Passive Anti-frosting Surface Comprised of Microscopic Wettability Patterns Containing Sacrificial Ice

Abstract

A method and device for reducing ice and frost on a surface comprising a wettable pattern on a surface. The pattern is wetted with water which is frozen into ice to create overlapping hygroscopic that cover the surface.

Claims

1. (canceled)

2. (canceled)

3. (canceled)

4. (canceled)

5. (canceled)

6. (canceled)

7. (canceled)

8. (canceled)

9. (canceled)

10. (canceled)

11. (canceled)

12. (canceled)

13. (canceled)

14. (canceled)

15. A method of reducing ice and frost on a surface comprising the steps of: providing a pattern of stripes on said surface; filling said stripes with water; freezing said water in said stripes to form ice in said stripes; said ice in said stripes creates overlapping hydroscopic zones; and said hydroscopic zones overlap to cover the entire surface of said substrate.

16. The method of claim 15 wherein said hydroscopic zones are in-plane.

17. The method of claim 16 wherein said hydroscopic zones keep siphoning nearby water vapor, keeping the rest of the surface completely dry from condensation and frost.

18. The method of claim 17 wherein said stripes are spaced apart, said spacing between stripes is less than twice the value of a hydroscopic zone.

19. The method of claim 17 wherein said stripes are formed into linear arrays.

20. The method of claim 19 wherein said linear arrays include fins having grooves thereon, said fins having varying heights and varying spaced apart distances.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0023] In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.

[0024] FIG. 1A is a top-down and side-view of a microscopic pattern of interconnected hydrophilic stripes for an embodiment of the present invention.

[0025] FIG. 1B is a top-down and side-view of the embodiment shown in FIG. 1A that has been wetted with water.

[0026] FIG. 1C is a top-down and side-view of the embodiment shown in FIG. 1B where the water has subsequently frozen into ice.

[0027] FIG. 1D is a top-down and side-view of the embodiment shown in FIG. 1C showing that the entire surface area between the ice stripes remain substantially dry from condensation and frost when the spacing between stripes is less than twice the value of a single dry zone for an embodiment of the present invention.

[0028] FIG. 2 is an isometric view of the microgrooves connected to the water reservoir which would fill the grooves to enable the freezing of the stripes together by forming contact with an external ice for an embodiment of the present invention.

[0029] FIG. 3 shows a side view of the elevated microgrooves, some being flush with the surface and some as high as 1 mm in height for an embodiment of the present invention.

[0030] FIG. 4 is an isometric view of the entire aluminum plate with microgrooves of different elevations and inter-stripe distances for an embodiment of the present invention.

[0031] FIG. 5 depicts frozen 10 m water stripes with 1 mm inter-stripe distances for an embodiment of the present invention.

[0032] FIG. 6A shows micro-grooves along the top of each fin for an embodiment of the present invention.

[0033] FIG. 6B shows how water preferentially wicks along the tops of the fins.

[0034] FIG. 6C shows how water freezes into ice.

[0035] FIG. 6D shows the hygroscopic ice stripes siphoning water vapor, keeping the rest of the finned surface dry from condensation and frost.

[0036] FIG. 7A is a micrograph of how a millimetric frozen droplet creates a dry zone around itself on a substrate kept at Tw=12.5 C., air temperature being T=17.4 C. and humidity RH=21%.

[0037] FIG. 7B shows a saturation vapor pressure of water and ice plotted against temperature. The saturation vapor pressure difference peaks at T=12.5 C. It is this vapor pressure difference that causes ice to behave as a humidity sink, creating the dry zone.

[0038] FIG. 8A depicts sacrificial fin tops on an aluminum surface with the fins acting to keep the aluminum surface completely dry from condensation and frost even after 24 hours of exposure to supersaturated conditions for an embodiment of the present invention.

[0039] FIG. 8B shows how a smooth aluminum surface is completely frosted over in under 1 hour under the identical environmental conditions.

[0040] FIG. 8C is a plot of frost-free area over time for regular aluminum surface (iii) and an embodiment of the present invention (i) and (ii). The data points of (i) correspond to flat bare aluminum surface between the ridges, which is a critical region of interest, as shown in (i). Note that this part of the surface develops no frost whatsoever since the data points are constant at 100%. The (ii) data points correspond to the top-down projected area including the ridges. The decreasing nature of the (ii) data points is because the frost atop of the ridges coarsen over time. However, this frost could not reach the aluminum floors at the bottom between the ridges, that is, region (i) at any point of time.

DETAILED DESCRIPTION OF THE INVENTION

[0041] Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.

[0042] As shown in FIGS. 1A and 2, in one embodiment, the present invention may utilize photolithography to pattern in substrate 100 arrays of microscopic hydrophilic (bare silicon oxide) stripes 110-112 onto a hydrophobic (silane monolayer) backdrop. The pattern may also be exposed to an oxygen plasma to restore the full hydrophilicity of the silicon oxide features, followed by a dry peel-off of the patterned parylene coating that was protecting the hydrophobic monolayer. The hydrophilic stripes may have a width of around 10-15 m and may be connected to a long hydrophilic water pad 130 at the edge of the substrate as shown which holds water 150 as shown in FIG. 1B.

[0043] The water pad 150 serves two purposes: a) it can be used to deposit a water drop which would then spread onto the hydrophilic stripes and b) when the water pad is frozen by touching it with a piece of ice it would also freeze all the water stripes together at the same time. Another possible way of obtaining ice stripes is to cool the substrate below the dew point so that condensation fills the hydrophilic stripes. Freezing may also be induced in multiple ways for the supercooled water pattern: by touching with ice, electrofreezing, or by spontaneous heterogeneous ice nucleation.

[0044] As shown in FIGS. 1A-1D, a microscopic pattern of interconnected hydrophilic stripes or channels 110-112 are wetted with water 150 and subsequently frozen into ice 170. The entire surface area between the ice stripes 171-173 should remain dry from condensation and frost when the spacing between stripes is less than twice the value of a single dry zone.

[0045] Physical microgrooves patterns are known to be more robust, durable, easy to fabricate and less expensive than chemical microfabrication and can be done in a number of other ways such as 3D printing, molding, etc. The distance between two microgrooves is varied with the maximum being 1 mm. These grooves are 15 m in width and 25 m in depth and are connected to a water reservoir. Some of these grooves are flush with the dry zone surface while others may be elevated off the ground by different heights ranging from 100 m to 1 mm. Elevating the microgrooves off the ground serves several purposes. First, elevating the microgrooves off the ground assists in preventing the ice stripes from growing in-plane over the dry regions. Also, elevating assists in pushing the boundary layer thickness higher, which in turn, serves to increase the dry zone length about each ice stripe.

[0046] As shown in FIG. 3, elevated microgrooves 310-312, some being flush with the surface (310) and some as high as 1 mm in height (312). It is expected that the taller the pillars are with the frost on top, the more pronounced would be the effect of dry zone.

[0047] In an alternate embodiment, as shown in FIG. 4, a substrate 400 is provided which may be an aluminum plate. For this embodiment of the present invention, a plurality of microgroove sets 410-416 in the form of linear arrays of channels is provided. The channels may have different elevations and varying inter-stripe, channel, or groove distances.

[0048] FIG. 5 depicts frozen 15 m water stripes with 1 mm inter-stripe distances for an embodiment of the present invention. The ice stripes keep the intermittent distances dry because of overlapping dry zones. Experiment shows near 90% dry surface with no frost or condensation at around T=8 C. and a supersaturation S=1.2.

[0049] In a preferred method, the nucleation energy barrier for condensation is lower for microgrooves than for flat surfaces which causes preferential condensation in the grooves. An alternative way for doing the same is by filling the water reservoir connected to the grooves with water. The next step is to freeze the water stripes all at the same time. This creates parallel arrays of ice stripes than can have overlapping dry zones that keep the surface macroscopically frost free.

[0050] A sample was put on a Peltier stage and placed inside a humidity chamber. In order to obtain microscopic arrays of ice on the hydrophilic regions, the temperature of the substrate was brought down to 10 C. Once supercooled condensation completely wetted the hydrophilic stripes, the temperature was sharply brought down to 30 C. Approximately 5 s later, all the hydrophilic stripes were frozen. The stage was brought to around 8 C., and the humidity was set to 21%. Corresponding to air temperature of 16.6 C., the supersaturation was S=p/pw=1.2, where P is the vapor pressure in the ambient and Pw is the saturation vapor pressure corresponding to the substrate temperature. The fact that S>1 implies that the substrate temperature was significantly below that the dew point and hence the surface should exhibit condensation and subsequent freezing. However, the entire sample barring the sacrificial ice stripes was observed to be frost free for 12 mins. The hydrophilic stripes were 20 m in width, while the edge to edge separation between two ice stripes was 1 mm. This implies that despite being in a subfreezing humid environment close to 90% of the substrate was completely dry without any observable condensation or frost whatsoever

[0051] FIGS. 6A-6D shown an alternate embodiment of the present invention. For this embodiment, microgrooves 600-603 may be machined into substrate 610. Reservoir 620 is also provided. By machining micro-grooves along the top of each fin 630-633, water preferentially wicks along the tops of the fins. Upon freezing into ice 650 in chilled conditions, these hygroscopic ice stripes siphon all nearby water vapor, keeping the rest of the finned surface completely dry from condensation and frost as shown in FIGS. 6C and 6D.

[0052] In other embodiments, the fins may have the same height and be equally spaced apart or be not equally spaced apart. In other embodiments, the fins may have varying heights and be equally spaced apart or be not equally spaced apart. The fins may also be arranged in linear arrays.

[0053] The embodiments of the present invention follow directly from the discovery that ice can evaporate liquid water droplets around itself, creating a dry zone 700, where no condensation or frost can grow as shown FIG. 7A. The underlying mechanism is that the saturation vapor pressure of ice is lower than that of water at the same subzero temperature, causing ice to behave as a humidity sink as shown in FIG. 7B.

[0054] FIG. 8A depicts sacrificial fin tops on an aluminum surface staying completely dry from condensation and frost even after 24 hr of exposure to supersaturated conditions for an embodiment of the present invention. FIG. 8B shows a smooth aluminum surface is completely frosted over in under 1 hr under the identical environmental conditions.

[0055] FIG. 8C is a plot of frost-free area over time for regular aluminum surface (iii) and an embodiment of the present invention (i) and (ii). The data points of (i) correspond to flat bare aluminum surface between the ridges, which is a critical region of interest, as shown in (i). Note that this part of the surface develops no frost whatsoever since the data points are constant at 100%. The (ii) data points correspond to the top-down projected area including the ridges. The decreasing nature of the (ii) data points is because the frost atop of the ridges coarsen over time. However, this frost could not reach the aluminum floors at the bottom between the ridges, that is, region (i) at any point of time because ice 811 and 813 create hygroscopic zones 815 and 817 that form overlap 819 to reduce ice formation on substrate 823.

[0056] In yet other embodiments of the present invention, micro-milling was employed to create an array of fins on an aluminum substrate that were 1 mm tall, about 200 m wide, and spaced 1 mm apart from each other. Subsequently, 15 m micro-grooves were cut into the middle of the top of each fin. By having all these grooves feed into a connecting mini reservoir, the array of water stripes could be easily produced by simply filling the reservoir with water. The surface was then chilled down to Tw=10+/1 C. on a Peltier stage to freeze the water into ice stripes, and the resulting anti-frosting behavior in a humid environmental chamber was observed using top-down and side-view microscopes. The air was both warm (T=17+/1 C.) and humid: both 30% and 16% relative humidities were tried, which corresponds to supersaturations of 1.5 and 1.1, respectively, relative to the saturation pressure of the Tw=10+/1 C. surface. It was observed that no matter how much time elapsed, all of the frost growth occurred solely atop the ice stripes, leaving the rest of the substrate completely dry from both supercooled condensation and frost.

[0057] While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.