Investigation of Droplet Evaporation on Copper Substrate with Different Roughness

In the present study, we investigated the evaporation process and deposition pattern of saline droplet on a copper substrate with different roughness under 40 °C ambient temperature. These four substrates are classified as smooth surface and rough surface based on their droplet contact angles. It has been found in this study that the evaporation pattern of droplets has a strong relationship to substrate roughness. The thickness boundary of the evaporation pattern on a smooth surface is larger than that on a rough surface and the particles are closer to boundary and the tendency is more obvious on a smooth surface. The below factors contribute to the result. On the smooth surface, the contact angle of droplet increases as the roughness decreases. On the rough surface, the contact angle increases as the roughness increases. With contact angle decreasing, the evaporation rate at the boundary increases leading to the particles at the boundary more easily sedimentate. Moreover, the capillary flow is hindered by increasing the substrate roughness, while the Marangoni flow remains constant, resulting in more particles remain in the center of the droplet on the rough surface. To sum up, the coffee-ring formation is suppressed by increasing the substrate roughness on a copper substrate under 40 °C temperature.