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.