Patterns from dried water-butanol binary-based nanofluid drops

In this work, the behavior of evaporating binary-based nanofluid sessile droplets deposited on a smooth silicon substrate at different temperatures is explored. The formation of deposition patterns during the evaporation is studied by tracking particle clusters using optical microscopy. Similarly to evaporation of pure water-based nanofluid droplets, three distinctive deposition patterns are left behind the complete evaporation: a relatively uniform coverage pattern (on a nonheated surface); a “dual-ring” pattern at higher temperature, i.e., 81 °C; and a “stick-slip” pattern at 99 °C. Infrared thermography technique was employed to visualize the evolution of thermal patterns on the surface of the drying droplets. Thermal imaging shows that the evaporation of binary mixture droplets can be classified into three regimes. In the first regime, multiple convection vortices can be observed at the droplet interface, corresponding to the chaotic motion of nanoparticles captured by video microscopy. This flow regime is believed to be driven by surface tension gradients arising from local concentration gradients. As evaporation time proceeds, the number of convection vortices decreases in regime I, and a few numbers of those are left in the second regime. The flow slows down and a rapid transition (the second regime) occurs; this is followed by the last regime. At the two highest temperatures of 81 and 99 °C, the end of the transition regime is associated with the existence of two distinctive counter-rotating vortices. For the third regime, the results from both infrared thermography and video microscopy show identical behavior to those of water-based nanofluid droplets at the same substrate temperatures. This reveals that most of the more volatile component (not all) has evaporated after the first two regimes; hence, the solutal Marangoni driven by local concentration gradients is significantly weakened and has no further role in the flow structure in the last regime. Instead, the thermocapillary effect and continuity are the underlying reasons for the internal flow structure of the evaporating droplets during the last regime.