When droplets containing non-volatile solutes dry on substrates, the fluid migrates outward via capillary force. Consequently, the solutes are carried to the periphery of the droplets and form “coffee rings” after drying. This is known as the coffee ring effect (CRE). The CRE is typically observed during printing, and it significantly affects the uniformity and quality of films, especially in droplet-based dispersed deposition methods, such as spray-coating.
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Source: Phys.org