Patent Application
20100034335
This invention relates to articles having enhanced wettability. More particularly, this invention relates to articles having surfaces engineered to promote increased wetting of the surfaces by liquids.
The “liquid wettability”, or “wettability,” of a solid surface is determined by observing the nature of the interaction occurring between the surface and a drop of a given liquid disposed on the surface. A surface having a high wettability for the liquid tends to allow the drop to spread over a relatively wide area of the surface (thereby “wetting” the surface). In the extreme case, the liquid spreads into a film over the surface. On the other hand, where the surface has a low wettability for the liquid, the liquid tends to retain a well-formed, ball-shaped drop. In the extreme case, the liquid forms spherical drops on the surface that easily roll off of the surface at the slightest disturbance.
The extent to which a liquid is able to wet a solid surface plays a significant role in determining how the liquid and solid will interact with each other. By way of example, so-called “hydrophilic” materials have relatively high wettability in the presence of water, resulting in a high degree of “sheeting” of the water over the solid surface. A high degree of wetting results in relatively large areas of liquid-solid contact, and is desirable in applications where a considerable amount of interaction between the two surfaces is beneficial, such as, for example, adhesive and coating applications, certain medical device applications, and applications involving boiling or evaporation heat transfer mechanisms.
In practice, techniques for increasing the wetting of surfaces often involve adding surfactants to the contacting fluid. However, in many applications it will be difficult or nearly impossible to add surfactants to a fluid. Therefore, there is a need to provide articles with durable surfaces having high liquid wettability.
Embodiments of the present invention meet these and other needs. One embodiment is an article having a surface configured for promoting a phase transformation from a liquid phase to a vapor phase. The article comprises an element comprising a surface disposed to be in contact with a liquid to be transformed to a vapor, and the surface comprises a plurality of surface features having a median feature size, a, and a median feature spacing, b, such that the ratio b/a is up to about 8. The surface comprises a material disposed to contact the liquid, and this material has a nominal wettability sufficient to generate a nominal contact angle of up to about 80 degrees with a drop of the liquid. In particular embodiments, the liquid is water.
Another embodiment is a fuel rod for a nuclear power reactor, comprising: a cladding portion surrounding a nuclear fuel material, wherein the cladding portion comprises a surface disposed to be in contact with a liquid flowing or impinging upon the rod, the surface comprising a plurality of surface features having a median feature size, a, and a median feature spacing, b, such that the ratio b/a is up to about 8; wherein the surface comprises a material disposed to contact the liquid, the material having an nominal wettability sufficient to generate a nominal contact angle of up to about 80 degrees with a drop of the liquid.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular feature of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
Referring to the drawings in general and to
One commonly accepted measure of the liquid wettability of a surface is the value of the static contact angle formed between the surface and a tangent to a surface of a droplet of a reference liquid at the point of contact between the surface and the droplet. Low values of the contact angle indicate a high wettability for the reference liquid on surface. The reference liquid may be any liquid of interest. In many applications, the reference liquid is water. In other applications, the reference liquid is a liquid that contains at least one hydrocarbon, such as, for example, oil, petroleum, gasoline, an organic solvent, and the like. Because wettability depends in part upon the surface tension of the reference liquid, a given surface may have a different wettability (and hence form a different contact angle) for different liquids.
Surface 110 comprises a plurality of surface features 120. The size, shape, and orientation of features 120 have a strong effect on the wettability of surface 110, and in embodiments of the present invention these parameters are selected such that the surface 110 has a high liquid wettability. The selection is based upon the physics underlying the interaction of liquids and rough solid surfaces.
The size of features 120 can be characterized in a number of ways. In some embodiments, as shown in
Numerous varieties of feature shapes are suitable for use as features 120. In some embodiments, at least a subset of the features 120 has a shape selected from the group consisting of a cube, a rectangular prism, a cone, a cylinder, a pyramid, a trapezoidal prism, and a hemisphere or other spherical portion. These shapes are suitable whether the feature is a protrusion 220, such as a pedestal, or a cavity 200, such as a groove or a pore. As an example, in particular embodiments, at least a subset of the features comprises nanowires, which are structures that have a lateral size constrained to tens of nanometers or less and an unconstrained longitudinal size. Methods for making nanowires of various materials are well known in the art, and include, for example, chemical vapor deposition onto a substrate. Nanowires may be grown directly on article 100 or may be grown on a separate substrate, removed from that substrate (for example, by use of ultrasonication), placed in a solvent, and transferred onto article 100 by disposing the solvent onto the article surface and allowing the solvent to dry.
Feature orientation is a further design consideration in the engineering of surface wettability in accordance with embodiments of the present invention. One significant aspect of feature orientation is the spacing of features. Referring to
In some embodiments, all of the features 120 in the plurality are disposed in a nonrandom distribution. In some cases features 120 have substantially the same respective values for h, a, and/or b (“an ordered array”), though this is not a general requirement. For example, the plurality of features 120 may be a collection of features, such as nanowires, for instance, exhibiting a random distribution of size, shape, and/or orientation. In certain embodiments, moreover, the plurality of features is characterized by a multi-modal distribution (e.g., a bimodal or trimodal distribution) in h, a, b, or any combination thereof. Such distributions may advantageously provide enhanced wettability in environments where a range of drop sizes is encountered. Estimation of the effects of h, a, and b on wettability are thus best performed by taking into account the distributive nature of these parameters. Techniques, such as Monte Carlo simulation, for performing analyses using variables representing probability distributions are well known in the art. Such techniques may be applied in designing features 120 for use in articles of the present invention.
Surface 110 is made of a material disposed to contact the liquid, and this material has a nominal wettability sufficient to generate a nominal contact angle of up to about 80 degrees with a drop of the liquid. As used herein, a “nominal contact angle” means the static contact angle measured where a drop of a reference liquid is disposed on a flat, smooth surface of the material. This nominal contact angle is a measurement of the “nominal wettability” of the material, and may be contrasted with the “effective wettability” of the surface, which is the wettability measured for surface 110 after the surface 110 has been provided with texture, such as features 120 as described above. Generally, the use of a material having a lower nominal contact angle results in a lower effective contact angle for a given disposition of surface features 120. In certain embodiments, the nominal contact angle is up to about 70 degrees, and in particular embodiments, the nominal contact angle is up to about 60 degrees.
A variety of materials meet these requirements for comparatively high nominal wettability. In some embodiments, the material comprises a metal, such as a metal comprising an element selected from the group consisting of iron, titanium, copper, zirconium, aluminum, and nickel. In certain embodiments the material is essentially completely metallic. In other embodiments, the material comprises a ceramic, such as an oxide typified by titanium oxide, silicon dioxide, and zirconium oxide. The material may be present as a coating disposed on article 100, or features 120 may be made of the material. Other mildly to very hydrophilic materials, such as, for example, certain polymeric materials, may be used in embodiments of the present invention. However, the low thermal conductivity of most polymers may make them unattractive for use in certain applications, and so in some embodiments the surface 110 is essentially free of polymeric material.
The present inventors have found that specific ranges and combinations of the surface parameters described above provide a regime in which the effective wettability of surface 110 may be driven to values significantly above that of the nominal wettability of the material used to form surface 110. For example, in certain embodiments, the surface has an effective wettability sufficient to generate an effective contact angle of up to about 15 degrees with a drop of the reference liquid. In some cases it has been shown that the effective contact angle may be reduced to near zero.
In embodiments of the present invention, surface 110 comprises a plurality of surface features 120 having a median feature size, a, and a median feature spacing, b, such that the ratio b/a is up to about 8. In particular embodiments, b/a is up to about 6, and in certain embodiments is up to about 3. A lower b/a indicates more closely spaced features, and as these features are more closely spaced, the surface area of surface 110 increases, providing more contact area for the liquid. However, in some situations there is a practical lower limit as to how closely features may be spaced, due in part to limitations in fabrication methods. Moreover, in certain applications, spacing features 120 too closely together may cause a situation in which droplets of liquid are suspended between features, without wetting the areas between features 120. Such a condition would reduce the effective wetting area. Accordingly, in some embodiments b/a is at least about 0.5, and in some cases at least about 2. In particular embodiments, b/a is in the range from about 0.5 to about 6, and in certain embodiments b/a is in the range from about 2 to about 4, though any range within the endpoint parameters described herein may be suitable in particular applications.
The aspect ratio (h/a) of features 120 also plays a role in determining the effective wetting behavior of surface 110. Generally, high aspect ratios, such as at least about 1 and, in some situations, at least about 4, are desirable because surface area increases as aspect ratio increases. In some high temperature heat transfer application conditions, such as, for instance, the type experienced by nuclear fuel rods, high aspect ratio (h/a at least about 4) features are desirably sized and spaced apart to give b/a in the range from about 0.5 to about 6. This combination of parameter values provides a surface that maximizes heat transfer by impinging droplets or by a flowing liquid film.
As stated above, in some embodiments at least a subset of the plurality of features is a plurality of cavities 200, such as, for example, pores, disposed in the article 100. By analyzing the interaction between liquids and surfaces having cavities, the present inventors have discovered certain texture parameter combinations that result in enhanced wettability of the surface 110.
In some embodiments, the cavities include a plurality of grooves.
The trend described above for cavities, in which the range of b/a which gives very low contact angles expands as the aspect ratio of surface features increases, is also found to hold true where the features 120 are protrusions. This trend is illustrated in
Features 120 can be fabricated and provided to article 100 by a number of methods. In some embodiments, features 120 are fabricated directly on surface 110. In other embodiments, features 120 are fabricated separately and then disposed onto article 100. Disposition of features 120 onto article 100 can be done by individually attaching features 120, or the features may be disposed on a sheet, foil or other suitable medium that is then attached to the article 100. Attachment in either case may be accomplished through any appropriate method, such as, but not limited to, welding, brazing, mechanically attaching, or adhesively attaching via epoxy or other adhesive.
The disposition of features 120 may be accomplished by disposing material onto the surface of the article, by removing material from the surface, or a combination of both depositing and removing. Many methods are known in the art for adding or removing material from a surface. For example, simple roughening of the surface by mechanical operations such as grinding, grit blasting, or shot peening may be suitable if appropriate media/tooling and surface materials are selected. Such operations will generally result in a distribution of randomly oriented features on the surface, while the size-scale of the features will depend significantly on the size of the media and/or tooling used for the material removal operation. General roughening of surfaces to promote enhanced wetting has been described previously. See, for example, U.S. patent application Ser. No. 11/206,565. However, certain embodiments of the present invention require control over specific parameters such as relative spacing and aspect ratio of features 120 to provide improved wetting performance. Many of the parameter ranges and combinations thereof are very difficult or impossible to achieve via the use of traditionally described roughening processes such as grit blasting, for example.
Lithographic methods are commonly used to create surface features on etchable surfaces, including metal surfaces. Ordered arrays of features can be provided by these methods; the lower limit of feature size available through these techniques is limited by the resolution of the particular lithographic process being applied. Lithography and other etching methods are generally not well-suited to the formation of high aspect ratio features on some metal surfaces, however, due to the tendency to “undercut,” i.e., to etch laterally as well as vertically.
Electroplating methods are also commonly used to add features to surfaces. An electrically conductive surface may be masked in a patterned array to expose areas upon which features are to be disposed, and the features may be built up on these exposed regions by plating. This method allows the creation of features having higher aspect ratios than those commonly achieved by etching techniques. In particular embodiments, the masking is accomplished by the use of an anodized aluminum oxide (AAO) template having a well-controlled pore size. Material is electroplated onto the substrate through the pores, and the AAO template is then selectively removed; this process is commonly applied in the art to make high aspect ratio features such as nanorods. Nanorods of metal and metal oxides may be deposited using commonly known processing, and these materials may be further processed (by carburization, for example) to form various ceramic materials such as carbides. As will be described in more detail below, coatings or other surface modification techniques may be applied to the features to provide even better wettability properties.
Micromachining techniques, such as laser micromachining (commonly used for silicon and stainless steels, for example) and etching techniques (for example, those commonly used for silicon) are suitable methods as well. Such techniques may be used to form cavities (as in laser drilling) as well as protruding features. Where the plurality of features 120 includes cavities 200, in some embodiments article 100 comprises a porous material, such as, for example, an anodized metal oxide. Anodized aluminum oxide is a particular example of a porous material that may be suitable for use in some embodiments. Anodized aluminum oxide typically comprises columnar pores, and pore parameters such as diameter and aspect ratio may be closely controlled by the anodization process, using process controls that are well known to the art to convert a layer of metal into a layer of porous metal oxide.
In short, any of a number of deposition processes or material removal processes commonly known in the art may be used to provide features to a surface. As described above, the features may be applied directly onto article 100, or applied to a substrate that is then attached to article 100.
Embodiments of the present invention may be particularly of use in heat transfer applications, particularly in those applications involving evaporative heat transfer or boiling heat transfer. Although the use of textured surfaces in boiling heat transfer applications has been described previously, the function of the texture in these previously described applications generally has been to provide nucleation sites for bubbles to form during the boiling process. In contrast, the function of the textured surfaces described herein is to enhance the wetting of the surface by the liquid. As a result of this difference in function, the size, shape, and orientation of the features 120 on the surface 110 varies from that described for bubble nucleation enhancement. For example, in U.S. Pat. No. 4,312,012, the surface is designed to discourage complete wetting by the boiling fluid so that bubble nucleation may be enhanced. Furthermore, in U.S. Pat. No. 4,767,497, submicron-sized features (pits) are described as being undesirable due to bubble nucleation and agglomeration concerns. In embodiments of the present invention, the use of submicron-sized features 120 may be suitable for many applications so long as the required ratios for b/a and h/a are met. In other embodiments, at least one of a and b is less than about 100 micrometers, such as less than about 50 micrometers.
Accordingly, as shown in
One particular embodiment of the present invention, as illustrated in
According to embodiments of the present invention, rod 400 comprises a cladding portion 420 surrounding fuel pellets 430. In some embodiments, cladding is a metal, such as an alloy containing zirconium. Surface 110 as described previously is disposed on cladding portion 420 so that contact between rod 400 and a liquid flowing or impinging upon rod 400 occurs at surface 110. In one embodiment surface 110 comprising surface features 120 (
The parameters characterizing surface 110 of rod 400 coincide with those described previously. In some embodiments, a is in the range from about 1 micrometer to about 25 micrometers, such as from about 5 micrometers to about 15 micrometers. In some embodiments, b is in the range from about 5 micrometers to about 75 micrometers, such as from about 15 micrometers to about 45 micrometers. In some embodiments, h is in the range from about 10 micrometers to about 100 micrometers. These values may give desirable levels of performance given the particular temperature and flow conditions encountered in the nuclear power reactor environment. Moreover, the present inventors have found, surprisingly, that high aspect ratio features, that is, those with h/a>1 and in some cases h/a>4, show measurable increases in Leidenfrost temperature, while low aspect ratio features (h/a below about 1) do not show this effect.
The following examples are set forth to further illustrate embodiments of the present invention, and are not meant to limit the scope of embodiments of the invention in any way.
To further illustrate embodiments of the present invention, silicon surfaces were given textures made of matrices of square pedestals 3 micrometers in width, having various respective b/a values and having h/a of about 3. The static contact angle of these surfaces with water was measured to determine the effect of relative spacing on the contact angle. The contact angle measured on a smooth (untextured) silicon surface was found to be about 45 degrees. Measurements on silicon wafers with square pedestals show that for a certain range of b/a the contact angle is as low as zero degrees. These results are shown in
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations, equivalents, or improvements therein may be made by those skilled in the art, and are still within the scope of the invention as defined in the appended claims.
