in Wiley Online Library (wileyonlinelibrary.com).Key microstructural properties of particulate coatings such as porosity and particle order are established during drying. Therefore, understanding the evolution of particulate distributions during drying is useful for designing coating properties. Here, a 1D model is proposed for the particle distribution through the coating thickness at different drying times and conditions, including Brownian diffusion, sedimentation, and evaporation. Effects of particle concentration on diffusion and sedimentation rates are included. Results are condensed onto a drying regime map which predicts the presence of particle surface accumulation or sediment based on two dimensionless numbers: the Peclet number and the sedimentation number. Cryogenic scanning electron microscopy (cryoSEM) is used to image the transient particulate distributions during the drying of a model system comprised of monodisperse silica particles in water. Particle size and evaporation rates are altered to access various domains of the drying map. There is good agreement between cryoSEM observations and model predictions.

Ceramic nanoparticle/monodisperse latex coatings with a nanoparticle-rich surface and a latex-rich body were created by depositing aqueous dispersions of monodisperse latex, approximately 550 nm in diameter, and nanosized ceramic particles onto substrates and drying. On the top surface of the dried coating, the latex particles are closely packed with nanoparticles uniformly occupying the interstitial spaces, and along the cross section, nanoparticles fill the spaces between the latex particles in the near surface region; a compacted latex structure, nearly devoid of nanoparticles, lies beneath. Cryogenic scanning electron microscopy images of partially dried coatings at successive drying stages reveal two important steps in forming this structure: top-down consolidation of latex particles and accumulation of nanoparticles in interstitial spaces among latex particles near the surface. A systematic study of the effect of processing conditions, including nanoparticle concentration, nanoparticle size, latex glass transition temperature, and drying conditions, on the final microstructure was carried out. The unique microstructure described above forms when the monodisperse latex is large enough to create pore channels for the transport of nanosized particles and the drying conditions favor "top-down" as opposed to "edge-in" drying.

Hollow latex particles are used as white pigments for paints and paper coatings. In the coating dispersion, each hollow particle is filled with water. As the coating dries, water vacates the latex, leaving an air-filled void sized to scatter light ($0.5 lm) within each particle. Examinations of dried coatings reveal that hollow particles can collapse, decreasing their light scattering efficiency. Cryogenic scanning electron microscopy (cryoSEM) was used to characterize the microstructure of coatings containing hollow latex during drying. Images suggest latex voids empty after air invades into the coating interstitial space and collapse occurs late in the drying process. The effects of temperature (10-60°C), humidity (20-80%), and binder concentration (0-30 wt%) on particle collapse were also studied through SEM of dried coating surfaces. High drying temperature, high humidity, and low binder concentrations promoted collapse. For hollow latex particles with porous shell walls, temperature and humidity had little effect, whereas binder increased collapse. From these results, a theoretical model is proposed. During drying, diffusion of water from the particle creates a vacuum inside the latex. The vacuum is either relieved by nucleation of a gas bubble from the dissolved air in the water-filled particle or it causes the particle to collapse by buckling.