Supporting Information
Closed-air Induced Composite Wetting on Hydrophilic
Ordered Nano Porous Anodic Alumina
Zhirong Li,1, 2 Jingxia Wang,1,* Youzhuan Zhang,1 Jianjun Wang,1 Lei Jiang,1 and Yanlin Song1,*
1Beijing
National Laboratory for Molecular Sciences (BNLMS), Laboratory of New Materials, Key
Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190,
P.R. China.
2Graduate
University of the Chinese Academy of Sciences, Beijing, 100049, P.R. China.
Experimental Section
Preparation of Nano porous anodic alumina (PAA). Nano PAA substrates are prepared by twostep anodization process.1 High purity aluminum (99.99 wt%) foil with thickness of 0.2 mm is used as
the starting material. Before anodization, the aluminum foil is firstly annealed at 500 C for 10 h to
remove mechanical stresses, and is immersed in the acetone (95 wt%) for 10 min to degrease, and is
immersed in sodium hydroxide solution (5 wt%) for 5 min to remove the oxide layer. Secondly, the
aluminum is electropolished in a solution of perchloric acid (70 wt%) and absolute ethanol (V:V/1:5) at
12 V for 5 min. Then a two-step anodization process of aluminum is developed: the substrate is firstly
anodized at fixed direct current (DC) voltage in electrolyte for 12 h, then the formed alumina films are
chemically removed in a mixture of phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 60 C for
12 h, the second anodization process is repeated under the same conditions. After the two-step
anodization of aluminum, PAA substrate with ordered nanopores on the remaining aluminum substrate
(called as PAA-closed) is obtained. The obtained PAA-closed could be continuously treated by removal
of aluminum substrate in a mixing solution of CuCl2 (0.2 M) and HCl (20 wt%) at 20 C, and then
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removal of the barrier layer and pore-widening process in phosphoric acid solution (5 wt%) at 34 C for
30 - 60 min. As a result, the substrate with ordered nanopore arrays without the aluminum substrate and
the barrier layer (called as PAA-open) is obtained.
PAA-closed substrates, with an average pore diameter of 25, 50, 90, 150, 200 nm are fabricated by
anodization at 25 V, 50 V, 60 V,100 V and 145 V in aqueous oxalic acid (0.2 M) at 3 C for 12 h
respectively. PAA-open substrates with an average pore diameter of 25, 50, 90, 150, 200 nm are
fabricated by anodization at 10 V, 30 V, 50 V, 60 V and 100 V in aqueous oxalic acid (0.2 M) at 3 C
for 12 h respectively, followed by the removal of the aluminum substrate and the barrier layer and porewidening process. Before characterization, the samples are cleaned with acetone, ethanol and deionized
water sequentially, and dried for 10 h at 120 C.
Characterizations. The SEM images are obtained with a field-emission SEM (JEOL JSM-6700,
Japan), after sputtering the samples with a thin layer of platinum. Water contact angle (CA) is measured
on an OCA20 machine (Data-Physics, Germany) at ambient conditions (25±2 C and 30 - 40% relative
humidity). The water drop with volume of 2L is placed gently onto the nanoporous substrate. The
average CA value is obtained from the same sample at five different positions. The CA is recorded when
the three-phase contact line (TCL) stops moving and keeps pinning, which is called as the equilibrium
water CA, eq. The value is record after the water drop placed on the PPA-closed at 1s and the PAAopen at 2s. The In-situ shape evolution with time of water drop on PAA is captured on an OCA20
machine (Data-Physics, Germany), which is equipped with a CCD video camera and drop shape analysis
software, operating at a frame rate of 25 images /s. Pores area fraction (fP) and solid area fraction (fS) are
average statistical value from five different regions of SEM image of the same sample by software of
image pro plus (IPP), which is based on optical principle of gray-scale (Media Cybernetics, Inc., Version
6.0).2-3
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Morphology Characterization of PAA.
FIG. S1. SEM images of ordered nanoporous PAA. PAA-closed with pore diameter of (a) 25, (c) 50 and
(e) 200 nm and PAA-open with pore diameter of (b) 90, (d) 150 and (f) 200 nm respectively.
Clearly, the nanopores are separated from each other by the hexagonal network. Obviously, pore
fraction ratio of PAA-open is larger than that of PAA-closed, which are resulted from the additional
pore-widening process.
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In-situ photo images of water drop on PAA.
FIG. S2. In-situ photo images of water drop on PAA evolution with time. (a) PAA-closed, (b) PAAopen. Pore diameter, 90 nm.
Clearly, when water droplet wets and spreads on PAA-closed, the three-phase contact line (TCL)
stops moving and keeps pinning about 1 s after the water drop placed on the PAA-closed. Subsequently,
some air bubbles emerge slowly underneath the water drop due to the water imbibes into the nanopores
around 5 s.
In contrast, when water droplet wets and spreads on PAA-open, the TCL stops moving and keeps
pinning around 2 s. No air bubble is observed as time going on, which indicates air entrapped in the
nanopores will be easily squeezed out of the PAA-open accompanying the water flows into the
nanopores.
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The possible wetting mechanism of water drop on PAA.
During a water drop wets, spreads on PAA substrate, there are mainly two forces affecting the wetting
process of water droplet on PAA-closed, which are capillary force and counter force produced from the
closed-air in the nanopore respectively. In this case, gravity force is relatively small and can be
neglected, and the capillary length (2.72 mm) is much larger than the naonopore diameter.4 So, the
capillary force, Fc, is defined by equation (1), 5
Fc  d p cos  0
(1)
where  is the liquid surface tension, 0 is the Young CA, and dp is the average pore diameter.
While counter force produced from the closed-air in the nanopore, Fv, is given by equation (2), 5
Fv 
P0 Hd p2
4( H  h)
(2)
where P0 is the atmospheric pressure, h is the invasion depth of water into the nanopore, and H is the
length of the nanopore. In addition, equation (2) is based on the assumption of FcFv, where no air is
expelled from the closed nanopore.
Wetting model of water drop on PAA.
In our experiments, equilibrium water CA (eq) on PAA-closed is bigger than the Young CA (0 is
35.62.4°, 0090), which indicates that the wetting state is not a totally wetting state of Wenzel
regime 6 but a partial wetting state of Cassie regime7. Generally, there are two limiting cases, one is that
nanopores are not filled with water, water drop on PAA-closed forms a composite interface consisting of
the solid-liquid (S-L) contact and the liquid-vapor (L-V) contact. According to Cassie-Baxter equation 7,
eq can be defined as follow,
coseq = fSLcos0 + fLVcosLV
(3)
where fSL is S-L fraction (S-L contact underneath the water drop equals solid area fraction fS, which is
obtained from SEM image) and 0 is Young’s CA, fLV is the L-V fraction (cosLV = -1, since LV = 180),
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Another one is that nanopores are filled with water, which forms a composite interface consisting of
S-L contact and liquid-liquid (L-L) contact. The eq can be written as follow,
coseq = fSLcos0 + fLLcosLL
(4)
where fLL is the L-L fraction (cosLL=1, since LL=0, corresponding to water-on-water contact).
Because the actually situation is much more complicated, herein, we proposed a generalized
formulation, which merged the aforementioned two limiting cases into one. The water drop could be
assumed resting on a composite surface, consisting of S-L, L-V and L-L contact, the eq will be written
as follow, 4,7
coseq = fSLcos0 + fLVcosLV + fLLcosLL
(5)
Notes and References
1
H. Masuda, M. Satoh, Jpn. J. Appl. Phys. 35, L126 (1996).
2
http://www.mediacy.com/index.aspx?page=Home.
3
M. Pennuto, D. Dunlap, A. Contestabile, F. Benfenati, F. Valtorta, Mol. Biol. Cell 13, 2706 ( 2002).
4
P.-G. d. Gennes, F. Brochard-Wyart and D. Quere, Capillarity and Wetting Phenomena: Drops,
Bubbles, Pearls, Waves, Springer-Verlag New York Inc.: New York, 2004; Chapter 2, Section 1;
Chapter 9, Section 2.
5
A. W. Adamson and A. P. Gast, Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New
York, 1997; Chapter II, Section 4.
6
R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1936).
7
A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944).
6