Evaporation-Triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures

Peichun Tsai, Rob G. H. Lammertink, Matthias Wessling, and Detlef Lohse

Phys. Rev. Lett. 104, 116102 – Published 18 March 2010

Abstract

When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and suggest the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.

Received 24 September 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.116102

Authors & Affiliations

Peichun Tsai^1,*, Rob G. H. Lammertink², Matthias Wessling², and Detlef Lohse¹

¹Physics of Fluids Group, Faculty of Science and Technology, Impact and Mesa+ Institutes, University of Twente, P.O. Box 217 7500AE Enschede, The Netherlands
²Membrane Science and Technology Group, Faculty of Science and Technology, Impact and Mesa+ Institutes, University of Twente, P.O. Box 217 7500AE Enschede, The Netherlands

^*Corresponding author.

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Vol. 104, Iss. 11 — 19 March 2010

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Images

Figure 1
Experiment of an evaporating water droplet on a hydrophobic microstructured surface shown by the SEM (scanning electron microscope) image on the right.Reuse & Permissions
Figure 2
Snapshots of the bottom view reveal the water infiltration dynamics at the transition from a Cassie-Baxter to a Wenzel wetting state triggered by evaporation. Here, the microstructures are arranged in a square lattice with a dimension of $a = 5$ , $w = 5$ , and $h = 10 μ m$ . The dark areas indicate the water imbibition, while the air pockets present in the bright areas enclosed by a rather bright circumference marked by the droplet base. These images are sequentially recorded at $t = 0$ , 30, 58, 72, 116, 180, 236 and 310 ms from the transitional point, determined by the first frame where a small initial infiltration point is observed (marked by an arrow).Reuse & Permissions
Figure 3
Snapshots of the side views of an evaporating droplet with the time interval 15 s between the images, revealing the decrease of the contact angle. The dashed lines, marking the drop base, separate the main droplet from its mirror image. The arrow shows the length scale of the critical diameter $2 B_{c}$ at the wetting transition, determined from the bottom views.Reuse & Permissions
Figure 4
(a) A typical contact angle dynamic of a water droplet evaporating on hydrophobic microstructure substrates of regular pillars of the width $w = 5 μ m$ , the interspacing $a = 5 μ m$ , and height $h = 6 μ m$ in a square lattice. The contact angle is calculated with the time evolutions of the drop height $H$ and the base radius $B$ , shown in the inset, by assuming a spherical cap. The symbol $*$ marks the transition point from CB to $W$ state. (b) The corresponding curvature radius $R$ reveals a power law in time for $R ≳ 200 μ m$ . The red line shows the best scaling fit of $C (t_{f} - t)^{α}$ , with the power-law exponent $α = 0.61$ .Reuse & Permissions
Figure 5
Representative global interfacial energies of the Cassie-Baxster and Wenzel states: $E_{CB}$ and $E_{W}$ , respectively, as a function of drop size. The inset shows the difference of these two energies, $Δ E = E_{CB} - E_{W}$ , giving the critical base radius $B^{*}$ for the wetting transition when $Δ E = 0$ .Reuse & Permissions
Figure 6
The critical base radius at the evaporation-triggered CB to $W$ transition vs the geometric parameter, $a^{2} / h$ , for different initial droplet sizes. Experimental data for $B_{c}$ , marked by ○ (□) for cylindrical (squared) micropillars in a squared lattice, agree well with the predicted $B^{*}$ shown by $*$ (◇) using the presented energy argument for the cylindrical (squared) micropillars.Reuse & Permissions

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