(657a) Printability, Pinch-Off Dynamics and Extensional Rheology of Complex Fluids (Invited Talk)

Liquid transfer and drop formation/deposition processes underlying printing, jetting, spraying and coating involve complex free-surface flows, and the formation of columnar necks that undergo spontaneous surface tension-driven instability, thinning and pinch-off. The progressive self-thinning of the neck is often characterized by self-similar profiles and scaling laws that depend on the relative magnitude of capillary, inertial and viscous stresses for simple (Newtonian and inelastic) fluids. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field that can orient and stretch macromolecules, contributing extra elastic stresses and extensional viscosity that change thinning and pinch-off dynamics. It is well established that polymeric complex fluids exhibit a much larger resistance to flow in an elongational flow field than Newtonian fluids with same shear viscosity and even O(~100) ppm amount of added polymers can lead to delayed pinch-off affecting printability of multicomponent fluids. However, extensional rheology measurements are far from routine and require bespoke instrumentation not available, or easily replicated, in most laboratories. 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 η < 20 mPa.s and relatively short relaxation time, λ < 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. We show that dripping-onto-substrate methodology can be implemented using an off-the-shelf digital camera to distinguish between pinch-off dynamics and printability of a wide variety of complex fluids. In this talk, we will elucidate how polymer composition, flexibility and molecular weight determine the kinetics of capillary-driven thinning and pinch-off in our experiments. Both effective relaxation time and transient extensional viscosity are found to be strongly concentration dependent even for dilute solutions. Lastly, we show how finite extensibility of polymers dramatically changes the kinematics from elastocapillary to viscocapillary under strong extensional flow fields that can lead to coil-stretch transition.