Spreading of a Water Drop Triggered by the Surface Tension Gradient Created by the Localized Addition of a Surfactant

The spreading of aqueous solutions of trisiloxane surfactants on solid surfaces has been studied extensively. Trisiloxane surfactants are used in pesticide delivery as adjuvants to promote spreading on leaves and provide a larger area for solute transfer. The spreading of a dew-drop on a leaf when a spray of pesticide is delivered is simulated by studying the spreading of a water drop on a hydrophobic surface when a small drop of aqueous trisiloxane surfactant is brought in contact with it. This study reveals many new features that differ from the spreading of an aqueous trisiloxane drop on a solid surface; the spreading of the substrate drop is characterized by an inertial rather than a viscous response, the imposed surface tension gradient dies out rapidly, and the spreading velocity is consistent with a balance of kinetic energy imparted to the substrate drop and the decrease in its surface energy.