Development of dark-field dynamic light scattering microscopy and its application: Tracking dynamics of particles in condensed slurries spreading on planar/nonplanar substrates.

Hypothesis: While dynamics of particles in slurries is usually evaluated by dynamic light scattering measurements, this technique had only been applicable to particles in the bulk slurries. Because this limitation is mainly owing to strong reflection of light, the dynamics of particles in slurries spreading/drying on solid substrates is to be obtained by spatially separating the reflection light from scattering (signal) light. This may allow us to track the particles in practical samples such as cosmetics or inks spreading on solid surfaces.

Experiment: We developed novel "dark-field dynamic light scattering microscopy". The system was evaluated with test samples of polystyrene beads dispersed in several viscosities of bulk glycerol aqueous solutions. This setup was then applied to slurries spreading/drying on planar and nonplanar substrates.

Findings: The results for planar surface indicate that origin of coffee-ring are the particles flowing into the edge of the droplet just before complete drying. On a skin-modelled nonplanar substrate, the slurry on bumps was found to maintain semi-dry condition longer than that at dents. This suggests that the dispersive medium was supplied to bumps from dents. This unique flow was explained as effective drying from the bumps increased surface tension at the bumps to pull up the liquid around.