Concentration Field Evolution during the Drying of a Thin Polymer Solution Film near the Contact Line A. Babaie † and B. Stoeber* ,‡,† † Department of Mechanical Engineering and ‡ Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada ABSTRACT: 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. ■ INTRODUCTION Solvent casting and inkjet printing of polymer solutions are widely used for controlled polymer deposition on surfaces in a variety of different applications; the fabrication of microdevices such as transistors 1 and microneedles 2 and the manufacturing of polymer LED displays 3 and photovoltaic cells 4 all involve the drying of a polymer solution through solvent casting or inkjet printing. The drying of a flat film of polymer solution has been the topic of theoretical study for many years; 5 however, the drying process near the contact line is not very well understood. The evaporation rate variation along the interface, in addition to evaporation-induced convection, complicates the drying process at the meniscus. Pinning and depinning of the contact line might also occur during the evaporation of a droplet on a flat substrate, which adds even more complexity to the drying process. Using microcavity structures or microliter wells instead of a flat substrate helps to decrease the complexity of the problem by fixing the contact line. For an evaporating film of a polymer solution that is pinned at the top rim of a cavity, capillary flow is generated toward the pinned contact line. 6 Due to the capillary transport and nonuniform evaporation at the meniscus, a polymer concen- tration gradient will exist along the surface with a high polymer concentration near the contact line. For many polymer solutions, the surface tension is a function of polymer concentration. In the case considered here, the surface tension decreases with the increase in polymer concentration (aqueous poly(vinyl alcohol) (PVA) solution 7 ), and the solutal Marangoni flow caused by the surface tension gradient 8 can oppose the capillary flow by moving the solution away from the contact line. Kajiya and Doi 9 used optical microscopy to study the effect of transport mechanisms on the surface profile of the dried film. They observed excessive polymer deposition near the side walls due to the capillary flow toward the contact line; however, the deposition profile became flat after adding surfactant to the solution due to the solutal Marangoni effect. Mansoor and Stoeber 10 visualized these two simultaneous flow phenomena during the drying of an aqueous poly(vinyl alcohol) (PVA) solution near the meniscus using microparticle image velocimetry (PIV) and confocal microscopy. In a similar study, Babaie et al. 11 studied the drying of an aqueous PVA solution near the side wall in cavities with different depths and with different initial polymer concentrations. They discovered that the competition between the capillary flow and the solutal Marangoni flow can lead to viscous flow separation 12 from the bottom wall of the cavity, resulting in flow recirculation. While velocimetry results provide information on transport mecha- nisms, the direct measurement of the polymer concentration is still necessary to fully understand the drying process. Microscopic dynamical studies performed with imaging techniques such as magnetic resonance imaging (MRI) on flat films of polymer solution reveal spatial structural heterogeneity across the polymer solution during the drying process. 13 The continuous increase in the polymer concen- Received: May 28, 2015 Revised: August 5, 2015 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.5b01960 Langmuir XXXX, XXX, XXX−XXX