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This paper describes low Reynolds number vortices that can occur during the evaporation of a polymer solution inside a cavity. Confocal microscopy combined with image processing, micro particle image velocimetry, and micro laser induced fluorescence are used to measure the unsteady evaporation-driven velocity field and the concentration field in a shallow liquid film inside a microliter cavity near a wall. In addition to evaporation-driven flow and Marangoni flow, the velocity field also reveals single and multiple vortices generated by the creeping flow induced by evaporation. Similar to other low Reynolds number vortices, it is seen that the geometry strongly affects the presence, endurance and size of these vortices during the evaporation process. The bulk shear stress of the solution affects the vortex behavior, and no recirculation is observed at high viscosity.

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An experimental study is performed for polymer concentration field measurements during the drying of an aqueous poly(vinyl alcohol) solution inside a shallow cavity near a vertical side wall. The measurements are based on optical techniques such as 3D confocal microscopy for laser-induced fluorescence analysis. The results reveal a significant concentration heterogeneity across the film near the meniscus during the drying process. The concentration at the solution-air interface remains higher compared to the bulk, and it increases toward the pinned contact line and also over time. A skin layer starts forming as the surface concentration reaches the glass-transition concentration, after which the evaporation rate starts decreasing. Regardless of the cavity depth and the initial polymer concentration, the drying film undergoes a similar concentration evolution during the evaporation process, although minor differences can be recognized. For instance, a low local capillary number at the surface is associated with a wavy surface concentration profile while at higher capillary numbers disturbances are damped and a much more uniform concentration profile is observed.