observed in the experiments. Our experiments show that, within the range of strain rates used, the response of the entangled polymer solution in planar extensional flow is identical to that observed in uniaxial extensional flow. Such a finding, if found to be generally valid, facilitates the comparison between simulations and experimental data, since planar flows are easier to simulate than uniaxial...

Control of the mixing time in vessels agitated by submerged recirculating jets

Submerged recirculating jet mixing systems are an efficient and economical method of agitating large tanks with a high hydraulic residence time. Much work has been carried out in developing design correlations to aid the predictions of the mixing time in such systems, with the first such correlation being developed nearly 70 years ago. In most of these correlations, the mixing time depends directly on the volume of the vessel and inversely on the injection velocity of the submerged jet. This work demonstrates, for the first time, that the distance between the injection and suction nozzles also significantly affects the mixing time and can be used to control this time scale. The study introduces a non-dimensional quantity that can be used as an adjustable parameter in systems where such control is desired.

ADMiER-ing thin but complex fluids

The Acoustics Driven Microfluidic Extensional Rheometer (ADMiER) utilises micro litre volumes of liquid, with viscosities as low as that of water, to create valid and observable extensional flows, liquid bridges that pinch off due to capillary forces in this case. ADMiER allows the study fluids that have been beyond conventional methods and also study more subtle fluid properties. We can observe polymeric fluids with solvent viscosities far below those previously testable, accentuating elastic effects. Also, it has enabled the testing of aqueous solutions of living motile particles, which significantly change fluid properties, opening up the potential for diagnostic applications.