Ring deposition of drying suspension droplets

Deposition of colloidal particles in a drying droplet is important in many scientific researches and technological applications. In this work, the ring deposition of drying droplets on a solid substrate is investigated experimentally at a microscopic level. A ring deposition is formed at the contact line as the water solution droplet containing SiO2 particles is drying, just like the formation of coffee ring. Contact line pinning is crucial to the ring deposition formation. There will be a replenish flow in the droplet towards the edge, and the particles are driven to the contact line, deposited on the substrate. As the particle mass fraction is large, the particles which are left inside the spot, when the droplet dries out, may form a single particle layer, packing in order. The contact angle of the droplet on glass substrate is very small, the SiO2 particles will gather at the rim of the droplet, which initially form a chain along the contact line. As more particles come to the rim, they are deposited in a line by line way to form a 2D close packing. Since the contact angle decreases with evaporation when the contact line is pinned, a capillary force between liquid surface and particles arises once the height of droplet surface near the contact line is lower than that of the particle, pushing the particles to move inward. The effect on the larger particles is more pronounced-it even leads to a separation of the particles, with the smaller ones at the outer side.