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Title: Residual Patterns of Alkyl Polyoxyethylene Surfactant Droplets after Water Evaporation

item Zhu, Heping

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2007
Publication Date: 3/10/2008
Citation: Pierce, S.M., Chan, K.B., Zhu, H. 2008. Residual Patterns of Alkyl Polyoxyethylene Surfactant Droplets after Water Evaporation. Journal of Agricultural and Food Chemistry. 56(1):213-319.

Interpretive Summary: Pest control efficiency is greatly influenced by plant fine surface structure and plant growing conditions. For the plants with thick, waxy or hairy leaf surfaces, the addition of surfactants in spray solutions can minimize variations in spray performance and improve pesticide effectiveness. The nonionic surfactants are widely used as spray solution additives to increase spray coverage in pest control. They can reduce surface tension of spray solutions, and improve spray droplets spreading and sticking on targets. Spreading, evaporation characteristics, evaporation dynamics characterizations and post-evaporation deposit formations of nonionic surfactant-added droplets with various concentrations on two artificial target surfaces were investigated under controlled environmental conditions. It is found that maximum coverage is achieved with a proper surfactant concentration, applied under high humidity conditions. These conditions have the added advantage of minimizing post-evaporation ring formations, affording the best opportunity for leaf exposure via a pattern of small islands of surfactant, evenly distributed over the maximum spread area.

Technical Abstract: Using a nonionic, alkyl polyoxyethylene surfactant (X-77®) in aqueous solutions, sessile droplet spreading, pinning, evaporation, contraction, and post-evaporation deposits are characterized. X-77® is widely used in the agricultural field as a spreader/adherent, intended to optimize pathenogenic agent coverage. Using a single droplet size under monitored temperature conditions, we alter humidity, substrate hydrophobicity and surfactant concentration to mimic varying agricultural conditions. For hydrophilic surfaces, the droplet spreads, reaching and retaining a maximum, stationary size. At this stage, a ring accretion occurs at the maximum spread diameter. During the final stage, the water film retracts, resulting in deposition of small islands of surfactant residue inside the ring. At lower concentrations of surfactant, we discover ring formations that break-up into ‘ring islands’ at late stage evaporation, accompanied by a distribution of the smaller islands in the interior portion of the substrate contact area. These are promoted by higher relative humidity. At higher concentrations, only a solid ring of surfactant remains, post-evaporation. Increasing surfactant concentration tends to increase the mean of the interior island size and broaden the overall island size distribution. On sufficiently hydrophobic surfaces, surfactant-laden droplets do not evidence pinning, ring formations, or post-evaporation interior islands. Interestingly, lower humidity increases spreading at higher surfactant concentrations. Such pattern formations of surfactant deposit are reported for the first time and are of significance in projecting how surfactants like X-77® distribute pesticides or other chemicals would have deposited on leaf surfaces.