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UBC Theses and Dissertations

Flow physics during the drying of a thin polymer solution film near the contact line Babaie, Ashkan


The drying process of a thin polymer solution film has been studied inside a micro-liter cavity near the contact line. Confocal microscopy along with particle image velocimetry and laser induced fluorescence are used for the real time measurement of velocity and concentration fields during the drying process. In addition to the capillary flow and the Marangoni flow, the velocity field also reveals the possible existence of a single vortex and multiple vortices inside the creeping flow induced by evaporation. These vortices appear soon after the beginning of the evaporation process, their size shrinks over time, and they disappear before the end of the evaporation process. This thesis includes a study of the effect of rheological and geometrical parameters on the presence, size and endurance of these vortices. Significant concentration heterogeneity is observed across the film during the drying process, in particular near the contact line. The concentration at the solution-air interface is higher compared to the bulk, and it increases towards the contact line and also over time. A skin layer starts forming as soon as the surface concentration reaches the glass transition concentration after which the evaporation rate starts decreasing. The drying film undergoes a similar concentration evolution during the evaporation process, regardless of the cavity depth and the initial polymer concentration; although, minor differences can be recognized that are associated with the flow recirculations that delay the concentration increase inside the vortex. Finally, a theory is developed based on experimental data which explains the existence and behavior of viscous vortices near the bottom wall of the cavity. The competition between the capillary flow and the Marangoni flow results in flow separation on the bottom wall which leads to such vortices. This study provides better understanding of the drying process of thin polymer solution films near the contact line. Furthermore, viscous flow separation adds to the current understanding of flow physics during the drying process, in addition to the well-known evaporation induced capillary transport and the Marangoni effect.

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