Smart responsive surfaces switching reversibly between super-hydrophobicity and super-hydrophilicity

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Original entry: Alexander Epstein, APPHY 226, Spring 2009

Smart responsive surfaces switching reversibly between super-hydrophobicity and super-hydrophilicity

Authors: Fan Xia, Ying Zhu, Lin Feng and Lei Jiang

Soft Matter, 2009, 5, 275–281

Soft matter keywords

superhydrophobicity, photowetting, electrowetting, thermowetting, pH-response, mechanowetting, multiresponsive surfaces


Abstract from the original paper

Super-hydrophilicity and super-hydrophobicity are fundamentally opposite properties of special wettability, which are governed by surface chemical composition and surface roughness. Smart responsive surfaces switching reversibly between super-hydrophobicity and super-hydrophilicity can be effectively fabricated by modification of stimuli-responsive materials on rough surfaces. The externally applied stimuli include light irradiation, electrical potential, temperature, pH or selected solvents, and mechanical forces. Such surfaces with controllable wettability are of great importance to both fundamental research and practical applications.

Soft matters

The authors, who have written a considerable body of papers on controllable wettability, discuss five principal systems of surface wetting control. There is clearly a good deal of progress to date in reversibly switching between superhydophobic and philic surface states. I found this paper interesting because it explains the different approaches to control surface chemistry and topography; the surface is not a black box, and its effect on the contact angle can be understood and exploited.

Fig. 1 (a) Water-drop profiles for the nano-structured V2O5. (b) SEM of a rose-garden-like nanostructure V2O5 substrate. (c) XPS of the O 1s level before and after UV irradiation. (d) Reversible wettability transitions through UV exposure and dark storage, respectively.
Fig. 2 (a) SEM image of the nanograss substrate. (b) Demonstration of electrically induced reversible transitions between different wetting states on a nanostructured substrate. (1) With no voltage applied. (2) With the application of about 35 V. (3) With a short pulse of electrical current.

Photo-response

As photogeneration increases carrier concentration in semiconductors, so do photo-responsive surfaces change their surface chemistry in the presence of UV radiation. The most well known class of photo-responsive materials are inorganic semiconductor oxides such as titania, zinc oxide, vanadium oxide, etc. These materials are normally hydrophobic, so texturing them with submicron roughness, as seen in Fig. 1, amplifies them to superhydrophobic. However, exposure of the surfaces to UV light renders them hydrophilic, and their roughness collaborates to give complete wetting, as predicted by the Wenzel relation. Now storage in the dark restores superhydrophobicity! As seen in Fig. 1, the contact angle was shown to change reversibly from 0 to 160 deg for at least 5 cycles without degradation.

The generation of superhydrophilicity by exposure to UV light is closely related to the preferential adsorption of water on photogenerated surface defect sites. UV exposure generates electron−hole pairs in, e.g., V2O5. The holes react with lattice oxygen, leading to the creation of surface oxygen vacancies, while the electrons react with lattice metal ions, inducing conversion of V5+ sites to V3+ defect sites. Hydroxyl adsorption on the defect sites is kinetically more favorable than oxygen adsorption, which leads to dissociative adsorption of water molecules at these sites. This renders the V2O5 films hydrophilic. [1]

Some organic compounds can also show the photoresponsive transition. Azobenzene and spiropyran molecules are typical examples. Irradiation by UV or visible light causes a cis-trans conformation change or cleavage of a C-O bond, respectively. The point is that the molecules switch between polar and nonpolar forms in response to light and switch back in the dark.

Electric

Electric control is desirable because it is a simple and familiar type of input. The most dramatic change in surface wettability is seen in the electrowetting approach by Krupenkin, et al [2]. Starting with superhydrophobic silicon nanograss etched by the Bosch process, as seen in Fig. 2, they set a droplet down and contacted it with an electrode pin from the top. Another electrode was in contact with the substrate. To induce a transition between rolling ball and immobile droplet, a voltage was applied between the droplet and the substrate. Note that this is not complete wetting, but is a transition from Cassie to Wenzel state. To reverse the transition a short pulse of electrical current was transmitted. The mechanism of the return to superhydrophobicity is a rapid vaporization of the droplet bottom, which propels the droplet upward in rocket fashion. Therefore some of the droplet is lost during each pulse. I had a notion recently to experiment with magnetic expulsion of ferrofluids, but learned that a Bell Lab researcher is already trying it.

Conducting polymers offer another approach to electric control. With a positively charged conjugated (alternating single and double bond) backbone and negatively charged dopants, conducting polymer wettability depends greatly on the type of dopant and the dopant concentration. Since the dopant concentration can be controlled by changing the electrical potential, it's possible to make reversibly switchable surfaces. Rough polypyrrole (PPy) films made by Xu, et al switch from superhydrophobic to philic by simply adjusting the voltage. [3]

Thermal

Thermal switching between extreme wetting states has been shown in special polymeric surfaces (PNIPAAm) over a temperature range of just 10 C and near room temperature. This property resulted from the combined effect of the chemical variation of the surface and surface roughness. Competition occurred between intermolecular and intramolecular hydrogen bonding below and above the lower critical solution temperature (LCST) of about 32–33 C (Fig. 3c). The switch could be reused when heating and cooling the as-prepared films (the interval is about 3 minutes) for 20 cycles, showing excellent reversibility (Fig. 3d). Contact angle varied from 0 to ~150 deg at 20 C and 40 C, respectively.

A fascinating recent application of this system was a a PNIPAAm-modified micro/nano-structured copper mesh film with thermally controlled water permeation--like a valve and filter combined [4]. At low temperatures (below 25 C), the film showed good water permeability because of the highly hydrophilic nature. As a result, water could easily penetrate through the film. At high temperatures (above 40 C), the film became impermeable to water because of its super-hydrophobicity and the large negative capillary effect induced by the micro- and nanostructures. Processes such as filtration, and water/oil separation could benefit from this kind of thermal control.

Fig. 3 (a) SEM image of the nanostructures on rough substrate modified with PNIPAAm. (b) Water drop profile for responsive surface at 25 C and 40 C. (c) Diagram of reversible formation of intermolecular hydrogen bonding between PNIPAAm chains and water molecules (left) and intramolecular hydrogen bonding between C]O and N–H groups in PNIPAAm chains (right) below and above the LCST, which is considered to be the molecular mechanism of the thermally responsive wettability of a PNIPAAm thin film. (d) CAs in at two different temperatures 20 C and 40 C for PNIPAAm-modified rough substrate.
Fig. 4 Reversibly change in structure and wettability of the triangular polyamide film during extension and unloading.

Mechanical

A different and equally clever approach to wettability control is to ditch chemical control in favor of geometric control. That is, to apply a mechanical stress to a micro/nanostructured surface to change the spacing and/or geometry of surface features. Roughness (projected area/actual area) therefore changes and feature aspect ratios may change. In Fig. 4 we see how the triangular microstructure of polyamide film changes under a biaxial stress. The film is superhydrophobic in the unstretched state but switches to superhydrophilic when stretched. The switch is due to the side length change of the triangular array, which (I would expect) either allows the droplet to enter the triangular cells or forces it to remain in a superhydrophobic Cassie state. [I plan to run some follow-up experiments using our nanograss and other structured surfaces.]

Multiresponsive and Common Theme

Obviously many people are interested in combining the different form of wettability control. In certain systems, such as the body if we are considering biomedical applications, more than one type of external cue might be needed for the material behavior to be specific enough for the task. For example, the temperature of the body is uniform, but the pH and glucose concentrations are localized. There have been some successes in fabricating multiresponsive surfaces, but there is still much room for research.

It is worth noting that all of these approaches share the common theme of surface roughness. Without the amplifying effect of micro/nanostructure, none of the systems mentioned here would be able to switch between extreme wettability states.

References

1. UV-Driven Reversible Switching of a Roselike Vanadium Oxide Film between Superhydrophobicity and Superhydrophilicity. Ho Sun Lim,, Donghoon Kwak,, Dong Yun Lee,, Seung Goo Lee, and, Kilwon Cho. Journal of the American Chemical Society 2007 129 (14), 4128-4129

2. T. N. Krupenkin, J. A. Taylor, T. M. Schneider and S. Yang, Langmuir, 2004, 20, 3824.

3. L. Xu, W. Chen, A. Mulchandani and Y. Yan, Angew. Chem., Int. Ed., 2005, 44, 6009.

4. W. L. Song, F. Xia, Y. B. Bai, F. Q. Liu, T. L. Sun and L. Jiang, Langmuir, 2007, 23, 327.