The first is that by adding a second solvent, usually a highly viscous polymer or molecule, the surface tension may change in addition to viscosity. Moreover, the volatility of the two co-solvents is typically not the same, leading to an enrichment of one solvent during the drying process. Thus, a gradient in solvent composition establishes, leading to a gradient in viscosity and surface tension, generating highly complicated Marangoni flow patterns. In some cases, a gradient in evaporative flux is generated as well, if the two solvents have varying evaporation rates.