US20140238645A1

US20140238645A1 - Hierarchically structural and biphillic surface energy designs for enhanced condensation heat transfer

Info

Publication number: US20140238645A1
Application number: US13/776,332
Authority: US
Inventors: Ryan M. Enright
Original assignee: Alcatel Lucent Ireland Ltd
Current assignee: Alcatel Lucent SAS
Priority date: 2013-02-25
Filing date: 2013-02-25
Publication date: 2014-08-28
Also published as: WO2014128563A2; WO2014128563A3

Abstract

An apparatus comprising a base layer, a distribution of separated micro-nucleation sites thereon. The apparatus also includes a distribution nanostructures located on the base layer, each of the micro-nucleation sites being adjacent to some of the nanostructures. Each of the micro-nucleation sites has a hydrophilic surface and the distribution of nanostructures form a superhydrophobic surface.

Description

TECHNICAL FIELD

The invention relates to in general, heat transfer devices, and methods for manufacturing the same.

BACKGROUND

This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
Condensation is an important process in a number of two-phase heat transfer apparatuses implemented for thermal management. Improving the efficiency of such condensation heat transfer processes has the potential to enable size reductions of heat transfer apparatuses while still achieving the same overall heat transfer performance.

SUMMARY

One embodiment is an apparatus comprising a base layer having a distribution of separated micro-nucleation sites thereon. The apparatus also includes a distribution of nanostructures located on the base layer, each of the micro-nucleation sites being adjacent to some of the nanostructures; and wherein each of the micro-nucleation sites has a hydrophilic surface and the distribution of nanostructures form a superhydrophobic surface.
In any of the above embodiments of the apparatus, the micro-nucleation sites and the nanostructures are located on a planar surface of the base layer. In some embodiments tops of the nanostructures are substantially coplanar with the hydrophilic surfaces of the micro-nucleation sites. In some embodiments, some of the nanostructures surround each of the micro-nucleation sites. In some embodiments, the hydrophilic surfaces of each of the micro-nucleation sites are substantially coplanar with bottoms of the nanostructures that contact the base layer. In some embodiments, the hydrophilic surface of each of the micro-nucleation sites has a smooth surface. In some embodiments, an area occupied by the hydrophilic surfaces of each of the micro-nucleation sites is in a range of 1 to 100 microns². In some embodiments, a separation distance between adjacent ones of the micro-nucleation sites is equal to or less than about 10 microns. In some embodiments, the nanostructures are located on a region of the base layer having a lower thermal conductivity than a region of the base layer having the micro-nucleation sites thereon. In some embodiments, wherein tops of the nanostructures include a reentrant angled ledge. In some embodiments, an area of the base layer having the nanostructures located thereon has a surface roughness where the following condition applies when a liquid droplet rests on the surface: −1/r*cos θa<1, wherein r is the surface roughness of the surfaces having the distribution of nanostructures located thereon and ea is an intrinsic advancing contact angle of the liquid droplet. In some embodiments, a distance between adjacent ones of the nanostructures is greater than a critical condensation radius for a nucleating liquid droplet on the surface.
One embodiment is a system. The system comprises heat generating equipment and a heat transfer apparatus configured to remove heat generated by the electronic equipment. The apparatus includes a base layer having a distribution of separated micro-nucleation sites thereon. The apparatus includes a distribution of nanostructures located on the base layer, each of the micro-nucleation sites being adjacent to some of the nanostructures. Each of the micro-nucleation sites has a hydrophilic surface and the distribution of nanostructures form a superhydrophobic surface.
In some embodiments of the system, the micro-nucleation sites are located on the surface of a condenser of the apparatus. In some embodiments, the condenser is part of a heat pipe or a vapor chamber.
Another embodiment is a method. The method comprises manufacturing a condenser, including: providing a base layer and forming a distribution of separated micro-nucleation sites on the base layer, wherein each of the micro-nucleation sites has a hydrophilic surface. Manufacturing the condenser also includes forming a distribution nanostructures on the base layer, wherein each of the micro-nucleation sites being adjacent to some of the nanostructures.
In any of the above embodiments of the method, forming the micro-nucleation sites includes forming a mask layer over the base layer and patterning the mask layer such that mask portions remain on the base layer in locations corresponding to the micro-nucleation sites. In some embodiments, forming the nanostructures on the base layer includes modifying portions of the base layer not covered by mask portions located on the base layer that correspond to locations of the micro-nucleation sites. In some such embodiments, forming the micro-nucleation sites includes removing the mask portions to uncover the micro-nucleation sites such that tops of the nanostructures are substantially coplanar with the hydrophillic surfaces of the micro-nucleation sites. In some embodiments, forming the nanostructures on the base layer includes forming a mask layer over the base layer, patterning the mask layer such that mask portions remain on the base layer corresponding to locations of the micro-nucleation sites, forming a material layer on the base layer and the mask portions and modifying the material layer on the base layer and the mask portions to form the nanostructures.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the disclosure are best understood from the following detailed description, when read with the accompanying FIGUREs. Some features in the figures may be described as, for example, “top,” “bottom,” “vertical” or “lateral” for convenience in referring to those features. Such descriptions do not limit the orientation of such features with respect to the natural horizon or gravity. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 presents a cross-sectional view of an apparatus;

FIG. 2 presents a plan view of the apparatus shown in FIG. 1 along view line 2-2 in FIG. 1;

FIG. 3A presents a cross-sectional view of an alternative apparatus, analogous to the view presented in FIG. 1;

FIG. 3B presents a cross-sectional view of an another alternative apparatus, analogous to the view presented in FIG. 1;

FIG. 4 presents a detailed cross-sectional view of one embodiment of nanostructures;

FIG. 5 presents a flow diagram of an example method of manufacturing a heat transfer apparatus, such as any of the apparatuses described in the context of FIGS. 1-4;

FIG. 6A presents a cross-sectional view of the apparatus shown in FIG. 1 at an intermediate stage of fabrication;

FIG. 6B presents a cross-sectional view of the apparatus shown in FIG. 1 at another intermediate stage of fabrication;

FIG. 6C presents a cross-sectional view of the apparatus shown in FIG. 1 at another intermediate stage of fabrication;

FIG. 7A presents a cross-sectional view of the apparatus shown in FIG. 3A an intermediate stage of fabrication;

FIG. 7B presents a cross-sectional view of the apparatus shown in FIG. 3A at another intermediate stage of fabrication;

FIG. 7C presents a cross-sectional view of the apparatus shown in FIG. 3A at another intermediate stage of fabrication;

FIG. 7D presents a cross-sectional view of the apparatus shown in FIG. 3A at another intermediate stage of fabrication;

FIG. 8 presents a block diagram of a system.

In the Figures and text, similar or like reference symbols indicate elements with similar or the same functions and/or structures.
In the Figures, the relative dimensions of some features may be exaggerated to more clearly illustrate one or more of the structures or features therein.
Herein, various embodiments are described more fully by the Figures and the Detailed Description. Nevertheless, the inventions may be embodied in various forms and are not limited to the embodiments described in the Figures and Detailed Description of Illustrative Embodiments.

DETAILED DESCRIPTION

The description and drawings merely illustrate the principles of the inventions. It will thus be appreciated that a person of ordinary skill in the relevant arts will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the inventions and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the inventions and concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the inventions, as well as specific examples thereof, are intended to encompass equivalents thereof. Additionally, the term, “or,” as used herein, refers to a non-exclusive or, unless otherwise indicated. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
The use of non-wetting surfaces can enhance heat transfer coefficients, in comparison to smooth surfaces. Surprisingly, when the non-wetting surface is a provided in the form of a surface covered with nanostructures, similar enhanced heat transfer coefficients have not been fully realized. In the earlier stages of a droplet's growth in-between and on the surfaces of nanostructures, growth is impeded by the droplet's adhesion (or pinning) to the nanostructures. Additionally, thermal resistance at the early stage of droplet growth is associated with the large curvature of small droplets. This leads to a lowering of the local saturation pressure of the water vapor, reducing the driving potential for water vapor condensing into liquid water. Impeded growth of the droplet presents sources of thermal resistance which decreases the efficiency of heat transfer.
It is believed that these sources of thermal resistance can be mitigated by providing a heat transfer apparatus with condensation surfaces having separate hierarchical structures thereon, i.e., separate structures with two different length scales, and, having biphilic surface energies, i.e., two different surface energies. The two different length scales of the hierarchically structures are micron-scaled structural features (“microstructures”) and nanometer-scaled structural features (“nanostructures”).
One embodiment is an apparatus (e.g., a heat transfer device). FIG. 1 presents a cross-sectional view of an embodiment of the apparatus 100. FIG. 2 presents a plan view, at a lower magnification scale, of the apparatus 100 shown in FIG. 1 along view line 2-2 in FIG. 1.
As illustrated in FIG. 1, in some cases the apparatus 100 includes or is a condenser 105. The condenser 105 can be part of a variety of different two-phase heat transfer apparatuses such as, but not limited to, heat pipes, vapor chambers, looped heat pipes or two-phase forced convection flow loops. For example, in some cases, the condenser 105 can be a portion of the heat transfer apparatus 100 configured as a heat pipe which further includes an evaporator portion 107. In still other embodiments the condenser can be used in heat transfer apparatuses such as compact condensers for electronics thermal management, e.g., in electronics thermal management, e.g., in telecommunications and data centers, industrial condensation heat exchangers, evaporator coils, dehumidifying coils, or water harvesting devices.
The apparatus 100, and in some cases the condenser 105, includes a base layer 110 having one or more micro-nucleation sites 115 thereon, wherein each of the micro-nucleation sites 115 has a smooth surface 117. The apparatus 100 also includes nanostructures 120 located on the base layer 110 and around the micro-nucleation site 115. Tops 125 of the nanostructures 120 (e.g., a group 122 of tops 125) has a lower surface energy than a surface energy of the smooth surface 120.
It is believed that the growth of droplets is promoted by providing the micro-nucleation sites 115 with a smooth, high energy surface 120. Droplet growth on such micro-nucleation sites 115 is promoted because the energy barriers associated with vapor condensing to a liquid state are low such that the nucleation rate is large. This is in contrast to the surrounding nanostructures that have a low intrinsic surface energy. Droplet growth from the high-surface-energy sites also significantly reduces the curvature resistance as compared to droplets growing among nanostructures or low-surface-energy regions in general.
As droplet grows on a micro-nucleation site 115 it will reach a large enough size to contact the nanostructures 120 around the site 115. Because the nanostructure tops 125 (e.g., groups 122 of tops 125) surrounding the micro-nucleation sites 115 has a relatively lower surface energy than the smooth surface 117 of the site 115, the growing droplet is prompted to reach a Cassie state on the tops 125. The low adhesion energy of the droplet on the tops 125 of the nanostructures 120 in turn facilitates the droplet to spontaneously leave, or jump off, e.g., when they coalesce with droplets formed on neighboring micro-nucleation sites. Furthermore, the spacing between the micro-nucleation sites is preferably above a minimum threshold to promote such droplet jumping. The spacing is determined by the relationship between the size of the micro-nucleation site 115 r and the spacing between the micro-nucleation sites 105.
As used herein, the term micro-nucleation site 115, refers to a structure that has at least linear one-dimension 127 adjacent to the base layer 110 (e.g., a base width or depth) that extends a distance across the micro-nucleation site 115 in a range of 1 to 1000 microns.
As used herein, the term nanostructure 120, refers to a structure that has at least one linear dimension (e.g. height, width, or depth) that extends a distance from one side to an opposing side (e.g., opposing lateral sides 130, 132, or, top 125 and bottom side 135) of the nanostructure 120 in a range from 1 to 1000 nanometers. Additionally, the one linear dimension of the nanostructure 120 is at least 10 times smaller than the one dimension 127 of the microstructure 115. As a non-limiting example, when the one dimension 127 of the microstructure 115 equals 1 micron, then the one dimension of the nanostructure 120 can be up to 100 nanometers. Consequently, in this example, the at least one linear dimension of the nanostructure 120 (e.g., height, width, or depth), can be in a range of 1 to 100 nanometers. As another non-limiting example, when the one dimension 127 of the microstructure 115 equals 100 microns, then the one dimension of the nanostructure 120 can be up to 1000 nanometers. Consequently, in this example, the one dimension of the nanostructure 120 (e.g., height, width, or depth), can be in a range of 1 to 1000 nanometers.
As used herein, the term smooth surface 117 is characterized as satisfying the following condition 1>cos θa>−0.65, where ea is the intrinsic advancing contact angle 140 of a liquid droplet 145 located on the micro-nucleation site. As illustrated for the embodiment shown in FIG. 1, the smooth surface 117 is free of nanostructures 120, that is, there are no fabricated nanostructures 120 on the smooth surface 117.
The surface energies of the smooth surface 117 and the group 122 of tops 125, are inversely proportional to the intrinsic advancing contact angle 140, ea, of a droplet on the surface of interest. That is, the lower the angle 140 of a liquid droplet on a surface, the higher the surface energy of that surface, and, the higher the apparent contact angle of the droplet on the surface, the lower the surface energy of the surface.
As used herein, the group 122 of tops 125 refers to the minimum number of top 125 needed to support a liquid droplet thereon in a Cassie state. A person of ordinary skill in the relevant arts would understand that Cassie state refers to wetting state of the droplet where the droplet rests on the tops 125 (e.g., group 122 of tops 125) of the nanostructures 120 in the vicinity of the droplet. For instance, in some cases, less than 10 percent of the nanostructure 120 nearest the tops 125 of the group supporting the droplet is in contact with the droplet when the droplet is in a Cassie state. When in a Cassie state, most of the droplet is not in contact with the nanostructures 120, so that the droplet's adhesion to the nanostructures 120 is reduced.
A person of ordinary skill in the relevant arts would understand how the minimum number of tops 125 in the group 122 would vary in accordance with the physical properties of the nanostructures 120, the droplet's composition and the dimension and spatial separation of nanostructures. In some embodiments, for example, the group 122 includes at least about 5 tops 125, and in some cases more preferably, at least about 10 tops 125 of nanostructures 120. The tops 125 in a group 122 are all in the same vicinity as each other, e.g., such that each nanostructure in a group 122 is at least adjacent to two other nanostructures in the group 122.
As illustrated in FIG. 1, in some embodiments, the micro-nucleation sites 115 and the nanostructures 120 are located on a planar surface 150 of the base layer 110. In some cases, the smooth surface 117 can be a planar surface that is parallel with the planar surface 150 of the base layer. In other case, the smooth surface 117 can be a planar surface that is non-parallel (i.e., sloped) with respect to the planar surface 150 of the base layer 110. In still other cases, the smooth surface 117 can be non-planar and the surface 150 of the base layer 110 can be non-planar.
As also illustrated in FIG. 1, in some embodiments of the apparatus 100, the tops 125 of the nanostructures 120 are substantially co-planar (e.g., within 10 percent of the height) with each other and with the smooth surface 117 of the micro-nucleation sites 115. However, in other embodiment, the top 125 are not all co-planar with each other, e.g., there can be local upward or downward sloping gradients of tops 125, provided by nanostructures 115 having different heights, in the vicinity of the micro-nucleation sites 115.
As illustrated in FIG. 2, in some embodiments of the apparatus 100, the nanostructures 120 surround each of the micro-nucleation sites 115 on the base layer 110. For instance, as illustrated, there can be a two-dimensional array of nanostructures 120 surrounding the micro-nucleation sites 115. As illustrated, in some cases, the nanostructures 120 of the two-dimensional array can be uniformly dimensioned and spaced apart from each other. In still other embodiments, however, there can be discrete groups 122 of nanostructures 120 around the micro-nucleation sites 115. In still other embodiments, there can be progressively (e.g., monotonically) increasing or decreasing distances separating the nanostructures 120 along one or more directions parallel to the base layer 110.
As illustrated in FIG. 3A, in some embodiments of the apparatus 100, the smooth surface 117 of each of the micro-nucleation sites 115 are substantial coplanar with bottoms 135 of the nanostructures 120 that contact the base layer 110. In some such embodiments, for instance, portions of the base layer 110 can serve as the micro-nucleation sites 115. In other cases, micro-nucleation sites 115 can be provided by a thin material layer 310 on the base layer 110, where the thickness 315 of the material layer is less than 10 percent of a height of the nanostructures 120 above the surface 150 of the base layer 110.
Referring again to FIGS. 1-3A, in some embodiments, to promote droplet nucleation and nascent growth at micro-nucleation sites 115, the smooth surface 117 of the micro-nucleation sites 115 is a hydrophilic surface, and, to promote mature droplet movement to the tops 125 of the nanostructures 115, the group 122 of the tops 125 of the nanostructures 120 is a superhydrophobic surface (synonymous with the term non-wetting surface as used herein).
As used herein, a surface is considered to be a hydrophilic surface (synonymous with the term wetting surface as used herein) when a liquid droplet 145 laying on the surface has a contact angle 140 of less or equal to about 90 degrees. As used herein, a surface is considered to be a superhydrophobic surface when a liquid droplet of the liquid laying on the surface has a contact angle 140 of less than or equal to about 90 degrees.
As illustrated in FIG. 2, in some embodiments of the apparatus 100, at least some of the micro-nucleation sites 115 are separated from each other by uniform distances 210. FIG. 2 depicts the sites 115 arranged in a square grid pattern. In other embodiments, for example, to optimize the packing density of sites 115 on the base layer 110, the sites 115 arranged in other patterns such as equilateral triangular, pentagonal, hexagonal or other patterns.
In some embodiments, the separation distance 210 between adjacent ones of the micro-nucleation sites 115 is less than a droplet diameter where droplet growth becomes heat conduction limiting. It is recognized that as droplets form and grow on a surface covered with nanostructures, as the droplet gets to a certain critical size, heat conduction through the bulk of the droplet begins to limit the heat transfer rate.
Consider, for instance, the case where for droplets 145 having a radius 155 (FIG. 1) of about 5 microns or greater, further droplet growth become heat conduction limited. Such may be the case for a water droplet under certain environmental conditions, for example. To promote droplet jumping in some such embodiments, the separation distance 210 between adjacent ones of the micro-nucleation sites 115 are designed to be equal to or less than about 10 microns.
Having a certain minimum separation distance 210 between the micro-nucleation sites 115 can promote droplet jumping, thereby enhancing heat transfer. For instance, there can be a characteristic droplet radius, below which viscous effects dominate to suppress droplet jumping. In such instances, if droplets growing on adjacent sites 115 are permitted to coalesce prematurely, if the sites 115 are too close to each other, then the jumping of the merged droplet may be deterred, e.g., because a high adhesion energy has to be overcome. On the other hand, keeping the sites 115 a certain minimum separation distance 210 can facilitate droplet growth on the individual sites 115 to a mature enough size, such that when the droplets grown on adjacent sites coalesce, the merged drop will spontaneously jump.
Consider, for instance, the case where it is undesirable to allow droplets to coalesce when the droplet radius is 0.5 microns or less. Such may be the case for a water droplet under certain environmental conditions, for example. Therefore in some such embodiments the separation distance between adjacent ones of the micro-nucleation sites 115 are preferably equal to or greater than about 1 micron.
Combining the above two considerations can help determine an optimal range for the separation distance 210 between sites 115 to promote efficient heat transfer. Continuing with the same example, in some embodiments the apparatus 100 the separation distance 210 between adjacent ones of the micro-nucleation sites 115 is in a range of about 1 to 10 microns.
It is desirable for each micro-nucleation site 115 to be size so as to provide as large an area as possible for droplet nucleation and growth, but not so large a surface area that the droplet will adhere to the surface as it grows beyond the area of the micro-nucleation site 115. For instance, in some embodiments, an area occupied by the smooth surface of the micro-nucleation sites is in a range of 1 to 100 microns². For instance in some embodiments when a micro-nucleation site 115 is circular in shape, the micro-nucleation site 115 has a radius 215 in a range of 1 to 10 microns, and can have an area range of 1 to 78 microns².
FIG. 3B presents a cross-sectional view of yet another embodiment of the apparatus 100 analogous to the view depicted in FIGS. 1 and 3A. The apparatus 100 depicted in FIG. 3B illustrates various features, which in combination with the micro-nucleation sites 115, are expected to be particularly advantageous for promoting condensation heat transfer of low surface tension liquids, such as organic liquids (e.g., unsubstituted or unsubstituted, alkanes, alkenes, alcohols and ketones) and refrigerants (e.g., hydrocarbons, fluorocarbons or halocarbons). Although the disclosed features are shown in combination, any of these features could used alone or in lesser combinations for any of the embodiments of the apparatus 100 discussed herein.
It is believed that when a liquid droplet 145 jumps from a micro-nucleation site 115 it is preferred that a remnant droplet portion 320 of the liquid remains on the site 115. For instance, the remnant droplet portion 320 can have a radius of curve in the micron scale, as compared with a newly formed (nascent) droplet, which may have a radius of curvature the nanometer scale. The remnant droplet portion 320 help bypass the early stages of nascent droplet growth associated with the large curvature of small droplets that present a high thermal resistance to droplet growth. Bypassing the early stage of droplet growth is therefore expected to further improve the heat transfer efficiency of the apparatus 100. In some embodiments, to promote such droplet growth occurring predominantly from remnant droplet portion 320 located at the micro-nucleation site 115, is it desirable to deter droplet nucleation in the vicinity of the nanostructures 120, e.g., on the nanostructures 120 and on the base layer 110 in-between the nanostructures 120.
It is believed that one way to deter droplet nucleation in the vicinity of the nanostructures 120 is to reduce the local supersaturation in the vicinity of the nanostructures 120. As used herein supersaturation is defined as the ratio of the vapor pressure to the saturation pressure at the condensing surface temperature. The local supersaturation in the vicinity of the nanostructures 120 can be reduced by locating the nanostructures 120 adjacent to a lower thermal conductively portion of the surface 150 of the base layer 110 as compared to the portion of the base layer that the micro-nucleation sites 115 are adjacent to. It is believed that in such embodiments, the temperature of the surface 150 having the low thermal conductively layer 327 will be higher than the temperature of the surface 117 of the micro-nucleation site 115 and therefore droplet formation and growth will preferably occur at the micro-nucleation site 115.
For instance, as illustrated in FIG. 3B, in some cases, it is advantageous for the base layer 110 to be composed of a high thermal conductivity material such as copper or aluminum of similar metals and for the micro-nucleation site 115 to be adjacent of such a portion 325 of the base layer 110. In some cases, it is also advantageous for the base layer 110 to further include a low thermal conductively portion (e.g., layer 327) located adjacent to the nanostructures 120 thereon. Suitable materials for the low thermal conductively layer 327 include thermally insulating organic polymers or inorganic oxides, such as silicon oxide. Such that the micro-nucleation site 115 is in direct contact with the high thermal conductivity base layer and the nanostructures 120 are insulated from the high thermal conductivity base layer by the insulating layer 327.
As illustrated in the expanded view shown in FIG. 3B another way to deter droplet nucleation in the vicinity of the nanostructures 120, is to configure the tops 125 of the nanostructures 120 to include a reentrant angled ledge 330. As used herein, the term reentrant angled ledge 330 refers to a layer on a post portion 335 of the nanostructure 120 that forms an interior angle 340 with the post portion 335 of greater than 180 degrees. A person of ordinary skill in the relevant arts would be familiar with how to fabricate such nanostructures, e.g., such a disclosed in Ahuja et al. “Nanonails: A Simple Geometrical Approach to Electrically Tunable Superlyophobic Surfaces” (Langmuir 2008, 24, 9-14) which is incorporated by reference herein in its entirety. It is believed that reentrant angled ledge 330 of such nanostructures 120 reduces a droplet's ability to de-pin and jump from the nanostructures 120 which in turn would deter new droplet nucleation on the nanostructure 120.
As illustrated in FIG. 3B, still another way to deter droplet nucleation in the vicinity of the nanostructures 120 is to infuse a low surface energy liquid 350 in the spaces between the nanostructures 120. Examples of suitable low surface energy liquid are described in Wong et al., “Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity” (Nature 2001, 477, 443) which is incorporated by reference herein in its entirety. It is believed that droplets on such liquid infused nanostructures 120 would have reduce ability to de-pin and jump from the nanostructures 120 which in turn would deters new droplet nucleation on the nanostructure 120.
FIG. 4 presents a detailed cross-sectional view of a portion of an apparatus 100, such as the apparatus 100 presented in FIG. 1, depicting example nanostructures 120 of the apparatus 100. As illustrated, the nanostructure 120 can be pillar-shaped, and, the pillars are spaced apart from each other. In other embodiments, the nanostructures 120 can be ridged-shaped and the ridges are spaced apart from each other.
The presence of nanostructures 120 beneficially provide a low-energy surface (e.g., non-wetting or super hydrophobic surface) in contrast to the higher surface energy of the smooth surface 117 of the micro-nucleation sites 115. This is advantageous over conventional condensing surfaces because droplet adhesion to the condensation surface can be reduced with the appropriate nanostructure configuration. Droplet adhesion to the condensation surface can be reduced with the appropriate nanostructure configuration. In particular, to reduce adhesion, it is desirable for nanostructures to be configured to facilitate the droplet taking on a Cassie state. There are several structural attributes that the nanostructures 120 can have to facilitate a droplet attaining a Cassie state.
For instance, in some preferred embodiments, it is desirable for the nanostructures 120 to provide the condensation surface with a certain surface roughness to deter a droplet from taking on an undesirable Wenzel state. A Wenzel state refers to a wetting state where the droplet substantially contacts the entire surfaces of the nanostructures 120 in the vicinity of the droplet. For example, in a Wenzel state, substantially the entire height of the droplet may contact the sides 130, 132 and tops 125 of the nanostructures 120 as well as the base layer 110. In various embodiments, it is often undesirable that a droplet take Wenzel states, because the large contact area of the droplet in such a state can provide a large adhesion that pins the droplet in-between the nanostructures 120. Wenzel state formation therefore impedes the droplet from moving away from its nucleation site to the apexes 135 of the microstructures 115, which in turn may reduce the efficiency of condensation heat transfer.
To help avoid growing droplets taking such a Wenzel state, it is desirable for the nanostructures 120 to have the following condition to satisfy the following condition when a liquid droplet 145 rests on the surface: −1/r*cos θa<1. Herein, the surface roughness factor, r, is defined as the total surface area, including the areas of the sides and tops and support surfaces 150 in between the nanostructures 120, divided by a projected surface area of the surfaces 150, e.g., the area support surfaces 150 with no nanostructures 120 thereon. Herein a is an intrinsic advancing contact angle 140 of the liquid droplet 145. The intrinsic advancing contact angle, ea, refers to the contact angle that the liquid droplet would have on a smooth surface, e.g., the support surfaces 150 with no nanostructures 120 thereon. In some embodiments, the smooth surface 117 of the nucleation sites 115 follow the condition 1>cos θa>−0.65, and the group 122 of tops 125 of the nanostructures follow the condition −1/r*cos θa<1.
Although the smooth surface 117 of the nucleation sites 115 are designed to promote droplet nucleation and nascent growth thereon, droplet nucleation and growth may still occur, e.g., concurrently, between the nanostructures 120. In some cases, it may be desirable for the nanostructures 120 to be spaced apart by a minimal separation distance 410 (e.g., the distance from the side 130 of one nanostructure 120 to the side 132 of an adjacent nanostructure 120) to promote droplets to form and grow in-between the nanostructures 120.
In some such cases, the distance 410 is preferably greater than a critical condensation radius 155, r_c, for a nucleating liquid droplet. The critical condensation radius can be estimated by the formula:
r _c=2γυ/(kTlnS),
Here, γ is the ratio of liquid to vapor surface tension, υ is a molecular volume of the liquid phase, k is the Boltzmann constant, and S, the saturation, is defined as the ratio of the vapor pressure pv to the saturation pressure at the condensing surface temperature T. For example, in some embodiments of the device 100, for a water droplet, the distance 410 separating adjacent nanostructures is equal to of greater than about 10 nanometers. For example, in some embodiments of the apparatus 100, the distance 410 is in a range of about 1 to 100 nanometers, and in some cases in a range of about 10 to 20 nanometers.
In some such cases, the distance 410 between adjacent ones of the nanostructures 120 preferably has a value that promotes a droplet to attain the Cassie state before the droplet radius 155, R, grows to size that is heat conduction limiting. For example, for water droplet this value of the radius 155 is about 5 microns or larger. The Cassie state, is promoted by spacing the nanostructures apart by preferred distance 320 and by having a height 420 that facilitates the droplet 145 have a receding contact angle 140, θr, of at least about 90 degrees.
For instance, in some preferred embodiments, the nanostructures follow a relationship:
cos θ_r<−(1+h/R)/(1+2h/w),
Here θ_ris a receding contact angle 140 of at least about 90 degrees for maximally desired size of droplet located on tops of the nanostructures, h is the uniform heights 420 of the nanostructures 120, R is a radius 155 of the liquid droplet and w is a uniform separation distance 410 between adjacent ones of the nanostructures 120.
As used herein, the term receding contact angle 140 is defined as the minimum stable angle that the droplet 145 achieves while on the nanostructures 120. A person of ordinary skill in the relevant arts would be familiar with methods to measure the receding contact angle 140 of a droplet (see e.g., “Condensation on. Superhydrophobic Surfaces: The Role of Local Energy Barriers and Structure Length Scale” and Supporting Information, by Enright et al., Langmuir pub. Aug. 29, 2012 (“Enright-1”), incorporated by reference herein in its entirety).
The receding contact angle 140 for a droplet to spontaneously achieve a Cassie state can be reduced by minimizing the ratio h/R and/or maximizing the ratio h/w. For example, in some embodiments, a ratio of a height 420 of the nanostructures 120 to a radius 155, R, of a maximally sized liquid droplet on the surface (e.g., the h/R ratio) is equal to or less than about 0.1:1. For example, in some embodiments, a ratio of a separation distance 410, w, to the height 420 of the nanostructures is in a range from about 0.5:1 to 30:1, and more preferably about 0.5:1 to 3:1.
For instance, consider a liquid whose critical size, where heat conduction through the bulk of the droplet begins to limit the heat transfer rate, and, that critical size corresponds to a droplet radius 155 of 5 microns or greater. Assuming a desired receding contact angle 140, θ_r, equal to 120 degrees, to make the ratio of h, the uniform height 420 of the nanostructures 305 to R, a radius 410 of the liquid droplet less than or equal to 0.1 (i.e., h/R≦0.1) requires h≦0.5 μm. For a h/R ratio equal to 0.1 and the height 420, h, equal to 0.5 μm, the separation distance 320 between microstructures, w, is then equal to 833 nanometers and the h/w ratio equals 0.6. Continuing with the same example, where the h/R ratio equals 0.1 and h equals 0.5 μm, for a receding contact angle 140, θ_r, equal to 110 degrees, w, is then equal to 451 nanometers and the h/w ratio equals 1.1, or, for a receding contact angle 140, θ_r, equal to 100 degrees, w, is then equal to 187 nanometers, and the h/w ratio equals 2.7, or, for a receding contact angle 140, θ_r, equal to 90 degrees, w, is then equal to 16 nanometers, and the h/w ratio equals 31.3.
In some embodiments of the apparatus 100, to reduce the adhesion of a de-wetted droplet (e.g., a droplet in a Cassie state) it is desirable to reduce the fraction of space occupied by the nanostructures 120 relative to the open space in-between adjacent ones of the nanostructures. For instance, in some embodiments, the solid fraction occupied by the nanostructures 120 is equal to or less than 0.1. As used herein the term solid fraction herein is equal to d/(d+w), where d is the width 430 of the nanostructure and w is the separation distance 410 between adjacent ones of the nanostructures 120.
As a non-limiting example, in cases where the separation distance 410 is equal to 833 nanometers, then the width 430 is preferably equal or less than 93 nanometers. Or, when the separation distance 410 is equal to 451 nanometers, then the width 430 is preferably equal or less than 50 nanometers. Or, when the separation distance 410 is equal to 451 nanometers, then the width 430 is preferably equal or less than 50 nanometers. Or, when the separation distance 410 is equal to 187 nanometers, then the width 430 is preferably equal or less than 21 nanometers. Or, when the separation distance 410 is equal to nanometers, then the width 430 is preferably equal or less than 1.8 nanometers.
Another embodiment is a method that comprises manufacturing an apparatus (e.g., a heat transfer apparatus).
With continuing reference to FIGS. 1-4 throughout, FIG. 5 presents a flow diagram of an example method of manufacturing a heat transfer apparatus, such as any of the apparatuses 100 described in the context of FIGS. 1-4. As illustrated in FIG. 5, the method includes a step 505 of manufacturing an apparatus 100, which in some cases can be or include a condenser 105. Manufacturing the apparatus (step 505) includes a step 510 of providing a base layer 110 and step 515 of forming micro-nucleation sites 115 on the base layer 110. Each of the micro-nucleation sites 115 has a smooth surface 117. The method also includes a step 520 of forming nanostructures 120 on the base layer 110 and around the micro-nucleation sites 115, wherein a group 122 of tops 125 of the nanostructures 120 have a lower surface energy than a surface energy of the smooth surface 117.
To further illustrate aspects of the method described in FIG. 5, FIGS. 6A-6C present cross-sectional views of an example apparatus 100, similar to the apparatus 100 shown in FIG. 1, at intermediate stages of fabrication for one embodiment of the method. FIGS. 7A-7D presents cross-sectional views of an example apparatus 100, similar to the apparatus 100 shown in FIG. 3A, at intermediate stages of fabrication for another embodiment of the method.
Referring to FIGS. 5, and 6A or 7A, in some embodiments of the method, providing the base layer 110 in step 510 can include providing a single material layer 610, e.g., a layer of copper, aluminum, or semiconductor material, upon which the micro-nucleation sites 115 are directly formed from in step 515. In some cases the use of a highly heat conductive material layer 610 such copper, aluminum or similar metals, is preferred. In other cases, the providing the base layer 110 in step 510 can include a step 525 of forming a second material layer 620 on the first material layer 610. In such cases, for example, the micro-nucleation sites 115 and/or nanostructures 120 can be formed from the second material layer 620. For instance, a second material layer 620 of copper or aluminum could be deposited on a first material layer 610 of steel, via electrolytic, electroless or other deposition processes familiar to a person of ordinary skill in the relevant arts. Or a material layer 620 of silicon oxide, silicon or other semiconductor material layer could be deposited or grown on silicon or other semiconductor layer 610. Or a low thermal conductivity material layer 620 could be selectively formed over a high thermal conductivity material layer 610. For instance, the low thermal conductivity layer 327 can be deposited so as to surround portions 325 of the high thermal conductivity base layer 110, such as depicted in FIG. 3B.
In some embodiments of the method, forming the one or more micro-nucleation sites 115 (step 515) includes a step 530 of forming a mask layer on the base layer 110 and step 532 of patterning the mask layer such that mask portions 625 (FIG. 6A 7A) remain on the base layer 110 in locations corresponding to the micro-nucleation sites 115. A person of ordinary skill in the relevant arts would be familiar with procedures for depositing a mask layer, such as a photoresist mask layer and how to pattern the mask layer, e.g., using standard photolithographic processes, to form the mask portions 625.
In some embodiments of the method, as illustrated in FIG. 6B, forming the nanostructures 120 (step 520) includes a step 535 of modifying the base layer 110 (e.g., one of layer 610 or layer 620 if present) not covered by the mask portions 625 that are located on the base layer 110 and that correspond to the locations of the micro-nucleation sites 115.
In some embodiments of the method, forming the one or more micro-nucleation sites 115 (step 515) can also include a step 540 of removing the mask portions 625 to uncover the micro-nucleation sites 115, e.g., such that tops 125 of the nanostructures 120 are substantially coplanar with the smooth surface 117 of the micro-nucleation sites 115, such as illustrated in FIG. 1. A person of ordinary skill in the relevant arts would be familiar with to remove a mask portion such as a mask portion composed of photoresist material.
As illustrated in FIG. 6C, in some embodiments of the method, forming the nanostructures 120 (step 520) include a step 545 of functionalizing outside surfaces of the nanostructures with a low surface energy material 630 as further illustrated in the expanded view presented in the figure. In some cases, it is preferable for step 545 to be performed before removing the mask portions 625 so that the smooth surface 117 of the sites 115 is not similarly functionalized.
In some embodiments of the method, as illustrated in FIG. 7B, forming the nanostructures 120 (step 520) includes, after the steps 525, 530 of forming a mask layer and patterning the mask layer to form the mark portions 625, a step 550 of forming a material layer (e.g., layer 620) on the base layer 110 (e.g., layer 610) and the mask portions 625. Forming the nanostructures 120 (step 520) can then include a step 555 of modifying the material layer 620 on the base layer 110 and mask portions 625 to form the nanostructures 120, such as illustrated in FIG. 7C.
As illustrated in FIG. 7D, forming the nanostructures 120 (step 520) in embodiments can also include the step 545 of functionalizing outside surfaces of the nanostructures 115 with the low surface energy material 630, as illustrated in the expanded view. Once again in some cases, it is preferable for step 545 to be performed before removing the mask portions 625 so that the smooth surface 117 of the sites 115 is not similarly functionalized.
Forming the one or more micro-nucleation sites 115 (step 515) can also include the step 540 of removing the mask portions 625 to uncover the micro-nucleation sites 115, e.g., such that bottoms 135 of the nanostructures 120 are substantially coplanar with the smooth surface 117 of the micro-nucleation sites 115, such as illustrated in FIG. 3A.
In some embodiments, modifying the base layer 110 (including layer 610 in some cases) or material layer 620 in accordance with step 535 or 555 can include exposing at least one these layers 110, 610, 620 to an oxidation process. For instance, a copper base layer 110 or copper material layer 620 thereon can be exposed to chemical oxidation conditions such as in “Condensation on Supersuperhydrophobic Copper Oxide Nanostructures,” by Enright et al. Proceedings of the 3rd Micro/Nanoscale Heat and Mass Transfer International Conference, Atlanta, Ga., Mar. 3-6, 2012 MNHMT2012-75277 (“Enright-2”), incorporated by reference herein in its entirety, to form the nanostructures 305 therefrom. For instance, an aluminum base layer 110 or aluminum material layer 620 thereon can be exposed to well-known hydrothermal oxidation processes form the nanostructures 305 therefrom.
In some embodiments, modifying the base layer 110 (including layer 610 in some cases) or material layer 620 in accordance with step 535 or 555 can includes exposing at least one these layers 110, 610, 620 to an etch process. For instance, layers 110, 620 composed of a semiconductor material, such as silicon, can be subjected to a reactive ion etching process to form the nanostructures 120, such as black silicon nanostructures. Other examples of etching process are presented in Enright-1.
In some embodiments, modifying the base layer 110 (including layer 610 in some cases) or material layer 620 in accordance with step 535 or 555 can include mechanically modifying portions of at least one of these layers 110, 610, 620. For instance, a layer of copper, aluminum of semiconductor material can be mechanically indented, machined, stamped, embossed or otherwise mechanically modified to form the nanostructures 120.
As used herein, the term low surface energy material 630, refers to a material having a surface energy of about 22 dynes/cm (about 22×10⁻⁵N/cm) or less as disclosed in U.S. Pat. No. 7,695,550 to Krupenkin et al. (“Krupenkin”), incorporated by reference herein in its entirety. A person of ordinary skill in the relevant arts in the art would be familiar with the methods to measure the surface energy of materials.
Non-limiting examples of functionalizing nanostructures in accordance with step 545 includes coating the nanostructures 120 with chlorosilanes, fluorosilanes or fluorinated polymers, such as disclosed in Krupenkin, Enright-1 or Enright-2.
FIG. 8 illustrates another embodiment of the disclosure, a system 800. In some embodiments, the system 800 can be communication system such as a telecommunication system or a system component (e.g., an electronic cabinet) of a communication system. The system 800 comprises heat generating equipment 810, such electrical equipment, e.g., circuit boards having heat generating components thereon. The system 800 also comprises a heat transfer apparatus 820. The heat transfer apparatus 820 can be configured to remove heat generated by the equipment 810 of the system 800.
The heat transfer apparatus 820 can be or include any apparatuses described herein. In some cases, for instance, referring to FIGS. 1-4, the apparatus 820 includes a base layer 110 having one or more micro-nucleation sites 115 thereon, wherein each of the micro-nucleation sites 115 has a smooth surface 117. The apparatus also includes nanostructures 120 located on the base layer 110 and around the micro-nucleation site 115. Tops 125 of the nanostructures 120 (e.g., a group 122 of top 125) have a lower surface energy than a surface energy of the smooth surface 120.
With continuing reference to FIGS. 1-4, in some embodiments of the system 800, the micro-nucleation sites 115 are located on the surface of a condenser 105 of the apparatus 100. In some embodiments, the condenser 105 is part of a heat pipe (e.g., serving as the base layer 110), while in other embodiments, the condenser 105 is part of a vapor chamber (e.g., serving as the base layer 110).
Although the present disclosure has been described in detail, a person of ordinary skill in the relevant arts should understand that they can make various changes, substitutions and alterations herein without departing from the scope of the invention.

Claims

1. An apparatus, comprising:

a base layer having a distribution of separated micro-nucleation sites thereon; and

a distribution of nanostructures located on the base layer, each of the micro-nucleation sites being adjacent to some of the nanostructures; and wherein each of the micro-nucleation sites has a hydrophilic surface and the distribution of nanostructures form a superhydrophobic surface.

2. The apparatus of claim 1, wherein the micro-nucleation sites and the nanostructures are located on a planar surface of the base layer.

3. The apparatus of claim 1, wherein tops of the nanostructures are substantially coplanar with the hydrophilic surfaces of the micro-nucleation sites.

4. The apparatus of claim 1, wherein some of the nanostructures surround each of the micro-nucleation sites.

5. The apparatus of claim 1, wherein the hydrophilic surfaces of each of the micro-nucleation sites are substantially coplanar with bottoms of the nanostructures that contact the base layer.

6. The apparatus of claim 1, wherein an area occupied by the hydrophilic surfaces of each of the micro-nucleation sites is in a range of 1 to 100 microns².

7. The apparatus of claim 1, wherein the hydrophilic surface of each of the micro-nucleation sites has a smooth surface.

8. The apparatus of claim 1, wherein a separation distance between adjacent ones of the micro-nucleation sites is equal to or less than about 10 microns.

9. The apparatus of claim 1, wherein the nanostructures are located on a region of the base layer having a lower thermal conductivity than a region of the base layer having the micro-nucleation sites thereon.

10. The apparatus of claim 1, wherein tops of the nanostructures include a reentrant angled ledge.

11. The apparatus of claim 1, wherein an area of the base layer having the nanostructures located thereon has a surface roughness where the following condition applies when a liquid droplet rests on the surface: −1/r*cos θa<1, wherein r is the surface roughness of the surfaces having the distribution of nanostructures located thereon and θa is an intrinsic advancing contact angle of the liquid droplet.

12. The apparatus of claim 1, wherein a distance between adjacent ones of the nanostructures is greater than a critical condensation radius for a nucleating liquid droplet on the surface.

13. A system, comprising:

heat generating equipment; and

a heat transfer apparatus configured to remove heat generated by the heat generating equipment, wherein the apparatus includes:

14. The system of claim 13, wherein the micro-nucleation sites are located on the surface of a condenser of the apparatus.

15. The system of claim 13, wherein the condenser is part of a heat pipe or a vapor chamber.

16. A method, comprising:

manufacturing an apparatus, including:

providing a base layer;

forming a distribution of separated micro-nucleation sites on the base layer, wherein each of the micro-nucleation sites has a hydrophilic surface; and

forming a distribution nanostructures on the base layer, wherein each of the micro-nucleation sites being adjacent to some of the nanostructures.

17. The method of claim 16, wherein forming the micro-nucleation sites includes:

forming a mask layer over the base layer; and

patterning the mask layer such that mask portions remain on the base layer in locations corresponding to the micro-nucleation sites.

18. The method of claim 16, wherein forming the nanostructures on the base layer includes:

modifying portions of the base layer not covered by mask portions located on the base layer that correspond to locations of the micro-nucleation sites.

19. The method of claim 18, wherein forming the micro-nucleation sites includes:

removing the mask portions to uncover the micro-nucleation sites such that tops of the nanostructures are substantially coplanar with the hydrophillic surfaces of the micro-nucleation sites.

20. The method of claim 16, wherein forming the nanostructures on the base layer includes:

forming a mask layer over the base layer;

patterning the mask layer such that mask portions remain on the base layer corresponding to locations of the micro-nucleation sites;

forming a material layer on the base layer and the mask portions; and

modifying the material layer on the base layer and the mask portions to form the nano structures.