No abstract

Tracking errors due to particles moving in and out of the focal plane are a fundamental problem of multiple particle tracking microrheology. Here, we present a new approach to treat these errors so that a statistically significant number of particle trajectories with reasonable length are received, which is important for an unbiased analysis of multiple particle tracking data from inhomogeneous fluids. Starting from Crocker and Grier's tracking algorithm, we identify particle displacements between subsequent images as artificial jumps; if this displacement deviates more than four standard deviations from the mean value, trajectories are terminated at such positions. In a further processing step, trajectories separated by a time gap Δτ max are merged based on an adaptive search radius criterion accounting for individual particle mobility. For a series of Newtonian fluids covering the viscosity range 6-1300 mPa s, this approach yields the correct viscosity but also results in a viscosity-independent number of trajectories equal to the average number of particles in an image with a minimum length covering at least two orders of magnitude in time. This allows for an unbiased characterization of heterogeneous fluids. For a Carbopol ETD 2050 solution we recover the expected broad variation of particle mobility. Consistent with the widely accepted structural model of highly swollen microgel particles suspended in a polymer solution, we find about 2/3 of the tracers are elastically trapped.

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.