Electrowetting Controls the Deposit Patterns of Evaporated Salt Water Nanodroplets

So-called "coffee-ring" stains are the deposits remaining after complete evaporation of droplets containing nonvolatile solutes. In this paper we use molecular dynamics to simulate the evaporation of salt water nanodroplets in the presence of an applied electric field. We demonstrate, for the first time, that electrowetted nanodroplets can produce various deposit patterns, which vary substantially from the original ringlike deposit that occurs when there is no electric field. If a direct current (dc) electric field with strength greater than 0.03 V/A is imposed parallel to the surface, after the water evaporates the salt crystals form a deposit on the substrate in a ribbon pattern along the field direction. However, when an alternating current (ac) electric field is applied the salt deposit patterns can be either ringlike or clump, depending on the strength and frequency of the applied ac field. We find that an ac field of high strength and low frequency facilitates the regulation of the deposit patterns: the threshold electric field strength for the transition from ringlike to clump is approximately 0.006 V/angstrom. These findings have potential application in fabricating nanostructures and surface coatings with desired patterns.