This work presents a simple method to create photonic microstructures via the natural evaporation of surfactant-laden colloidal sessile droplets on a flat substrate. In the absence of dissolved surfactant, the evaporating colloidal droplet forms a well-known coffee ring deposition. In contrast, the presence of surfactant leads to the formation of multiple ring structures due to the repetitive pinning-depinning behavior of the droplet contact line (CL). It is found that the multiring structure shows vibrant iridescent structural colors while the coffee ring lacks a photonic nature. This difference in the structural color for the presence and absence of the surfactant is found to be dependent on the arrangement of the particles in the deposition structure. The particle arrangement in the multirings is monolayered and well-ordered. The ordering of the particles is strongly influenced by the particle dynamics, contact angle (CA), and CL dynamics of the evaporating colloidal solution droplet. Furthermore, the iridescent nature of the multiring deposition is demonstrated and explained. The dependence of the multiring deposition structure on the concentration of the dissolved surfactant and the suspended particles is also studied. The findings demonstrate that an intermediate surfactant concentration is desirable for the formation of a multiring structure. Further, the pinning-depinning CL dynamics that causes the formation of the multiring deposition structure is discussed. Finally, we demonstrate the applicability of the approach to smaller droplet volumes.

中文翻译：

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表征表面活性剂蒸发胶体固着液滴中的微粒沉积结构及其光子性质

这项工作提出了一种简单的方法，通过在平坦基底上负载表面活性剂的胶体固着液滴的自然蒸发来创建光子微结构。在没有溶解的表面活性剂的情况下，蒸发的胶体液滴形成众所周知的咖啡环沉积。相反，由于液滴接触线（CL）的重复钉扎-脱钉行为，表面活性剂的存在导致多个环结构的形成。研究发现，多环结构显示出鲜艳的虹彩结构颜色，而咖啡环缺乏光子性质。发现存在和不存在表面活性剂时结构颜色的这种差异取决于沉积结构中颗粒的排列。多环中的颗粒排列是单层且有序的。颗粒的排序很大程度上受到蒸发胶体溶液液滴的颗粒动力学、接触角 (CA) 和 CL 动力学的影响。此外，还演示并解释了多环沉积的虹彩性质。还研究了多环沉积结构对溶解的表面活性剂和悬浮颗粒的浓度的依赖性。研究结果表明，中间表面活性剂浓度对于形成多环结构是理想的。此外，还讨论了导致多环沉积结构形成的钉扎-脱钉 CL 动力学。最后，我们证明了该方法对较小液滴体积的适用性。