An article can have a surface with selected wetting properties for various liquids.
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This application is a continuation of U.S. application Ser. No. 12/599,465, filed Aug. 23, 2010, now U.S. Pat. No. 10,202,711, which claims priority to PCT Application No. PCT/US2008/060176, filed Apr. 14, 2008, which claims priority to provisional U.S. Patent Application No. 60/917,012, filed May 9, 2007, titled “Tunable Surfaces,” each of which is incorporated by references in its entirety.FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with government support under Grant No. FA9300-06M-T015 awarded by the Air Force Office of Scientific Research. The government has certain rights in this invention.TECHNICAL FIELD
This invention relates to surfaces having tunable surface energy.BACKGROUND
Surfaces having a nanotexture can exhibit extreme wetting properties. A nanotexture refers to surface features, such as ridges, valleys, or pores, having nanometer (i.e., typically less than 1 μm) dimensions. In some cases, the features can have an average or rms dimension on the nanometer scale, even though some individual features may exceed 1 μm in size. The nanotexture can be a 3D network of interconnected pores. Depending on the structure and chemical composition of a surface, the surface can be hydrophilic, hydrophobic, or at the extremes, superhydrophilic or superhydrophobic.SUMMARY
An article can have a surface with selected wetting properties for various liquids. The surface can include a protruding portion configured to protrude toward a liquid and a re-entrant portion opposite the protruding portion. The re-entrant surface can have negative curvature relative to the space adjacent that portion of the surface. The protruding portion and the re-entrant portion can be surfaces of a fiber or surfaces of microstructures, for example, micronails or reverse micronails. The microstructures can include a surface texture selected to influence contact angle hysteresis.
In general, an article can include a superoleophobic surface. The superoleophobic surface can include nanoparticles. A nanoparticle can have a diameter of less than 100 nm, less than 50 nm, less than 40 nm, less than 30 nm, less than 20 nm, or less than 10 nm. The surface of the nanoparticle can be treated with a hydrophobic material. For example, the nanoparticles can be halogenated, perhalogenated, perfluorinated, or fluorinated nanoparticles, for example, perfluorinated or fluorinated silsesquioxanes. In certain embodiments, the concentration of nanoparticles can be less than 0.1 mass fraction nanoparticles, greater than 0.1 mass fraction nanoparticles, greater than 0.15 mass fraction nanoparticles, greater than 0.2 mass fraction nanoparticles, or greater than 0.25 mass fraction nanoparticles.
In another aspect, a method of manufacturing a fabric having tunable wettability can include selecting a concentration of nanoparticles to create a superhydropilic, a superhydrophobic, a superoleophilic, or a superoleophobic surface, forming a fiber from a mixture including a polymer and the concentration of nanoparticle, and assembling a plurality of the fibers to form a fabric. The step of selecting a concentration of nanoparticles can include choosing the concentration to create a superhydrophilic and superoleophobic surface or a superhydrophobic and superoleophilic surface. The fiber can be formed by electrospinning.
In another aspect, a method of modifying the wetting properties of a surface includes introducing a component onto the surface having a protruding portion configured to protrude toward a liquid and a re-entrant portion opposite the protruding portion. The step of introducing the component can include depositing a fiber including a polymer and a plurality of nanoparticles on the surface or forming a plurality of microstructures on the surface. The microstructures can be micronails or can include nanoparticles.
In another aspect, a method of modifying the wetting properties of a surface comprising exposing the surface to a liquid composition including a plurality of nanoparticles.
Exposing the surface to a liquid composition can include, for example, chemical solution deposition, or dip coating. The surface can include a surface of a fabric. The method can include stretching the fabric.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Surface geometry can create superoleophobic surfaces. It is believed that any superoleophobic surface has to make use of a geometry in which the surface has a protrusion portion and a re-entrant portion. Referring to
In addition, fabrics with tunable wettability, produced in a single step by electrospinning two components, a polymer and a fluorinated nanoparticle. The process can be used to create superhydrophilic, superhydrophobic, superoleophilic or superoleophobic surfaces (i.e., surfaces having a contact angle greater than 150° with alkanes such as hexadecane, decane and octane) by only changing the concentration of the nanoparticles. In general, higher the nanoparticle concentration, the lower the surface energy. This flexibility can allow surfaces having multiple desirable properties to be produced, for example, a surface that is both superhydrophobic and superoleophilic. Such a surface has been produced and is an excellent oil-water separator.
The produced fabrics can also be used as coatings on a wide range of rigid substrates such as metals, ceramics or bricks and glass, as well as, flexible substrates like paper and plastic. The fabric can be formed on directly the surface of the substrate or formed on a transfer medium and subsequently transferred to the surface of the substrate. The surface energy of the coating can be controlled to provide resistance or repellency to all liquids including water and alkanes or to specifically repel only a few liquids like water or alcohols.
The methods and surfaces described here can have certain advantages and improvements over other methods of surface modification. For example, super-oleophobic surfaces, i.e., surfaces which are resistant to even the lowest surface tension liquids like decane and octane, can be produced. A re-entrant surface curvature can be an essential feature for creating a superoleophobic surface. It is likely that any super-oleophobic surface produced by any method will have to make use of this geometry.
Fabrics with tunable wettability can be produced in a single step by electrospinning. The wettability of the fabric is easily controlled by changing the concentration of the nanoparticles. This flexibility allows for the production of surfaces having multiple desirable properties, for example a surface that is both superhydrophobic and superoleophilic.
There are a number of different commercial applications for the various types of surfaces produced in this work. The surfaces can be a portion of any article, including a vehicle, equipment, a tool, construction material, a window, a flow reactor, a textile, or others. A few applications for each surface include the following.
Superhydrophobic surfaces can be used to produce articles having anti-icing and/or anti-fogging properties, which can make them an ideal coating for airborne and ground-borne vehicle applications. Also, the superhydrophobic surfaces can be self-cleaning, i.e., water droplets simply roll of them, dissolving and removing any dust or debris present on the surface. Hence, they would be ideal as coating on windows, traffic lights etc. Other applications include prevention of adhesion of snow to antennas, the reduction of frictional drag on ship hulls, anti-fouling applications, stain-resistant textiles, minimization of contamination in biotechnological applications and lowering the resistance to flow in microfluidic devices.
Superhydrophobic and superoleophilic surfaces can be ideal for oil-water separation, which has a number of useful applications, including waste water treatment and cleaning up oil spills. Other applications include cleaning of ground water, oil well extractions, biodiesel processing, mining operations and food processing.
Superoleophobic surfaces can be resistant to dust, debris and fingerprints. This would make them ideal as coating on lenses, computer screens, tablet computers, personal data assistants and other handheld devices. Superoleophobic surfaces can also be used as anti-graffiti self-cleaning surfaces. Superoleophobic surfaces can also be of great use in the petroleum industry. For example, various surfaces that are attacked by the petroleum products could be lined with these superoleophobic coatings, preventing their degradation, for example, providing swell resistance to organic materials on fabrics. Also, superoleophobic linings can be used as a drag reducer in various pipelines.
A number of surfaces in nature use extreme water repellency for specific purposes; be it water striding or self-cleaning. A number of surfaces encountered in nature are superhydrophobic, displaying water (surface tension γ=72.1 mN/m) contact angles (WCA) greater than 150°, and low contact angle hysteresis. The most widely-known example of a superhydrophobic surface found in nature is the surface of the lotus leaf. It is textured with small 10 μm to 20 μm sized protruding nubs which are further covered with nanometer size epicuticular wax crystalloids. See, for example, W. BARTHLOTT et al., “Purity of the sacred lotus, or escape from contamination in biological surfaces,” Planta, Vol. 202 (1997) 1-8. Numerous studies have shown that it is this combination of surface chemistry plus roughness on multiple scales—micron and nanoscale that imbues super hydrophobic character to the lotus leaf surface. The effects of surface chemistry and surface texture can be controlled to create high levels of oil-repellency and superoleophobic behavior.
Two distinct models, developed by Cassie and Wenzel, are commonly used to explain the effect of roughness on the apparent contact angle of a drop sitting on a surface. See, for example, A. B. D. CASSIE et al., “Wettability of porous surfaces,” Trans. Faraday Soc., Vol. 40 (1944) 546-551; and R. N. WENZEL, “Resistance of solid surfaces to wetting by water,” Ind. Eng. Chem., Vol. 28 (1936) 988-994. The Wenzel model recognizes that surface roughness increases the available surface area of the solid, which geometrically increases the contact angle for the surface according to:
cos θ*=r cos θ (1)
here θ* is the apparent contact angle, r is the surface roughness, and θ is the equilibrium contact angle on a smooth surface of the same material. The Cassie model, on the other hand, proposes that the superhydrophobic nature of a rough surface is caused by air remaining trapped below the water droplet. This results in a composite interface with the drop sitting partially on air. Thus, the contact angle is an average between the value of the fluid-air contact angle (i.e., 180°) and θ. If ϕs is the fraction of the solid in contact with water, the Cassie equation yields:
cos θ*=−1+ϕs(1+cos θ) (2)
Thermodynamic arguments can be used to determine whether a rough hydrophobic surface will stay in the Wenzel or the Cassie state. See, for example, A. MARMUR, “Wetting on Hydrophobic Rough Surfaces: To Be Heterogeneous or Not To Be?” Langmuir, Vol. 19 (2003) 8343-8348; and M. NOSONOVSKY, “Multiscale Roughness and Stability of Superhydrophobic Biomimetic Interfaces,” Langmuir, Vol. 23 (2007) 3157-3161. Previous work has shown that if a series of substrates with progressively increasing equilibrium contact angles is considered, a transition from the Wenzel to the Cassie state should ultimately be observed on the corresponding rough surfaces. See, for example, A. LAFUMA et al., “Superhydrophobic states,” Nat. Mater., Vol. 2 (2003) 457-60. The threshold value of the critical equilibrium contact angle (θc) for this transition can be obtained by equating eqns. 1 and 2:
cos θc=(ϕs−1)/(r−ϕs) (3)
Because r>1>ϕs, the critical angle, θc, is necessarily greater than 90°, and thus θ>90° is required to create superhydrophobic surfaces. This is readily achievable using siloxanes or fluorinated surfaces and a wide variety of superhydrophobic surfaces have now been created. However, these arguments also explain why researchers so far have not been successful in making superoleophobic surfaces, i.e., surfaces with contact angles about 150° for mobile alkane oils such as decane (γ=23.8 mN/m) or octane (γ=21.6 mN/m). For a smooth surface to have an equilibrium contact angle about 90° with a liquid alkane, the surface would need to have a surface energy about 5 mN/m. See, for example, K. TSUJII et al., “Super oil-repellent surfaces,” Angewandte Chemie-International Edition in English, Vol. 36 (1997) 1011-1012. Zisman et al. reported that the surface free energy decreased in the order —CH2>—CH3>—CF2>—CF2H>—CF3, and the lowest solid surface energies reported to date are in the range of approximately 6 mN/m (for a hexagonally closed pack arrangement of —CF3 groups on a surface). See, for example, W. A. ZISMAN, “Relation of the equilibrium contact angle to liquid and solid construction,” In: Contact Angle, Wettability and Adhesion, ACS Advances in Chemistry Series. (ed. Fowkes, F. M.) (American Chemical Society, Washington, D C., 1964); and T. NISHINO et al., “The lowest surface free energy based on —CF3 alignment, Langmuir, Vol. 15 (1999) 4321-4323.
Surface curvature can be used as a third factor, apart from surface energy and roughness, to modify surface wettability. The surface curvature (apart from surface chemistry and roughness), can be used to significantly enhance liquid repellency, as exemplified by studying electrospun polymer fibers containing very low surface energy perfluorinated nanoparticles (FluoroPOSS). Increasing the POSS concentration in the electrospun fibers can systematically transcend from superhydrophilic to superhydrophobic and to the superoleophobic surfaces (exhibiting low hysteresis and contact angles with decane and octane greater than 150°).
A surface has a re-entrant portion surface (or negative curvature) as shown in
where γ refers to the interfacial tension and s, l, and v refer to the solid, liquid and vapor phases, respectively, is satisfied at the air-liquid-solid interface (contact angle=equilibrium contact angle) even if 0<90°. See, for example, M. OWEN et al., “Surface active fluorosilicone polymers,” Macromol. Symp., Vol. 82 1994) 115-123; A. MARMUR, supra; and M. NOSONOVSKY, supra. Thus, the re-entrant surface leads to the drop sitting partially on air with high overall contact angles (Cassie state). This Cassie state is however metastable as the total energy of the system decreases significantly when the liquid advances and completely wets the surface leading to a homogeneous interface. See, for example, M. NOSONOVSKY, supra. It should be mentioned that the lower the value of θ, the more the liquid wets the curved surface, leading to higher contact angle hysteresis, even with the composite interface. Thus, a surface in the Cassie state does not necessarily have low hysteresis, as is widely believed. Surfaces without curvature or having only a protruding surface cannot lead to a composite interface if θ<90°, as the Young's equation is not satisfied at any point, other than for complete wetting.
Consider the schematics shown in
The presence of re-entrant texture (or ω<90°) in the surface illustrated in
Nosonovsky analyzed the stability of composite interfaces on a range of surfaces having different roughness profiles and suggested that the creation of a stable composite interface on any rough surface requires a local minimum in the overall free energy diagram and dAsldθ<0. See M. NOSONOVSKY, supra, which is incorporated by reference in its entirety. Here dAsl is the change in solid-liquid contact area with the advancing or receding of the liquid, accompanied by a change in the local contact angle, dθ. Based on this criterion, Nosonovsky proposed a liquid-repellent structure of rectangular pillars, covered with semi-circular ridges and grooves as shown in
Based on the above considerations, oleophobic surfaces were prepared electrospinning polymer-nanoparticle composite fibers. The fibers possess the re-entrant surface by virtue of their curvature, and hence have enhanced resistance to wetting by liquids. The details for the materials and the process used are as follows.
Nanoparticles can include inorganic nanoparticles. One or more of the nanoparticle can be modified to have a hydrophobic surface. The nanoparticles can be halogenated, perhalogenated, perfluorinated, or fluorinated nanoparticles, for example, perfluorinated or fluorinated silsesquioxanes. The halogenated, perhalogenated, perfluorinated, or fluorinated nanoparticles can be surface modified with organic moieties having between 1 and 20 carbon atoms, in particular, C2-C18 alkyl chains, which can be substituted or unsubstituted. The nanoparticles can have an average diameter of less than 50 nm, less than 40 nm, less than 30 nm, less than 20 nm, between 1 nm and 10 nm, or between 1 nm and 5 nm, inclusive. The nanoparticles can have a surface area to volume ratio of greater than 1 nm−1, greater than 2 nm−1, or greater than 3 nm−1.
A new class of hydrophobic fluorinated polyhedral oligomeric silsesquioxanes (POSS) molecules has been developed in which the rigid silsesquioxane cage is surrounded by fluoro-alkyl groups (details for the synthesis are provided as supplementary information). A number of different molecules with different organic groups including 1H,1H,2H,2H-heptadecafluorodecyl (referred to as fluorodecyl POSS) and 1H,1H,2H,2H-tridecafluorooctyl (fluorooctyl POSS) have now been synthesized, and this class of materials is denoted generically as fluoroPOSS. The fluoroPOSS molecules contain a very high surface concentration of fluorine containing groups, including —CF2 and —CF3 moieties. The high surface concentration and surface mobility of these groups, as well as the relatively high ratio of —CF3 groups with respect to the —CF2 groups results in one of the most hydrophobic and lowest surface energy materials available today. See, for example, M. J. OWEN, supra. A spin coated film of fluorodecyl POSS on a Si wafer has an advancing and receding contact angle of 124.5±1.2°, with an rms roughness of 3.5 nm. Blends of a moderately hydrophilic polymer, poly(methyl methacrylate) (PMMA, Mw=540 kDa, PDI˜2.2) and fluorodecylPOSS can be used in various weight ratios to create materials with different surface properties. Other polymers can be used in place of or in combination with other polymers. By varying the mass fraction of fluoroPOSS blended with various polymers, the surface energy of the polymer-fluoroPOSS blend can be systematically changed. This ability can afford control over the equilibrium contact angle of the blends and provide a mechanism for systematically studying the transition from the Wenzel to the Cassie state on rough surfaces made from the blends.
Smooth surfaces (maximum rms roughness of about 4.4 nm; maximum advancing water contact angle=123°) can be created by spin coating. The corresponding rough surfaces for the system can be created by electrospinning (see, for example, M. L. M A et al., “Electrospun poly(styrene-block-dimethylsiloxane) block copolymer fibers exhibiting superhydrophobicity,” Langmuir, Vol. 21 (2005) 5549-5554) solutions of fluorodecyl POSS and PMMA from Asahiklin-AK225 (Asahi Glass Co.) solvent. The density of fibers can be modified, selected, or otherwise adjusted to allow fluid to contact one or more fibers at one time depending on the sag of the bottom of a drop of fluid.
A number of different researchers have seen similar effects with unusual hydrophobicity or oleophobicity obtained from rough materials whose corresponding smooth surfaces are hydrophilic or oleophilic, and have so far been unable to explain these unexpected results (the surfaces should be in the Wenzel state leading to contact angles less than θ). See, for example, K. TSUJII, supra; S. SHIBUICHI et al., “Super water- and oil-repellent surfaces resulting from fractal structure,” J. Colloid Interface Sci., Vol. 208 (1998) 287-294; W. CHEN et al., “Ultrahydrophobic and Ultralyophobic Surfaces: Some Comments and Examples,” Langmuir, Vol. 15 (1999) 3395-3399; and Z. MEIFANG et al., “Superhydrophobic surface directly created by electrospinning based on hydrophilic material,” J. Mater. Sci., Vol. 41 (2006) 3793. This unusual effect is further explored in
This effect is further explored in the form of a general wetting diagram,
The electrospinning process is described in more detail here. PMMA was purchased from Scientific Polymer Products, Inc., while the fluorodecyl POSS nanoparticles were obtained. See, for example, J. M. MABRY et al., “Hydrophobic Silsesquioxane Nanoparticles and Nanocomposite Surfaces,” IN: ACS Symposium Series, The Science and Technology of Silicones and Silicone-Modified Materials, Eds: S. J. Clarson et al. (2006) 290-300. Both the polymer and the nanoparticle were dissolved in a common solvent, Asahiklin AK-225 (Asahi glass co.) in this case, at a concentration of about 5 wt %. The solution was then electrospun using a custom-built apparatus as described previously (see, for example, S. SHIBUICHI, supra) with the flow rate, plate-to-plate distance and voltage set to 0.05 mL/min, 25 cm, and 20 kV, respectively.
The re-entrant surfaces of the electrospun fibers can also be used to make extremely oleophobic surfaces (in the metastable Cassie state), (i.e., these electrospun surfaces are also strongly oleophobic (with advancing contact angles about 140° and receding contact angles greater than 100° for Octane)), even though all of the corresponding spin coated surfaces are oleophilic, at all POSS concentrations.
An interesting application for the electrospun materials can be derived by studying the data in
The metastability strength for the electrospun fiber surfaces is directly measured by electrospinning the PMMA and POSS fibers directly on to a steel wire mesh (with pore size of 1 mm2) and measuring the height of liquid required to ‘breakthrough’ the metastable Cassie surface of the fibers. This breakthrough height is shown in
Herminghaus first pointed out that many leaves in nature display superhydrophobic properties, even though their flat contact angles are less than 90°, recognizing this unusual effect to be a direct result of the re-entrant surfaces (he refers to them as surfaces with overhangs, like the micronail structure described below). See, for example, S. HERMINGHAUS, supra. Herminghaus also contended that the superhydrophobic state of the leaves was not the true equilibrium state (which should be the Wenzel state), and a transition from this ‘metastable’ state to the true equilibrium state could be made by submerging the leaf in water to a certain depth. Based on the re-entrant geometry, as well as the metastability of the re-entrant electrospun fibers, SiO2 micronails, i.e., pillars with large flat caps (
As an alternative to micronails, the microstructure can be a reverse micronail, in which the base is broader than the top, and the top has a re-entrant portion on the surface. The microstructures can be spaced periodically, for example, in square or hexagonal patterns. The spacing between microstructures and height can be selected to avoid liquid contact with the substrate upon with the microstructures are built. In certain circumstances, the re-entrant portion of the surface has negative curvature relative to the space between microstructures. In an alternative method of forming the microstructures, a material can be used as a template or porophore to create microstructures on a surface of a substrate. The microstructures can be patterned in a periodic or aperiodic manner.
To demonstrate the importance of re-entrant curvatures in the electrospun fiber mats, model SiO2 micropillars with large flat caps were also fabricated using lithographic chemical etching. A number of different pillar surfaces with inter-pillar spacing varying between 10 μm-40 μm were fabricated, in order to vary the fractional surface coverage, ϕs. The pillar height and cap width were held fixed at 7 μm and 20 μm, respectively.
As the SiO2 nails were fabricated on flat Si wafers (covered with a layer of SiO2), the contact angles can be measured for the rough (with nails) and smooth (without nails) surfaces on the same wafer.
Next, the capped SiO2 pillars were treated with vapor phase tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane, to lower the substrate surface energy chemically.
It can also be seen from the figure that the receding contact angles for the surfaces decrease with increasing ϕs. This is due to the additional resistance offered to the receding liquid, which is expected to be proportional to the total number of pillars on the air-liquid-solid contact line, as explained above. However, decreasing ϕs also decreases the breakthrough height (metastability strength). Thus, there is an inverse relationship between contact angle hysteresis and the stability of the composite interface which needs to be considered while designing any superoleophobic surface. Electrospun fiber mats can contain as little as 2 wt % POSS are strongly hydrophobic, even though spin coated surfaces with the same fluorodecylPOSS/PMMA composition remain hydrophilic. At higher concentrations of the fluoroPOSS it is also possible to create highly oleophobic substrates with low contact angle hysteresis; however, these surfaces are metastable. The critical role of re-entrant surface curvature in controlling the ability to generate Cassie surface states is demonstrated by lithographically fabricating a model surface of micronails covered with a fluorosilane chemical coating. These model surfaces couple low surface energy with a re-entrant surface geometry and lead to the first truly superoleophobic surfaces.
The combination of surface chemistry and roughness on the micron and nanoscale imbues enhanced repellency to many natural surfaces, like the lotus leaf, when in contact with a high surface tension liquid, such as water (surface tension γlv=72.1 mN/m). This understanding has led to the creation of a number of biomimetic superhydrophobic surfaces (water contact angles greater than 150°, low hysteresis). However, researchers so far have been unsuccessful in producing superoleophobic surfaces for liquids with much lower surface tensions; for example, alkanes such as decane (γlv=23.8 mN/m) or octane (γlv=21.6 mN/m).
Here, we have developed a new class of fibers which are resistant to both water and hexadecane.
To further elucidate the significance of re-entrant curvature in the formation of a metastable composite interface, the variation in the specific Gibbs free energy caused by the propagation of the liquid-air interface on various rough surfaces was calculated. These calculations are based on the formulation described elsewhere (see, e.g., A. MARMUR, supra; and A. TUTEJA supra; which are incorporated by reference in their entirety).
As an introductory example, the Gibbs free energy density variation for water (
Similar calculations can be performed for the propagation of water (
Estimation of Solid Surface Energy (gsv)
Previous work by Shibuichi et al. argued that for a chemically homogeneous, smooth surface to exhibit θ>90° with any liquid, its solid surface energy (γsv) must be less than one-fourth the liquid surface tension, (γlv/4 (see, for example, K TSUJII, supra; and S. SHIBUICHI, supra; each of which is incorporated by reference in its entirety). Careful studies of monolayer films by W. A. ZISMAN, supra; which is incorporated by reference in its entirety) show that the contributions to the overall magnitude of surface energy of a flat surface decreased in the order —CH2>—CH3>—CF2>—CF2H>—CF3, and based on this analysis, the lowest solid surface energy is estimated to be about 6.7 mN/m (for a hexagonally closed packed monolayer of —CF3 groups on a surface) (see, e.g., T. NISHINO, supra, which is incorporated by reference in its entirety). Taken in conjunction, these studies explain the absence of non-wetting surfaces displaying equilibrium contact angles about 90° with decane and octane, as a solid surface would need to have a surface energy of about 5 mN/m to display θ>90° with these liquids (see, for example, A. TUTEJA, supra; K. TSUJII, supra; S. SHIBUICHI, supra; and W. CHEN, supra; each of which is incorporated by reference in its entirety).
However, recently a few groups have reported extremely low γsv values; for example, Coulson (S. R. COULSON et al., “Ultralow surface energy plasma polymer films,” Chem. Mater., Vol. 12 (2000) 2031; and S. R. COULSON, “Plasmachemical functionalization of solid surfaces with low surface energy perfluorocarbon chains,” Langmuir, Vol. 16 (2000) 6287; each of which is incorporated by reference in its entirety) report surface energy values as low as 1.5 mN/m for coatings created by pulsed plasma polymerization of 1H,1H,2H-perfluoro-1-dodecene.
Thus, the issue of the minimum surface energy seems to be a bit controversial and unresolved in the literature. Measurement of equilibrium contact angles only provides an indirect estimate of the surface energy, and typically involves extrapolation or assuming an additive decomposition of γsv into dispersive and H-bonding/polar contributions. The most accurate determination of surface energies requires the measurement of the work of adhesion, and this is infrequently done (see, e.g., M. J. OWEN, supra, which is incorporated by reference in its entirety).
Indeed, Coulson et al. also report two different measures of surface energy. They obtain values of γsv=1.5 mN/m (on a smooth glass substrate coated by pulsed plasma polymerization of 1H,1H,2H-perfluoro-1-dodecene) and 4.3 mN/m (on a smooth glass substrate coated by pulsed plasma polymerization of 1H,1H,2H,2H-heptadecafluorodecyl acrylate) using the Zisman analysis, or γsv=8.3 mN/m and 10 mN/m using the Owens-Wendt method for the same two surfaces. See S. R. COULSON, Chem. Mater., supra; and S. R. COULSON, Langmuir, supra; each of which is incorporated by reference in its entirety. It is therefore unclear as to which method provides a more accurate value for γsv. An indication that the Zisman analysis might be providing a γsv value lower than the actual value for their surface comes from the values of octane contact angles obtained by Coulson et al. As mentioned above, if γsv<γlv/4, the equilibrium contact angle θ measured experimentally should be greater than 90°. In contrast, Coulson et al. report values of advancing contact angle, θadv=74° and receding contact angle, θrec=35°, respectively, on their coatings of 1H,1H,2H-perfluoro-1-dodecene when using octane (γlv=21.7 mN/m).
We have also computed the surface energy of the various spin coated PMMA and fluoroPOSS surfaces (r.m.s roughness for all spin coated surfaces was less than 4 nm) using the Zisman and the Owens-Wendt methods. For a spin coated surface containing 44.4 wt % POSS we obtain values of γsv=−3 mN/m and γsv=7.8 mN/m (with the dispersive component of surface energy, γd=6.6 mN/m and the polar component, γp=1.2 mN/m) using the Zisman and the Owens-Wendt method respectively.
Although the negative value of the surface energy obtained from the Zisman analysis of our surfaces were spurious (and arose solely from the extrapolation process employed), however, these calculations again point out the limitations of the various methods that use measurements of equilibrium contact angles to compute γsv. It was clear from the data in
The presence of re-entrant texture is not a sufficient condition for producing robust superhydrophobic or superoleophobic surfaces as in many cases the activation energy required to irreversibly transition from a composite interface to a fully wetted interface can be extremely small. Further, even though a Gibbs free energy approach can reliably predict the existence of a composite interface, its ability to estimate the robustness of the regime is limited as the analysis typically assumes a locally flat liquid-vapor interface. See, e.g., A. TUTEJA, supra; and A. MARMUR, supra; each of which is incorporated by reference in its entirety. With actual droplets, possessing significant internal pressure or under externally applied pressure, considerable sagging of the liquid-vapor interface can occur and the actual failure of the composite regime typically originates not from the activation energy required to transition between the composite and fully-wetted states, but from the sagging of the liquid-vapor interface. Hence the robustness of a composite interface can be significantly lower than the values obtained using Gibbs free energy calculations.
To provide a relative measure of the pressure required to cause the breakdown of a composite interface, we have developed the robustness parameter, H*, which relates to the sagging of the liquid-vapor interface as a result of pressure (Laplace pressure, external pressure or gravity). H* compares the maximum pore depth (h2 in
Consider the idealized fiber mat surface shown schematically in
The system transitions from a composite interface to a fully wetted interface when the sagging height (h1) becomes equal to the original clearance between the liquid-vapor interface and the next level of fibers (pore depth), h2=R(1−cos θ) (neglecting any shift in contact angle due to sagging). When D=1/κ≈Icap (which is true for most micro or nano scale textures), sin(Dκ) Dκ. Thus, h1≈κ−1(1−cos(Dκ))≈κD2/2.
Therefore, the ratio,
H*=h2/h1≈2(1−cos θ)RIcapD2 (4)
The robustness parameter for the micro-nail geometry (
Thus, a rough structure possessing a high pore depth (h2) will have an extremely high value of H*. However, even if the composite interface on a surface is expected to be extremely resistant to failure with its high pore depth, it can still readily fail due to a shift in the local contact angle as a result of the sagging liquid-vapor interface. Initially, on any rough surface (for example consider
by assuming D<<Icap (as done for the derivation of H*). Therefore,
Note that for both the electrospun and the micro-nail surfaces, re-entrant curvature leads to ψ=0°, which maximizes the value of (θ−ψ) for any liquid. Geometries with ψ<0° (for example a spade geometry) can lead to even higher values of T*. Given a fixed value of ψ, T* can be maximized by increasing the value of the equilibrium contact angle (θ), which can be accomplished by lowering the surface energy of the structure. This is the reason why various low surface energy molecules are applied as coatings on various re-entrant geometries, thereby simultaneously increasing the values of both the design parameters H* and T*.
The design parameter T* can be considered to be a robustness angle, while H* is a robustness height. A composite interface can therefore transition irreversibly to a fully-wetted interface by either of the two mechanisms discussed above, and it is expected that the robustness of any composite interface will be proportional to the minimum between the values of the two robustness parameters.
A third design parameter (D* or the spacing ratio) relates the surface texture parameters to the obtained apparent contact angles with any liquid. The apparent contact angles for a composite interface are determined by ϕs, as defined through the Cassie relation. For any given equilibrium contact angle θ, the fraction ϕs on the electrospun fiber surface (see
To achieve both extremely high apparent contact angles and a robust composite interface, the design parameters D*, H*, and T* are preferably simultaneously minimized. In the case of the electrospun fibers, the three design parameters are inherently coupled. Increasing the spacing between the fibers (D) leads to higher D* values, however, this also leads to lower values of both T* and H* corresponding to more severe sagging of the liquid-air interface. This, in turn, allows for easier liquid penetration through the structure. For the micro-nail geometry, on the other hand, the spacing ratio takes the new form
As the nail spacing (W) and height (H) can be varied independently (see
These design parameters therefore provide a mechanism for designing surfaces that are able to support super-repellency, with both high apparent contact angles and a robust composite interface. Further, they also provide a tool to rank-order various super-hydrophobic or oleophobic surfaces discussed in the literature.
Many natural and commercial surfaces such as woven and non-woven fabrics, feathers, plant leaves, spheres, cylinders etc. already have intrinsic re-entrant geometries and these surfaces can be rendered oleophobic through various simple surface treatments. These treatment are described in further detail below:
Chemical Vapor Deposition (CVD):
CVD is a chemical process used to coat a substrate with uniformly deposited high-purity, high-performance solid material. In a typical CVD process, the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to deposit the desired coating. Micro-nail structures become oleophobic after a CVD process using various fluoro-silanes as reactive, volatile precursors (see, for example,
Chemical Solution Deposition (CSD):
CSD uses a liquid precursor, usually dissolved in an organic solvent, which reacts and thereby adheres conformably to any surface. This is a relatively inexpensive, simple process that is able to produce uniform and conformal thin coatings. Unlike CVD, which is carried out in a highly controlled environment (such as in a vacuum chamber), CSD allows for producing a coating with less rigorous/stringent environmental conditions.
Dip coating refers to the immersing of a substrate into a tank containing the coating material, removing the coated substrate from the tank, and allowing it to drain. The coated substrate can then be dried, for example, by convection or baking. Dip coating can be, generally, separated into three stages (see
- Immersion: the substrate is immersed in the solution of the coating material at a constant speed. Preferably the immersion is judder free—in other words, the substrate is lowered into the solution in a smooth motion.
- Dwell time: the substrate remains fully immersed and motionless to allow for the coating material to apply itself to the substrate.
- Withdrawal: the substrate is withdrawn, again avoiding judders. Coating thickness can be influenced by the withdrawal speed: the faster the substrate is withdrawn from the tank, the thicker the coating.
We have dip-coated various naturally occurring and synthetic surfaces that inherently possess re-entrant curvature, to make them superoleophobic. A few examples are shown in
Mechanical durability of the dip-coated fabrics (obtained by dip-coating with pure fluoroPOSS and fluoroPOSS-polymer mixtures) was tested by stretching the fabric multiple times and mechanically rubbing the fabric surface by hand. All of these experiments did not damage the coating (this was confirmed by imaging the microstructure of the fabric using a scanning electron microscope) or reduce performance (as determined by measuring the contact angles with various liquids, before and after testing).
One application of the dip-coated fabrics is separation of liquids having different surface tensions. Stretching of the fabric changes the pore size within the fabric (leading to a change in the value of the design parameters, H* and T*, for different liquids). This then allows for some liquids to wet the fabric and permeate through it, while other liquids remain unable to wet the surface. Generally, liquids with lower surface tensions begin to wet the surface first as the pore size increases. Wetting liquids are able to pass through the fabric. This is illustrated in
Controlling Contact Angle Hysteresis.
Although apparent contact angles on any surface are governed by fraction of solid in contact with a liquid (ϕs), the amount of contact angle hysteresis (i.e., the difference between the advancing and receding contact angles) can vary significantly depending on the details of each individual surface texture. Hence a surface that supports a robust composite interface can also be tailored to enhance or reduce contact angle hysteresis. Low hysteresis results in very small roll off angles, corresponding to easy movement of the liquid droplets on the surface. On the other hand, high hysteresis implies that a significant amount of energy needs to be expended in moving the liquid droplet (see, e.g., W. CHEN, supra, which is incorporated by reference in its entirety). This in turn can be used to adhere the liquid droplet at a particular spot on the surface.
To achieve both these aims, we have fabricated two kinds of micro-nail structures, with different surface textures, as shown in
The texture shown in
The texture shown in
Another structure (
All three designs discussed above are expected to be useful for different applications. Concentric circles can enhance contact angle hysteresis. Such samples can be used to position and confine liquid drops at preferred locations, with the preferred shape. Surface texture-directed liquid immobilization can be useful for cell culturing, localizing liquid droplets on quartz crystal microbalances, or in chemical or biological sensors.
A spiral texture (as in
A texture of parallel lines, or stripes, leads to anisotropic hysteresis. Such surfaces can be useful in developing structures with directional wettability. These surfaces also allow for easy control over the path that any liquid follows on these surfaces, which could be very useful in controlling the movement of small volumes of liquid, for example in micro-fluidic channels.
Each reference cited herein is incorporated by reference in its entirety. Other embodiments are within the scope of the following claims.
43. A method of manufacturing a fabric having tunable wettability, the method comprising:
- selecting fluorinated nanoparticles to create a superhydrophobic, a superhydrophobic, a superoleophilic, or a superoleophobic surface;
- coating a polymer with the fluorinated nanoparticles;
- forming fibers from the mixed polymer and fluorinated nanoparticles, the fluorinated nanoparticles forming microstructures on the fibers configured to influence contact angle hysteresis;
- assembling a plurality of the formed fibers into a fabric.
44. The method of claim 43, wherein selecting the fluorinated nanoparticles further comprises:
- selecting a concentration of the fluorinated nanoparticles, wherein the concentration determines whether the fabric is superhydrophilic and superoleophobic surface or superhydrophobic and superoleophilic surface.
45. The method of claim 44, wherein the concentration is less than 0.1 mass fraction nanoparticles.
46. The method of claim 44, wherein the concentration is greater than 0.1 mass fraction nanoparticles.
47. The method of claim 44, wherein the concentration is greater than 0.15 mass fraction nanoparticles.
48. The method of claim 44, wherein the concentration is greater than 0.2 mass fraction nanoparticles.
49. The method of claim 44, wherein the concentration is greater than 0.25 mass fraction nanoparticles.
50. The method of claim 43, wherein the fluorinated nanoparticles include a fluorinated silsesquioxane.
51. The method of claim 43, wherein forming the fiber includes electrospinning.
52. The method of claim 43, wherein coating the polymer with the fluorinated nanoparticles includes chemical vapor deposition, dip coating, or chemical solution deposition.
53. A method of modifying the wetting properties of a surface, the method comprising:
- exposing the surface to a mixture comprising a plurality of fluorinated nanoparticles and an organic solvent, the fluorinated nanoparticles of the plurality configured to form microstructures on the surface the influence contact angle hysteresis.
54. The method of claim 53, wherein exposing the surface to a liquid composition includes chemical vapor deposition, dip coating, or chemical solution deposition.
55. The method of claim 53, wherein the fluorinated nanoparticles include a fluorinated silsesquioxane.
56. The method of claim 53, wherein a concentration of the plurality of fluorinated nanoparticles in the mixture is less than 0.1 mass fraction nanoparticles.
57. The method of claim 53, wherein a concentration of the plurality of fluorinated nanoparticles in the mixture is greater than 0.1 mass fraction nanoparticles.
58. The method of claim 53, wherein a concentration of the plurality of fluorinated nanoparticles in the mixture is greater than 0.15 mass fraction nanoparticles.
59. The method of claim 53, wherein a concentration of the plurality of fluorinated nanoparticles in the mixture is greater than 0.2 mass fraction nanoparticles.
60. The method of claim 53, wherein a concentration of the plurality of fluorinated nanoparticles in the mixture is greater than 0.25 mass fraction nanoparticles.
61. The method of claim 53, wherein the surface includes a surface of a fabric.
62. The method of claim 61, further comprising:
- stretching the fabric.
Filed: Feb 8, 2019
Publication Date: Dec 5, 2019
Applicants: MASSACHUSETTS INSTITUTE OF TECHNOLOGY (Cambridge, MA), Government of the United States as represented by the Secretary of the Air Force (WRIGHT-PATTERSON AIR FORCE BASE, OH)
Inventors: Anish Tuteja (Cambridge, MA), Wonjae Chol (Cambridge, MA), Gareth H. McKinley (Acton, MA), Robert E. Cohen (Jamaica Plain, MA), Joseph Mark Mabry (Lancaster, CA)
Application Number: 16/271,770