The effect of colloidal particles on the evaporation is of considerable importance in a number of processes. Variation in particle concentration is found to affect the droplet wetting state that governs the evaporation rate . The accumulation of particles at the droplet periphery is accompanied by a thin liquid layer, termed as the nanofluid thin layer (NFTL) . It is important to examine the extended thin film of the bulk liquid possibly with a constant-curvature region . It has been established that the heat exchange is maximum in this region . Recent studies have achieved visualization of this evaporating thin layer in nanofluid droplets using confocal microscopy  and the effect of particle on meniscus curvature for partially wetting nanofluid films  using image analysis interferometry. We attempt to study the effect of particle size on the characteristics of this layer using optical interferometry.
Polystyrene latex beads are obtained from Sigma-Aldrich having an average diameter of 0.055 μm and 0.46μm representing the nano and sub-micron regimes respectively. These suspensions are diluted in deionized (DI) water (1 w/w %) and used hereafter as the working fluid. solid Glass slides are used as the substrates and they are thoroughly rinsed in acetone and water to remove surface contaminants, followed by plasma treatment for 60 seconds to impart hydrophilicity. Colloidal droplets (1μl) are placed on these substrates using a micropipette. Contact angles between the colloidal droplets and substrate are measured using a goniometer and found to be ~ 50. The droplets are placed on the automated stage of a Leica DM6000M microscope (20× objective). The light source is monochromatic light of wavelength 543.5 nm and interferometric fringes are readily observed. These droplets are allowed to evaporate at constant temperature and humidity while real time videos are captured. Images (as shown in Figures 1 and 2 below) are extracted at different instants of time and analyzed.
Figure 1 Image sequence depicting the alterations in fringe patterns during evaporation of nanofluid droplets.
Figure 2 Image sequence depicting the alterations in fringe patterns during evaporation of sub-micron colloidal droplets. The red line clearly demarcates between the thin and thick fringes.
Evaporation of the solvent (water) results in the accumulation of the colloidal particles at the droplet edge, well known as the coffee ring effect . We focus on the evolution of the fringe pattern accompanying this phenomenon. Figures 1 and 2 depict the various stages of fringe pattern evolution in presence of nanoparticles and sub-micron particles respectively. Narrow spaced (higher curvature) fringes are observed for nanofluid droplets. Sub-micron particles, however, show two distinct regimes of narrow spaced (higher curvature) and wider spaced (lower curvature) fringes. This is closely followed by a rapid extension region' characterized by widely separated and quick-forming fringes. It is thus evident that the timescale of appearance of these patterns is a function of the colloidal particle size. The data are analyzed further to probe the evaporation from colloidal droplets.
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