Long-Term Repellency of Liquids by Superoleophobic Surfaces

Applications of superoleophobic surfaces depend on the stability of the air cushion formed under liquid drops. To analyze the longevity of air cushions we used reflection-interference contrast microscopy (RICM) for drops on a porous fractal-like structure of sintered nanoparticles. RICM permits us to monitor the height of the air cushion with nanometer resolution. Whereas the air cushion under all investigated liquids was stable on a time scale of a few seconds to minutes and liquids rolled off, liquids with low surface tension penetrated the coating on the time scale of hours and longer. The penetration speed showed a power law dependence on time, $dz/dt\sim t^p$, the exponent $p$ varying from $-0.5$ to $-1.2$. Thus, penetration is qualitatively different from the Lucas-Washburn law that governs spontaneous capillary filling of porous structures.

Figure 1

Superoleophobic surfaces. (a) A coating based on soot templating. Scanning electron microscope (SEM) side view image of a cross section. Candle soot deposited on glass was covered with a silica shell about 20 nm thick, soot was combusted at $500°\mathrm{C}$ and the remaining transparent silica shell was hydrophobized. (b) Top-view SEM image. (c) Confocal laser scanning microscope (CLSM) vertical cross section of a fluorescently labeled water drop (yellow) on this coating. Reflection shown in cyan. (d) A fluorescently labeled $n$-hexadecane drop on the same coating. (e) Model of sintered spheres for calculation of the energy barrier for liquid penetration. Equilibrium position, where the liquid-air interface forms the Young’s contact angle ${\theta }_0$ with the solid. (f) Local energy maximum and next equilibrium position (dashed line). Depth $z$ is measured from the top of the pillars. (g) Cassie wetting state of a water drop on this coating. (h) A mixed Cassie-Wenzel state and (i) the Wenzel state.

Figure 2

Material’s contact angles ${\theta }_{\text{adv}}$, ${\theta }_{\text{rec}}$ (squares) and respective apparent contact angles ${\mathrm{\Theta }}_{\text{adv}}^{\text{app}}$, ${\mathrm{\Theta }}_{\text{rec}}^{\text{app}}$ on the superoleophobic surface (triangles). Advancing and receding values shown by full and hollow symbols, respectively. Water, alkanes, PDMS are marked red, black, green, respectively. Unless explicitly shown, the error bars are smaller than the symbols.

Figure 3

Penetration depth of liquids on the superoleophobic surface extracted from RICM. (a) Interference from reflections at the glass-coating and coating-liquid interfaces. The volume of the drop was kept constant at $5\mu \mathrm{l}$. (b) RICM image of a water drop on the same superoleophobic surface as Figs. 1 and 2. (c) RICM image of an $n$-decane drop 0.13, (e) 800, and (g) 2500 s after deposition. (d) Time dependence of reflected intensity of a pixel in the series of RICM images (red, marked with a circle in (c), (e), (g). (f),(h) Penetration depth corresponding to (e),(g). Depth at $t=0$ is defined as zero. The points marked as 1–8 are used for analysis in Fig. 4.

Figure 4

Wetting of the same superoleophobic surface (Fig. 1) by different $n$-alkanes and PDMS. (a) Penetration depth $z(t)$ of $n$-decane at different spots of the surface for the same drop [Fig. 3]. The mean of all eight curves is shown with the thick solid black line. (b) Time dependence of the average depth (averaged over $0.01{\mathrm{mm}}^2$) for different alkanes and PDMS. (c) Penetration velocity showing a power-law dependence on time after deposition. (d) Exponent $p$ of the power law dependence on the number of carbon atoms of the alkane. Error bars from three independent measurements. (e) Effect of Laplace pressure. A curved interface forms the angle ${\theta }_0$ with the solid at a lower position than a horizontal interface. At larger spacing, the effect is more pronounced (dotted line). (f) Capillary condensation at the neck.