Microscale Electrokinetic Desalting and Salting of Water-in-Oil Droplets

Several microscale analytical and bioanalytical applications have been revolutionized through droplet microfluidic technologies, which embrace and necessitate the creative modification of droplet compositions. Droplet capabilities such as merging, mixing, sorting, and splitting enable fluidic precision and control in modifying droplet compositions. However, such whole-droplet manipulation may result in volumetric changes. Limited technologies exist to modify the in-droplet ionic composition with minimal changes to the original droplet volumes. Previously, our group demonstrated droplet-based ion concentration polarization (ICP) as an electrokinetic technique to introduce and exchange ions across nanoliter-scale water-in-oil droplets. This presentation reports the development of a technique called Droplet Ion Manipulation and Exchange (DIME), which utilized two distinctly selective ion-exchange membranes to electrokinetically introduce ions into (salt) or extract ions from (desalt) water-in-oil droplets. Ionic distribution within the droplets under these desalting and salting regimes as well as the ion transport mechanisms were visualized and monitored using a model precipitation reaction and charged fluorescent tracers. An in-situ current measurement setup was developed to determine the extent and rate of droplet desalting and salting. The rates of ion transport were found to linearly correlate with applied voltage and the ionic strength of the droplets. Specifically, up to 98% desalting efficiency was achieved with the capability to reconcentrate the droplet back to its original concentration by applying the voltage in salting mode (distinct voltage configuration relative to membrane functionality). DIME technique is a simple way to alter and control the ionic compositions of droplets at the microscale.