(343h) Viscous Fingering in Drying Colloid-Polymer Solutions

We study experiments of drying colloid-polymer films similar to those used in many chemical and pharmaceutical applications. While flow and drying stresses set the colloidal microstructure of these films, after this microstructure is set we observe various modes of air entrainment into the matrix following various capillary, viscous, and elastic modes of instabilities. In this system, Haines jumps, capillary fingering, and crack formation at the bottom surface of this film follow their general understood physical phenomena. However, the predominant mode of air entrainment at moderate polymer concentrations shows micron-scale viscous fingering reminiscent of the Saffman-Taylor instability, yet at length scales much smaller than expected. In controlled drying in periodic systems using the ordered porosity in colloidal crystals, the viscous fingers are highly ordered and scale with the particle size and porosity. The onset and structure of this instability is characterized and a semi-elastic model driving the instability may be responsible for the short wavelength of the instability.