Nanofluids are suspensions of nanoparticles with specific interfacial properties that can be tuned by the addition of surface active molecules. Due to the presence of nanoparticles, nanofluids raise fundamental questions when investigating their properties. One still challenging problem in this context is the description of the overall evaporation kinetics of modified nanofluid droplets when deposited on solid substrates. In this work, droplets consist in a mixture of 15 nm diameter silica nanoparticles (SiO2) modified by a surfactant (CTAB). When surfactant free, nanoparticles are soluble in water up to 300 g/L. For large values of CTAB concentrations, they become hydrophobic and change the nanofluid solid/liquid/air interfacial properties. The considered droplets have approximately 1.4 µL volume and are deposited on a copper substrate. Due to their small size, they appear initially as spherical caps. Their evolution is studied under controlled temperature (T=22±1°C) and relative humidity (40±2%) conditions with an optical microscope. For all the experiments, the initial height (resp. contact line diameter) is such that h=0.8±0.1 mm (resp. L=2.0±0.2 mm). For a given CTAB concentration, the evaporation time is shown to be weakly dependent of the nanoparticles concentration as long as [SiO2] < 20 g/L. This indicates that the addition of a low amount of nanoparticles does not modify significantly the evaporative flows. Conversely, for a given SiO2 concentration, evaporation becomes faster when increasing [CTAB]. Finally a critical mixing ratio for which evaporation is the fastest is evidenced.
m.antoni@univ-amu.fr
