Self-Retraction of Surfactant Droplets on a Superhydrophilic Surface

In the present work, an interesting droplet self-retraction phenomenon of myristyltrimethylammonium bromide (C₁₄TAB) 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 C₁₄TAB 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/m² (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.