Journal of Non-Newtonian Fluid Mechanics, Vol.221, 55-65, 2015
The effect of shear-thickening on liquid transfer from an idealized gravure cell
Gravure printing is an economical roll-to-roll processing technique with potential to revolutionize the fabrication of nano-patterned thin films at high throughput. In the present study, we investigated the impact of shear-thickening on the liquid transfer from an idealized gravure cell by a combination of experiments and numerical computations. We chose as a model system fumed silica nanoparticles dispersed in polypropylene glycol; these dispersions exhibit shear and extensional thickening as verified by steady shear and filament stretching extensional rheometry. Model gravure printing experiments were conducted using a linear motor to pick out the fluid vertically up from a truncated conical shaped idealized gravure cell cavity; the cell size is large enough that gravity is important, and therefore experiments were also conducted to pickout the fluid vertically down from the cavity-on-top. The amount liquid transfer from the cavity was studied with varying stretch velocities and dispersion concentrations. The filament profile evolution during the pickout process was examined using a high speed camera. Beyond a critical stretch rate, shear-thickening of the fluid, manifested by the formation of long stable filaments, exacerbates gravitational drainage during pickout. Beyond a second critical stretch rate, shear-thinning induces conical profile evolutions that result in pickout insensitive to stretch rate. All of these observations were qualitatively predicted by finite element computations using a generalized Newtonian fluid model, where the shear rheology was modeled explicitly. We showed that under the influence of gravity, wetting/de-wetting may be a critical phenomenon in determining pickout at low stretch rates. (C) 2015 Elsevier B.V. All rights reserved.
Keywords:Gravure printing;Coating;Shear thickening;Colloids;Extensional rheology;Finite element method