RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S. C. § 119(e) to co-pending United States Application Ser. No. 61/792,455, filed March 15, 2013.

INCORPORATION BY REFERENCE

[0002] All patents, patent applications and publications cited herein are hereby incorporated by reference in their entirety in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described herein.

TECHNICAL FIELD

[0003] This technology relates generally to paper based devices. In particular, this invention relates to paper based devices having a low friction or high slip, omniphobic surface.

BACKGROUND

[0004] The design of devices that handle liquids, or control the transport of gases, would benefit from new materials, and from repurposing currently available materials to have new properties and functions. To channel or restrict the flow of liquids requires non-permeable materials; to transport gases requires porous media. Porous, water-repellent materials based on expanded polytetrafluoroethylene (ePTFE, Gore -Tex®, Nafion®, Teflon®) and other polymers have been useful in a wide range of applications, from high performance fabrics and membrane filters to fuel cells, surgical implants and lung-assist devices. The relatively high cost of these materials has, however, limited their utilization for applications requiring a low-cost or single- use format.

[0005] Paper is a useful substrate in applications that require low cost, flexibility, disposability, porosity, and adaptability to large-scale manufacturing. In recent years, it has become increasingly popular as a material for the construction of "high-tech" devices in consumer electronics, chemical and physical microelectromechanical systems (MEMS) sensors, user interfaces, electronic displays, cell-based assays and microfluidic devices. The tendency of paper to absorb liquids (including water), however, limits its adoption as a substrate in liquid- handling applications in which wicking is not desirable, or in which moisture and humidity can cause deleterious effects (especially changes in mechanical and electrical properties). For such applications, paper must be made resistant to wetting by liquids, and to adsorption of liquids (especially water) from the atmosphere.

[0006] Various techniques have been used to minimize the tendency of paper to adsorb liquids. Methods to render paper hydrophobic include spraying alcohol suspensions of Si0 ₂ nanoparticles on surface of the paper, soaking in polystyrene solutions, patterning using photolithography with SU-8, wax printing, plasma processing, and treatment with silanizing reagents.

[0007] Organosilanes with hydrophobic organic groups have been used to make the hydrophilic hydroxyl-rich surfaces of cellulose-based materials hydrophobic following both gas- phase and solution immersion reactions. Most methods, however, require long reaction and processing times (usually longer than one hour), and immersion in solvents requires pre- or post- treatment steps (washing cycles to remove excess reagents or side products, drying, etc.); these processes typically produce surfaces that have limited hydrolytic stability, or limited repellency to liquids with surface tensions lower than that of water. They also often cause the paper to buckle or warp.

SUMMARY

[0008] A rapid, simple method for altering the surface chemistry of paper by treatment with organosilanes in the gas phase is described. In certain aspects, paper is transformed into an omniphobic material by exposure to vapors of a fluoroalkyl trichlorosilane. The process is simple (e.g., conducted in a single step), and rapid (~5 min to transform paper to either

H20

hydrophobic and oleophobic). The treated paper repels water (θ _αρρ >140°), organic liquids with surface tensions as low as 28 mN/m, aqueous solutions containing ionic and non-ionic surfactants, and complex liquids such as blood (which contains salts, surfactants, and biological material such as cells, proteins, and lipids). When impregnated with a perfluorinated oil, fluoroalkylated paper forms a "slippery" surface (paper slippery liquid-infused porous surface, or "paper SLIPS") capable of repelling liquids with surface tensions as low as 15 mN/m. The foldability of the paper SLIPS allows the fabrication of channels and flow switches to guide the transport of liquid droplets without adhesion to the paper surface.

[0009] In one aspect, a device having a low friction, low adhesion surface includes a cellulosic substrate comprising a covalently functionalized surface comprising perfluorocarbon or fluorinated groups in an amount sufficient to provide an omniphobic surface; and an lubricating liquid infused into and coating over the cellulosic substrate to form an immobilized layer of the omniphobic liquid, wherein the cellulosic substrate comprises as least one fold, said fold selected to provide an article capable of transporting or holding liquid having a surface tension of greater than 15 mN/m without adhesion.

[0010] In one or more embodiments, the at least one fold forms a V-shaped channel.

[0011] In one or more embodiments, the at least one fold forms a switch that is capable of directing the direction of fluid flow.

[0012] In one or more embodiments, the at least one fold forms a well that is capable of holding a liquid.

[0013] In any of the preceding embodiments, the article is gas permeable.

[0014] In any of the preceding embodiments, the cellulosic substrate is selected from the group consisting of paper, cellulose derivatives, woven cellulosic materials, and non-woven cellulosic materials.

[0015] In any of the preceding embodiments, the paper is selected from the group consisting of chromatography paper, card stock, filter paper, vellum paper, printing paper, wrapping paper, ledger paper, bank paper, bond paper, blotting paper, drawing paper, fish paper, tissue paper, paper towel, wax paper, and photography paper.

[0016] In any of the preceding embodiments, covalently functionalized surface comprises a fluorinated group linked to the cellulosic surface through a siloxane linker.

[0017] In another aspect, a method of making a device having a low friction, low adhesion surface includes providing a cellulosic substrate comprising a covalently functionalized surface comprising perfluorocarbon or fluorinated groups in an amount sufficient to provide an omniphobic surface; infusing an lubricating liquid into and over the cellulosic substrate to form an immobilized layer of the omniphobic liquid, and folding the cellulosic substrate to provide a structure selected to provide an article capable of transporting or holding liquid having a surface tension of greater than 15 mN/m.

[0018] In one or more embodiments, the at least one fold forms a V-shaped channel, or forms a switch that is capable of directing the direction of fluid flow, or forms a well that is capable of holding a liquid.

[0019] In one or more embodiments, providing a cellulosic substrate comprising a covalently functionalized surface occurs before folding the cellulosic substrate.

[0020] In one or more embodiments, providing a cellulosic substrate comprising a covalently functionalized surface occurs after folding the cellulosic substrate.

[0021] The ability to resist wetting by liquids with a wide variety of surface tensions, combined with mechanical flexibility, foldability, light weight, biocompatibility (e.g. lack of wetting by blood), and gas permeability, make paper SLIPS a valuable new material, and a possible alternative to polymer-, glass-, and silicone- based materials now used as substrates for biomedical and bioanalytical applications, micro fluidics, and MEMS. For example, the functionalized paper can be used as a separating membrane. The membrane can include a cellulosic substrate comprising a covalently functionalized surface comprising fluorinated groups in an amount sufficient to provide an omniphobic surface; and a lubricating liquid infused into and coating over the cellulosic substrate to form an immobilized layer of the omniphobic liquid, a first liquid having a surface tension of greater than 15 mN/m disposed adjacent to a first side of the membrane, and a second liquid comprising water disposed against a second side of the membrane, wherein the membrane prevents mixing of the first and second liquids.

BRIEF DESCRIPTION OF THE DRAWING

[0022] The invention is described with reference to the following figures, which are presented for the purpose of illustration only and are not intended to be limiting. [0023] FIG. 1 is a schematic illustration of (a) the folds introduced into a cellulosic substrate functionalized to be omniphobic and infused with an omniphobic liquid to form paper SLIPS and (b) the folded device according to one or more embodiments.

[0024] FIG. 2 shows (a) representative images of water droplets on silanized paper; (b) static contact angles θ _αρρ , and contact angle hysteresis (Θ _α -Θ _Γ ) of water on silanized paper for three types of paper with different surface functionalization (identified on the x-axis as Ci ^H (methyl), Cio ^H (decyl), C ₈ ^F (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl), Ci ₀ ^F (3,3,4,4,5,5,6,6,

7,7,8,8,9,9, 10,10, 10-heptadecafiuorodecyl), Ci2 ^F (3,3,4,4,5,5,6,6,7,7,8,8,9,9, 10,10,

1 1 , 1 1 ,12, 12,12-henicosafluorododecyl), in which solid square symbols represent angles with Gel Blot paper (Blot), open round symbols represent angles with Whatman#l paper (Wl), and solid diamond symbols represent angles with Whatman #50 paper (W50); the volume of each drop is 10 μί; error bars: standard deviations for N=30 measurements; and (c) SEM images showing the topography of the different paper surfaces.

[0025] FIG. 3 is a plot illustrating the dependence of static contact angles on surface tension for 10 μΐ, droplets of liquid for variously functionalized (a) blot paper, (b) Whatman #1 filter paper and (c) Whatman #50 paper; test liquids are shown on top plot for all figures; error bars: standard deviation for N=30 measurements..

[0026] FIG. 4 is a representation of 3-D "slippery" structures fabricated by folding and creasing omniphobic paper impregnated with a perfluoropolyether lubricant (Krytox® GPL 105), demonstrating that (a) a slippery "channel" formed by successive V-pleats in a paper SLIP can guide the transport of liquid droplets of dyed methanol (right) and dyed toluene (left) using S and (b)-(c) a fluidic switch formed by folding in a pre-defined geometry can direct the flow liquid in different direction by tilting.

[0027] FIG. 5 shows the crease pattern for the fabrication of channels and switches shown in FIG. 4.

[0028] FIG. 6 is a representation of omniphobic microtiter plates fabricated by creasing and folding of fluorinated paper to form (a) a square array of re-entrant honeycomb cells able to stably hold in each well 500 of (b) aqueous solutions (Dulbecco's Modified Eagle Medium), and (c) a negative Poisson ratio structure based on a triangular array of re-entrant honeycomb cells able to stably hold in each well 500 of (d) organic liquids (toluene dyed with Sudan I). [0029] FIG. 7 shows the crease pattern for the fabrication of a multiwell plate shown in FIG 6.

[0030] FIG. 8 provides a schematic illustration of the system and process used for silanization according to one or more embodiments.

[0031] FIG. 9 shows static contact angles (filled symbols) and hysteresis (9 _a -9 _r ) (hollow symbols) of several biological fluids: a) whole blood, b) plasma, c) artificial saliva, d) artificial urine, e) a solution of protein (5% BSA in PBS), and f) a solution of DNA (500μΜ of DNA in TE buffer) on functionalized Gel Blot (square symbols), Whatman #1 (circular symbols) and Whatman #50 (triangular symbols); error bars: standard deviation for N=30 measurements.

[0032] FIG. 10 shows time-sequence images of a drop of heparinized human blood rolling down on Gel Blot paper functionalized with Ci ₀ ^F (side view: left; front view: right) at (a) t=0, (b) t=140 ms, (c) t=210 ms and (d) t=350 ms; the rolling drop does not leave a stain visible to the unaided eye.

[0033] FIG. 11 shows (a) images obtained with a fluorescence gel scanner from variously functionalize silanized paper after the drop of blood released and rolled off on tilted paper (the dark spots are dried blood); (b) the angles of incline at which the droplets of blood rolled off the silanized papers); and (c) the amount of blood adhering to the paper after the blood droplet had rolled off, quantified as the area of the blood stain left on the paper; bars are standard deviations for N=7 measurements.

[0034] FIG. 12 shows static contact angles (filled symbols) and hysteresis (hollow symbols) for several buffers commonly used in biological applications (a) PBS (pH 7.2), (b) Tris (pH 7.4), (c) LB, (d) DMEM, (e) Tris-Gly Buffer (pH 8.3), and (f) lx OneTaq MasterMix (typical PCR buffer) on functionalized Gel Blot, Whatman #1 and Whatman #50 paper; error bars represent standard deviation (N=30).

[0035] FIG. 13 shows static contact angles (filled symbols) and hysteresis (hollow symbols) of aqueous solutions of detergent commonly used in molecular biology (a) SDS, (b) IGEPAL, (c) Triton X-100 and (d) Tween 20 on functionalized Blotting, Whatman#l and Whatman#50 paper; error bars represent standard deviation (N=30). [0036] FIG. 14 shows (a) the fabrication of paper slippery porous liquid-infused surfaces (paper SLIPS) from omniphobic paper impregnated with a perfluoropolyether, and time- sequence images showing rolling droplets of (b) heparinized human blood (volume -30 μί), (c) diethyl ether dyed with Sudan Red (yzr=17 mN/m, volume -30 ) and (d) toluene dyed with Sudan Blue { LV ⁼ 28 mN/m, volume -30 μί) on a paper SLIPS at -5° tilting.

[0037] FIG. 15 provides (a) a comparison of apparent contact angles and contact angle hysteresis {9 _a -9 _r ) as a function of surface tension of test liquids (indicated) for SLIPS fabricated from Gel Blot, Whatman #1 and Whatman #50 paper silanized with Cio ^F and impregnated with a perfluoropolyether lubricant (Dupont™ Krytox® GPL 105)1 and (b) a plot of the cosine of the predicted equilibrium contact angle— calculated with the assumption that the liquids wet a hypothetical flat surface composed solely of perfluoropolyether lubricant— versus the cosine of the measured apparent angle of liquids with the paper SLIPS; the diagonal dashed line is drawn to guide the eye to show the case of perfect correlation.

DETAILED DESCRIPTION

[0038] Low friction cellulose-based surfaces for use in a variety of devices are described. Cellulose is a polysaccharide consisting of glucose units. The hydroxyl groups are reactive and lend themselves to functionalization with a variety of agents. Cellulose can be rendered omniphobic by formation of a perfluoro or fluoroalkyl surface.

[0039] As used herein perfluorocarbon group is meant to include chemical species or moieties that are made up of only carbon and fluorine. For example, a perfluoroalkyl group is an alkyl group in which all hydrogens have been replace by fluorine.

[0040] As used herein fluorinated group is meant to include chemical species or moieties that include carbon and fluorine, but which may also include other atoms. Most commonly the additional atoms are hydrogen. For example, a fluoroalkyl group is an alkyl group in which some of the hydrogens have been replace by fluorine. The presence of additional atoms is selected to maintain the omniphobic properties typically imparted by the fluorine atoms in the groups. Fluorinated is a more general description that include partial and/or complete substation by fluorine. [0041] As used herein "omniphobic" means a surface or material that is both hydrophobic and oleophobic. A material is considered to be hydrophobic when it exhibits a contact angle greater than 90 ° with water. A material is considered to be oleophobic when it exhibits an angle higher than 90 ° with hexadecane.

[0042] In one exemplary method, paper is transformed into an omniphobic material by exposure to vapors of a fluoridated or perfluorocarbon containing species, e.g., fluoroalkyl trichlorosilane, which react with the hydroxyl groups of the paper to form long fluoridated or perfluorocarbon chains of grafted siloxane molecules. It is simple (single step), rapid (~5 min to completion) and low-cost. The combined effects of the long fluoridated or perfluorocarbon chains of grafted siloxane molecules with the micro-scale roughness and porosity of paper (typical papers have a -30-45% void volume), yield an omniphobic material that preserves the properties of mechanical flexibility and low resistance to transport of gas of the untreated paper.

[0043] Part of the omniphobicity of organosilane-functionalized paper reflects its textured surface— a mixture of small fibers and voids— which enables it to form metastable composite solid-liquid-air interfaces. This architecture— one comprising voids and solid structures— is the basis of the omniphobic surface. The propensity of the treated paper to resist wetting by liquids with a wide range of surface tensions correlates with the length and degree of fluorination of the organosilane, and with the roughness of the paper.

[0044] Next, the omniphobic paper is wet with a lubricating liquid. By 'lubricating', it is meant that the liquid is able to spread out and wet, e.g., lubricate, the omniphobic paper. In one or more embodiments, the lubricating liquid is a low surface tension, e.g., <15 mN/m, liquid, in which some but not necessarily all C-H bonds have been replaced with C-F bonds. Treatment of the omniphobic paper in this way further increases the ability of fluorinated paper to resist wetting by low surface tension liquids (<15 mN/m). In contrast to water, which has a high contact angle with the treated paper, the low surface tension fluorinated liquid spreads readily and is absorbed into the paper. Sufficient low surface tension fluorinated liquid is used to completely cover (but not overwhelm) the paper surface. The combination of omniphobic paper infused with an low surface tension fluorinated liquid forms an immobilized film of a low surface tension liquid at the "solid"-air or "solid"-liquid interface. This structure nearly eliminates pinning of the contact line for both high- and low-surface-tension liquids, and leads to a remarkably low contact angle hysteresis. Paper slippery liquid infused porous surfaces or "paper SLIPS" have the advantage of being mechanically flexible, and thus can be folded into three-dimensional structures to serve as elementary flow switches and channels for guided transport of drops of liquid.

[0045] Paper SLIPS can be used to prepare functional devices by taking advantage of their ability to be manipulated— e.g., folded, creased, wound or rolled— into structures that can be used as fluid flow channels, well plates, and switches. In one or more embodiments, paper SLIPS can be folded into simple channels for directing the passage of fluids from one location to another. The fluid channels can be simple V-shape folds, as in the shape of a fan as shown in FIG. 1. FIG. 1A illustrates the folds introduced into the paper SLIPS, in which dashed fold lines indicate a crease line in the paper away from the viewer and the solid fold lines indicate a crease line in the paper towards the viewer. The resultant folded paper SLIPS is shown in FIG. IB. The V-channels can serve as simple fluid conduits. Because the paper SLIPS exhibits minimal adhesion and sticking of water-based fluids, such as biological fluids, the paper SLIPS channels are useful conduits for fluid transport without loss or sticking.

[0046] Paper SLIPS possesses several distinguishing and advantageous properties:

- The ability to change form and function by creasing

- The ability to create hydrophilic areas in hydrophobic sheets simply by using a craft cutter to carve out specific regions and replacing them with untreated paper to assemble all paper micro fluidic systems.

- The high transparency to gas of paper SLIPS based since the permeability of gasses through fluorocarbons and silicones is very rapid

- The ease of replenishing the liquid in the SLIPS from the back side

- Low cost/light weight, making them amenable to reel-to-reel processing

- The ability to have different fluids on both sides (so, for example, paper SLIPS with fluorocarbon could be a membrane separating water on one side and a low surface tension liquid on the other)

- Potential for use in wound-care due to its anti-biofouling properties Cellulose Substrate

[0047] Paper SLIPS devices are prepared using a cellulosic substrate that has been covalently modified to increase its hydrophobicity and omniphobicity. The cellulosic substrate can be covalently modified using any suitable methodology, as discussed below. Cellulose is a polysaccharide including a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units. Cellulose is mainly used to produce paperboard and paper. A cellulosic substrate includes articles of manufacture such as paper and cardboard that are made primarily of cellulose. It also includes modified cellulose, for example where the hydroxyl groups of cellulose can be partially or fully reacted with various reagents to afford derivatives with useful properties such as nitrocellulose, cellulose esters and cellulose ethers.

[0048] Generally, the cellulosic substrate is flexible. In preferred embodiments, the cellulosic substrate can be bent through its thinnest dimension, rolled around a cylindrical rod with a diameter of no more than two inches, and return to a flat configuration without damaging the integrity of the substrate. Due to this flexibility, paper SLIPS devices fabricated from the cellulosic substrate can be treated in this fashion without damaging the integrity and/or functionality of the resultant folded device. For certain applications, it is preferable that the cellulosic substrate can be folded, creased, or otherwise mechanically shaped to impart structure and function to a device formed from the cellulosic substrate.

[0049] Examples of suitable substrates include cellulose; derivatives of cellulose such as nitrocellulose or cellulose acetate; paper (e.g., craft paper, card stock, filter paper,

chromatography paper); woven cellulosic materials; non-woven cellulosic materials; and thin films of wood that have been covalently modified to increase their omniphobicity, as discussed below.

[0050] Preferably, the cellulosic substrate is paper. Paper is inexpensive, widely available, readily patterned, thin, lightweight, and can be disposed of with minimal environmental impact. Furthermore, a variety of grades of paper are available, permitting the selection of a paper substrate with the weight (i.e., grammage), thickness and/or rigidity and surface characteristics (i.e., chemical reactivity, hydrophobicity, and/or roughness), desired for the fabrication of a particular microfluidic device. Suitable papers include, but are not limited to, chromatography paper, card stock, filter paper, vellum paper, printing paper, wrapping paper, ledger paper, bank paper, bond paper, blotting paper, drawing paper, fish paper, tissue paper, paper towel, wax paper, and photography paper.

[0051] Exemplary paper includes cardstock paper, which is particularly suitable as the cellulosic material is lightweight and flexible, sufficiently smooth to create a tight seal with tape and inexpensive; it is also thick enough (300 μιη) to retain mechanical stability while

accommodating folds and creases introduced during the forming process. Thinner, more flexible paper can be used when more complex or more highly folded and/or creased structures are desired.

[0052] In certain embodiments, the cellulosic substrate is paper having a grammage, expressed in terms of grams per square meter (g/m ² ), of greater than 50, 60, 70, 75, 85, 100, 125, 150, 175, 200, 225, or 250.

Covalent Modification of the Cellulosic Surface

[0053] The cellulosic substrates can be covalently modified to provide an omniphobic surface using any suitable synthetic methodology. For example, hydroxyl groups present on the surface of the cellulosic substrate may be covalently functionalized by silanization, acylation, or by epoxide, aziridine, or thiirane ring opening. In preferred embodiments, the cellulosic substrate is treated with a volatile reagent to increase its oleophobicity.

[0054] Examples of suitable groups include linear, branched, or cyclic perfluoro or fluoroalkyl groups; linear, branched, or cyclic perfluorinated or fluorinated alkenyl groups; linear, branched, or cyclic perfluorinated or fluorinated alkynyl groups, aryl groups, optionally substituted with between one and five substituents individually selected from linear, branched, or cyclic perfluoro or fluorinated, linear, branched, or cyclic perfluorinated or fluorinated alkenyl, linear, branched, or cyclic perfluorinated or fluorinated alkynyl, perfluorinated or fluorinated alkoxy, perfluorinated or fluorinated amino, halogen, nitrile, CF ₃ , ester, amide, aryl, and heteroaryl. In preferred embodiments, the omniphobic group is an aryl ring substituted with fluorine atoms and/or trifluoromethyl groups, or a linear or branched alkyl group substituted with one or more halogen atoms. The introduction of halogenated functional groups via glycosidic linkages increases the omniphobicity of the cellulosic surface. [0055] Examples of suitable hydrophobic groups include linear, branched, or cyclic alkyl groups; linear, branched, or cyclic alkenyl groups; linear, branched, or cyclic alkynyl groups, aryl groups, heteroaryl groups, optionally substituted with between one and five substituents individually selected from linear, branched, or cyclic alkyl, linear, branched, or cyclic alkenyl, linear, branched, or cyclic alkynyl, alkoxy, amino, halogen, nitrile, CF ₃ , ester, amide, aryl, and heteroaryl. The hydrophobic group may also be a fluorinated or perfluorinated analogs of any of the groups described above.

[0056] In some cases, the surface hydroxyl groups of the cellulosic substrate (i.e., the cellulose fibers) are reacted with volatile, omniphobic silanes to form surface silanol linkages. Suitable silanes include linear or branched fluoroalkyl or perfluoroalkyl trihalosilanes, and fluoroalkyl or perfluoroalkyl aminosilanes. In certain embodiments, the cellulosic substrate is reacted with one or more fluoroalkyl or perfluoroalkyl trichlorosilanes, such as (tridecafluoro- 1,1,2,2-tetrahydrooctyl) trichlorosilane, to form a fluorinated, highly textured, omniphobic surface on the cellulosic substrate. The surface groups may have all or a portion of the hydrogen atoms replaced by fluorine atoms. If there are sufficient C-F bonds in the molecule, the surface free energy of the solid can be lowered sufficiently to render it omniphobic. Other functional groups can be present, but high surface energy functional groups (OH, C=0, etc.) may be incompatible for stability reasons and their presence should be limited.

[0057] The silanization treatment does not degrade the physical properties of the paper and does not require pre- or post- treatment steps (e.g. washing to remove reagents or side products, drying, etc.). Exemplary commercially available silanes, (3,3,4,4,5,5,6,6, 7,7,8,8,9,9,10,10,10- heptadecafluorodecyl) trichlorosilane, CF ₃ -(CF ₂ )7-CH ₂ -CH ₂ -SiCl ₃ (CIOF), and decyl

trichlorosilane, CH ₃ (CH ₂ ) ₉ SiCl ₃ (CIOH), are volatile and reactive towards the hydroxyl groups of cellulose. This silanization reaction generates highly omniphobic surfaces on the cardstock paper (static contact angles of water 9 _S (CIOF) =137° ± 4°, n=20 and θ ₃ (CIOH) =131° ± 5°, n=20). Paper functionalized with CIOF is also omniphobic (contact angle with hexadecane (¾ (CIOF) =93 ± 3°, N=10). In contrast, paper functionalized with CIOH is wet by hexadecane. The paper can be silanized before or after forming the device into its final form, e.g., by folding and creasing. Silanizing after folding can avoid damaging the organosilane layer or exposing cellulose fibers that had not come in contact with vapors of organosilane; however, the more complex surfaces may be more difficult to obtain complete coverage. [0058] Treatment of paper with an organosilane (either R ^H SiCl3 or R ^F SiCl ₃ ) in the vapor phase renders paper highly repellent to pure water. Paper treated with an organosilane (either R ^H SiCl ₃ or R ^F SiCl ₃ ) can be referred herein to as R ^H paper or R ^F paper, respectively. FIG. 2A shows that silanization rendered paper highly hydrophobic. Water was no longer wicked into the paper, but instead formed droplets on the surface with apparent static contact angles, θ _αρρ , between 130° and 160°, and with contact angle hysteresis from 7° to 20°. The apparent contact angle of each type of functionalized paper increased (modestly) for the most part with the chain length and degree of fluorination of the organosilane, while the hysteresis did not show any noticeable trends (FIG.2B).

[0059] In another embodiment, instead of being silanized, the surface hydroxy 1 groups of the cellulosic substrate are acylated by reaction, for example, with one or more perfluoro or fluoroalkyl groups functionalized with an acid chloride.

[0060] The cellulosic substrate can also be covalently modified by treatment with a hydrophobic group substituted with one or more epoxide or thiirane rings.

[0061] The omniphobicity/oleophilicity of the covalently modified cellulosic substrate can be quantitatively assessed by measuring the contact angle of a water droplet on the substrate surface using a goniometer. The omniphobicity/oleophilicity of the covalently modified cellulosic substrate can be qualitatively assessed by rolling droplets of water and/or hexane on the surface of the modified paper to evaluate the wettability of the surface.

[0062] Generally, the covalently modified omniphobic cellulosic substrate (prior to introduction of a low surface tension perfluorinated liquid) is substantially impermeable to aqueous solutions. In preferred embodiments, the covalently modified cellulosic substrate has a contact angle with water, as measured using a goniometer, of more than 90° {i.e., it is hydrophobic). In particular embodiments, the covalently modified cellulosic substrate has a contact angle with water of more than about 95°, 100°, 105°, 110°, 115°, 120°, 125°, 130°, 135°, 140°, 145°, 150°, or 155°. An omniphobic surface exhibits contact angles higher than 90 ° with both water and hexadecane.

[0063] Covalent attachment of the modifying reagent to the cellulosic substrate can be confirmed using appropriate molecular and surface analysis methods, including reflectance FTIR and XPS. In certain embodiments, at least 5%, more preferably at least 25%, more preferably at least 35%, more preferably at least 50%>, most preferably at least 75% of the pendant -OH groups present on the cellulosic backbone are covalently modified. In certain embodiments, more than 80% of the pendant -OH groups present on the cellulosic backbone are covalently modified.

Low Surface Tension Lubricating Liquids

[0064] The covalently modified paper is rendered repellant to liquids and other objects by infusing the paper with a low surface tension lubricating liquid. The fluid used to impregnate the omniphobic paper to make the paper SLIPS can be any kind of low surface tension fluorinated oil, such as perfluoropolyether, perfluorocarbons, perfluoroalkylether, perfluoropolyalkylether. Other lubricants can also be used to impregnate omniphobic paper to create paper SLIPS like fluorinated grease, Teflon lubricants and greases. The liquid molecules may have all or a portion of the hydrogen atoms replaced by fluorine atoms. In some embodiments, might be desirable to retain some C-H bonds in order that the compound is stable. If there are sufficient C-F bonds in the molecule, the surface free energy can be lowered sufficiently to render it wetting to the omniphobic paper. Other functional groups can be present, but high surface energy functional groups (OH, C=0, etc.) may be incompatible for stability reasons and their presence should be limited. Fluorinated is a more general description that include partial and/or complete substation by fluorine.

[0065] The lubricating layer can be prepared from a variety of fluids. Perfluorinated organic liquids, in particular, are suitable for use in folded applications. In some aspects, the lubricating layer is perfluorinated oil, non-limiting examples of which include PFC oils such as FC-43, FC- 70, perfluorotripropylamine, perfluorotripentylamine, perfluorotributylamine, perfluorodecalin, perfluorooctane, perfluorobutane, perfluoropropane, perfluoropentane, perfluorohexane, perfluoroheptane, perfluorononane, perfluorodecane, perfluorododecane, perfluorooctyl bromide, perfluoro(2-butyl-tetrahydrofurane), perfluoroperhydrophenanthrene, perfluoroethylcyclohexane, perfluoro(butyltetrahydrofuran), In other aspects, the lubricating layer is fluorinated hydrocarbon oil, non-limiting examples of which include oils such as 3-ethoxy-l, 1,1,2,3,4,4, 5,5, 6,6,6- dodecafluoro-2-trifluoromethyl-hexane, trifluoromethane, difluoromethane, pentafluoroethane, hydrofluoroether, etc. [0066] In some aspects, the viscosity of the lubricating layer can be chosen for particular applications. For example, the viscosity of the lubricating oil can be < 1 cSt, <10 cSt, < 100 cSt, < 1000 cSt, or < 10,000 cSt.

[0067] In some aspects, the lubricating layer has a low freezing temperature, such as less than -5 °C, -25 °C, or -50 °C. A lubricating layer with a low freezing temperature allows the layer to remain liquid in low temperatures to maintain the ability of the combination of the lubricating layer and functionalized surface to repel a variety of liquids or solidified fluids, such as ice and the like.

[0068] This liquid fills the interstitial pores of the paper and is locked into place by the roughness and high surface area of the paper substrate. The lubricating layer can be applied in a thickness sufficient to cover the surface of the cellulosic substrate. The amount of low surface tension perfluorinated liquid used is sufficient to completely wick into the pores of the paper and to cover the upper surface of the substrate. Large excesses of the low surface tension perfluorinated liquid is not needed, as the low friction slippery surface arises from the interaction of the immobilized liquid located immediately above the paper upper surface. In some embodiments, the lubricating layer is applied at a thickness sufficient to form a

monomolecular layer on the substrate. In other embodiments, the lubricating layer is applied at a thickness of 10 nm to 10 μιη on the substrate. In other embodiments, the lubricating layer is applied at a thickness of 10 μιη to 10 mm on the substrate. The lubricating layer applied in a typical thickness, assumed to be a monomolecular layer.

[0069] The dependence of static contact angles on surface tension for 10 uL droplets of liquid is shown in FIG. 3(A)-3(C) for blot paper, Whatman #1 and Whatman #50, respectively. The liquids used in these experiments and their respective surface tensions (γιν) at 20°C in mN/m are (literature values: pentane (15.5), anhydrous ethanol (22.3), hexadecane (27.4), DMF (37.1), ethylene glycol (46.3), thiodiglycol (54.0), glycerol (63.7), water (72.8), 6m NaCl (82.6). The grey area indicates liquids that spontaneously spread onto the functionalized substrate through capillary wicking. Test liquids with lower surface tensions wicked into the paper.

Hydrophobic paper was able to resist wetting by liquids with surface tensions as low as 27 mN/m (hexadecane). This value likely reflects the lower energy of the paper fibers functionalized with fluoroalkyl siloxane chains relative to those functionalized with alkyl siloxanes. [0070] In some aspects, the lubricating layer has a low evaporation rate or a low vapor pressure. For example, the vapor pressure of the lubricating liquid can be less than 10 mmHg at 25 °C, less than 5 mmHg at 25 °C, less than 2 mmHg at 25 °C, less than 1 mmHg at 25 °C, less than 0.5 mmHg at 25 °C, or less than 0.1 mmHg at 25 °C. , can remain liquid repellant for a long period without requiring replenishing. By way of example, the surface can remain liquid repellant for a period longer than 1 hour, or longer than 6 hours, or longer than 24 hours, longer than a week, or longer than a year or more.

[0071] The lubricating liquid can be applied to the surface by wicking, spin coating, pipetting drops of lubricating liquid onto the surface, or dipping the surface into a reservoir containing the lubricating liquid. In some embodiments, any excess lubricating liquid can be removed by spinning the coated article or by drawing a squeegee across the surface or flushing and rinsing with another liquid.

Surface Properties of Paper SLIPS

[0072] Paper SLIPS is highly omniphobic. Both water and hydrophobic organic liquids are not adsorbed on the surface, but instead formed droplets on the surface that roll off when the surface is tilted. Furthermore, the paper SLIPS surface remains visually free of residue after contact with a range of liquids. Blood, toluene, and diethyl ether all slide off the paper SLIPS when the surface is tilted. The non-wetting of silanized paper by biological fluids is important for its use in applications such as bioanalysis, cell culture, and drug discovery and development. Device made of paper SLIPS may provide advantageous anti-bioadhesion and anti-biofouling properties.

[0073] In addition to demonstrating low adhesion of fluids, the paper SLIPS is permeable to gasses and therefore can be used as a gas sensor or be used in applications for which gas permeability is desirable (such as the elimination of volatile materials from a mixture by evaporation, removing gas contaminants from a sample, or in reactions that require gas exchange).

Folded Devices [0074] Gas-phase silanization does not affect the mechanical properties of the paper substrate. Thus, three-dimensional functional structures can be built by creasing or folding the paper, either before or after silanization.

[0075] The paper SLIPS can be folded, creased, rolled or wound into 2 dimensional and 3 dimensional structures that can be used to direct the flow of liquid. The principals of origami can be used to prepare structures with a wide range of complexity and functions. The mechanical flexibility and foldability of the paper SLIPS allows it to be formed into a range of shapes, such as microtiter plates, from single sheets of paper using the principles of origami.

[0076] Complex 3-D "slippery" structures of paper SLIPS can be easily fabricated using techniques based on (for example) origami directly from paper SLIPS, or from omniphobic paper folded, then impregnated with a perfluoropolyether. Paper SLIPS thus differ from low friction surfaces developed on rigid surfaces in the ease with which (originally) planar sheets can be transformed into structures with complex topographies by folding.

[0077] To demonstrated their mechanical flexibility and foldability, paper SLIPS can be folded into more complex shapes such as V-pleats that form "slippery" channels that can be used to guide droplets of toluene and methanol. FIG. 4A is a paper SLIPs v-pleated device demonstrating the ability to hold liquid (in the valleys of a V-pleat) and direct it along a corrugated channel defined by a series of adjacent V-pleats. FIGS. 4B and 4C illustrate a simple switch made by combining tilting with a pre-designed folded geometrical path. When the structure is tilted towards the left (FIG.4B), the liquid droplet moves along the left hand channel, while when the structure is tilted to the right, the path the liquid droplet takes is changed and the droplet moves along the right-hand channel (FIG. 4C). The fold lines for the paper SLIPS used to create the V-channel design are shown in FIG. 5, where solid lines and dashed lines indicate a crease line away from and towards the viewer, respectively.

[0078] Folding can also be used to design three-dimensional structures, such as the microtiter plates illustrated in FIGs. 6A-D. FIG. 6A shows a square array of re-entrant honeycomb cells; the same microtiter plate filled with aqueous fluid is shown in FIG. 6B. FIG. 6C shows a negative Poisson ratio structure based on a triangular array of re-entrant honeycomb cells; the same microtiter plate filled with hydrophobic organic fluid is shown in FIG. 6D. Origami principles can be applied to SLIPS to make a great variety of structures using paper SLIPS as a substrate. Structures like reservoirs and channels of different geometries can be fabricated like this. Stacking different pieces of paper folded with different patterns 3 dimensional devices can be made. The mechanical structure of the SLIPS allows formation of functional devices that rely on pressure, folding, or tilting. Button-like structures like pumps or valves can be made by appropriate folding or stacking SLIPS. FIG. 7 shows the crease patterns used in making the microwell plate structures shown in FIG. 6 from paper SLIPS, where solid lines and dashed lines indicate a crease line away from and towards the viewer, respectively.

[0079] The invention is illustrated in the following examples, which are presented for the purpose of illustration only.

Example 1. Fabrication of fluorinated omniphobic paper via silanization

[0080] Whatman chromatography paper was used as the starting material because it is uniform in structure and free of hydrophobic binders or coatings that could interfere with, or mask, the effect of silanization. Three types of papers were investigated: Whatman Gel Blot paper, Whatman #1 paper, and Whatman #50 paper. The root mean square surface roughness, R _R . _M .S. of these papers, as measured by optical profilometry, was R _R . _M .S =24 ± 1 μιη for Gel Blot paper, R _R . _M .S. ⁼ 18.0 ± 0.4 μιη for Whatman #1 paper, and R _R . _M .S =14.5± 0.7 μιη for Whatman #50 paper. FIG. 2C shows SEM photomicrographs of the paper porous structure. The height RMS roughness was measured with a Taylor-Hobson CCI HD Optical Profiler according to the IS025178 norm. The CCI method is based on the cross-coherence analysis of two low-coherence light sources, the beam reflected from the sample and a reference beam reflected from a reference mirror. Paper has a low reflectance that hinders the application of interferometry techniques. To improve the reflectance of paper a conformal thin layer of Au (~4 nm) was deposited using a Cressington 208HR sputter coater. In each experiment, at least seven 0.4 μιη x 0.4 μιη areas were analyzed.

[0081] Gas-phase silanization rendered paper hydrophobic; the procedure did not require pre- or post- treatment steps and the processing can be completed within minutes. This method is scalable and compatible with large-scale processing (that is, it will be able to generate virtually unlimited areas of silanized paper, using a standard reel-to-reel processing technique).

[0082] Five commercially available organosilane reagents (RS1CI3) of varying chain length and fluorination of the organosilane were investigated: i) methyltricholorosilane (CH3S1CI3, "Ci "); ϋ) decyltrichlorosilane (CH ₃ (CH ₂ ) ₉ SiCl3, "Cio "); iii) (3,3,4,4,5,5,6,6,7,7,8,8,8- tridecafluorooctyl) trichlorosilane (CF ₃ (CF ₂ ) ₅ CH2CH ₂ SiCl ₃ , "C ₈ ^F "); iv)

(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10, 10,10-heptadecafluorodecyl) trichlorosilane

(CF ₃ (CF ₂ ) ₇ CH ₂ CH ₂ SiCl ₃ , "Ci ₀ ^F ") ; v) (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10, 10, 1 1 ,1 1 , 12,12, 12- henicosafluorododecyl) trichlorosilane (CF ₃ (CF ₂ ) ₉ CH ₂ CH ₂ SiCl ₃ , "Ci/"); C _i2 ^F is the

trichlorosilane with the longest fluorinated alkyl chain that is commercially available.

[0083] The silanization reaction was performed in a vacuum oven at 95 °C and 30 mbar using a solution of the organosilane (-10 mL of a -30 mM solution in toluene) to supply a useful concentration of the silanizing reagent in the vapor phase. The organosilane was allowed to react with the hydroxyl-rich cellulose paper surfaces for 5 min (this time and temperature are not optimized). Each experiment typically required approximately 100 mg of silane in 5 mL of anhydrous toluene. Diffusion inside the reaction chamber is sufficient for an even distribution of the silane within the chamber. This process makes it possible to functionalize the areas of paper (>100 cm ² ) required for experimental work rapidly, using low quantities of organosilane and solvent. FIG. 8 provides a schematic illustration of the system and process used for silanization.

Example 2. Characterization of the wettability of silanized paper

[0084] We examined, by means of apparent static (θ _αρρ ), advancing ((¾), and receding (9 _r ) contact angle measurements, the wettability of paper substrates modified by gas-phase silanization by a wide range of liquids: organic liquids with different surface tensions, biological fluids, and aqueous solutions of ionic and non-ionic surfactants above the critical micelle concentration (cmc). To ensure that the shape of the droplet is determined by surface forces and is not due to gravitational deformation, we used 10 μΐ, droplets of liquid for the contact angle measurements. For a liquid of density p and surface tension y _L v, the resulting droplets had radii of -1.4 mm, below the capillary length _c :

[0085] For the liquids we selected, X _c ranged between 1.6 mm (for pentane) and 2.7 mm (for water). In addition, we tested the compatibility of the different silanized papers with buffers relevant for immunoassays, electrophoresis, magnetic levitation, polymerase chain reactions (PCR), and culture of mammalian cells or bacteria.

[0086] The contact angle measurements were performed using a contact angle measurement system (Rame-Hart model 500-F1 , Rame-Hart Instrument Co.) at room temperature (20 - 25 °C) with -20% relative humidity. The droplet volume for the measurement was ~10 (unless otherwise specified). The advancing and receding contact angles were measured by the sessile drop method, which involves expanding or contracting a contact angle droplet by adding or withdrawing fluid in 1 μΐ _^ increments. The droplet profile was fitted to a spherical profile using the software provided by the system. The advancing contact angles are measured at the leading droplet edge when the value of the contact angle remained constant and solid/liquid interface started to increase; the receding angles were measured at the trailing droplet edge when the value of the contact angle remained constant and the solid/liquid interface started to decrease.

Example 3. Comparison of Hydrophobic and Omniphobic Paper With Biological Fluids

[0087] This study determined that omniphobic treated paper is superior to hydrophobic treated paper for applications requiring minimal interactions with biological fluids. The wetting of silanized paper by biological fluids is important for its use in applications such as bioanalysis, cell culture, and drug discovery and development. Viscous solutions with a high content of protein or DNA, such as artificial saliva, a 5% (mass-to-mass ratio) solution of bovine serum albumin (BSA) in PBS, and a 500 μΜ solution of DNA in Tris-EDTA (TE) buffer, formed higher contact angles on fluorinated than on non-fluorinated papers of corresponding roughness (FIGs. 9A-9F. In all cases, the longer the fluorinated alkyl chains, the higher the static contact angle.

[0088] Whole blood (fresh whole human blood, treated with an anticoagulant-preservative solution containing sodium heparin) and plasma formed contact angles larger than 150° on both hydrophobic and omniphobic treated papers. Droplets of whole blood and plasma exhibit the lowest contact angle hysteresis (<15°) with fluorinated surfaces (FIG. 10). A drop of whole blood rolling on the surface of a strip of paper functionalized with Cio ^F did not leave a visible trace behind. The lack of staining suggests a low level of cell and protein adhesion on the omniphobic paper.

[0089] For applications involving blood, it might be important to minimize the amount of trace liquid left behind on the surface (i.e. the surface should be self-cleaning). To test the performance of silanized paper, we deposited 50 μΐ, drops of blood on the surface of various silanized papers that were initially horizontal. We then increased the tilt angle until the drop of blood rolled off the surface of the paper. FIG. 1 1 A shows representative images of the paper surfaces after the blood had rolled off. There are significantly more traces of blood left on the R ^H papers than on the omniphobic papers. Papers treated with the organosilanes with the longer fluoroalkyl chains (Cio ^F and Ci ₂ ^F ) showed no detectable trace of blood on the surface after roll- off. FIG. 1 1C is a bar graph showing the stain area (mm ² ) for each of the treated papers shown in FIG. 1 1A. FIG. 1 IB shows the roll-off angles measured for these papers. Hydrophobic papers had significantly higher roll-off angles than omniphobic papers. The blood drops adhered so strongly to Cio ^H treated surfaces that the droplets did not fall off even when the paper was turned upside down (i.e. roll-off angle > 180°). The state characterized by high adhesion and high contact angles has been termed the "petal effect" and is attributed to hierarchical roughness (multiple length-scales of features) on surfaces.

Example 4. Investigation into products formed on incineration of omniphobic paper

[0090] Bioanalytical devices fabricated using silanized paper can be disposed of by incineration; we wished to estimate the environmental impact of burning omniphobic paper. The elemental analysis of the fluorinated papers, suggests that the incineration of a 1 cm ² device at T < 1500 °C can produce at most 34 μg of a perfluoroalkyl carboxylic acid; under more stringent conditions (temperatures above 1500 °C), this content of fluorine could lead to the formation of a maximum of ca. 29 μg of HF, or a maximum of ca. 49 μg of COF ₂ .

[0091] Combustion of fluoroalkanes occurs at temperatures above 1500°C under

atmospheric pressure. The distribution of products includes COF ₂ , CF ₄ , CO, and C0 ₂ , with COF ₂ and C0 ₂ being the most abundant when the combustion occurs with 20% 0 ₂ . The toxic volatile compounds, COF ₂ and HF, have threshold limits for short-term exposure of 2ppm (5.4 mg/m ³ ) for COF ₂ and 2 ppm (1.7 mg/m ³ ) for HF.

[0092] We used a FLIR (Forward Looking Infrared) camera (B400, FLIR Systems Inc.) to record the burning of paper with different surface treatments, and recorded the maximum temperatures in each individual image. In the case of untreated paper, paper treated with Ci ^H , and paper treated with Cio ^H , the maximum temperatures are ~ 630°C between ~ 670°C, respectively. In the case of paper functionalized with Cs ^F , Cio ^F and Ci ₂ ^F , the maximum temperatures are between ~ 450°C and ~ 490°C. The omniphobic papers burned more slowly, and generated flames of lower temperatures, than untreated paper and R ^H paper.

[0093] If the omniphobic paper is burned in a simple set-up, with no high-temperature combustion catalyst present in the system when the paper is burned, the temperature of the flame is likely not high enough to allow the decomposition of the fluoroalkyl chains. It is, however, sufficiently high ^ to allow the breaking of the C-Si bond and the oxidation of the terminal carbon atom to yield terminally oxidized fluoroalkyl species.

[0094] The amounts of perfluoroalkylcarboxylic acid, HF and COF ₂ , released as by-product upon burning paper functionalized with Ci ₂ ^F are estimated based on the elemental analysis (wt % C, F, Si) performed by the Intertek QTI Laboratory (Whitehouse, NJ). The results of the elemental analysis are summarized in Table 1.

Table 1: Elemental analysis (wt%) of papers functionalized with Ci ₂ ^F

Basis weight

Type of paper wt % C wt % F wt % Si

(mg/cm ² )

Whatman #50 ^9 42.7 ± 0.3 0.5 ± 0.5 0.0018 ± 0.0005

Whatman #1 -8.6 42.8 ± 0.3 0.6 ± 0.5 0.0019 ± 0.0005

Gel Blot paper -49.3 42.3 ± 0.3 0.6 ± 0.5 0.0041 ± 0.0005

[0095] We can estimate, based on the Si content, that one cm ² of Gel Blot paper

functionalized with Ci ₂ ^F contains ~7 ^χ 10 ^"8 moles Ci ₂ ^F , corresponding to ~4 ^χ 10 ¹⁶ molecules Ci ₂ ^F per cm ² of paper. Similarly, one cm ² of functionalized Whatman #1 and Whatman #50 papers contain~6 ^χ 10 ^"9 moles or ~4 ^χ 10 ¹⁵ molecules Ci ₂ ^F per cm ² of paper.

[0096] Thus, the incineration of 1 cm ² of Gel Blot paper functionalized with Ci ₂ ^F will produce at most 34 μg of perfluoroalkylcarboxylic acid (C ₁₂ H ₃ F ₂₁ O ₂ ), corresponding to a maximum of ca. 29 μg of HF, or a maximum of ca. 49 μg of COF ₂ . The amounts of fluorinated products released by the incineration of 1 cm ² of Whatman #1 and Whatman #50 papers are maximum ca. 3 μg of perfluoroalkylcarboxylic acid, or ca. 2.6 μg of HF and ca. 4.3 μg of COF ₂ .

Example 5. Compatibility of omniphobic paper with buffers commonly used in bioassays

[0097] The wettability of silanized paper by common buffers was investigated, since the surface tension of an aqueous buffer can be dramatically altered by the addition of surfactants or other solutes. The buffers surveyed include phosphate-buffered saline (PBS) buffer, Tris buffer, lx Taq Buffer (used for polymerase chain reactions), Tris-Gly buffer (typically used in capillary electrophoresis); Lysogeny broth (LB) or Dulbecco's Modified Eagle Medium (DMEM)— buffers typically used for mammalian or bacterial cell culture. The lx OneTaq Mastermix is an aqueous solution that notably wets the non-fluorinated surfaces. Buffers containing amines, amino acids, or dissolved salts form contact angles that are indistinguishable from that formed by pure water, as is illustrated in FIG. 12A-12F.

[0098] A distinction between the hydrophobic and omniphobic surfaces however, is in their ability to resist wetting by aqueous solutions of nonionic surfactants. These surfactants reduce the surface tension of pure water to ~ 30 mN/m when present at or above the critical micelle concentration (cmc). Nonionic surfactants are present in standard buffers used for PCR reactions, such as the lx Taq Buffer. When used above the cmc, nonionic surfactants containing

polyethylene oxide chains, (e.g. IGEPAL CA@630, Triton X-100 and Tween 20), wetted R ^H , but not the R ^F paper surfaces, as is illustrated in FIG. 13-13D in the plots of contact angles for variously treated paper. The non-fluorinated surfaces are wetted by 0.05% TritonX, 0.07% Tween 20 and 0.05% IGEPAL@CA630. Therefore, omniphobic papers provide the additional advantage that they are compatible with applications that require buffers containing nonionic surfactants.

Example 6. Formation of Omniphobic Paper Infused with a Perfluorinated Liquid

[0099] Omniphobic paper Infused with a perfluorinated liquid forms an omniphobic "SLIPS" surface exhibiting very low contact angle hysteresis. We introduced perfluoropolyether lubricant (Dupont™ Krytox® GPL 105) onto omniphobic paper surfaces.

CF ₃ (CF ₂ ) ₂ 0-[CF-CF ₂ 0] _n CF ₂ CF ₃ F3 n= 10-60

Dupont™ Krytox® GPL 105 : molecular structure

[0100] Lubricating fluid (Dupont™ Krytox® GPL 105), was added to the surfaces (-50 by pipette to impregnate the paper and form a coating film. The fluid spontaneously spread onto the whole substrate through capillary wicking. Tilting the surface and mildly applying compressed air removed the excess of lubricating fluid. The liquid spontaneously spreads onto the whole substrate through capillary wicking, and the large pores in paper facilitate the infiltration and retention of the lubricating perfluoropolyether to form a continuous overlying film. FIG. 14A shows how omniphobic paper can serve as a substrate for SLIPS— "Slippery Liquid-infused Porous Substrates" with remarkably low hysteresis towards most liquids. Time- sequence images in FIGS. 14B-14D show rolling droplets of: heparinized human blood (volume -30 μί), diethyl ether dyed with Sudan Red (yz^l7 mN/m, volume -30 ) and toluene dyed with Sudan Blue { LV ⁼ 28 mN/m, volume -30 μί) on a paper SLIPS at -5° tilting, respectively. Blood, toluene, and diethyl ether all slide off the paper SLIPS when the surface is tilted.

[0101] FIG. 15A demonstrates that paper SLIPS show omniphobic behavior, with contact- angle hysteresis of the surface below 10° for all liquids we tested, and below 5° for most of them. FIG. 15A provides a comparison of apparent contact angles and contact angle hysteresis (θα-dr) as a function of surface tension of test liquids (indicated) for SLIPS fabricated from Gel Blot, Whatman #1 and Whatman #50 paper silanized with Cio ^F and impregnated with a perfluoropolyether lubricant (Dupont™ Krytox® GPL 105). Paper SLIPS are also able to resist wetting by pentane, which has a surface tension of -15 mN/m. Since the wetting characteristics (apparent contact angles θ _αρρ , and hysteresis, θ _α - Θ ) of the paper SLIPS did not vary for the three types of paper substrates, we hypothesized that the film of perfluoropolyether oil dominates the wetting characteristics of this material. [0102] To test this hypothesis, we measured the surface tension of the perfluoropolyether oil and the liquid-liquid interfacial tension between the perfluoropolyether oil and the test liquids using the pendant drop method. FIG. 15B is a plot of the cosine of the predicted equilibrium contact angle— calculated with the assumption that the liquids wet a hypothetical flat surface composed solely of perfluoropolyether lubricant— versus the cosine of the measured apparent angle of liquids with the paper SLIPS. The diagonal dashed line is drawn to guide the eye to show the case of perfect correlation. These values, along with the liquid-air surface tensions, provide all three surface energy components for calculating the equilibrium contact angle, θγ of our test liquids resting on a hypothetical smooth— solid surface that consisted only of the perfluoropolyether oil. We plot cos θ _γ calculated using Equation (2) versus the cos θ _αρρ in Figure 9b, and find that indeed these values are strongly correlated. Thus, the wetting behavior of paper SLIPS can be reasonably predicted from the interfacial tension of a liquid of interest with the lubricating oil with Young's equation.

Example 7. Formation of Paper SLIPS Folded Structures

[0103] A useful feature of paper SLIPS is their foldability, which can be exploited for low-cost fabrication of structures with desired functionalities. Complex 3-D "slippery" structures of paper SLIPS can be easily fabricated using techniques based on (for example) origami directly from paper SLIPS, or from R ^F paper folded, then impregnated with a perfluoropolyether. To demonstrated their mechanical flexibility and foldability, we folded paper SLIPS to make V- pleats and form "slippery" channels that can be used to guide droplets of toluene and methanol, as shown in FIG. 4. The fold lines for the paper SLIPS used to create the V-channel design is shown in FIG. 5. Mountain and valley folds are indicated by solid and dashed lines,

respectively. The mechanical flexibility of the omniphobic paper allowed us to form 3-D structures by folding the sheet of paper before or after functionalization. We created a simple switch by combining tilting with a pre-designed folded geometrical path. When the structure is tilted towards the left, the liquid droplet moves along the left hand channel, as shown in FIG. 4B. When the structure is tilted to the right, the path the liquid droplet takes is changed and the droplet moves along the right-hand channel (FIG. 4C).

[0104] Paper SLIPS allows (originally) planar sheets to be transformed with each into structures with complex topographies by folding. FIG. 7 shows the crease patterns used in making the microwell plate structures shown in FIG. 6 from paper SLIPS. Mountain and valley folds are indicated by green and red dashed lines, respectively. The mechanical flexibility of the omniphobic paper allowed us to form 3-D structures by folding the sheet of paper after impregnation with a perfluoropolyether (Krytox® GPL 105).

[0105] What is claimed is: