The evaporating liquid droplet with suspended particles on a solid substrate may leave a ring-like structure along the droplet circumference under certain conditions, which is commonly known as the coffee ring phenomenon. This phenomenon has many interesting applications related to microfluidic problem such as self-assembly of colloidal particle, particle and biomolecule separation and concentration and ink-jet printing. Although much work has been done on this topic, not very much work focused on the fluid flow inside a droplet. In order to investigate the inner flow inside of the droplet, water solutions with the fluorescing or non-fluorescing particles as the solute are dropped on surfaces with different hydrophilicity. The whole process are recorded using high dynamic CCD camera system and analyzed using Particle Image Velocimetry (PIV) methods. It is found that in all the droplets that form coffee ring, a central symmetric outward radial flow is observed. The velocity of the flow is zero at the center of the droplet. Along with radius outside, the flow velocity increases. For the same droplet, the flow velocity increases as the evaporation goes on and the velocity reaches its maximum at the end the evaporation. For different droplets, with the higher concentration, smaller droplet size and more hydrophilic surface, the flow velocity is larger. Comparisons between the experimental data and the published convection models show that the flow can be considered as part of the flow caused by the maximum evaporation rate at the pinned wetting line.

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