Evaporation-driven dewetting of a liquid film

We study the dynamics of evaporating ethanol films deposited by a receding liquid meniscus. The films are surrounded by pure vapor in a capillary heated above the saturation temperature. We observe the substrate dewetting with the dewetting ridge in spite of the complete wetting at equilibrium. The dewetting is caused by a high contact angle ($\sim {30}^{\circ }$) induced by evaporation. The obtained values agree with a theory proposed earlier. The film shape is measured with both grid deflection technique and interferometry. The phenomenon is convenient to observe inside a capillary with an axial thermal gradient. When the capillary is closed at one end and open at another to a constant pressure reservoir, the meniscus oscillations are known to appear spontaneously. Such a system is the simplest version of an industrial device called a pulsating heat pipe. The effect is general and can be used in any system to control the wetting properties.

Figure 1

(a) Schematic of the experiment. (b) Zoomed-in upper part with the optical deflectometry setup. (c) Raw camera image of the retracting trapezoidal film at the front porthole [in (b)]. Drops and rivulets remain from the rear film. Grid image discontinuity indicates the CL position; grid image distortion indicates the dewetting ridge. The coordinate system is also shown. (d) Time evolution of the meniscus position $y_m$, topmost CL position $y_{\text{CL}}$, and vapor pressure $p$ measured for $p_r=23.7$ kPa, $T_c=0{}^{\circ }\mathrm{C}$, and heating powers 4 and 60 W at the front and rear portholes [in (b)], respectively. The oscillation frequency is 1.45 Hz. The vertical dash-dotted line indicates the time of the image in (c) [18].

Figure 2

Film thickness measured by interferometry (symbols) just after the film deposition at different heights to obtain different meniscus velocities. It is compared to Eq. (2) (line, calculated with $D=2$ mm corresponding to the cell thickness; $\gamma =22.3\times {10}^{-3}$ N/m, and $\eta =7.9\times {10}^{-4}\mathrm{Pa}\mathrm{s}$).

Figure 3

Time evolution of the central film part. The film slope is smaller than $0.{01}^{\circ }$ (note the scale difference of the axes $y$ and $h$). For one profile, the upper ridge is shown schematically to indicate its relative size and position. The inset shows the time evolution of the upper ridge profile determined with the grid deflection technique.

Figure 4

Three-dimensional reconstruction of the film profile corresponding to the image in Fig. 1. The surface is obtained by interpolation between reconstructed points (symbols) along the images of grid edges; ${\theta }_{\mathrm{app}}=7\pm 1^{\circ }$.

Figure 5

(a) Plot of ${\theta }_V$ (squares) and $T_{\mathrm{sat}}$ (solid line) [cf. $p\left(t\right)$ shown in Fig. 1] temporal variations. Also shown are $U_{\text{CL}}$ (circles) and ${\theta }_{\mathrm{app}}$ (crosses), both measured at the right inclined portion of the CL at $y=3$ cm height. The precision for $\theta$ is $\pm 2^{\circ }$. (b) Plot of ${\theta }_V$ versus $\mathrm{\Delta }T=T_s-T_{\mathrm{sat}}$. The symbols denote ${\theta }_V$ and $T_{\mathrm{sat}}$ data extracted from the graph (a) for the moving (depinned) CL; $T_s=48{}^{\circ }\mathrm{C}$ is assumed. The dotted line is the theoretical curve calculated for ethanol with the approach described in Ref. [9].

Figure 6

Dimensionless contact line receding velocity. The theoretical curve is the capillary dewetting velocity calculated with Eq. (3) using the ${\theta }_V\left(t\right)$ and $w\left(t\right)$ data from Figs. 5 and 3, respectively. The experimental values (circles) correspond to the $U_{\text{CL}}$ data of Fig. 5.