Evaporation of nanofluid droplets with applied DC potential.

A considerable growth of interest in electrowetting (EW) has stemmed from the potential exploitation of this technique in numerous industrial and biological applications, such as microfluidics, lab-on-a-chip, electronic paper, and bioanalytical techniques. The application of EW to droplets of liquids containing nanoparticles (nanofluids) is a new area of interest. Understanding the effects of electrowetting at the fundamental level and being able to manipulate deposits from nanofluid droplets represents huge potential. In this work, we study the complete evaporation of nanofluid droplets under DC conditions. Different evolutions of contact angle and contact radius, as well as deposit patterns, are revealed. When a DC potential is applied, continuous and smoother receding of the contact line during the drying out of TiO2 nanofluids and more uniform patterning of the deposit are observed, in contrast to the typical "stick-slip" behavior and rings stains. Furthermore, the mechanisms for nanoparticle interactions with the applied DC potential differ from those proposed for the EW of droplets under AC conditions. The more uniform patterns of particle deposits resulting from DC potential are a consequence of a shorter timescale for electrophoretic mobility than advection transport driven by evaporation.