CONCENTRATION GRADIENTS INSIDE MICRODROPLETS.

Small water microdroplets in microfluidic systems have a high surface charge density resulting from charged surfactants. As a result, an electric double layer forms inside the droplet. Depletion of ions from the center of the droplet to form the double layer can shift the concentration of ions dramatically from that of the microdroplet precursor solution. Here we show numerical solutions to the Gouy-Chapman model in spherical coordinates. Some notable effects include: 1) large percentages of the microdroplet volume experience very large DC electric fields; 2) many ions get forced into a Stern layer giving dramatically different conditions from the bulk.