Electrowetting controls the deposit patterns of evaporated salt water nanodroplets

The so-called “coffee-ring” stains are usually deposited after complete evaporation of droplets containing non-volatile solutes. In this paper we use molecular dynamics to simulate the evaporation of salt water nano droplets in the presence of an applied electric field. We demonstrate, for the first time, that electrowetted nano droplets can produce various deposit patterns, which vary substantially from the original ring-like deposit when there is no electric field. If a direct current (DC) electric field with strength greater than 0.03 V/Å 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 crystal deposits vary between the original ring-like and clump patterns depending on the strength and frequency of the applied AC fields. We find that an AC field of high strength and low frequency facilitates the regulation of deposit patterns: the threshold of the electric field strength for the transition from ring-like to clump is approximately