The critical nanofluid concentration as the crossover between changed and unchanged solar-driven droplet evaporation rates

Highlights: • Solar-driven evaporation of nanofluid droplet is investigated. • A critical nanofluid concentration distinguishes two regimes of droplet evaporations. • The boundary layer effect is identified near droplet contact line during evaporation. • Coffee-ring nanoparticles dominate the light energy absorption in visible region. • The two regimes of drop dynamics correspond to insufficient and extra coffee-ring nanoparticles. -- Abstract: The challenge of the nanofluid solar energy harvesting lies in the strong coupling between nanoparticles generated heat and flow/temperature fields in various length and time scales. In this paper, the solar-driven evaporation of nanofluid droplet is investigated. The droplet evaporation rates behave a sharply increased regime and a constant regime when crossing a critical nanofluid concentration of c_i,cr= 10 ppm. We decouple the total solar energy absorption into two parts in terms of the optical bands, and the whole droplet into two spatial domains. Our study reveals a small length scale starting from contact line to behave higher temperature and larger temperature gradient there, newly recognized as a boundary layer effect. Correspondingly, a contact line region CLR is defined, while the remain is the bulk volume region BVR. The observed coffee-ring behaves a self-assembly of nanoparticles in CLR. We found that even though CLR is two magnitudes smaller compared to the whole droplet, it dominates almost all of the electromagnetic energy absorption, while BVR contributes almost all of the infrared energy absorption. Our finding successfully explains the two regimes of droplet evaporations. With increase of initial nanofluid concentrations, the droplet evaporation rates are raised due to more heat generation induced by enhanced nanoparticles deposition in CLR. The increased evaporation rates stop at a critical concentration of 10 ppm, corresponding to sufficient nanoparticles deposition. Beyond the critical condition, extra nanoparticles are hidden beneath the effective nanoparticles layers to cause unchanged evaporation rate. The present work is useful to develop novel solar steam generation device. The evaporation induced coffee-ring can be regarded as a novel "fabrication" method of nanostructure for plasmonic heating. The pure water evaporation of liquid film or droplet on a prepared coffee-ring array offers great convenience for solar energy utilization.