Transfer and Filling of Non-Newtonian Liquids in Printing and Coating Processes

Abstract

Non-Newtonian liquids are omnipresent in a variety of industrial settings and natural systems. While complex rheology is commonly encountered for multi-component inks in liquid-based printing and coating applications, much remains to be explored regarding their influence on the flow dynamics. In this thesis we study several model problems to advance our understanding of how non-Newtonian behavior affects (i) transfer of liquid between two surfaces and (ii) filling of liquid into a cavity, which are two important processes in printing and coating techniques and are relevant to numerous technologies. For liquid transfer dominated by the relative vertical motion between two surfaces, we study the stretching of a liquid bridge between either two flat plates or a flat plate and a cavity using finite-element simulations. We find that the influence of rate-dependent rheology primarily occurs near the less-wettable surface for cases of two plates and mainly occurs near the flat plate for cases involving a cavity due to stronger interface deformation there. We further examine the influence of shear thinning and strain hardening on liquid transfer between two flat plates using flow visualizations. We observe that shear thinning can be exploited to significantly enhance liquid transfer from a less-wettable plate to a more-wettable one, and that strain hardening results in a stabilized thin liquid thread but has little effect on the amount of liquid transferred. For liquid transfer dominated by the relative horizontal motion between a flat plate and a cavity, our numerical results suggest that the fraction of liquid left in the cavity collapses onto a master curve with three regimes distinguished by the ratio of the driving forces for flow to the resistance controlling contact-line motion. For Newtonian liquids, we find that the second regime is characterized by a power-law relationship similar to that observed for liquid-film withdrawal. We find that shear thinning improves cavity emptying compared to Newtonian liquids by aiding contact-line motion through reduced viscosities and has larger power-law exponents in the second regime, and that shear thickening leads to the opposite. To study cavity filling we consider liquid confined between a flat plate and a cavity. We find that shear thinning reduces entrapped air compared to Newtonian liquids in general. For flows driven purely by a pressure gradient, shear thinning improves filling by producing a difference of viscosity gradients near the contact line between two surfaces. For flows driven by a pressure gradient and horizontal plate motion, shear thinning benefits filling by enhancing contact-line motion along the cavity.