Abstract:
Understanding the evaporation of a multi-component droplet has found immense importance in
various technological applications. This study investigates the evaporation behaviour of a colloidal binary
droplet system comprising of the ethanol-water mixture and polystyrene nanoparticles. The wetting and
evaporation dynamics were studied with an emphasis on the collective influence of ethanol and nanoparticle concentrations. The temporal behaviour of the contact angles, shapes and volumes of the droplets
was monitored in order to analyse the evaporative behaviour. With increase of ethanol concentrations,
the binary droplet volumes were found to decrease nonlinearly with time. Ethanol being more volatile
evaporated in the initial stage. Towards the end of the evaporation process, the evaporation characteristics
mimics the behaviour of pure water. Our study shows that the initial contact angle decreases monotonically
with increased concentration of ethanol in the mixture. The contact angle is maximum for a particular
nanoparticle concentration. Droplets with higher ethanol concentration show higher wettability which in its
turn is maximum for low nanoparticle concentrations. This trend shows the interconnected effect of ethanol
and nanoparticle concentrations on evaporation. Rim width of the final deposition pattern increases with
nanoparticle concentration although it is almost independent of ethanol concentration. Finally, it is noticed
that fast evaporation of a relatively more volatile component in a binary mixture droplet leads to nanoparticle segregation for low nanoparticle concentrations. Thus for binary mixtures, the evaporation of the
more volatile component, ethanol for our case, offers characteristic differences in the resulting evaporation
dynamics from that of pure water which finds applicability for multi-component evaporation processes.
