Thursday, February 23, 2012
106 Spalding Lab, Hartley Memorial Seminar Room
Chemical Engineering Seminar
Designing complex fluids for jetting and spraying applications: associative polymers as rheology modifiers
Vivek Sharma, Postdoctoral Research Associate, Mechanical Engineering, Massachusetts Institute of Technology
Associative polymer solutions are used extensively in the formulations of functional soft materials like inks, paints, food, nasal sprays, cosmetics, insecticides, fertilizers and bioassays to control the rheology and processing behavior of multi-component complex fluids. These complex dispersions are processed and used over a broad range of shear rates and extensional rates, and their non-Newtonian behavior displayed under processing conditions is often not measurable in the conventional rheometric measurements. In this talk, we use the example of a cellulose ether - ethyl-hydroxyethyl-cellulose (EHEC) - and its hydrophobically modified analog (HMEHEC) to illustrate the influence of hydrophobic stickers on the linear and nonlinear rheology of their aqueous solutions. We show that a constitutive model based on fractional calculus can be used to characterize the physical gelation in HMEHEC solutions, and we use microfluidic rheometry to extend viscosity measurements to high shear rates. The transient extensional viscosity response of the associative polymer solutions dictates the complex pinch-off dynamics during jetting and spraying, where inertial, viscous and elastic effects moderate capillary break-up of cylindrical fluid elements into droplets. We compare the responses of EHEC and HMEHEC solutions to extensional deformations during capillary break-up using (a) capillary break-up extensional rheometer (CABER) and (b) Rayleigh Ohnesorge Jetting Extensional Rheometer (ROJER), an imaging-based technique capable of attaining high extension rates that has been developed as a part of this study. Furthermore, we measure extensional viscosity using excess pressure drop and flow induced birefringence in the vicinity of a stagnation point in a cross-slot extensional rheometer. This study highlights a number of complimentary techniques that are critical for the design and characterization of functional soft materials used in complex processing operations such as jetting and spraying.