(230u) Free Surface Flows and Extensional Rheology of Polymer Solutions

Free-surface flows â?? jetting, spraying, atomization during fuel injection, roller-coating, gravure printing, nanoimprint hot embossing, several microfluidic drop/particle formation techniques, and screen-printing â?? all involve the formation of axisymmetric fluid elements that spontaneously break into droplets by a surface-tension-driven instability. The growth of the capillary-driven instability and pinch-off dynamics are dictated by a complex interplay of inertial, viscous and capillary stresses for simple fluids. Additional contributions by elasticity, extensibility and extensional viscosity play a role for complex fluids. We show that visualization and analysis of capillary-driven thinning and pinch-off dynamics of the columnar neck in an asymmetric liquid bridge created by dripping-onto-substrate can be used for characterizing the extensional rheology of complex fluids. Using a particular example of dilute, aqueous PEO solutions, we show the measurement of both the extensional relaxation time and extensional viscosity of weakly elastic, polymeric complex fluids with low shear viscosity (< 20 mPa.s) and relatively short relaxation time, (< 1 ms). Characterization of elastic effects and extensional relaxation times in these dilute solutions is beyond the range measurable in the standard geometries used in commercially available shear and extensional rheometers (including CaBER, capillary breakup extensional rheometer). As the radius of the neck that connects a sessile drop to a nozzle is detected optically, and the extensional response for viscoelastic fluids is characterized by analyzing their elastocapillary self-thinning, we refer to this technique as dripping-onto-substrate (DoS) extensional rheometry. Using DoS experiments and theory of polymer dynamics, we elucidate how polymer composition, flexibility, and molecular weight as well as coil-stretch transition determine the thinning and pinch-off dynamics, and thus influence printability, spinnability, sprayability and jettability of polymeric complex fluids.