Colloidal Particle Tracking in Drying Droplets

For direct tracking of individual particle motions with confocal laser microscopy, poly(methyl methacrylate) (PMMA) colloidal particles labeled with a fluorescent dye were used49 50 . As standard solvents for the colloidal droplets, a mixture of cis- and trans-decalin (Decahydronaphthalene, 99%, Sigma-Aldrich), were employed which has widely been used in drying experiments49 51 52 (link). Each droplet was gently deposited onto a clean cover glass (VWR, 22 × 30 mm², No. 1.5) in all experiments. The initial drop volume was controlled to be V₀ ≈ 0.5 μl, resulting in the initial contact radius of R₀ ≈ 1.4 mm, which is smaller than the capillary length (≈1.9 mm for decalin): therefore, the droplet shape was assumed to be spherical. The PMMA colloids were prepared and supplied by A. Schofield (University of Edinburgh) according to the procedure described by Antl et al.53 , and thus were expected to show hard-sphere-like behaviors. The colloid radii were r_S = 100 nm (small colloids) and r_L = 1000 nm (large colloids) with ~5% polydispersity in size (previously determined by dynamic light scattering33 (link)). The density difference between colloids (1.19 g/cm³) and decalin (0.897 g/cm³) was small enough to prevent any sedimentation issues for the microscopic colloids54 .