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

Abstract

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.

A comparison of spreading behaviors of Silwet L-77 on dry and wet lotus leaves

“…In the previous studies, it was shown that the superspreading behavior of surfactant solutions on a hydrophobic solid surface like polystyrene surface was driven by surface tension gradient at the expanding air-liquid interface [6,9,25,26]. At very low concentrations, increasing the surfactant concentration will lead to a larger decrease in surface tension and a larger surface tension gradient.…”

“…For example, the wetting behaviors of surfactants were studied on superhydrophobic surfaces prepared from alkylketene dimer (AKD) [8]. To simulate a spray of pesticide caused a dew drop to spread on a leaf surface, Chengara et al [9] studied the spreading of water drop on a hydrophobic surface when a small drop of trisiloxane surfactant was brought in contact with it. We recently reported a group of siloxane surfactants [10] and investigated their spreading behaviors on fresh wheat and cabbage surfaces [11].…”

Comparison of spontaneous wetting and drop impact dynamics of aqueous surfactant solutions on hydrophobic polypropylene surfaces: scaling of the contact radius

“…Even with a low concentration (0.1 wt%), the surface tension of aqueous trisiloxane solutions is significantly lower (~22 mN/m) than that of pure water (~72 mN/m) and therefore the wetting ability of such solutions is dramatically improved [28]. A number of studies have been performed to investigate spontaneous wetting of hydrophobic surfaces by aqueous trisiloxane solutions [28][29][30][31]. During the drop impact process, surfactants could prevent the drop from recoiling and bouncing, thus enhancing the deposition efficiency.…”

Superspreading

“…This prevention of superspreading can be achieved by using surfactant concentrations that are too low (i.e., below the so-called cwc, or critical wetting concentration; Chengara et al, 2007) and/or a substrate that is too hydrophobic (e.g., perfluoroalkane; . There are several reasons why one should have serious doubts about the relevance of some recent papers when it comes to providing new insights into the superspreading phenomenon.…”