Abstract:
An article includes a body having at least one viscous drag-reducing surface defining a multiplicity of macro-scale depressions separated and enframed by ridges, at least a portion of the surface being phobic to a preselected liquid so that gas bubbles may be trapped in the macro-scale depressions in order to reduce viscous drag between the article and the liquid.

Description:
[0001]     The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0002]     Specifically referenced and incorporated herein by reference are: U.S. patent application Ser. No. 10/900,248 filed on Jul. 27, 2004 by Brian R. D&#39;Urso and John T. Simpson entitled “Composite, Ordered Material Having Sharp Surface Features”; and U.S. patent application Ser. No. 10/900,249 filed on Jul. 27, 2004 by Brian R. D&#39;Urso and John T. Simpson entitled “Composite, Nano-Structured, Super-Hydrophobic Material”.  
       FIELD OF THE INVENTION  
       [0003]     The present invention relates to materials having surfaces that are designed to reduce friction drag at a solid-liquid interface, and more particularly to such materials having appropriately phobic microscale characteristics in combination with macroscale depressions on a surface thereof.  
       BACKGROUND OF THE INVENTION  
       [0004]     A water-repellant but non-super-hydrophobic surface (by the generally accepted contact angle &gt;150 degrees definition) was shown to decrease drag in a pipe (K. Watanabe et al, “Drag reduction of Newtonian fluid in a circular pipe with a highly water-repellant wall”, J. Fluid Mechanics, vol. 381, p. 225, 1999).  
         [0005]     There are few currently available materials which efficiently provide significant viscous drag reduction over large areas for applications such as watercraft and ducts. Specifically referenced and incorporated herein by reference is U.S. Patent Application Publication No. US 2005/0061221 A1, Ser. No. 10/493,054, PCT filed on Oct. 29, 2002 by Mariusz Paszkowski entitled “Superhydrophobic Coating”.  
         [0006]     Some super-hydrophobic materials may decrease drag on a microscopic length scale, which could be useful for micro-fluidics applications, but none are expected to give significant drag reduction on a macroscopic scale, as would be needed for drag reduction for watercraft (T. Min et al, “Effects of hydrophobic surface on skin-friction drag”, Physics of Fluids, vol. 16, no. 7, 2004).  
         [0007]     Decreased drag in watercraft will enable faster, more fuel-efficient, and/or decreased signature watercraft. Such benefits will be particularly useful for defense and commercial applications. Even a reduction of a few percent in drag can result in significant speed increase and/or fuel savings for sundry watercraft and water ducts. The present invention is directed toward such needs.  
       OBJECTS OF THE INVENTION  
       [0008]     Accordingly, objects of the present invention include provision of friction drag reducing surface features suitable for watercraft applications, ducts, and other applications where friction between a solid material and a liquid material need to be reduced. Further and other objects of the present invention will become apparent from the description contained herein.  
       SUMMARY OF THE INVENTION  
       [0009]     In accordance with one aspect of the present invention, the foregoing and other objects are achieved by an article that includes a body having at least one viscous drag-reducing surface defining a multiplicity of macro-scale depressions separated and enframed by ridges, at least a portion of the surface being phobic to a preselected liquid so that gas bubbles may be trapped in the macro-scale depressions in order to reduce viscous drag between the article and the liquid.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a photomicrograph of a microscale, disordered hydrophobic base material in accordance with some embodiments of the present invention.  
         [0011]      FIG. 2  is a photomicrograph of a microscale, ordered hydrophobic base material in accordance with some embodiments of the present invention.  
         [0012]      FIG. 3  is a schematic, oblique, isometric, cutaway view of a material having super-hydrophobic macroscale depressions in a surface thereof in accordance with some passive embodiments of the present invention.  
         [0013]      FIG. 4  is a schematic top view of the material shown in  FIG. 3 .  
         [0014]      FIG. 5  is a schematic lateral view through section A-A of the material shown in  FIG. 4 .  
         [0015]      FIG. 6  is a schematic lateral view through section B-B of the material shown in  FIG. 4 .  
         [0016]      FIG. 7  is a schematic, oblique, isometric, cutaway view of a material having super-hydrophobic depressions in a surface thereof in accordance with some active embodiments of the present invention.  
         [0017]      FIG. 8  is a schematic top view of the material shown in  FIG. 7 .  
         [0018]      FIG. 9  is a schematic lateral view through section A-A of the material shown in  FIG. 8 .  
         [0019]      FIG. 10  is a schematic lateral view through section B-B of the material shown in  FIG. 8 .  
     
    
       [0020]     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The invention preferably uses a material having a multi-scale surface feature to trap a gas layer at the surface of the material to significantly reduce the drag on a macroscopic scale. A multiscale surface feature includes at least one microscale surface feature and at least one macroscale surface feature. The macroscale feature is a depression on the surface for trapping a discrete gas bubble, and the microscale feature renders the surface hydrophobic for holding the gas bubble in the depression. An appropriately phobic surface (phobic to a particular liquid material in which the surface is immersed) which is not necessarily characterized by a microscale surface feature is also contemplated to fall within the scope of the present invention.  
         [0022]     Embodiments of the present invention involve a multiscale material of a highly hydrophobic base material with surface depressions enframed (i.e., surrounded) by sharp ridges for drag reduction. The surface depressions trap air or other gas, forming, in effect, a layer of the gas over much of the surface of the material which decreases viscous or friction drag on the surface moving through a liquid (typically water), or on the liquid traveling across the surface (as in a pipe). The gas is held in the depressions by the combination of the hydrophobic material and surface depressions enframed by sharp ridges. The sharp ridges which enframe the depressions prevent the gas layer from being forced off by pressure gradients in the liquid. In passive embodiments of the material, more gas may be picked up from the surrounding liquid. In active embodiments of the material, the depressions are filled with gas and pressurized from behind the surface through holes leading into the depressions.  
         [0023]     Microscale Feature  
         [0024]     A microscale surface feature, as used herein, has general dimensions as defined in Table 1.  
                                                                                         TABLE 1                                       Dimension                Length   Width   Depth            Range   Min.   Max.   Min.   Max.   Min.   Max.               Operable    1 nm    1 μm    1 nm    1 μm    1 nm   10 μm       Preferable   10 nm   500 nm   10 nm   500 nm   10 nm   10 μm       More   20 nm   100 nm   20 nm   100 nm   20 nm   500 μm        Preferable            Most   50 nm   50 nm   250 nm       Preferable                  
 
         [0025]     The base material in which there are depressions has hydrophobic or preferably super-hydrophobic microscale features in applications where water is the liquid in which the surface is immersed. A super-hydrophobic material may comprise a hydrophobic material or hydrophobic coated material with micrometer-scale or nanometer-scale features.  
         [0026]     Referring to  FIG. 1 , particularly easily adapted super-hydrophobic materials include disordered super-hydrophobic materials produced by etching a composite produced by spinodal decomposition of a parent material. The materials are described in detail in U.S. patent application Ser. No. 10/900,249 filed on Jul. 27, 2004 by Brian R. D&#39;Urso and John T. Simpson entitled “Composite, Nano-Structured, Super-Hydrophobic Material”, the entire disclosure of which incorporated herein by reference.  
         [0027]     Referring to  FIG. 2 , another easily adapted material is described in detail in U.S. patent application Ser. No. 10/900,248 filed on Jul. 27, 2004 by Brian R. D&#39;Urso and John T. Simpson entitled “Composite, Ordered Material Having Sharp Surface Features”, the entire disclosure of which incorporated herein by reference.  
         [0028]     A microscale surface can include nanoscale features and can be multiscale, as described in the above referenced and incorporated patent applications.  
         [0029]     The skilled artisan will recognize that, in applications where a liquid other than water is the liquid in which the surface is immersed, a material that has appropriate phobic characteristics is used. All such applications are considered to fall within the scope of the present invention.  
         [0030]     Macroscale Feature  
         [0031]     A macroscale feature, as used herein, has general dimensions as defined in Table 2.  
                                                                                         TABLE 2                                       Dimension                Length   Width   Depth            Range   Min.   Max.   Min.   Max.   Min.   Max.               Operable    1 μm   100 cm    1 μm   10 cm    1 μm   10 cm       Preferable    10 μm    10 cm    10 μm    1 cm    10 μm    1 cm       More   100 μm    1 cm   100 μm    5 mm   100 μm    5 mm       Preferable            Most   10 mm   1 mm   1 mm       Preferable                  
 
         [0032]     Referring to  FIGS. 3-6 , a base material  10  defines macroscale depressions  12 , which may be in the form of an array that is open to the surface  20 . The exact shape of the depressions  12  is not critical; they can be rectangular, pentagonal, hexagonal, rounded, elliptical, irregular, or any shape that is convenient. The depressions  12  are separated and enframed by ridges  14 . It is preferable, but not critical, that the depressions  12  are elongated in the direction of the liquid flow, as evidenced by the longer lengths given in Table 1. Gas bubbles  16  are shown trapped in the depressions  12 . The gas bubbles  16  provide minimal contact of the surface  20  with liquid  22 .  
         [0033]     Ridges  
         [0034]     The ridges  14  protrude from the surface  20  to separate and enframe (define) each of the discrete depressions  12 . It is preferable that the ridges  14  are sharp as shown to minimize the contact between the surface  20  and the liquid  22 . The ridges  14  can be flattened, rounded, or otherwise shaped (not illustrated).  
         [0035]     In cases where the ridges  14  are wedge-shaped as shown, the ridge half angle Φ should preferably satisfy the relationship: 
 
Φ&lt;Θ−90°
 
         [0036]     In the above relationship, Θ is the contact angle between the hydrophobic material and the liquid. The ridges  14  may also comprise narrow protrusive features with vertical sidewalls (in which case Φ=0 degrees). For the purposes of describing the present invention, a ridge  14  is defined as including up to the top 50%, preferably up to the top 20%, more preferably up to the top 10%, most preferably up to the top 5%, of the distance from the surface  20  to the bottom of a depression  12 . The top of a ridge is the surface  20 .  
       Passive Material Embodiments  
       [0037]     In passive embodiments of the invention, such as that illustrated in  FIGS. 3-6 , for example, the ridges  14  inhibit the flow of gas between the depressions  12 . This inhibits the movement of the gas  16  across the surface  20 , which prevents a pressure gradient in the liquid  22  from forcing the gas  16  off of the surface  20 . In order to inhibit the flow of gas  16  between depressions  12 , the microscale surface features of the hydrophobic base material  10  may be absent or minimal on the tops of the ridges  14 . This minimizes the thickness of the gas  16  layer on top of the ridge  14  which could otherwise allow transfer of gas  16  between depressions  12 .  
         [0038]     If gas is lost from the surface, it may be replenished by picking up gas from the surrounding liquid. Gas in the surrounding liquid may be naturally present or may be intentionally forced into the surrounding liquid.  
       EXAMPLE I  
       [0039]     In accordance with the present invention, sodium borosilicate glass comprising 68.5 molecular % SiO 2 , 23.6 molecular % B 2 O 3 , and 7.9 molecular % Na 2 O is heat treated at 700° C. for 1 hour, resulting in phase separation via spinodal decomposition. The glass is cooled, and the surface is machined with a diamond cutter to produce macroscale depressions and ridges. The surface of the material is subsequently etched with an aqueous solution of HF, etching back the recessive phase and revealing the protrusive phase to produce microscale surface features. The microscale features are mechanically removed from the ridge tops by lapping. The surface is then coated with a hydrophobic self-assembled monolayer by immersing the material in a solution of (tridecafluoro-1,1,2,2 tetrahydrooctyl) trichlorosilane in hexanes.  
       EXAMPLE II  
       [0040]     In accordance with the present invention, differentially etchable glasses are drawn into a bundled array. The bundle is cut and the surface is machined with a diamond cutter to produce macroscale depressions and ridges. The surface of the material is subsequently etched with an aqueous solution of HF, etching back the recessive phase and revealing the protrusive phase to produce microscale surface features. The microscale features are mechanically removed from the ridge tops by lapping. The surface is then coated with a hydrophobic self-assembled monolayer by immersing the material in a solution of (tridecafluoro-1,1,2,2 tetrahydrooctyl) trichlorosilane in hexanes.  
       EXAMPLE III  
       [0041]     In accordance with the present invention, the material described in Example II is used as a mold for casting a hydrophobic polymer. A photocurable perfluoropolyether (See J. Rolland et al, J. Am. Chem. Soc. 126, pp. 2322-2323, 2004) is poured over the material in Example II and cured by exposure to ultraviolet light. The polymer is peeled off and used as a mold for casting a curing a second (possibly the same composition) hydrophobic polymer. The second casting, curing, and peeling step creates a duplicate of the original glass structure in a hydrophobic polymer, creating a drag reducing polymer structure.  
       Active Material Embodiments  
       [0042]     In active embodiments of the invention, such as that illustrated in  FIGS. 7-10 , for example, the gas  16  is replenished by forcing gas  16  through holes  24  which penetrate into the depressions  12  through the base material  10 . The gas  16  on the surface  20  can thus be actively maintained and pressurized. Since gas  16  may be constantly added to the surface  20 , it may not be as important to prevent the movement of gas among depressions  12  across the surface  20  as in passive material embodiments of the invention described above.  
         [0043]     The invention is applicable to all types of sundry military, industrial, sport, and consumer watercraft applications such as, for example, cargo and passenger ships, boats, military ships, submarines and weapons, sailboats, motorboats, rowboats, personal watercraft, canoes, kayaks, surf boards, and the like. The invention is also applicable to non-watercraft applications such as, for example, ducts, pipes, swim suits, swim fins, microfluidic devices, and the like.  
         [0044]     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the inventions defined by the appended claims.