Research ArticleMATERIALS SCIENCE

Not spreading in reverse: The dewetting of a liquid film into a single drop

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Science Advances  28 Sep 2016:
Vol. 2, no. 9, e1600183
DOI: 10.1126/sciadv.1600183
  • Fig. 1 Experimental imaging of the dewetting of a liquid droplet from a smooth solid surface.

    (A) A liquid droplet (TMPTGE; Ω = 1.45 μl, μ = 180 mPa s, γ = 43 mN m−1, T = 22°C, θe = 70°) is forced to wet a circular Teflon patch using a dielectrowetting setup. The resulting pancake-shaped liquid film, which corresponds to the initial configuration of the experiment, has a radius R0 = 2.5 mm. At time t = 0 ms, the dielectrowetting voltage is removed. The dewetting dynamics that follows is tracked by recording the instantaneous film radius, R(t), and the apparent contact angle averaged between the left and right film edges, θ(t). Scale bars, 2 mm. (B and C) At intermediate times, an annular rim forms close to the contact line. The rim is visible from both the top and side views of the film. (D and E) At long times, the rim merges and the film relaxes to a spherical cap shape. (F) Equilibrium state of the droplet, where the radius and contact angle reach constant values, Rf and θe. (G and H) Representative curves for the base radius and apparent contact angle. The formation of the rim correlates with a linear decrease in the radius and a plateau in the contact angle. The merging of the rim into a spherical cap gives way to a relaxation stage where the radius and contact angle relax to their final equilibrium values.

  • Fig. 2 Base radius and apparent angle as a function of time during the dewetting of liquid films.

    (A) At intermediate times, the base radius decreases linearly in time, with a dewetting speed that increases with increasing temperature and decreasing volume. (B) The apparent contact angle, θ, is normalized using the equilibrium contact angle, θe. The linear dewetting regime, where the speed of the contact line is constant, corresponds to the first plateau. At longer times, there is a crossover to a second plateau, corresponding to the equilibrium state of the drop. The data collapse to a single master curve upon rescaling time by the relaxation time of the rim, τrim. The solid line corresponds to the theoretical prediction for the first plateau (see text).

  • Fig. 3 Lattice Boltzmann simulations of a dewetting film.

    (A to F) Instantaneous liquid profiles relaxing from an initial film configuration of aspect ratio h0/R0 = 0.02 on a surface where the equilibrium contact angle is θe = 70°. In agreement with experiments, the droplet forms a rim (B and C), which eventually merges to form a spherical cap (D and F). (G) The speed of the dewetting rim increases with increasing surface tension and decreasing viscosity (insets). a.u., arbitrary units. Simulation data collapse onto the same scaling curve dR/dt ~ γ/μ.

  • Fig. 4 Lattice Boltzmann simulations of dewetting films of different initial shapes.

    (A to F) Instantaneous liquid profiles relaxing from slab (left) and spherical cap (right) initial configurations. In both cases, the aspect ratio of the initial liquid shape is set to h0/R0 = 0.02, and the final equilibrium contact angle is θe = 70°. (G) Apparent contact angle as a function of time for different initial aspect ratios of slab and spherical cap–shaped films. The time axis is rescaled by the time scale τrim = 9μ(R0Rf)/γθe3. The good collapse of the data shows that the rim speed scales with the capillary speed, UCa = γθe3/9μ, and that the duration of the linear dewetting regime scales with the amplitude of the lateral distortion, R0Rf.

  • Fig. 5 The two dynamic dewetting regimes of a liquid film.

    (A) Schematic shape of the cross-sectional profile of a dewetting film. The cross section of the rim corresponds to two independent structures of width w, connected by a thin film of thickness h0. The shape of the rim is described by the apparent contact angles θ and θf. (B) Schematic of the quasi-equilibrium shape of a dewetting droplet.

  • Fig. 6 Scaling of the interface speed in the linear regime of a dewetting film.

    Speed of the receding rim as a function of the capillary speed UCa = γθe3/9μ. The solid line is a visual guide. The inset shows a direct comparison of the experimental data (symbols) with the hydrodynamic theory (solid line). Error bars correspond to 1 SD of the sample. Experimental parameters for each symbol are summarized in Table 1.

  • Fig. 7 Exponential approach to equilibrium of a dewetting droplet.

    (A) Difference between the equilibrium and apparent contact angles as a function of time. The solid line corresponds to a fitted exponential function. (B) Scaling of the relaxation time in the exponential regime. The inverse relaxation time is compared to the theoretical prediction. Error bars correspond to 1 SD of the sample. Experimental parameters for each symbol are summarized in Table 1.

  • Fig. 8 Teflon-coated IDEs.

    A circular electrode patch is defined from two interdigitated coplanar metal (Ti/Au) stripe arrays. The electrodes are covered by an insulating SU8 layer (1 μm), which is overcoated with a thin oleophobic layer of Teflon. The resulting pattern has a circular envelope 5 mm in diameter.

  • Table 1 Summary of experimental conditions.

    Errors for the volume, surface tension, and viscosity correspond to the accuracy of the measuring apparatus. The error in the contact angle is the SD of the sample.

    HandlenΩ (μl)T
    (°C)
    γ (mN m−1)μ (mPa s)θe (°)Symbol
    D94 521.20 ± 0.03544.78 ± 0.25686 ± 2071 ± 1Embedded Image
    D96 511.93 ± 0.03544.78 ± 0.25686 ± 2071Embedded Image
    D94 1021.20 ± 0.031044.28 ± 0.25437 ± 2071 ± 1Embedded Image
    D96 1021.93 ± 0.031044.28 ± 0.25437 ± 2071 ± 1Embedded Image
    D97 1541.71 ± 0.031543.77 ± 0.25299 ± 2072 ± 1Embedded Image
    D96 2021.93 ± 0.032043.27 ± 0.25210 ± 2071 ± 1Embedded Image
    D89 2251.35 ± 0.032243.06 ± 0.25180 ± 2073 ± 2Embedded Image
    D92 2271.45 ± 0.032243.06 ± 0.25180 ± 2075 ± 2Embedded Image
    D97 2541.71 ± 0.032542.76 ± 0.25149 ± 2075 ± 2Embedded Image
    D94 3021.20 ± 0.033042.25 ± 0.25109 ± 2074 ± 1Embedded Image
    D96 3021.93 ± 0.033042.25 ± 0.25109 ± 2072 ± 1Embedded Image

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1600183/DC1

    movie S1. Top view of a 1.45-μl thin liquid film dewetting from a Teflon-covered dielectrowetting 5-cm-wide circular patch at room temperature.

    movie S2. Side view of a 1.45-μl thin liquid film dewetting from a Teflon-covered dielectrowetting 5-cm-wide circular patch at room temperature.

  • Supplementary Materials

    This PDF file includes:

    • Legends for movies S1 and S2

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.avi format). Top view of a 1.45-μl thin liquid film dewetting from a Teflon-covered dielectrowetting 5-cm-wide circular patch at room temperature.
    • movie S2 (.avi format). Side view of a 1.45-μl thin liquid film dewetting from a Teflon-covered dielectrowetting 5-cm-wide circular patch at room temperature.

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