Abstract: Regulation of the deposition morphology after sessile drop evaporation has numerous applications and potential benefits. The drop evaporation of particle suspension with polystyrene particles and sodium dodecyl sulfate (SDS) was experimentally observed and analyzed. The effect of SDS with a concentration of 0wt%~1wt% on the evaporation process of polystyrene particle suspension and the deposition morphology after evaporation was studied. The results demonstrated that the droplet contact angle decreased nonlinearly with the increase of SDS concentration. When SDS was added, spontaneous spreading driven by surface-tension gradient occurred. It was found that the maximum spread coefficient was positively correlated with SDS concentration. The spreading process took only 1/5~1/3 of the total evaporation process time. The gray level co-occurrence matrix (GLCM) was used to analyze the effect of adding SDS on the deposition morphology inside the coffee ring. The SDS could not only regulate the ring width, but also reduce the aggregation of particles in the middle of a deposition, and the deposition was more uniform (the most obvious when the concentration of SDS was 0wt% compared with 0.1wt%). Marangoni flow in the droplet would become more ferocious as SDS concentration rose, and the ring grew broader. There was an upper limit for SDS to regulate the ring width of the annular deposition, as the annular deposition was not readily apparent when the concentration of SDS was higher than 0.75wt%. Most of the particles were uniformly distributed throughout the entire deposition area, forming a disc-like deposition. The maximum percentage of annular deposition area for particles were 58.6% for diameter of 5.7 μm and 44.6% for diameter of 10 μm. It demonstrated that SDS affected the deposition pattern of particles of various sizes in distinct ways.

Key words: drop evaporation, coffee ring effect (CRE), surfactant, Marangoni flow

摘要： 调节液滴蒸发后的沉积形态存在许多应用与潜在价值。以聚苯乙烯颗粒配置悬浮液，加入表面活性剂十二烷基硫酸钠(SDS)，进行液滴蒸发的观察与分析，探究浓度为0wt%~1wt%的SDS对聚苯乙烯颗粒悬浮液液滴蒸发的过程及蒸发后沉积形态的影响。结果表明，实验的浓度范围内，随SDS浓度增加，液滴接触角非线性减小；加入SDS后，受表面张力梯度的驱动，液滴会自发向外铺展，最大铺展系数与SDS浓度呈正相关。液滴铺展过程较短，大约只占液滴整个蒸发过程的1/5~1/3。采用灰度共生矩阵分析加入SDS后对咖啡环内部的沉积形态的影响可知，SDS不仅能调节液滴沉积的环宽，还会减少沉积中间部分颗粒的聚集，沉积纹理更均匀(SDS浓度为0wt%与0.1wt%时两种沉积形态对比最明显)；SDS浓度的增加会使液滴内Marangoni流动更加剧烈，沉积的环宽随SDS浓度增加而变宽，当SDS的浓度高于0.75wt%后，环状沉积不再明显，颗粒大部分均匀分布在整个沉积区域，形成盘状沉积，说明SDS对环形沉积的环宽调节存在上限；比较直径为5.7和10 μm颗粒沉积的环宽随SDS浓度变化以及与咖啡环沉积面积占比的最大值(直径为5.7 μm时约为58.6%，直径为10 μm时约为44.6%)可知，SDS对不同直径的颗粒的沉积形态的影响存在差异。

关键词: 液滴蒸发, 咖啡环效应, 表面活性剂, Marangoni流