(591d) Interferometric Investigation of Evaporating Thin Layer of Colloidal Droplets

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 [1]. The
accumulation of particles at the droplet periphery is accompanied by a thin
liquid layer, termed as the nanofluid thin layer (NFTL) [2]. It is important to
examine the extended thin film of the bulk liquid possibly with a
constant-curvature region [3]. It has been established that the heat exchange
is maximum in this region [4]. Recent studies have achieved visualization of
this evaporating thin layer in nanofluid droplets using confocal microscopy [5]
and the effect of particle on meniscus curvature for partially wetting
nanofluid films [6] 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 [7]. 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.

References:

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T.; Ito, K.; Kitano, H. Particle adsorption in evaporating droplets of polymer
latex dispersions on hydrophilic and hydrophobic surfaces. Colloid Polym. Sci. 1998, 276, 810-815.

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2014, 72, 336-344.

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[5] Shin.
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an Evaporating Thin Layer during the Evaporation of a Nanofluid Droplet. Langmuir 2015, 31, 1237-1241.

[6] Chakraborty,
M.; Chatterjee, R.; Ghosh, U. U.; DasGupta, S. Electrowetting of Partially Wetting
Thin Nanofluid Films Langmuir 2015,
31(14), 4160-4168

[7] Deegan, R. D.;
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