Superspreading Wetting of Nanofluid Droplet Laden with Highly Dispersed Nanoparticles

Abstract

Wetting of nanofluids containing highly dispersed nanoparticles of single-nanometer size was investigated, as these nanoparticles can persist within a nanometer-scale liquid film near contact line, potentially causing significant changes in wetting characteristics. We discerned distinctive superspreading wetting, featured by temporal indices (0.29 to 0.46) in the relationship between contact radius and time. We employed a phase-shifting imaging ellipsometer to measure droplet shape, including the nanometer-scale liquid film and nanoparticle layer after drying. The liquid film shapes differed from pure liquids at micrometer-scale but not at nanometer-scale. Furthermore, surface tension measurements and substrate surface energy control contributed to unraveling these characteristics. These findings differentiated the observed superspreading wetting from the mechanisms proposed in existing studies of aqueous surfactant solutions.

Figure 1

Schematic of experimental apparatus equipped with phase-shifting imaging ellipsometer capable of measuring thickness of liquid films on nanometer-micrometer scale. Sealed container with a liquid tank that suppresses evaporation of droplets was set at the test section of ellipsometer.

Figure 2

Visualization images near contact line of n-heptane droplet and 5 wt % nanofluid droplet composed of n-heptane and decanoic acid-modified CeO₂ nanoparticles deposited on Si substrate. Gray scale color denotes phase difference between p- and s-polarization states in ellipsometry and represents contour line of the liquid film thickness.

Figure 3

Micrometer-thick liquid film shapes of droplets in radial direction r from droplet center. All samples have n-heptane as organic solvent. Nanoparticle concentrations are 0, 1, and 5 wt %, respectively, and substrates are either clean or silanized substrates.

Figure 4

Relationship between dimensionless contact radius R/R₀ and time t for nanofluids. (a) Variations in nanoparticle concentration and substrate surface energy using n-heptane as organic solvent. (b) Effect of individual organic solvents on clean substrate.

Figure 5

Nanometer-scale liquid film profiles of droplets along radial direction r. All samples correspond to those depicted in Figure 3, wherein n-heptane serves as organic solvent, nanoparticle concentrations range from 0 to 5 wt %, and substrates include both clean and modified ones. The thickness distribution of the nanoparticle layer subsequent to solvent evaporation, as measured by unsealing and evaporating the liquid 5000 s postdroplet deposition, is also shown as red lines.