Precursor film of self-propelled droplets: Inducing motion of a static droplet

Acetic and propionic acids on glass show a total wetting behavior with a large exponent beyond Tanner's law. However, incorporating the Silwet L-77 surfactant into the droplet of acids causes the wetting behavior to change from spreading to random motion. The directed motion of the self-propelled droplet can be achieved on a linear runway constructed by coating polysilazane on glass. The absence of contact line pinning can be attributed to a thin film extending from the edge of the bulk droplet. Two simple methods are proposed to observe the precursor film associated with the spreading droplet and the self-running Silwet-laden droplet: (i) the color change of nearby water droplets containing a pH indicator, and (ii) the noncontact interaction observed using an optical microscope. Because of the precursor film, the self-running droplet can induce the motion of a static droplet without direct contact between their contact lines. This interesting finding may be explained using the interfacial tension gradient associated with the Marangoni stress due to the contact between the precursor film and the static droplet.