A mildly hydrophobic substrate is used for examining the post-evaporation residues of an agricultural adjuvant, X-77, due to sessile droplet deposition and drying.  0.5µl droplets of five concentrations in aqueous solution are tested over a 5x range.  Top view images taken during and after evaporation are analyzed using a contrast-based, edge detection algorithm.  Due to internal droplet flow and surface tension gathering, resulting deposits evidence “coffee ring” formations, accompanied by thousands of micron scale “islands” distributed throughout the interior portion of the original droplet's area.  Increasing concentration results in a larger mean “island” diameter and a broader distribution of sizes.  Fitting the size distribution's histogram reveals the general equation: where C is the concentration, y the frequency and α a constant having vales between 2.55 and 3.00.  Over a 10x size range, log-log plots are linear.  In addition, post-spread area is compared to concentration.  Contrary to intuition, increasing concentration does not increase total post-evaporation residue coverage area in a uniformly linear fashion.  Instead, a transition point is noted at approximately 0.15%.  Below and above this value, the relationships between area covered and concentration appear linear, but with differing slopes.  Additional X-77 above 0.15% results in a more gradual increase in total area covered.

Acknowledgement:

Funding was made possible (in part) by 5P20MD0001085-04 from the National Center on Minority Health and Health Disparities. Views expressed are the presenter(s)', and do not constitute endorsement by DHHS.