Simulations of extensional flow in microrheometric devices

We present a detailed numerical study of the flow of a Newtonian fluid through microrheometric devices featuring a sudden contraction–expansion. This flow configuration is typically used to generate extensional deformations and high strain rates. The excess pressure drop resulting from the converging and diverging flow is an important dynamic measure to quantify if the device is intended to be used as a microfluidic extensional rheometer. To explore this idea, we examine the effect of the contraction length, aspect ratio and Reynolds number on the flow kinematics and resulting pressure field. Analysis of the computed velocity and pressure fields show that, for typical experimental conditions used in microfluidic devices, the steady flow is highly three-dimensional with open spiraling vortical structures in the stagnant corner regions. The numerical simulations of the local kinematics and global pressure drop are in good agreement with experimental results. The device aspect ratio is shown to have a strong impact on the flow and consequently on the excess pressure drop, which is quantified in terms of the dimensionless Couette and Bagley correction factors. We suggest an approach for calculating the Bagley correction which may be especially appropriate for planar microchannels

Poly(Alkylene oxide) Ionomers II: Solution Co- and Terpolymerization of Trioxane for the Preparation of Polyoxymethylene Ionomers

Solution polymerization of trioxane (TO) with l,3-dioxolane {DO) gave copolymers of various comonomer compositions. TO was also copolymerized with ethyl glycidate (EG) and terpolymerized with DO and EG to polymers of low EG content. The copolymers were acetate endcapped or base treated to give stable co- and terpolymers. The treatment of EG containing polyoxymethylene (POM) copolymers with sodium hydroxide in methanol at elevated temperatures gave POM ionomers. The polymers were characterized by their infrared and PMR spec­tra for their general structure and co-and terpolymer composition. POM co­polymers with only EG as comonomer showed low base stability but those which contained ethylene oxide (EO), e.ither alone or in combination with EG, gave polymers of high base stability