COMMENT pubs.acs.org/Langmuir
Comment on Water Droplet Motion Control on Superhydrophobic Surfaces: Exploiting the Wenzel-to-Cassie Transition
his comment concerns the article entitled “Water Droplet Motion Control on Superhydrophobic Surfaces: Exploiting the Wenzel-to-Cassie Transition” by Liu et al.18 Wetting transitions (WTs) occurring on rough surfaces were studied intensively in the past decade.118 Various wetting states may coexist on the rough surfaces. These states become possible because of the multiple minima of free energy of a droplet deposited on a rough surface.19 The main wetting regimes are described by Cassie and Wenzel equations and their extensions.20,21 It could be shown that the Wenzel state implying the total penetration of liquid in the grooves constituting the relief is energetically favorable compared to the pure Cassie vapor trapping (“fakir”) state, when the inequality (eq 1) holds:
T
cos θ >
fS 1 r fS
ð1Þ
θ is the Young equilibrium contact angle, established at the atomically flat, homogeneous surface, r is the surface roughness, defined as the ratio of the real surface in contact with liquid to its projection onto the horizontal plane, and fS is the fraction of the solid surface wet by the liquid. The inequality (eq 1) usually takes place on the artificial superhydrophobic surfaces reported by researchers. Thus, it is assumed that WTs start from the Cassie fakir state and proceed to the Wenzel state. However, one more wetting state is possible, namely, the Cassie impregnating state. In this case, liquid penetrates the grooves of the solid, and the drop finds itself on a substrate viewed as a patchwork of solid and liquid (solid ‘‘islands’’ ahead of the drop are dry).22,23 It should be mentioned that the Cassie impregnating wetting usually supplies the lowest possible apparent contact angle and consequently the lowest possible free energy of the droplet. Hence, the final wetting regime could be the Cassie impregnating state and not the Wenzel impregnating state.23 Both Wenzel and Cassie impregnating states are “sticky” and are characterized by their high contact angle hysteresis, whereas a low contact angle hysteresis is inherent for true Cassie (vapor trapping) wetting, allowing a high mobility of droplets. The WT promoted by external stimuli such as vibrations, bouncing, or evaporation of the droplet are well known to be irreversible.117 The irreversibility of WT is explained by the fact that the energy barrier from the side of the metastable (higher-energy) wetting state is always much lower than that from the side of the stable state. Liu et al. reported in their recent very interesting paper that reverse Wenzel-to-Cassie transitions are possible for heated droplets deposited on superhydrophobic surfaces.18 Moreover, they observed two distinct wetting states, the first of which is the r 2011 American Chemical Society
sticky wetting characterized by the high apparent contact angle and the second of which is the true Cassie (fakir or vaportrapping) state featured by the high apparent contact angle and low contact angle hysteresis leading to the high mobility of the droplet. This comment proposes the mechanism of the reverse WT observed by Liu et al.18 The proposed explanation is illustrated in Figure 1. Liu et al. started from the sticky state. (The initial apparent contact angle could be acute or obtuse; it does not matter for our treatment.) First, it should be mentioned that this sticky state could be a Wenzel or Cassie impregnating state, and it is not simple to distinguish between them without additional experiments. The droplet, when heated to 190 °C, proceeded to the sticky wetting state, accompanied by a high apparent contact angle. How is this possible? This comment proposes the analysis of the wetting situation that could not be resolved with a microscope or high-speed camera. It is reasonable to suggest that the evaporation of the heated droplet starts from the area adjacent to the triple line, as depicted in Figure 1A, whatever the initial wetting state: Wenzel or Cassie impregnating. Thus, the situation in which grooves constituting the relief that are adjacent to the triple (three-phase) line are already filled with vapor and grooves far from the triple line are still filled with water is shown in Figure 1B. In this case, the droplet will have a high Cassie (vapor trapping) apparent contact angle and will be simultaneously sticky from water “bridges” connecting the droplet with the central area of the substrate underneath the droplet (Figure 1B). The results reported by Liu et al. illustrate a very important idea that the apparent contact is totally dictated by the area of the substrate that is in the vicinity of the triple line.2427 The primary importance of the wetting regime occurring in the neighborhood of the triple line has recently been intensively discussed; however, the dominance of the threephase adjacent area in constituting apparent contact angles was explicitly stated by Gray in 1965.28 The sticky wetting featured by high apparent contact angles exemplifies the so-called “rose petal effect”.2931 The further heating of the substrate reported in Liu et al.18 emptied all of the grooves underneath the droplet and promoted the formation of the true Cassie vapor trapping state characterized by a high apparent contact angle and a low contact angle hysteresis leading to easy sliding of the droplets as shown in Figure 1C. This comment proposes the mechanism of the reverse Wenzel-to-Cassie WT observed for heated droplets placed on superhydrophobic surfaces and emphasizes the importance of the area Received: August 2, 2011 Published: September 06, 2011 12769
dx.doi.org/10.1021/la203010d | Langmuir 2011, 27, 12769–12770
Langmuir
COMMENT
Figure 1. Analysis of the wetting regime occurring upon heating the droplet. (A) The initial sticky state, where evaporation starts from the triple line. (B) The sticky state characterized by the high apparent contact angle, where the grooves in the vicinity of the triple line are filled with vapor and the grooves far from the triple line are filled with water. (C) The true Cassie state, where all of the grooves underneath the droplet are filled with vapor, the apparent contact angle is high, and the contact angle hysteresis is low.
of the rough substrate adjacent to the triple line, governing the apparent contact angle. Edward Bormashenko Department of Physics, Ariel University Center of Samaria, POB 3, Ariel 40700, Israel
’ AUTHOR INFORMATION Corresponding Author
Tel: +972-3-9066134. Fax: +972-3-9066621. E-mail: edward@ ariel.ac.il.
’ ACKNOWLEDGMENT I am thankful to Mrs. Ye. Bormashenko and Mrs. A. Musin for their help in preparing this comment. ’ REFERENCES
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