Superspreading: Role of the Substrate Surface Energy

Abstract

Trisiloxane surfactants are a class of nonionic surfactants that have the unique ability to spread quickly on difficult-to-wet surfaces (i.e., hydrophobic surfaces). There are numerous studies on the spreading of trisiloxane surfactants, but the mechanism of spreading is not fully understood. The trisiloxane surfactant containing 7.5–8.0 mol of ethylene oxide groups (e.g., Silwet L-77) has been found to be one of the best spreading agents for substrates with an intermediate wettability, and is an excellent herbicide adjuvant, causing an aqueous solution to completely spread on leaf surfaces (e.g., “velvetleaf”). It was observed that, at any substrate wetting angle between 35^∘ and 115^∘ (referred to as the “water phase”), the spreading rate vs. concentration had a maximum. The value of the concentration corresponding to the maximum rate of spreading did not depend on substrate’s wetting angle. The spreading rate had an optimum at a substrate wetting angle between 60–65^∘. The optimum in the spreading rate for L77 reported in the literature had a maximum at a surfactant concentration of 0.45 wt% and a substrate wettability of 60–65^∘. Here, we present a simple model based on the Marangoni flow over a curved surface illustrating why the rate of spreading vs. substrate wettability has an optimum at a wetting angle of 60–65^∘. The model predicts well the value of the spreading rate and the optimum value of the substrate wetting angle.