State Key Laboratory of Tribology , Tsinghua University , Beijing 100084 , China.
Langmuir. 2018 Dec 18;34(50):15388-15395. doi: 10.1021/acs.langmuir.8b03130. Epub 2018 Dec 3.
In the present work, an interesting droplet self-retraction phenomenon of myristyltrimethylammonium bromide (CTAB) solution on the superhydrophilic mica surface was observed. The self-retraction could only occur within a concentration range from 0.01 to 16 cmc. The maximum variation of the contact angle (from 24.99 to 76.85°) was observed in droplets with a concentration of 0.1 cmc. The self-retraction mechanism was studied based on high-speed photography, surface analyses, surface energy calculation, and model fitting. It was proved that there was a monolayer of CTAB molecules adsorbed on the mica surface, which formed through the electrostatic interaction between the negative mica and the positively charged headgroups. The formation of this monolayer took only a short time of <67 ms since the droplet touched the surface, and the monolayer was well-distributed and hydrophobic with a surface free energy of 76.041 mJ/m (20% of mica's surface free energy). The reduction of surface tension led to an imbalance at the borderline of the droplet, causing self-retraction. A model was established to simulate the self-retraction process and it agreed with the experimental data well. This study has significant implication in understanding the surface modification caused by surfactant molecules.
在本工作中，观察到了溴化十六烷基三甲铵（CTAB）溶液在超亲水云母表面上的有趣的液滴自缩回现象。自缩回仅能在 0.01 到 16cmc 的浓度范围内发生。在浓度为 0.1cmc 的液滴中观察到接触角的最大变化（从 24.99 度到 76.85 度）。基于高速摄影、表面分析、表面能计算和模型拟合研究了自缩回机制。证明在云母表面上吸附有一层 CTAB 分子，这是通过带负电荷的云母和带正电荷的头基之间的静电相互作用形成的。该单层的形成仅需液滴接触表面的 <67ms 时间，且单层分布均匀且疏水，表面能为 76.041mJ/m（云母表面能的 20%）。表面张力的降低导致液滴边界处的不平衡，导致自缩回。建立了一个模型来模拟自缩回过程，该模型与实验数据吻合较好。这项研究对于理解表面活性剂分子引起的表面改性具有重要意义。
