Spatially-Resolved Characterization of Oil-in-Water Emulsion Sprays

TL;DR

This paper investigates how pressure, location, and oil emulsion application affect droplet size, shape, and velocity in sprays, revealing that emulsions suppress small droplets, alter eccentricity, and increase droplet velocity, impacting spray dispersion.

Contribution

It provides detailed spatial and size-dependent droplet characterization using digital inline holography, highlighting the effects of oil-in-water emulsions on droplet dynamics in sprays.

Findings

Oil-in-water emulsions suppress small droplet formation.

Emulsions increase droplet velocity compared to water droplets.

Droplet eccentricity decreases with emulsion application, especially at the spray center.

Abstract

This study concerns the effects of pressure, spatial location, and application of oil emulsions on the resulting droplet size, eccentricity, as well as velocity distributions, all of which are crucial information in determining the dispersion dynamics of the droplets during the spray applications. Experiments were conducted with the abovementioned droplets information measured using digital inline holography (DIH). Results show that the volumetric droplet size distributions (VDSD) span widely from sub-200 um to over 2 mm in size. The application of an oil-in-water emulsion results largely in the suppression of smaller droplets, while the VDSD is relatively insensitive to increasing the oil volume fraction beyond a critical level. DIH additionally enables the determination of size-dependent droplet eccentricity and velocity measurements. Interestingly, the application of oil-in-water emulsion generally decreases the eccentricity, more significantly at the center than at the edge of the spray fan. We attribute this decrease to the increase in lamella sheet thickness and thus decrease in characteristic shrinkage rates, consistent with the observation using high-speed shadowgraphs. In all instances, oil-in-water emulsion droplets have higher velocities than equivalent sized water droplets. We attribute this to the earlier action of the spray breakup process in the oil-in-water emulsion, reduced surface energy generation during a breakup (larger droplets), and reduce energy dissipation during breakup with oil-in-water emulsions, leading to increased translational energy after the breakup process. Therefore, it appears that oil-in-water emulsion application simultaneously suppresses small droplet formation and increases droplet velocity, and hence spray penetration in agricultural application.