NON-ICING SURFACES

Abstract

A coating applied to a surface to minimize or prevent the ability of drops of super-cooled water that impact on the surface from transforming into ice, comprises one or more layers collectively providing: a) thermal insulation between the surface to be protected and the drop; and b) a superhydrophobic surface on the top.

Claims

1. A coating applied to a surface to minimize or prevent the ability of drops of super-cooled water that impact on the surface from transforming into ice, the coating comprising one or more layers collectively providing: a) thermal insulation between the surface to be protected and the drop; and b) a superhydrophobic surface on the top.

2. The coating of claim 1, wherein the insulation layer comprises a rigid foam.

3. The coating of claim 2, wherein the rigid foam is comprised of microballoons within an epoxy matrix.

4. The coating of claim 1, wherein the superhydrophobic surface is a coating that is sprayed onto the insulation.

5. The coating of claim 1, wherein the surface is operative on air, ground or sea, and is one of: an electricity line; an antenna; an external surfaces of a ship including decks, rails, weapons, and antennas; and a wing or control surface of an air vehicles.

6. The coating of claim 1 having coefficient of thermal conductivity (K) no more than 0.15 W÷(m° k), contact angle hysteresis (CAH) no more than ten degrees, and roll off angle no more than five degrees.

7. A method of minimizing or preventing the ability of drops of super-cooled water that impact on a surface from transforming into ice, the method comprising applying to the surface a coating comprising one or more layers that collectively provide: a) thermal insulation between the surface to be protected and the drop; and b) a superhydrophobic coating on the insulating layer.

8. The method claim 7, wherein the insulation is provided by using rigid foam.

9. The method of claim 8, wherein the rigid foam is comprised of microballoons within an epoxy matrix.

10. The method of claim 7, wherein the superhydrophobic coating is sprayed onto the insulation.

11. The method of claim 7, wherein the surface is one of: an electricity line; an antenna; an external surfaces of a ship including decks, rails, weapons, and antennas; and a wing or control surface of an air vehicle.

12. The method of claim 7, wherein the surface to be coated is selected from among motor vehicles, buildings, including elevators and windows, glass surfaces and large refrigerators.

13. A method for preventing ice from adhering to a surface, the method comprising providing on said surface a coating that, when coming into contact with drops of super-cooled water, prevents them from transforming into ice by providing thermal insulation between the surface to be protected and said drops, the upper surface of said coating being superhydrophobic.

14. An ice-phobic coating that, when coming into contact with drops of super-cooled water, prevents them from transforming into ice, the ice-phobic coating comprised of a thermal insulation between the surface to be protected and said drops, and an upper surface which is superhydrophobic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 schematically shows droplets of water on hydrophylic, hydrophobic, and superhydrophobic surfaces;

[0046] FIGS. 2 schematically shows the method used for measurement of the contact angle hysteresis (CAH);

[0047] FIG. 3A schematically shows the Cassie-Baxter model that underlies the phenomenon of superhydrophobicity; FIG. 3B schematically shows the Wentzel model used to describe the phenomenon of superhydrophobicity;

[0048] FIG. 4 schematically shows a drop of water spread on a superhydrophobic substrate on impact and the force of “retraction” on the drop pulling it back towards its original shape;

[0049] FIG. 5 shows typical results for the measured slide/roll-off angle for different substrates.

[0050] FIG. 6 shows typical results for measurements of CAH for different substrates;

[0051] FIG. 7 shows the response of water droplets impacting different surfaces at the time of impact and at different periods of time after impact;

[0052] FIG. 8 show the normalized diameter of the drop as a function of time from the instant of impact with a surface tilted at angles of 25°;

[0053] FIG. 9 shows the normalized velocity of a drop of water as a function of time after impacting a surface tilted at 25°; and

[0054] FIG. 10 schematically shows the different stages of ice formation within a drop of super cooled water.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0055] As mentioned previously, at present there is no passive technological solution (i.e., surface coating) for dealing with the problem of ice formation on external surfaces such as electricity lines, antennas of all types including dish antennas, external surfaces of ships including decks, rails, weapons, and antennas, and on the external surfaces of air vehicles in flight. The inventor's research efforts that culminated in the invention were directed to the specific problem of icing on air vehicles, however the results described herein are applicable to any surface, such as those described above, on which ice can form as a result of the impact of drops of supercooled water.

[0056] In an effort to find a practical solution to this problem the inventors investigated various substrates and the ability of several commercial hydrophobic or superhydrophobic coatings to prevent or limit ice formation under icing conditions. The most effective solution was found to be a combination of a thermally insulative layer (for example, rigid foam) and a superhydrophobic coating for example, Ultra Ever Dry (UED).

[0057] As mentioned earlier, the parameters that describe the ability of a water drop to wet a surface are the angle at which a water drop will slide or roll off the surface (designated as the roll-off angle) and the contact angle hysteresis (CAH) of the drop on the surface.

[0058] In order to measure the slide/roll-off angle the inventors used a tilt stage and measured the tilt angle with a digital angle meter. At least five drops were measured for each substrate. The measurements were carried out by starting at a random tilt angle between zero and thirty degrees. If the drop slid or rolled off the surface, then the angle was reduced gradually until the angle was reached at which no sliding or roll off occurred. If the drop didn't slide or roll off the surface, then the angle was increased gradually until the angle was reached at which sliding or roll-off occurred.

[0059] Typical results for the measured slide/roll-off angle for different substrates as a function of their coatings are shown in FIG. 5. Surfaces for which the slide/roll-off angle is less than ten degrees are classified as superhydrophobic. It can be seen that the substrates coated with UED have an especially low slide/roll-off angles.

[0060] A goniometer was used to measure the contact angle (CA) and hysteresis of the contact angle (CAH) on the substrates and the coatings. The measurements were made by photographing the drops on the surface and processing the images. The hysteresis was measured by adding and removing water from a drop in a horizontal orientation, as shown in FIG. 2.

[0061] At least five measurements were made for each type of substrate. FIG. 6 shows typical results for measurements of CAH for different substrates and coatings. Seen in FIG. 6 are the CAH indicated by the solid bars, the advancing angle represented by the squares, and the receding angle represented by triangles. In FIGS. 5 and 6 the hydrophobic coating (2.sup.nd bar from the right) is applied to a carbon-epoxy composite substrate.

[0062] One of the most desirable characteristics of an icephobic surface is a low value of CAH. This parameter is connected with the ability of surfaces to repel water and as a measure of the adhesion of ice to the surface. From the results shown in FIG. 6, it is apparent that the surfaces coated with UED (Superhydrophobic coating on foam) stand out for having an especially low CAH.

[0063] In order to characterize the formation of ice from water drops preliminary experiments were carried out. In these experiments coated and uncoated substrates were cooled to −20° C. and water drops with a diameter of 1-2 mm at a temperature of 4° C. were dripped onto the substrates from a height of 40 cm. The actual temperature of the substrates was measured to be −8° C. The experiments were filmed with a high speed camera. From the results of the experiments it was seen that ice formed on all of the substrates within a very short time with the exception of substrates coated with UED (Superhydrophobic coating on foam). These experiments were a feasibility study for finding a surface that would delay formation of ice from water droplets that impact upon it with speed. FIG. 7 shows the response of water droplets impacting different surfaces at the time of impact and at different periods of time after impact.

[0064] In FIG. 7 it can be seen the behavior of the drop at different stages from the instant of impact until it stabilizes into a new state after impact, attached to the surface and turned into ice or rebound from the surface. In the first stage after impact similar behavior can be seen for all substrates—stretching and spreading of the drop on the substrate until it reaches a maximum radius. After this waves develop within the drop that in most of the cases causes concentration of the mass of water around the circumference. However with UED (Superhydrophobic coating on composite) the drop isn't stable enough to withstand the waves and the drop breaks up into small round drops, which begin to separate from the substrate. It is important to note that beginning when the drop impacts the surface and all of the time that it is in contact with the surface there is a process of transfer of heat from the drop to the substrate. In the next stage there begins movement of water towards the center of the drop. For each material this process is governed by its surface energy, the morphology of the surface, and the kinetic energy that remains in the drop. It can be seen that on the reference substrate of the composite material (Composite) a minimal shrinking is obtained and ice with a very low contact angle is formed. On the substrates coated with hydrophobic coatings there is movement towards the center and a larger contact angle, however ice still formed. On the substrate coated with UED the water drops exploded into smaller droplets and rebounded from the surface. In addition there occurs a unique phenomenon that, after rebounding from the surface, the droplets recombined into a single large drop. This phenomenon was not known to the inventors from any of their own previous work or from any of the prior art work by other researchers with which the inventors are familiar.

[0065] The high speed images of the experiment served for quantitative analysis of the dynamics of water drops during and after impacting on the surfaces. The speed of impact was measured from the images to be ˜3 m/s (˜11 kmh). FIG. 8 show the normalized diameter of the drop as a function of time from the instant of impact with a surface tilted at angles of 25° respectively. The temperature of all of the substrates was −8° C. For the substrates not coated with Superhydrophobic coating (UED), at first the diameter grew (spreading of the drop on the surface of the substrate) and some time after that began to shrink asymptotically to a value D/D.sub.0>1, where D is the instantaneous diameter and Do is the starting diameter of the drop. For the substrates coated with UED the diameter grew after impact and afterwards shrank rapidly (less than 5 msec) until complete separation from the surface.

[0066] Another parameter that was measured was the normalized speed of motion of the drop on the substrate after impact. In this experiment the x-axis was defined to be in the plane of impact. FIG. 9 shows the normalized velocity (V/V.sub.0, where V is the instantaneous velocity and V.sub.0 the velocity at impact) of a drop of water as a function of time after impacting a surface tilted at 25°. The temperature of all of the substrates was −8° C. For the substrates that were not coated with UED, the velocity of the drop fell rapidly to zero, that is the drops lost all of their kinetic energy on impact—apparently as a result of friction—stopped, and froze. For the surfaces with low CAH, i.e. those that were coated with UED, the loss of kinetic energy was very low such that it didn't prevent the drop from returning to its original shape and separating from the surface. On some of the substrates drops were observed that changed the direction of their velocity, but they lacked sufficient energy to complete the process. These surfaces had a CAH of 25°-35°. As opposed to these substrates, on those that were coated with UED the spread out to a maximum value (V=0), changed the direction of their velocity, returned to the shape of a drop, and rebounded from the surface, That is the drops didn't lose much kinetic energy on impact.

[0067] In order to investigate the behavior of water drops impinging on surfaces under conditions that more closely resemble those encountered by air vehicles in flight experiments were carried out in an environmental test chamber, which is capable of reaching temperatures of −40° C. and water content in the air of between 0.25 g/m.sup.3 and 2.7 g/m.sup.3 depending on the temperature. The environmental test chamber has an opening in its top that allows dripping drops of water into it and temperature and humidity controls that allow accurate measurement of both of these parameters.

[0068] Experiments were carried out by dripping drops of water having diameters between 0.4-1.5 mm from a height of 55 cm onto coated and uncoated substrates supported inside the environmental test chamber. The speed of the drops at impact, as measured from the high speed film, was 10-12 Kph. The experiments were carried out at temperatures between −10° C. to −40° C. They were filmed with at a speed of 5000 frames/sec. For example, a neat aluminum substrate showed ice formation at all temperatures, UED-coated aluminum prevented ice formation down to −10 C. A composite substrate coated with UED showed no ice formation up to −20° C., in contrast to an untreated identical substrate, upon which ice formed at all conditions tested.

[0069] From the results of all of the experiments described herein above, both in the laboratory and in the environmental test chamber, it was observed that the foam substrates coated with UED succeeded to repel water drops at temperatures down to −30° C. It appeared to the inventors that the substrate upon which the coating is applied influences the ability to prevent formation of ice, apparently because of the rate of transfer of heat between the surface and the drop.

[0070] From the results of the experiment the influence of several phenomena related to formation of ice and/or the ability to prevent ice formation was observed. Thus, the combination of an insulating surface (i.e. foam) with a superhydrophobic coating (i.e. UED) succeeded together to repel the drops of water and prevent formation of ice on various surfaces under the conditions described above. The work done to find a low density material with a low thermal transfer coefficient that can serve as a substrate for the superhydrophobic coating will be described herein below. The ice that formed on the substrates coated with the superhydrophobic coating froze with a high contact angle (CA) from which the inventors concluded that the superhydrophobic coating maintains its superhydrophobic property even at low temperatures. Additionally, freezing at a high CA results in a small contact area between the ice and the surface; therefore it was reasonable to assume that the adhesion between the ice and the surface is low.

[0071] Another significant insight gained from the work described above is that there is no direct relationship between superhydrophobicity of a surface and its ability to prevent and/or delay ice formation. The indicators of super hydrophobicity are CAH and the roll-off angle. FIGS. 5, and 6 shows that a random fiber surface without a coating and foam substrates coated with UED have low roll-off angles and CAH; however, experiments showed that each surface has a different ability to prevent ice formation.

[0072] The next stage of the work was to measure the ability of different surfaces to delay or prevent ice formation under conditions that would be encountered by an air vehicle flying at speeds of 100 to 200 kph. These experiments were carried out in an ice tunnel in which the flow of air reaches 112 kph, the size of the water drops and amount of water in the air (according to the temperature) is controlled, and the temperature reached −20° C. (although at high speeds the temperature was lower). By this stage in the research it was clear that a superhydrophobic coating would form part of the solution to the problem that the inventors were trying to solve. Therefore, various substrates, with and without superhydrophobic coatings were glued to aluminum plates and attached to support rods inside the tunnel. In addition, a control surface made of composite material was also attached to the support rods.

[0073] From the results of the experiments it is seen that the superhydrophobic coating was effective, providing an insulating substrate was incorporated as well. In some of the cases, this combination prevented ice formation almost completely, in others it reduced the percent of coverage of ice on the substrate and in others it reduced the thickness of the layer of ice. In the cases in which ice formed, it formed at a high contact angle and the adhesion of the ice to the substrate was considerably weaker than to the reference substrates. The adhesion of the ice to the substrates was tested qualitatively by spraying compressed gas at low pressure and measuring the time and visually observing the way in which the ice was detached from the substrate. An additional affirmation to the importance of the combination of an insulating layer with a superhydrophobic coating was received when testing a composite wing, consisting of structural foam within.

[0074] It appears that the speed of the drop/air vehicle has a large influence on the ability to form ice on a surface. In the environmental test chamber, the speed of the drops was 10 kph and ice didn't form at −10° C. on any of the substrates, with or without surface morphology, that were coated with a superhydrophobic coating. In the ice tunnel at similar temperatures at drop velocities of 110 kph ice formed on all of the substrates without surface morphology, under all conditions. Indeed a reduction in the coverage and adhesion was achieved when compared to the reference substrates without the superhydrophobic coating; however, ice did form. The main conclusions of the investigations carried out by the inventors at this stage that are relevant to solving the problem of preventing the formation of ice on the surfaces of air vehicles in flight are: [0075] Slow transfer of heat between the drops and the surface raises the chances of delaying/preventing formation of ice in the super cooled drops. [0076] Rebound of the drop, caused by superhydrophobicity of a surface, is essential for minimizing the contact time between drop and surface, thereby limiting the time available for the heat transfer to occur. [0077] Neither heat transfer nor superhydrophobicity by themselves, are sufficient to delay/prevent ice formation.

[0078] The essence of the invention can be understood by referring to FIG. 10, which is a graph of temperature vs. time that schematically shows the different stages of ice formation within a drop of super-cooled water. In the transition between stages B and C, the latent heat is released, enabling a phase transfer from liquid to solid, which is thermodynamically more stable. This leads to ice formation. Most of the latent heat is transferred from the liquid drop to the surface that it impacts. A property of the surface that influences the rate of transfer of this heat is the heat transfer constant K. D marks the end of melting and crystal formation. Crystal incubation takes place between D and E, and cooling at a constant rate occurs between E and F where crystal burst takes place.

[0079] The solution to the problem of preventing the formation of ice by drops of super cooled water impacting on the surface of a substrate that is provided by the inventors is to intervene in the stage between B and C. The intervention comprised delaying the transfer of heat by placing a “generic coating” that has a low value of K between the drop and the surface. In addition a super hydrophilic coating, which very strongly repels the water drop, is applied on top of the generic coating to shorten the time of contact of the drop with the surface.

[0080] The results of all of the above experiments show that a thermally insulating medium that repels water (i.e. superhydrophobic) constitutes a solution to the issue of ice formation on surfaces. An example for such an implementation is a combination of insulating foam with a superhydrophobic coating on it. This implementation can be provided in a coating comprised of one layer, i.e. an insulating material whose surface is superhydrophobic; in a coating comprised of two separate layers, i.e. an insulating material on top of which is applied a superhydrophobic coating; or a coating comprised of several layers, which constitute a combination of insulating and superhydrophobic materials.

[0081] This structure combines two central features: [0082] 1. Shortening the time the drop is on the surface and reducing the area of contact of the drop with the surface by means of the superhydrophobic coating. [0083] 2. Preventing the transfer of enough heat from the drop to the surface to form ice by means of the insulating layer between the body of the object to be protected and the drop.

[0084] This approach provides a solution applicable to all substrates, whether metal, composite materials, or any other, and is referred to as a “Generic Coating”.

[0085] The concept of a generic coating was investigated by producing samples consisting of a substrate (i.e., composite or aluminum) with an insulating layer (e.g., rigid foam), and a superhydrophobic coating (e.g., UED). This was characterized by means of static and dynamic experiments.

[0086] Two commercial rigid foams Rohacell© RIMA 51 and Rohacell© HP 31 were used in the experiments as well as a syntactic foam prepared by the inventors that comprised microballoons within an epoxy matrix foam, the components of which were mixed together and subsequently cast onto a substrate after which UED was sprayed onto the foam after its curing.

[0087] An experiment was performed to determine the minimum thickness of the prepared insulating layer on aluminum that would show improved ice prevention. Three different thicknesses were examined, and it was concluded that 1.15 mm is the ideal thickness of the insulating layer on a foundation of aluminum to prevent icing.

[0088] After the static experiments, dynamic experiments were carried out in the ice tunnel. Samples of carbon-epoxy composite material similar to those used in air vehicles were coated with the insulating layer described herein above and a top coating of UED. Samples of composite material coated only with UED were also used in the experiments.

[0089] The dynamic experiments were carried out under the following conditions: temperature −18° C.; drops of super cooled water with diameters of tens of microns; wind speed 200 kph, and angle between the wind direction and the surfaces 45°.

[0090] Very little ice formed on the samples with the generic coating and UED and no ice piled up, although continuous layers of ice formed on the supports holding the samples. The experiment was halted after one minute since no ice piled up on the samples. The technician who entered the ice tunnel to remove the samples was able to remove the ice that had formed by blowing on the surface.

[0091] On the samples coated with UED only, ice formed in a discontinuous manner in the form of islands in a similar manner to that observed in earlier experiments in the ice tunnel described herein above.

[0092] Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, without exceeding the scope of the claims.

BIBLIOGRAPHY

[0093] [1] Karl Borg, Vincent Cregan, Andrew Fowler, Mark McGuinness, Stephen B. G. O'Brien, Leonard W. Schwartz and Vladimir Zubkov, “Partial Wetting Phenomena in Superhydrophobic Microchannels”, study group report. [0094] [2] Vaibhhab Bahadur, L. Mischenko, Benjamin Hatton, J. Ashley Taylor, Joanna Aizenberg and Tom Krupenkin, “Predictive Model for Ice Formation on Superhydrophobic Surfaces”, Langmuir, Sept. 07, 2011, 14143-14150. [0095] [3] N. Cohen, A. Dotan, H. Dodiuk, and S. Kenig, Thermomechanical Mechanisms of Reducing Ice Adhesion on Superhydrophobic Surfaces, Langmuir 2016, 32, 9664-9675 [0096] [4] Lindamae Peck, Charles C. Ryerson, and C. James Martel, “Army Aircraft Icing”, ERDC/CRREL TR-02-03, September 2002. [0097] [5] Alidad Amirfazli and Carlo Antonini, “Fundamentals of Anti-Icing Surfaces”, chapter 11 [Please provide the bibliographic details for the book.]