Alkyl Polyoxyethylene Surfactant Residue Distribution after Sessile Droplet Evaporation.*

Post-evaporation residues of an Alkyl Polyoxyethylene surfactant on a slightly hydrophilic surface are studied. ~An edge-detecting contrast recognition algorithm is used to measure the areas of small (7-30$\mu $m diameter) surfactant ``islands'' which form during the evaporation process as a result of flows within the droplet and surface tension gathering. ``Island'' distributions broaden with increased concentration, accompanied by higher mean interior island size. Fitting the histogram of island sizes beginning with the principle peak reveals a general form: $y=(6\times 10^8)Cx^{-\alpha}$ where y is frequency, $C$ the concentration and \textit{$\alpha $ }is a constant having value between 2.55 and 3.00. Log-log plots evidence a linear behaviour over two orders of magnitude. Total area covered does not increase in a linear fashion with concentration, as one may expect. Rather, a ``critical'' concentration is achieved at approximately 0.15{\%}, above which area increase is less pronounced.