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.

中文翻译：

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不同粗糙度的铜基板上的液滴蒸发研究

在本研究中，我们研究了在40°C环境温度下，盐滴在具有不同粗糙度的铜基板上的蒸发过程和沉积模式。根据它们的液滴接触角，将这四个基板分为光滑表面和粗糙表面。在这项研究中发现，液滴的蒸发方式与基材粗糙度有很强的关系。光滑表面上的蒸发图案的厚度边界比粗糙表面上的大，并且颗粒更接近边界，并且在光滑表面上的趋势更加明显。以下因素有助于结果。在光滑表面上，液滴的接触角随着粗糙度的降低而增加。在粗糙表面上，接触角随粗糙度的增加而增加。随着接触角的减小，边界处的蒸发速率增加，导致边界处的颗粒更容易沉淀。此外，通过增加基底的粗糙度来阻碍毛细管流动，而马兰戈尼流动保持恒定，从而导致更多的颗粒保留在粗糙表面上的液滴中心。总之，通过增加40℃温度下的铜基板上的基板粗糙度来抑制咖啡环的形成。