Measuring the Dynamic Material Properties of Drying Paint through Microrheology*

Studying rheological properties during drying of paint is important for controlling defect formation in the coating as well as connecting alterations in formulation to final paint performance. Passive microrheology, the tracking of the Brownian motion of tracers to determine the fluid material properties, is used to study both quiescent and drying thin films of a model paint where conditions such as molecular weight and composition are modified. Drying paint can experience convection due to Marangoni stresses, where the random fluctuations of tracers are superimposed on the kinematic motion of the fluid. This convective flow can be detrimental to the analysis of tracers’ mean squared displacement (MSD). Different methods are used to eliminate these effects, such as de-drifting algorithms, where the mean displacements were subtracted from a particle’s trajectory, or calculating the relative mean squared displacement (rMSD), where fluctuations of particles’ position are tracked in separation distance rather than absolute MSD. MSD, rMSD, and de-drifting are compared across various convection profiles to determine the appropriate approach to characterize the evolving material properties during drying.