WO2017062712A1

WO2017062712A1 - Methods for creating a porous layer on a substrate

Info

Publication number: WO2017062712A1
Application number: PCT/US2016/055898
Authority: WO
Inventors: Kim Philseok
Original assignee: Slips Technologies, Inc.
Priority date: 2015-10-07
Filing date: 2016-10-07
Publication date: 2017-04-13
Also published as: EP3359501A1

Abstract

Methods of making a porous layer on a permanent or temporary substrate. The methods can include coating a surface of a substrate with a liquid composition in a single step, where the liquid composition includes suspended nanoparticles, a porogen, and a solvent. Subsequently, the methods can include removing the solvent, and then removing the porogen and annealing the nanoparticles to form the porous layer on the substrate. The substrate can be an optically transparent substrate. The nanoparticles can be metal oxide nanoparticles.

Description

METHODS FOR CREATING A POROUS LAYER ON A SUBSTRATE

FIELD

The present teachings relate to methods of creating porous layers on substrates. More specifically, the present teachings relate to methods of creating an optically transparent porous layer on a permanent or temporary substrate.

BACKGROUND

Optically transparent coatings having self-cleaning properties (e.g., a repellant surface) are important for many different applications such as medical devices (e.g., endoscopes), plumber's scopes, underwater optics, solar panels, windows, and lab ware. Many methods of creating optically transparent coatings are known.

However, the creation of a repellant surface on an optical component, which typically is a hard, smooth, optically-clear substrate, usually first requires the creation of a porous layer on the substrate. Various methods for creating a porous layer on a substrate exist, including application of materials to the substrate such as textiles, polysiloxanes and fluorogels, and plating, chemical deposition from solutions, spin coating, chemical vapor deposition, atomic layer deposition, electrochemical deposition, thermal evaporation, sputter coating, pulsed layer deposition, cathodic arc deposition, electrospray and layer-by-layer ("LbL") deposition. The latter technique is the most promising where highly optically-transparent surfaces are desired. However, layer-by-layer deposition is a multi-step process that is repeated for each layer to be added to the substrate and as such, can be laborious, time-consuming, and expensive. In addition, the scalability of the LbL deposition process is challenging.

Thus, there is a need to improve methods for the creation of porous layers on substrates, particularly optically transparent substrates, which methods can be time and cost efficient as well as scalable to large substrates.

SUMMARY

In light of the foregoing, the present teachings provide methods and materials that can address various deficiencies and/or shortcomings of the state-of-the-art, including those outlined above. That is, the present teachings provide methods of making a porous layer on a permanent or temporary substrate such as an optically-transparent substrate using a single application or single coating of a liquid composition on the substrate to create a multi-layered nanoparticle porous layer. Subsequently, the porous layer can be chemically functionalized and a lubricating liquid added to create a stable, immobilized liquid repellant surface. The present teachings also provide articles of manufacture made by the methods of the present teachings.

Thus, in one aspect, the present teachings provide methods for creating a porous layer on a substrate such as optically-transparent substrate. The methods of the present teachings generally include coating a surface of a substrate with a liquid composition in a single step, where the liquid composition includes suspended nanoparticles, a porogen, and a solvent.

Subsequently, the methods can include removing the solvent, and then removing the porogen and annealing the nanoparticles to form the porous layer on the substrate. In certain methods of the present teachings, the porous layer can be removed from the substrate, for example, where a temporary substrate is used.

In various embodiments, the nanoparticles are metal oxide nanoparticles. Although the nanoparticles need not be metal oxide nanoparticles, for brevity and simplicity, the discussion and description herein will focus on metal oxide nanoparticles as an exemplary material for creating a porous layer according to the present teachings. Nevertheless, the scope of the present teachings should not be limited to only metal oxide particles and other materials can be substituted for "metal oxide nanoparticles" in the description and practice of the present teachings, unless otherwise understood from the context, description, or particular application.

The methods can include making a porous layer of nanometer scale thickness. The methods can include controlling the thickness of the coating to influence the thickness of the porous layer. The methods can include making a porous layer or surface having an increased surface to planar projection area ratio on the surface of the substrate.

Removing the porogen and annealing the metal oxide nanoparticles can form or result in a coherent porous layer with inter-particle binding on the substrate and multiple levels of the metal oxide nanoparticles above the substrate. The porous layer, which includes or is a randomly-packed, multi-layered metal oxide nanoparticle porous structure (i.e., not a monolayer of metal oxide nanoparticles), can be particularly useful for the creation of a repellant surface, for example, a slippery liquid-infused porous surface as described herein and elsewhere. That is, the formed porous layer can have sufficient porosity to receive and hold a liquid stably within and above the thickness of the porous, multiple-level nanoparticle structure.

More specifically, methods of the present teachings can include functionalizing chemically the porous layer on the substrate to provide a functionalized porous layer; and introducing a lubricating liquid to wet spontaneously and adhere to the functionalized porous layer to form a stabilized liquid overlayer immobilized in, on and over the functionalized porous layer, without dewetting from the substrate, to form a repellant surface. The repellant surface can be a slippery liquid-infused porous surface ("SLIPS"™).

To that end, chemical functionalization or modification of the porous layer, lubricating liquids, and their use to form a slippery liquid-infused porous surface are well known and already described in detail elsewhere and will not be repeated here. For example, U.S. Patent Application Publication No. 2014/0147627 is incorporated by reference herein for all purposes, and particularly for its description and teachings relating to chemical

functionalization of surfaces, lubricating liquids, and their use in forming a slippery liquid- infused porous surface.

In various embodiments of the methods of the present teachings, the liquid

composition includes a porogen that can be non-ionic and/or water-soluble, for example, a water-soluble polymer or a water-soluble non-ionic polymer. The liquid composition can include nanoparticles such as silica in its various forms (e.g., fumed, precipitated, and condensation-polymerized (colloidal)) and sized in the nanometer range. The solvent of the liquid composition can include water. The solvent of the liquid composition can be a solvent system, for example, include a first solvent and a second solvent such as an aqueous ethanol or propanol solution. The liquid composition can further include a surfactant, such as non- ionic surfactant having hydrophilic characteristics and properties. When a surfactant is present in the liquid compositions, the methods can include removing the surfactant.

In some embodiments of the methods of the present teachings, the coating of the surface of a substrate can be accomplished using a single application or single coating.

Various techniques can be used such as spraying, dip coating, drop casting, spin coating, screen printing, inkjet printing, slit coating, and draw down casting the liquid composition. The thickness of the coating can be controlled using these techniques thereby influencing the thickness of the resulting porous layer.

Removing the solvent after deposition of the coating of the liquid composition on the substrate can be achieved by evaporating the solvent. To maintain a relatively consistent composition across the surface of the substrate, evaporating the solvent should be slow to avoid the "coffee ring" effect. To that end, evaporating the solvent can be at a temperature that is less than or lower than the boiling point of the solvent (or solvent system). Removing the porogen can be accomplished using different techniques. For example, the porogen can be removed by combusting the porogen, which a high temperature is also favorable for concurrently annealing the metal oxide nanoparticles.

In certain embodiments, the methods of making a porous layer can include coating a surface of a optically-transparent substrate with a liquid composition in a single step, where the liquid composition includes suspended silica nanoparticles, a water-soluble polymer, and water and/or ethanol. Subsequently, the methods can include removing the water and/or ethanol, and then removing the water-soluble polymer and annealing the silica nanoparticles to form the porous layer on the substrate.

In particular embodiments, the methods of making a porous layer can include coating a surface of an optically-transparent substrate with a liquid composition in a single step, where the liquid composition includes colloidal silica nanoparticles, a water-soluble polymer, a surfactant, water, and ethanol. Subsequently, the methods can include removing the water and ethanol, and then removing the water-soluble polymer and surfactant, and annealing the colloidal silica nanoparticles to form the porous layer on the substrate.

In another aspect, the present teachings include an article of manufacture including the porous layer made by a method of the present teachings. The article of manufacture can be or include the porous layer adhered to a substrate on which the porous layer was formed. The article of manufacture can be or include the porous layer itself, apart from the substrate, for example, where a temporary substrate is used in the method of manufacture. The article of manufacture can include a repellant surface such as a slippery liquid-infused porous surface. The article of manufacture can be or include an optical component.

The foregoing as well as other features and advantages of the present teachings will be more fully understood from the following figures, description, examples, and claims. DESCRIPTION OF DRAWINGS

It should be understood that the drawings described below are for illustration purposes only. Like numerals generally refer to like parts. The drawings are not necessarily to scale, with emphasis generally being placed upon illustrating the principles of the present teachings. The drawings are not intended to limit the scope of the present teachings in any way.

FIG. la- Id are SEM images of a porous layer made by a single application liquid composition of the present teachings (FIGS, la and lc) and by an LbL deposition method (FIGS, lb and Id).

FIG. 2 is a schematic diagram of the application of a cover slip having a slippery liquid-infused porous layer to a plumber's scope.

FIG. 3a-3d are images taken using a plumber's scope, where FIGS. 3a and 3b show the plumber's scope without a slippery liquid-infused porous layer covering its optical lens initially and after dipping in motor oil three times, respectively; and FIGS. 3c and 3d show the plumber's scope with a slippery liquid-infused porous layer covering its optical lens initially and after dipping in motor oil three times, respectively.

DETAILED DESCRIPTION

It now has been discovered that a porous layer of metal oxide nanoparticles can be created on a substrate using a single coating or single deposition process thereby avoiding the laborious and expensive multi-step LbL deposition procedure. That is, a liquid composition including suspended metal oxide nanoparticles and a porogen in a solvent can be deposited on a substrate in a single step, followed by removal of the solvent, then removal of the porogen and annealing of the metal oxide nanoparticles to form the porous layer on the substrate. In addition to simplifying the overall process for creating a porous layer, the present teachings provide a scalable process for larger and more complex substrates as the liquid composition of the present teachings can be deposited or coated on to a substrate by a variety of techniques such as spraying or spray coating.

Moreover, the present teachings provide methods for creating a transparent porous layer that can have sufficient porosity to receive and hold a lubricating liquid stably within, on and above the thickness of the porous multi-layer nanoparticle structure. Accordingly, a transparent porous layer on an optically-transparent substrate can be chemically functionalized and contacted with a lubricating liquid to create an immobilized, stable liquid repellant surface such as a slippery liquid-infused porous surface. Such a repellant surface can advantageously provide self-cleaning, anti-fogging, and/or anti-reflective or anti-glare properties to various optical components such as endoscopes, plumber's scopes, and underwater optical devices.

Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including, or comprising specific process steps, it is contemplated that compositions of the present teachings also consist essentially of, or consist of, the recited components, and that the processes of the present teachings also consist essentially of, or consist of, the recited process steps.

In the application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that the element or component can be any one of the recited elements or components, or the element or component can be selected from a group consisting of two or more of the recited elements or components.

Further, it should be understood that elements and/or features of a composition, an apparatus, or a method described herein can be combined in a variety of ways without departing from the spirit and scope of the present teachings, whether explicit or implicit herein. For example, where reference is made to a particular structure, that structure can be used in various embodiments of apparatus of the present teachings and/or in methods of the present teachings, unless otherwise understood from the context. In other words, within this application, embodiments have been described and depicted in a way that enables a clear and concise application to be written and drawn, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the present teachings and invention(s). For example, it will be appreciated that all features described and depicted herein can be applicable to all aspects of the invention(s) described and depicted herein.

It should be understood that the expression "at least one of includes individually each of the recited objects after the expression and the various combinations of two or more of the recited objects unless otherwise understood from the context and use. The expression "and/or" in connection with three or more recited objects should be understood to have the same meaning unless otherwise understood from the context.

The use of the term "include," "includes," "including," "have," "has," "having," "contain," "contains," or "containing," including grammatical equivalents thereof, should be understood generally as open-ended and non-limiting, for example, not excluding additional unrecited elements or steps, unless otherwise specifically stated or understood from the context.

The use of the singular herein, for example, "a," "an," and "the," includes the plural (and vice versa) unless specifically stated otherwise.

Where the use of the term "about" is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term "about" refers to a ±10% variation from the nominal value unless otherwise indicated or inferred.

Where a percentage is provided with respect to an amount of a component or material in a structure or composition, the percentage should be understood to be a percentage based on weight, unless otherwise stated or understood from the context.

Where a molecular weight is provided and not an absolute value, for example, of a polymer, then the molecular weight should be understood to be an average molecule weight, unless otherwise stated or understood from the context.

It should be understood that the order of steps or order for performing certain actions is immaterial so long as the present teachings remain operable. Moreover, two or more steps or actions may be conducted simultaneously.

At various places in the present specification, values are disclosed in groups or in ranges. It is specifically intended that the description include each and every individual subcombination of the members of such groups and ranges and any combination of the various endpoints of such groups or ranges. For example, an integer in the range of 0 to 40 is specifically intended to individually disclose 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,

16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, and

40, and an integer in the range of 1 to 20 is specifically intended to individually disclose 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20. The use of any and all examples, or exemplary language herein, for example, "such as" or "including," is intended merely to illustrate better the present teachings and does not pose a limitation on the scope of the invention unless claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the present teachings.

The present teachings provide methods of creating a porous layer of metal oxide particles on a permanent or temporary substrate, which can be an optically-transparent substrate. The porous layer of metal oxide particles can be transparent making the use of the present teachings particularly relevant for optical -related devices. The methods can generally include depositing, for example, coating, a liquid composition in a single step on a surface of a substrate, where the liquid composition includes suspended metal oxide nanoparticles, a porogen, and a solvent. Subsequently, the solvent can be removed such as by evaporation, and then the porogen can be removed, for example, by combustion or calcination, and the metal oxide nanoparticles can be annealed, often under the same conditions for porogen removal, to form the porous layer on the substrate. The porous layer can be anti -reflective or have anti-glare properties with or without the presence of a slippery liquid-infused porous surface.

The process of depositing the liquid composition can be coating the surface of the substrate with the liquid composition in a single step, i.e., using a single application or a single coating of the liquid composition. The thickness of the coating, among other parameters, can determine the thickness of the resulting porous layer. Accordingly, the methods of the present teachings can include controlling the thickness or the height of the multi-layered metal oxide nanoparticle-structured porous layer similar to that achieved by the multi-step layer-by-layer deposition procedure but using only a single deposition step. In practice, the methods can include controlling the thickness of the coating to influence the thickness of the porous layer. In this regard, the methods of the present teachings can permit similar control over thickness of the resulting porous layer compared to LbL deposition processes.

The substrate onto which the porous layer can be made typically is a hard, smooth substrate, for example, having a hard and/or smooth surface. The substrate can be an optically-transparent substrate, for example, glass, fused silica, quartz, sapphire, tin-doped indium oxide or indium tin oxide ("ITO"), aluminum-doped zinc oxide ("AZO"), indium- doped cadmium oxide, fluorine-doped tin oxide ("FTO"), and combinations thereof. In certain methods of the present teachings, the porous layer can be removed from the substrate such as when a temporary substrate is used to make the porous layer. The temporary substrate can be removed after or during the formation of the porous layer, for example, concurrent with annealing the nanoparticles. A temporary substrate can be optically-transparent or can be translucent or opaque.

Nanoparticles in the liquid composition of the presenting teachings can include metal oxides, metals, carbides, chalcogenides, nitrides, sulfides, selenides, telurides, antimonides, arsenides, borides, carbonitrides, hydrides, and combinations thereof, where these materials are sized in the nanometer range. Metal oxide nanoparticles can include silicon dioxide (silica), ITO, AZO, indium-doped cadmium oxide, FTO, doped zinc oxide, titanium dioxide (titania), zirconium dioxide (zirconia), alumina, iron oxides, nickel oxides, zinc oxides, and combination thereof, where these metal oxides are sized in the nanometer range.

Nanoparticles useful for creating a porous layer can be include certain metals and other materials; however, for optical transparency, the choice of other metals and materials is limited and average particles sizes may need to be lower than those of metal oxides.

Average particle sizes of the nanoparticles such as metal oxide nanoparticles generally can be from about 1 nm to about 35 nm, such as between about 3 nm to about 30 nm, or between about 5 nm to about 25 nm, or from about 5 nm to about 20 nm, or from about 3 nm to about 15 nm. The metal oxide nanoparticles can be present in the liquid composition in a weight percentage based on the total weight of the liquid composition of about 20%, less than or equal to about 20%, about 15%, less than or equal to about 15%, about 10%, less than or equal to about 10%, about 8%, less than or equal to about 8%, about 5%, less than or equal to about 5%), about 4%, less than or equal to about 4%, about 3%, less than or equal to about 3%, about 1%), less than or equal to about 1%, about 0.5%, less than or equal to about 0.5%, about 0.3%), less than or equal to about 0.3%, about 0.1%, or less than or equal to about 0.1%.

The metal oxide nanoparticles typically are charged or ionic, for example, having ionic character. The specific form of the metal oxide nanoparticles is not important, provided the metal oxide nanoparticles are able to remain suspended in the liquid composition. That is, the metal oxide nanoparticles should not aggregate and/or crash out of the liquid composition but should be stably suspended in the liquid composition. The suspended metal oxide nanoparticles can be uniformly suspended in the liquid composition. The suspended metal oxide nanoparticles can be or include colloidal silica. Although the metal oxide nanoparticles can be chelated to assist with suspension in the liquid composition, the complex or resulting coating may not be transparent and appropriate for optical applications.

The choice of metal oxide nanoparticles can be made based on the substrate on which a porous layer is made. For example, where glass is the substrate, silica can be used as the metal oxide nanoparticles. Where sapphire is the substrate, alumina can be used as the metal oxide nanoparticles. Indeed the choice of the (metal oxide) nanoparticles for a particular substrate (as well as the choice of porogen) should take into account the thermal

characteristics and properties of the nanoparticles, the substrate and the porogen, and the conditions to which they will be subjected during the methods of the present teachings, for example, to provide for the removal of the porogen and annealing of the nanoparticles into a porous layer while maintaining the integrity of the underlying substrate.

The porogen of the liquid composition is a sacrificial or fugitive material that can be removed after deposition of the liquid composition on the substrate. The porogen should be soluble in the liquid composition, for example, in the solvent or solvent system of the liquid composition. A porogen can be non-ionic or in certain cases, can be ionic. The porogen often is water-soluble, for example, a water-soluble polymer or a water-soluble non-ionic polymer. Water solubility of the porogen is a desired characteristic where the liquid composition includes water. Examples of porogens include poly(vinylpyrrolidone) ("PVP"), polyvinyl alcohol ("PVA"), polyacrylic acids, polyacrylamides, PVA-PEG co-polymers, waterborne polymers such as polyurethanes, polyamide epoxies, and acrylics, and biopolymers such as polysaccharides, alginates, and carrageenan. The porogen can be present in the liquid composition in a weight percentage based on the total weight of the liquid composition of about 35%, less than or equal to about 35%, about 30%, less than or equal to about 30%, about 25%), less than or equal to about 25%, about 20%, less than or equal to about 20%, about 15%), less than or equal to about 15%, about 10%, less than or equal to about 10%, about 5%), less than or equal to about 5%, about 3%, less than or equal to about 3%, about 2%, or less than or equal to about 2%. The solvent of the liquid composition can be a single solvent such as water or ethanol, or can be a solvent system including at least a first solvent and a second solvent such as water and ethanol. Solvents for the methods of the present teachings can include water, ethanol, methanol, propanol, acetone, parachlorobenzotrifluoride and combinations thereof. The solvent should be able to solubilize the porogen and suspend the metal oxide particles. The amounts or ratios of solvents in a solvent system can be adjusted to provide the proper balance of solubility of porogen, disperability of metal oxide nanoparticles, and removability such as evaporability of the solvent system. For example, for an aqueous ethanol solvent system for a silica nanoparticle and PVP system, the weight ratio of water to ethanol can be between about 0.01 to about 0.5, between about 0.02 to about 0.4, between about 0.03 to about 0.35, between about 0.05 to about 0.3, or between about 0.075 to about 0.25, such as between about 0.1 to about 0.22, or between about 0.1 to about 0.2. The amount or weight percentage of solvent in a liquid composition can vary but is usually considered to be the balance of the composition depending on the amounts of other components added.

The solvent should evaporate at a relatively low temperature but should contain a low amount of or no volatile organic compounds ("VOCs"). In particular embodiments, the solvent is a non-VOC solvent, for example, a VOC exempt solvent. The choice of solvent is dependent on the solubility of the porogen and its ability to form stable suspensions of the metal oxide particles. For example, for PVP, either ethanol, propanol, or acetone can be used because these solvents are generally safe and in particular, the latter is a non-VOC, and suspensions formed with colloidal silica as the metal oxide nanoparticles have been observed to be stable.

Related to solvent choice, a prolonged drying time of a coating of a liquid composition can result in the "coffee ring" effect. This phenomenon is an evaporation effect that occurs when a liquid drop on a surface has a non-zero contact angle and a pinned contact line, such that the liquid lost from evaporation at the contact line is replenished by the outward capillary flow from the interior of the droplet. This replenishing action results in the transportation of particulates to the outer edge of the coating, where the particulates are deposited. The mitigation of this effect while expediting the drying time is a trade-off, which can be assisted with the use of a surfactant. In addition, despite the choice of solvent to mitigate the coffee ring effect, coatings or films deposited on glass substrates can still display some iridescence effects, which can indicate that the film may not have uniform thickness over its entirety. A surfactant can assist in reducing the surface tension of the liquid composition, and also can assist in reducing surface tension gradients to further mitigate the presence of the coffee ring effect.

Accordingly, a liquid composition of the present teachings can further include a surfactant. A surfactant can assist with the drying time of the solvent, with creating a uniform coating, and/or with maintaining a stable suspension of metal oxide nanoparticles and avoiding aggregation. Because of the aqueous nature of the liquid composition, a surfactant having hydrophilic characteristics and properties can be useful. Examples of surfactants include polypropylene glycols ("PPGs"), polyethylene glycols ("PEGs") such as PEG-300 and PEG-600, having an average molecular weight of 300 and 600, respectively. In certain embodiments, the PEG or PPG has an average molecular weight less than about 1000 such as less than about 800. The surfactant can be present in the liquid composition in a weight percentage based on the total weight of the liquid composition of about 35%, less than or equal to about 35%, about 30%, less than or equal to about 30%, about 25%, less than or equal to about 25%, about 20%, less than or equal to about 20%, about 15%, less than or equal to about 15%, about 10%, less than or equal to about 10%, about 5%, less than or equal to about 5%), about 4%, less than or equal to about 4%, about 3%, less than or equal to about 3%), about 2%, less than or equal to about 2%, about 1%, less than or equal to about 1%, about 0.5%), or less than or equal to about 0.5%.

Selection of a surfactant or surfactant system can be based on the hydrophilic- lipophilic balance ("HLB") value. An HLB value is a method used to categorize emulsifiers based on hydrophilic-hydrophobic properties, for example, high values (> 10) indicate more hydrophilic character and low values (< 10) indicate more hydrophobic character.

For use with water or an aqueous solvent system, higher hydrophilicity is generally desired. For example, in various embodiments, the HLB value of the surfactant in the liquid composition is great than about 10, greater than about 15, greater than about 20, greater than about 25, or greater than about 30. The surfactant also can assist in maintaining a stable liquid composition, for example, increasing the shelf life of the liquid composition. A liquid composition can be deposited or coated onto a surface of a substrate by a variety of techniques, depending on the materials to be coated and parameters of the overall process. Such deposition or coating techniques can include, among others, at least one of spraying, dip coating, drop casting, spin coating, screen printing, inkjet printing, slit coating, and draw down casting the liquid composition. These techniques can be further defined, for example, spray coating of the liquid composition can be done using an aerosol spray bottle or can, or using an ultrasonic sprayer.

Removing the solvent from the liquid composition after deposition or coating on the surface of the substrate typically is accomplished via evaporation. Accordingly, control of the temperature at which the solvent is removed or evaporated can be important to reduce the coffee ring effect whereby a collared shoulder can be formed near and/or at the edges of the porous layer. Such control and reduction of the migration of metal oxide nanoparticles to the edges can increase the uniformity of the coated films and resulting porous layer.

Accordingly, evaporating the solvent can occur at a temperature less than the boiling point of the solvent. Removing or evaporating the solvent can be done at atmospheric pressure or at a reduced pressure, i.e., under a vacuum. For water and aqueous ethanol solvents, evaporating can be conducted at a temperature between about 40 °C and about 75 °C such as at a temperature between about 45 °C and 65 °C, or between about 50 °C to about 55 °C.

Removing the porogen from the coating on a surface of a substrate typically occurs after removing the solvent. In some embodiments, the removal of solvent and porogen can be accomplished concurrently, or removal of the porogen can begin before removal of the solvent is complete. Removing the porogen can include removing selectively the porogen while leaving the metal oxide nanoparticles distributed on the surface of the substrate, such as uniformly distributed on the surface of the substrate. Removing the porogen typically includes combusting or calcinating the porogen such that the metal oxide particles can be annealed concurrently. However, annealing, calcinating, and/or sintering the nanoparticles can be conducted at a different temperature such as a higher temperature and/or under a different atmospheric environment. The combusting or calcinating can be conducted in the presence of oxygen. The combusting or calcinating can be conducted at a temperature between about 250 °C and 800 °C such as between about 450 °C and 550 °C. When present, a surfactant typically is removed from the coating on a surface of a substrate by combustion or calcination. For example, removal of the surfactant can be concurrent with removal of the porogen. However, depending on the properties of the surfactant, it can be concurrently removed partially or completely with the solvent.

Methods of making a porous layer can include coating a surface of an optically- transparent substrate such as glass with a liquid composition in a single step, where the liquid composition includes suspended silica nanoparticles, a water-soluble polymer, and water and/or ethanol. Subsequently, the methods can include removing the water and/or ethanol, and then removing the water-soluble polymer and annealing the silica nanoparticles to form the porous layer on the substrate. The suspended silica nanoparticles can be colloidal silica nanoparticles.

The silica or colloidal silica nanoparticles can have an average size of between about 15 nm to about 25 nm such as about 20 nm. The silica or colloidal silica nanoparticles can be present in the liquid composition at a weight percentage based on the total weight of the liquid composition of between about 1% to about 10% such as about 5%.

The water-soluble polymer can be PVP. The PVP can be present in the liquid composition at a weight percentage based on the total weight of the liquid composition of between about 5% to about 35%, or between about 10% and 30%, such as about 25%.

In particular embodiments, the methods of making a porous layer can include coating a surface of an optically-transparent substrate with a liquid composition in a single step, where the liquid composition includes colloidal silica nanoparticles, a water-soluble polymer such as PVP, a surfactant, water, and ethanol. Subsequently, the methods can include removing the surfactant, water and ethanol, and then removing the PVP and annealing the silica nanoparticles to form the porous layer on the substrate.

The characteristics of the components of the liquid composition can be as described herein and particularly, as described above. The surfactant can be a PEG, such as PEG-600 or PEG-300. The PEG can be present in the liquid composition at a weight percentage based on the total weight of the liquid composition of between about 5% to about 35%, or between about 10%) and 30%, such as about 25%.

Subsequent to the formation of a porous layer, the porous layer can be chemically functionalized and a lubricating liquid introduced to the functionalized porous layer such as by wicking where the lubricating liquid wets spontaneously and adheres to the functionalized porous layer to form a stabilized liquid overlayer immobilized in, on and over the

functionalized porous layer, without dewetting from the substrate, thereby forming a repellant surface such as a slippery liquid-infused porous layer as described herein and elsewhere.

In an exemplary method, a liquid composition of colloidal silica particles, PVP, PEG-

300, water and ethanol is spray cast on to glass. The coating or film formed on the glass is dried for about 15 minutes at atmospheric pressure and about 50 °C. The resulting structure is calcinated for about 2 hours at 500 °C. The porous layer is modified using chemical vapor deposition and a lubricating liquid is wicked on to the modified porous layer to create a repellant surface.

The following examples are provided to illustrate further and to facilitate the understanding of the present teachings and are not in any way intended to limit the invention.

The materials used include: Colloidal silica, Ludox™ 40 (0 = 20 nm);

polyvinylpyrrolidone (MW of about 40,000); poly(ethylene glycol) (MW of about 300); and poly(diallyldimethylammoniumchloride) ("PDADMAC") (20% in water) was purchased from Sigma Aldrich. Reagent grade ethanol (> 99%) was purchased from VWR.

DuPontKrytox™ perfluoropoly ether GPL 100 ("K100") was purchased from Miller- Stephenson. Motor oil was used as the viscous hydrocarbon to test repellency of the coatings. Deionized ("DI") water was obtained using a MilliQ™ lab water system (Millipore, Billerica, MA, USA) and was used for all experiments and testing.

Silica particles were chosen as the metal oxide nanoparticles because they can create nanoscale roughness on substrates and, in the case of a glass substrate, can be strongly adhered to the surface of the glass after annealing at high temperature. PVP was chosen as the porogen because it has good solubility in aqueous systems and is already an FDA approved material, which is widely used in many industries such as cosmetics and

pharmaceuticals. PVP also was chosen because it can temporarily adhere the nanoparticles onto the substrate, and when removed at high temperature, PVP leaves behind a porous structure. Finally, PEG was used as the surfactant as it can reduce the surface tension of the solutions and can mitigate the drying effects, which can cause inhomogeneity in coating film thickness and thus, the resulting porous layer. The characterization methods included scanning electron microscopy ("SEM"), which was performed using a Zeiss Ultra55 (to avoid charging effects, the SEM specimens were coated with a 5 nm platinum/palladium coating). The wetting properties of the samples were measured using a goniometer. Static water contact angles (10 μΐ.) and contact angle hystereses were determined using built-in regression best-fits models for non-lubricated samples. For lubricated samples, the same parameters were measured.

Example 1. Creating porous layer using single deposition step

A single coating or single application liquid composition used for spray casting was prepared by first adding 1.9 g of PVP to 7.3 g of ethanol. The solution was mixed by ultrasonication until the PVP was fully dissolved. Subsequently, 0.4 g of PEG was added to the mixture and sonicated for an additional 5 minutes. Finally, 0.4 g of colloidal silica was added to the mixture and sonicated for 5 minutes. The solution was applied to 3" x 2" glass slides using an aerosol propellant spray can. The glass substrates were cleaned prior to application by oxygen plasma treatment for 1 minute. After application, the films were dried at 50 °C for 30 minutes. The PVP and PEG were removed by combustion at 500 °C.

The use of the single application liquid composition resulted in a multilayered silica nanoparticle porous layer with an interpenetrated (or porous) network. The resulting porous layer had excellent mechanical stability similar to that of a porous layer made by an LbL deposition process.

Example 2. Creating porous layer by layer-by-layer deposition (comparative example)

As a comparison, samples also were prepared using an immersive LbL assembly process. Oxygen plasma treated glass substrates (2" x 1") were submerged in a 0.1 wt % solution of PDADMAC for 5 minutes, followed by rinsing in DI water for 1.5 minutes, followed by immersion in a 0.1 wt % solution of colloidal silica for 5 minutes and then rinsed again in DI water for 1.5 minutes. This cycle was repeated 10 times to deposit a multilayered film. The polyelectrolyte was removed by combustion at 500 °C (ramped up from room temperature to 500 °C for 4 h, held at 500 °C for 2 h and ramped down from 500 °C to room temperature).

Example 3. Creating liquid repellant surface on substrates having a porous layer

Chemical functionalization of the porous layer was accomplished by applying a thin plasma-polymerized layer of octafluorocyclobutane on the surface of the substrate of Example 1 using a STS ICP-RJE for 1 minute. Subsequently, K100, a lubricating liquid, was applied to the functionalized surface by drop casting (0.44 μΙ ιηιη²). Excessive lubricating liquid was removed by allowing the lubricating liquid to drain vertically for 12 h to 24 h.

Surface treatment (chemical functionalization) and lubrication of the porous layer created in Example 2 was performed in the same manner as for the surface in Example 1. Example 4. Comparison of present teachings to LbL deposition method

The porous layer formed using the single coating of the present teachings were compared to those formed using the LbL assembly process in FIGS, la- Id. The SEM images show that the nanostructures and the homogeneity over a large length of the films formed are comparable. Surface treatment of the samples via plasma treatment reduces the surface energy of the substrate and create favorable conditions for the lubricant to wick into the porous nanostructured films. When this film is contacted by other liquids, the lubricant is not displaced, indicating that the conditions to form a thermodynamically stable SLIPS are satisfied.

Prior to the application of lubricating liquid, the water contact angle of the substrates was about 140°; however, the contact angle hysteresis could not be measured. After the application of the lubricating liquid, the apparent contact angle decreased to about 120° and the contact angle hysteresis was about 5°.

To demonstrate the applicability of the single coating process as an alternative method to LbL assembly, glass cover slips (No. 1.5) cut into circular pieces (0 = 6 mm) were spray coated with the liquid composition and a porous layer formed. Subsequent chemical functionalization and introduction of a lubricating liquid was performed to form a slippery liquid-infused porous layer. These treated glass cover slips were attached to a plumber's scope (low precision optical device) using polydimethylsiloxane as an adhesive (see FIG. 2 for schematic diagram of assembly process). Their repellency against motor oil was tested.

The attachment of the treated cover slips to the plumber's scope showed no observable effects on the optical clarity of the image seen from the plumber's scope lens as shown in FIGS. 3a and 3c. After multiple dips in a viscous hydrocarbon (motor oil), an obvious difference in the image quality was seen between the plumber's scope lens without any coating (FIG. 3b) and the treated cover slips, where there was no observable difference (FIG. 3d). The motor oil readily was shed off the lens having the slippery liquid-infused porous layer, even after multiple passes through the air-liquid interface, indicating that the lubricating liquid was retained within the nanostructure of the porous layer.

The present teachings encompass embodiments in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the present teachings described herein. Scope of the present invention is thus indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

CLAIMS What is claimed is:

1. A method of making a porous layer on a permanent substrate or a temporary substrate, the method comprising:

coating a surface of a substrate with a liquid composition in a single step, wherein the liquid composition comprises suspended nanoparticles, a porogen, and a solvent;

removing the solvent; and

removing the porogen and annealing the nanoparticles to form a porous layer on the surface of the substrate.

2. The method of claim 1, wherein the nanoparticles are metal oxide nanoparticles.

3. The method of claim 2, wherein the metal oxide nanoparticles comprise silica, tin- doped indium oxide, aluminum-doped zinc oxide, indium-doped cadmium oxide, fluorine- doped tin oxide, doped zinc oxide, titania, zirconia, alumina, iron oxides, nickel oxides, zinc oxides, and combination thereof.

4. The method of claim 1, wherein the suspended nanoparticles comprise colloidal silica.

5. The method of any one of claims 1-4, wherein the liquid composition further comprises a surfactant, and removing the porogen comprises removing the porogen and the surfactant.

6. The method of any one of claims 1-5, wherein the porogen is non-ionic.

7. The method of any one of claims 1-6, wherein the porogen comprises a water-soluble polymer.

8. The method of any one of claims 1-7, wherein the solvent is a solvent system comprising at least a first solvent and a second solvent.

9. The method of any one of claims 1-8, wherein coating a surface comprises controlling the thickness of the coating to influence the thickness of the porous layer.

10. The method of any one of claims 1-9, wherein coating the surface of the substrate comprises at least one of spraying, dip coating, drop casting, spin coating, screen printing, inkjet printing, slit coating, and draw down casting the liquid composition.

11. The method of any one of claims 1-10, wherein removing the solvent comprises evaporating the solvent at a temperature less than the boiling point of the solvent.

12. The method of any one of claims 1-11, wherein removing the porogen and, when present, the surfactant, comprises combusting the porogen and the surfactant.

13. The method of any one of claims 1-12, wherein the substrate is an optically- transparent substrate.

14. The method of claim 13, wherein the optically-transparent substrate is glass, fused silica, quartz, sapphire, tin-doped indium oxide, aluminum-doped zinc oxide, indium-doped cadmium oxide, fluorine-doped tin oxide, and combinations thereof.

15. The method of any one of claims 1-14, wherein the method further comprises:

functionalizing chemically the porous layer on the substrate to provide a

functionalized porous layer; and

introducing a lubricating liquid to wet spontaneously and adhere to the functionalized porous layer to form a stabilized liquid overlayer immobilized in, on and over the

functionalized porous layer, without dewetting from the substrate, to form a repellant surface.

16. An article of manufacture comprising the porous layer made by the method of any one of claims 1-15.

17. An article of manufacture comprising the repellant surface of claim 16. The article of manufacture of claim 16 or 17, wherein the article comprises an optical