@article{976b89191d0b4f0aba6c0cd8773e5ab0,
title = "Molecular dynamics simulation of nanodroplet spreading enhanced by linear surfactants",
abstract = "We utilize molecular dynamics simulations to probe the surfactant-mediated spreading of a Lennard-Jones liquid droplet on a solid surface. The surfactants are linear hexamers that are insoluble in the liquid and reduce the surface tension of the liquid-vapor interface. We study how the interaction of the surfactant hexamers with the solid substrate influences spreading, as well as the dependence of spreading on surfactant concentration. We find that the spreading speed is strongly influenced by the attraction of the hydrophobic surfactant tail to the solid surface. When this attraction is sufficiently strong, surfactant molecules partition to the liquid-solid interface and facilitate spreading. This partitioning can lead to an inhomogeneous distribution of surfactant over the liquid-vapor interface, which could drive the Marangoni convection. We also observe that the surfactant molecules can assemble into micelles on the solid surface. The repulsion between micelles at the liquid-solid interface can lead to break-off and migration of the micelles from the liquid-solid to the gas-solid interface and spreading is facilitated in this way. Our model system contains features that are believed to underlie superspreading in experimental studies of droplet spreading.",
author = "Kim, \{Hye Young\} and Yong Qin and Fichthorn, \{Kristen A.\}",
note = "Funding Information: This work was sponsored by the NSF Grant No. CCR-0303976. Table I. Pair interaction parameters C i j adopted from Refs. 44 and 45 . Here H , T , and D are for head, tail, and solvent monomers, respectively. C i j H T D H 1.0 1.0 3.0 T 1.0 0.2 0.6 D 3.0 0.6 1.15 Table II. Numbers of solvent and surfactant molecules in the small liquid droplet for each concentration probed. Surfactant concentration (\%) Number of solvent Number of surfactant 0 5511 0 5 5184 44 10 4929 90 15 4740 117 20 4496 151 FIG. 1. The effect of the surfactant tail-solid ( C T S ) interaction strength on the height (a) and the base radius (b) of the small droplets for various values of the surfactant head-solid interaction ( C H S ) . The droplets contain a 20\% surfactant concentration and the droplet-solid interaction is C D S = 0.6 . The dotted lines represent the initial values. All data are taken at 600 000 time steps. FIG. 2. The effect of surfactant concentration on the height (a) and the base radius (b) of the surfactant-bearing small nanodroplets as a function of time with C D S = 0.6 , C H S = 0.0 , and C T S = 3.0 . The symbols represent various surfactant concentrations: dotted curve (0\%), open diamonds (5\%), solid curve (10\%), closed circles (15\%), and open triangles (20\%). FIG. 3. Snapshots of the small nanodroplets at a surfactant concentration of 20\%, with C D S = 0.6 and C H S = 3.0 for various values of C T S = 0.0 (a), 1.0 (b), and 3.0 (c). All the configurations are taken at 600 000 time steps. The dark and light blue (on line) balls represent head and tail atoms in the surfactant molecules. For clarity, the solvent monomers are not shown. FIG. 4. The effect of the surfactant tail-solid ( C T S ) interaction strength on the height of the large droplets for various values of the surfactant head-solid interaction ( C H S ) . The droplets contain a 15\% surfactant concentration and the droplet-solid interaction is C D S = 0.6 . The dotted lines represent the initial values. All data are taken at 1 000 000 time steps. FIG. 5. The time dependence of the height of the large nanodroplets at a surfactant concentration of 15\%, with C D S = 0.6 and C H S = 3.0 for various values of C T S . FIG. 6. Snapshots of the large nanodroplets at a surfactant concentration of 15\%, with C D S = 0.6 and C H S = 3.0 for various values of C T S = 0.0 (a), 1.0 (b), and 2.0 (c). All the configurations are taken at 1 000 000 time steps. The dark and light blue (on line) balls represent head and tail atoms in the surfactant molecules. For clarity, the solvent monomers are not shown. FIG. 7. Numbers of solvent (a) and surfactant (b) molecules for the initial, large hemispherical droplet, a large droplet with C T S = 1.0 , and a large droplet with C T S = 2.0 as function of vertical distance ( z ) from the substrate. The results for C T S = 1.0 and 2.0 are taken at 1 000 000 time steps. Note the logarithmic scale used in the vertical axis in (b). FIG. 8. Ratio ( R ) of the numbers of solvent monomers (a) and surfactant molecules (b) in the monolayer ( z ⩽ 2.5 σ ) to the total number of each of these respective species in the system as function of time for various values of C T S . The large nanodroplets have a surfactant concentration of 15\%, with C D S = 0.6 and C H S = 3.0 . ",
year = "2006",
doi = "10.1063/1.2364484",
language = "English (US)",
volume = "125",
journal = "Journal of Chemical Physics",
issn = "0021-9606",
publisher = "American Institute of Physics",
number = "17",
}