Wettability Switching Techniques on Superhydrophobic Surfaces
© to the authors 2007
Received: 27 June 2007
Accepted: 22 October 2007
Published: 13 November 2007
The wetting properties of superhydrophobic surfaces have generated worldwide research interest. A water drop on these surfaces forms a nearly perfect spherical pearl. Superhydrophobic materials hold considerable promise for potential applications ranging from self cleaning surfaces, completely water impermeable textiles to low cost energy displacement of liquids in lab-on-chip devices. However, the dynamic modification of the liquid droplets behavior and in particular of their wetting properties on these surfaces is still a challenging issue. In this review, after a brief overview on superhydrophobic states definition, the techniques leading to the modification of wettability behavior on superhydrophobic surfaces under specific conditions: optical, magnetic, mechanical, chemical, thermal are discussed. Finally, a focus on electrowetting is made from historical phenomenon pointed out some decades ago on classical planar hydrophobic surfaces to recent breakthrough obtained on superhydrophobic surfaces.
KeywordsMicrofluidic Superhydrophobic surfaces Wettability switching Electrowetting
Biological surfaces, like lotus leaves, exhibit the amazing property for not being wetted by water leading to a self cleaning effect. The lotus leaves capability to remain clean from dirt and particles is attributed to the superhydrophobic nature of the leaves surface. The latter is composed of micro and nano structures covered with a hydrophobic wax, creating a carpet fakir, where water droplets attained a quasi spherical shape. In order to mimic these properties, artificial superhydrophobic surfaces have been prepared by several means, including the generation of rough surfaces coated with low surface energy molecules [1–6], roughening the surface of hydrophobic materials [7–9], and creating well-ordered structures using micromachining and etching methods [10, 11].
However, the modification of the liquid droplets behavior and in particular of their wetting properties on these surfaces is still a challenging issue. Functional surfaces with controlled wetting properties, which can respond to external stimuli, have attracted huge interest of the scientific community due to their wide range of potential applications, including microfluidic devices, controllable drug delivery and self cleaning surfaces.
In this review, after a brief overview on superhydrophobic states definition, we will discuss the techniques leading to the modification of wettability behavior on superhydrophobic surfaces under specific conditions: optical, magnetic, mechanical, chemical, thermal… Finally, a focus on electrowetting will be made from historical phenomenon pointed out some decades ago on classical planar hydrophobic surfaces to recent breakthrough obtained on superhydrophobic surfaces.
At the equilibrium state, using energy minimization (dE = 0), the Young relation (1) is found. This approach will be used thereafter to determine the relations of Wenzel and Cassie–Baxter on superhydrophobic surfaces.
Concretely, following the rule of Zisman [13, 14], wetting surfaces are surfaces of high energy (∼500–5,000 mN m−1), where the chemical binding energies are about an eV (ionic, covalent, metal connections). The wetting materials are typically oxides (glass), metal oxides,… On the other hand, nonwetting surfaces are characterized by low surface energy (∼10–50 mN m−1). For these materials, the binding energies are about kT (ex: crystalline substrates and polymers) .
Wetting on Superhydrophobic Surfaces: Wenzel and Cassie–Baxter States
The common point between all these surfaces is their roughness. Indeed, the surfaces are composed of nanometric structures limiting the impregnation of the liquid and pushing back the drop. Most of the time, the surfaces are made of a second scale of roughness, consisting of micrometric size. In order to minimize its energy, a liquid droplet forms a liquid pearl on the microstructured surface. The superhydrophobicity term is thus used when the apparent contact angle of a water droplet on a surface reaches values higher than 150°.
Previously, the studied substrates were regarded as smooth surfaces, i.e. the roughness of the substrate was sufficiently low and thus does not influence the wetting properties of the surface. In this case, the relation of Young (1) gives the value of the contact angle θ on the surface (which we will henceforth call angle of Young). However, a surface can have a physical heterogeneity (roughness) or a chemical composition variation (materials with different surface energies). In this case, a drop deposited on the surface reacts in several ways. A new contact angle is then observed, called apparent contact angle and generally noted θ*. It should be noticed that locally, the contact angle between the liquid droplet and the surface are always the angle of Young. Two models exist: the model of Wenzel [17, 18] and of Cassie–Baxter .
For a superhydrophobic surface, the fundamental difference between the two models is the hysteresis value. The first experiment on this subject was conducted by Johnson and Dettre (1964) who measured the advancing and receding contact angles, according to the surface roughness . For a low roughness, a strong hysteresis being able to reach 100° (Wenzel) is observed and attributed to an increase in the substrate surface in contact with the drop. Starting from a certain roughness (not quantified in their experiment), the hysteresis becomes quasi null resulting from the formation of air pockets under the drop. The receding angle approaches the advancing angle.
In the following two paragraphs, we will discuss in detail the two models. Then we will show that the reality is more complex, in particular in the presence of metastable states in the Cassie–Baxter model.
In this type of behavior, the liquid/solid interface and the hysteresis are strongly increased. The drop sticks to the surface and the Wenzel state contrasts with the superhydrophobicity idea i.e. the rolling ball effect.
However, when a drop is deposited on a rough surface, a Cassie–Baxter regime occurs even when θ < θ c (for water, θ < 120°) [27–29]. This state is metastable, i.e. by applying a pressure to the drop, for example, it is possible to reach the Wenzel regime: stable and displaying an important hysteresis . This state is problematic, in particular in microfluidic microsystems where the displacement of a drop with a hysteresis of 100° is not easily realizable. An ideal configuration is the rolling ball or fakir effect i.e. the Cassie–Baxter state.
Neinhuis and Barthlott studied in detail the superhydrophobic properties of almost 200 plants, the famous lotus effect. In most cases, the surface comprises two different roughness scales: one is micrometric and the other one is nanometric.
When Φ s < 1, cosθ* is smaller than in the case of a simple roughness, the contact angle increases.
Preparation of Superhydrophobic Surfaces
From a technological point of view, there are currently several possibilities to mimic and prepare artificial superhydrophobic surfaces, including generating of rough surfaces coated with low surface energy molecules, roughening the surface of hydrophobic materials, and creating well-ordered structures using micromachining and etching methods. Some examples will be seen in the next part of this review.
Wettability Switching Techniques on Superhydrophobic Surfaces
Carbon Nanotubes Anisotropic Structures
Carbon nanotubes (CNTs) are naturally hydrophilic. However, their wetting behavior is highly dependent on their arrangement and can vary from hydrophilic to hydrophobic and even superhydrophobic with in addition isotropic to anisotropic CA hysteresis. Two strategies have been developed to reach a stable superhydrophobic state. First a chemical modification of CNTs with a low surface energy compounds [mainly fluoropolymers like poly(tetrafluoroethylene) and silanes] leading to a CA as high as 171° with a roll off behavior, consistent with a quasi null hysteresis . Second, hierarchical structures inspired by the ‘lotus effect’ were fabricated by CVD on a patterned quartz substrate, giving a CA of 166° with a CA hysteresis of 3°. Using an anisotropically rough surface, leading to an anisotropic CA, Jiang et al. have prepared a surface mimicking the rice leaf (a two dimensional anisotropy) showing that a droplet can roll along a determined direction . As predicted by Jiang , three-dimensional anisotropic structured carbon nanotubes (ACNTs) can be designed with a gradient roughness distributed in a particular direction where the gradient wettability is predetermined and therefore the droplet may move spontaneously, driven by the wettability difference.
A superhydrophobic surface was used for reversibly oriented transport of superparamagnetic microliter-sized liquid droplets with no lost volume in alternating magnetic fields. The surface consists of an aligned polystyrene (PS) nanotube layer prepared via a simple porous alumina membrane template covering method . This surface displays a superhydrophobic behavior (CA of about 160°) with a strong adhesion force to water, as compared to traditional superhydrophobic surfaces. Instead of estimating the hysteresis of the surface, the authors measured the adhesive force. According to their results, adhesive forces of the surfaces were 10 times higher than that of a surface displaying a water CA hysteresis of 5°, proving the Wenzel state of the droplet. They used a super paramagnetic microdroplet (for an intensity of external magnetic field ranging from 0.3 to 0.5 T) placed on an ordinary superhydrophobic surface (CA of 160°), separated from the PS surface with 2 mm in height .
When the upper magnet was applied, the microdroplets were magnetized, fly upward and stick to the PS surface due to its strong hysteresis. On the other hand, when the magnetic force was reversed, the microdroplet fell down onto the initial surface. The principal key point of this application is that the reversible transport is made without any lost of liquid.
A two-level structured surface (SAS) of polymer has been synthesized by Zhou and Huch . The first level of roughness (∼1 μm) was obtained by plasma etching of a rough polymer film (PTFE). Then surface hydroxyl and amino functional groups have been introduced by plasma treatment in order to form a grafted mixed brush consisting of two carboxyl-terminated incompatible polymers PSF-COOH and P2VP-COOH. After exposure to toluene, an advancing contact angle of 160° was measured with no angle hysteresis (rolling ball state). After immersion of the sample in an acid (pH 3) bath for several minutes and its subsequent drying, a drop of water spreads on the surface. The authors clearly indicate that the superhydrophobic state is time dependant. Up to a few minutes after exposure to toluene, the surface was superhydrophobic with quasi null hysteresis, while the hysteresis increases dramatically with time due to the slow switching of the surface composition to a more hydrophilic state.
The first demonstration on thermal reversible switching behavior between superhydrophilicity and superhydrophobicity was reported by Sun et al. . They used a thermo responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) that exhibit, when deposited on a flat surface, a CA modification from 63.5° for a temperature of 25 °C (hydrophilic state due to the formation of intermolecular hydrogen bonding between PNIPAAm chains and water molecules) to 93.2° at 40 °C (hydrophobic state due to intramolecular hydrogen bonding between C=O and N–H groups of the PNIPAAm chains). The roughness effect on the wetting properties was further investigated by depositing the polymer on rough surfaces (obtained by a laser cutter on a silicon wafer) formed of a regular array of square silicon microconvexes (grooves of about 6 μm width, 5 μm depth and spacing from 31 to 6 μm). The obtained results clearly show that when the substrate is sufficiently rough (i.e. when groove spacing is smaller or equal to 6 μm), the thermally responsive switching between superhydrophilicity and superhydrophobicity can be realized: from a CA of 0° below T = 29 °C to 149.5° above 40 °C, indicating that a combination of the change in surface chemistry and surface roughness can enhance stimuli-responsive wettability.
Fu et al.  have developed a slightly different approach based on porous anodic aluminum oxide (AAO) template with nominal pore sizes from 20 to 200 nm. The grafting of PNIPAAm on the template was obtained by surface-initiated atom transfer radical polymerization (ATRP) leading to a reproducible and uniform brush film (15 nm thick) on the textured surface. According to the authors, the macroscopic wettability is not due only to the change of the polymer hydrophobicity, but also to the nanoscopic topography of the surface associated with expansion and contraction of the grafted polymer. Nonetheless, these surfaces led to a maximum contact angle of 158° at 40 °C (for 200 nm pore size) starting from a CA of 38° at 25 °C, comparable to the contact angles reported by Sun et al. .
The first example showing that the wetting characteristics of polymer surfaces doped with photochromic spiropyran molecules can be tuned when irradiated with laser beams of properly chosen photon energy was reported by Athanassiou et al. . The hydrophilicity was enhanced upon UV laser irradiation since the embedded nonpolar spiropyran molecules were converted to their polar merocyanine isomers. The process is reversed upon green laser irradiation. To enhance the hydrophobicity of the system, the photochromic polymeric surfaces were structured using soft lithography. Water droplets on the patterned features interact with air trapped in the microcavities, creating superhydrophobic air–water contact areas. Furthermore, the light-induced wettability variations of the structured surfaces are enhanced by a factor of 3 compared to those on flat surfaces. This significant enhancement is attributed to the photoinduced reversible volume changes of the imprinted gratings, which additionally contribute to the wettability changes induced by the light. In this work, it was demonstrated how surface chemistry and structure can be combined to influence the wetting behavior of polymeric surfaces. However, the contact angle values after the UV and green light irradiation are limited to the first two UV–green irradiation cycles. The aging and degradation of the system upon multiple irradiation cycles is the major drawback of such a polymeric system.
Theory and History
Although, Young–Laplace pressure works in prediction of droplet shape modification by EWOD, different theories have been proposed to explain the real nature of the movement. Historically, electrowetting was explained by the variation of interfacial energies: the increase of the voltage leads to a solid–liquid interfacial energy diminution . More recently, it has been proved that EWOD can be interpreted as an electromechanical effect: pressure exerted by electrical field on the drop surface acts on the contact line [58–60]. While this last view seems to be the correct one, both of them predict the same contact angle variation [61, 62].
Furthermore, according to Eq. 14, it is theoretically possible to obtain a total wetting of the drop by increasing the applied voltage. However, a saturation of the contact angle is observed starting from a certain voltage. The literature brings many assumptions for the comprehension of this saturation like an increase in the electric field to the level of the three phase contact line due to pick effect , trapping of charges in or on the dielectric layer [64, 65], ionization of air on the level of the triple line , leakage on the dielectric layer, . Nevertheless, while reasons for this saturation are not clearly established by the scientific community, in practice the maximum tension Vmax to be applied for electrowetting is always observed.
Optical Applications of EWOD
This part of the review, which is not exhaustive deals with the potential applications of the EWOD technique. For more detailed state of the art as well from the theoretical point of view, refer to recent reviews by Mugele and Baret  (which in addition contains an English version of the thesis of Lippmann on electrocapillarity), and by Fair .
EWOD for Microdroplets Displacement
The force F m drives the drop on the electrode under applied voltage. Until now, all the calculations were applied for perfect surfaces. However, certain forces such as hysteresis or viscous forces can hinder the displacement of the drop. Fouillet showed by digital simulation that the movement of the drop is related to the interfacial forces and not to the viscous forces . Concretely, it is necessary that the driving force is higher than the force of hysteresis in order to obtain a displacement of the drop. Within the framework of real surfaces, it is thus necessary that the driving force is higher than the force of hysteresis.
Although the industrial applications of the EWOD are in the field of optics, several groups are also interested in the possible applications in biotechnology. For this purpose, it is necessary to displace biological liquids and to realize microfluidic elementary operations for the development of Lab-on-chip, LoC. The LoC based on EWOD were initiated by Pollack et al., from the Duke University [79, 80]. By carrying out a series of electrodes, it is possible to move by EWOD effect the drop from one electrode to its neighbor by successive polarization. In this case, the electrodes are made of chromium; the dielectric is parylene C (700 nm thick) covered with Teflon (200 nm thick). The counter-electrode is a covered blade of glass ITO and Teflon. The gap between the two substrates is 300 μm for electrodes of 1.5 mm2. The displacement of drops of KCl (100 mM) was carried out under a tension of 120 VDC. In 2004, the same team has developed a Lab-on-Chip based on EWOD allowing the determination of the concentration of glucose in a drop of plasma, serum, urine and saliva . The detection scheme was based on the change of absorbance of the sample mixture/reactive versus time.
The hysteresis effect and the saturation phenomenon limit the interval of tension to be used for EWOD. Concretely, the voltage allowing displacement must lie between Vmin (related to hysteresis) and Vmax (related to saturation). The microsystems have most of the time vocation to be embarked. It is thus necessary to reduce the tensions of actuation. One of the solutions is the development of 1 plan microsystems, i.e. without counter-electrode . In this case, the force related to hysteresis is only reduced by a factor which is still not very practical in an embarked system. Moreover, such microsystems are definitely more sensitive to evaporation and do not allow microfluidic operations like drop scission. Another solution consists to reduce the thickness of the dielectric layer or to increase the permittivity of this one. However, a reduction in the dielectric layer involves an increase in the electric field. Under a certain thickness, the electric field is higher than the dielectric rigidity and involves a breakdown of the layer. There is thus a limit in the reduction of tension. The increase in the permittivity of the dielectric layer is limited by the weak permittivity of the hydrophobic layer. Thus, there is a breakdown even when a voltage of only few volts was applied .
The last possibility is the reduction of the hysteresis by using superhydrophobic surfaces (with hysteresis lower than Teflon).
Nonreversible Electrowetting on Superhydrophobic Surfaces
The same group brings in 2005 a first solution for the reversible wetting on such surfaces . A very short electrical current impulse applied to the substrate leads to the surface heating. The temperature can then reach 240 °C, causing liquid boiling and droplet expelling from the surface. Even though this technique is easy to implement, it is hard to imagine such an integrated system within a Lab-on-Chip. The heating would cause significant damage to biological material within the drop. Moreover, this expulsion creates satellite droplets.
It is interesting to notice that an oil environment prevents the Wenzel effect. However, the question of the applicability of such a surface is not clearly explained since a water drop in an oil environment has already a very high contact angle , even on a planar surface.
Thus a planar surface allows at the same time a total wetting but also a complete reversibility.
Recently, Heikenfeld has reported electrowetting applied to textiles . Two electrowetting textiles were prepared. The first one is made of a polyethylene naphthalate (PEN) film containing holes coated with Al (50 nm) (conductive layer). The second one was fabricated from wood microfibers on which a polymer (PEDOT-PSS and PEI) was deposited to render it electrically conductive. In both case parylene C (1 μm) and a fluoropolymer solution were used to insure a hydrophobic dielectric surface coating. The textile surfaces investigated are highly irregular and their electrowetting behavior was predicted, in first approximation by Cassie Baxter equation. For both textiles, irreversible electrowetting was observed with a contact angle varying from 120° to 70° in air. Here again, reversible electrowetting occurs in an oil environment.
Reversible Electrowetting on Superhydrophobic Surfaces
Growth conditions of silicon nanowires (Q = 40 sccm, T = 500 °C)
To achieve surface superhydrophobicity, the SiNWs were coated with a fluoropolymer C4F8 (60 nm thick), deposited using a plasma technique. All the resulting surfaces displayed liquid contact angle θ* around 164° for a saline solution (100 mM KCl) in oil (undecane) with almost no hysteresis, confirming that the droplet is in a Cassie state. Electrowetting in oil was performed on all surfaces, but a reversible behavior was only observed for the surface prepared using the process 8. When a voltage of 150 Vrms was applied, the apparent contact angle decreased down to 106° for a saline solution (100 mM KCl). When the tension was cut off, the effect is completely reversible. The drop returns to its initial position. Applied voltage leads to nonreversible wetting on the other surfaces (droplet trapped in a Wenzel configuration).
We have shown for the first time that reversible electrowetting is possible on superhydrophobic surfaces that display specific geometrical criteria as predicted by Bico . Due to low hysteresis of the surface, we assume that small voltages could be sufficient for droplet displacement. We have previously demonstrated the possibility to use such surfaces as EWOD ground electrodes with hydrophobic electrodes for matrix-free mass-spectrometry analysis (DIOS analysis). The main advantages associated are a simple realization of hydrophilic and functionalized pads in the superhydrophobic surface, allowing analytes trapping with an enhancement of the liquid/surface interaction, and a subsequent analysis by matrix-free desorption/ionization MS-DIOS on these pads.
Integration of the superhydrophobic electrodes inside a microfluidic microsystem, allowing low voltage actuation of a biological analyte and DIOS analysis is currently under investigation in our laboratory. Furthermore, the utilization of textured surfaces could prevent from nonspecific sticking of bio particles, leading to an easy and efficient removal operation as compared to planar surface. Application such as particle sampling, concentration and analysis on superhydrophobic surfaces should be dedicated to environment control.
Among all the superhydrophobic surfaces displaying high roughness combined with low surface energy coating, trapping of air between the substrate and the liquid droplets is necessary to obtain a rolling ball effect (i.e. a quasi null hysteresis). Associated to an effective way to switch the wettability properties of the surface, control of droplet displacement on superhydrophobic surface seems to be possible. Unfortunately, only few techniques based on optical, electrical, mechanical or magnetic phenomenon, lead to a reversible modification of surface wettability. Among these techniques, electrowetting on classical surfaces (i.e. hydrophobic) seems to be the more mature technology. This is particularly emphasized by recent results on EWOD droplet liquid pixel and by the very last improvement concerning optical lenses integrated inside commercialized cellular phones (varioptic.com). Combining the amazing properties of superhydrophobic surfaces with reliable EWOD devices will open new opportunities for designing systems with potential applications based on specific properties of theses surfaces, in particular in the field of lab-on-chip (preparation of highly functional microfluidic devices), optical devices and controlled self cleaning surfaces. Concerning lab-on-chip devices, the most important effect expected, due to the quasi null hysteresis of these surfaces, is the liquid manipulation at very low tension voltage.
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