Investigation of the flow field in thin polymer films due to inhomogeneous drying

Abstract

In the field of organic and printed electronics (e.g., polymer solar cells, OLEDs, or Li-ion batteries), there is a growing demand for thin functional layers with highly homogeneous surface topology. If these layers are coated from the liquid phase, the coating and drying steps affect the surface quality. As a result of inhomogeneous drying rates, the solvent concentration can vary along the top surface and the thickness of a solidifying solution, leading to local differences in surface tension. In turn, Marangoni convection, as the balancing mechanism, can occur and cause surface inhomogeneity. The in situ reconstruction of the free surface during drying has been presented elsewhere. During this investigation phenomena occurred that could not be completely understood without knowledge of the respective flow field. In the present work, the visualization of the flow field in thin polymer films [methanol-poly(vinyl acetate) solution with 67 wt% methanol] due to inhomogeneous drying is presented. To resolve the flow field, we apply fluorescent particle tracking (lPTV). Since both measurement techniques cannot easily be applied at the same time, the boundary conditions were adapted to the way of observation of each experimental setup. In the case of the setup for surface reconstruction of the free surface, locally different evaporation rates were realized by drying on a structured substrate (varying material). To force similar variation of the drying conditions in the case of the lPTV setup, the drying film was partially covered. As expected, both boundary conditions result in a propagating wave front towards regions of high surface tension. Combining both experimental setups, we were able to visualize the free surface and the flow structures up-and downstream of the wave front and found different flow regimes.

Calibration Routine for Quantitative Three-Dimensional Flow Field Measurements in Drying Polymer Solutions Subject to Marangoni Convection

“…A simple drying experiment as proposed in [4] was performed to demonstrate a quantitative flow field reconstruction. A solution of poly(vinyl acetate) (PVAc, Carl Roth, 9154.1) and methanol (MeOH, Carl Roth, 4627.1) with tracer particles and an initial solvent load of X 0 = 2 g MeOH /g PVAc was blade-coated onto a transparent glass substrate (t g = 144 µm, T = 20 • C) and observed with µPTV from below.…”

“…Inhomogeneous drying conditions were induced by partially covering the drying film (see Figure 1b). A Marangoni flow from the covered area towards the uncovered region was expected [4]. The 60× objective was used with its correction collar set to t * g = 110 µm.…”

“…Large area coatings may also suffer from deformations induced by inhomogeneities in the drying boundary conditions. Either lateral variations of the heat conductivity of the substrate [3] or lateral variations of the solvent mass transport in the gas phase above the drying film cause the liquid-gas interface to deform [4].…”

“…In our group, we have established a measurement setup avoiding the shortcomings of previous PIV measurements. It is based on microscopic particle tracking velocimetry (µPTV) with only one optical access that was implemented using an inverse microscope and a transparent substrate, thus observing the flow field from below [4]. The third velocity component of the tracer particles can be obtained from the convoluted optical response.…”

Nonuniform heating of a substrate in evaporative lithography

“…In the case of a predominant Marangoni flow, the precipitation structures are inverted relative to the holes in the mask [5]. External sources of airflow [6] and composite substrates with variable thermal properties [7,8] can also be useful for evaporative lithography applications. If the melting point of the particles is higher than room temperature, the resulting precipitation is brittle.…”