Salt Based Etching of Metals and Alloys for Fabricating Superhydrophobic and Superoleophobic Surfaces

Abstract

A process to etch hierarchical, re-entrant texture into the surface of metals and their alloys using salt-based etching solutions. The process imbues superhydrophobic, oleophobic or superoleophobic, omniphobic or superomniphobic properties by further imparting a low surface energy coating onto the etched surfaces by chemical functionalization by low surface energy hydrophobilizing compounds.

What is claimed is: 
     
         1 . A method for creating a hierarchical re-entrant texture of a metallic or metallic alloy surface comprising: etching the metal surface with an etching solution having 1 to 500 grams per liter of an at least one salt including an inorganic salt or organic salt, and water. 
     
     
         2 . The method of  claim 1  wherein the at least one salt is a weak acid-strong base, weak base-strong acid, weak acid-weak base, or strong acid-strong base salt. 
     
     
         3 . The method of  claim 2  wherein the etching solution is comprised of a salt having a strong acid-weak base in solution, or having a weak acid-strong base in solution, or a combination of two or more salts having either a weak acid-weak base with another salt that is a promoter or catalyst for etching, or a combination of two or more salts having a first salt of a strong acid-strong base and a second salt that is a promoter or a catalyst. 
     
     
         4 . The method of  claim 1  wherein the at least one salt is operable to dissociate a cation and an anion in the etching solution and wherein the at least one salt contains potassium hydrogen carbonate, potassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium carbonate, sodium dihydrogen carbonate, ammonium bromide, sodium carbonate, sodium acetate, potassium carbonate, ammonium acetate, iron chloride, iron sulfate, iron nitrate, cobalt chloride, cobalt sulfate, cobalt nitrate, pyridine hydrochloride, ammonium, sodium, potassium salts of phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, or phosphovanadates, diisopropylfluorophosphate. 
     
     
         5 . The method of  claim 1  further comprised of a promoter or catalyst wherein the promoter or catalyst is operable to alter dissociation of the at least one salt in the etching solution and wherein the at least one salt is an oxide, hydroxide, peroxide or persulfate salt of Group I and II cations including lithium, sodium, potassium, magnesium, calcium or barium. 
     
     
         6 . The method of  claim 1  wherein the process is further comprised of adding external energy including electricity, heat, mechanical agitation or pressure. 
     
     
         7 . The method of  claim 1  further comprised of a promoter or catalyst wherein the promoter or catalyst is operable to alter dissociation of the at least one salt in the etching solution and wherein the at least one salt is an oxide, hydroxide and peroxide salts containing cations of ammonium, pyridinium and hydrogen. 
     
     
         8 . The method of  claim 1  wherein the etching solution further comprises a solvent whereby the solvent has at least one of a polar solvent or a non-polar solvent. 
     
     
         9 . The method of  claim 1  wherein the etching solution has an acidic pH. 
     
     
         10 . The method of  claim 1  wherein the etching solution has an acidic pH between 1 and 4.5, or a basic pH between 8.5 to 14. 
     
     
         11 . The method of  claim 1  further comprised of a step wherein the surface is washed with at least one solvent being a neutral, apolar or polar solvent including water, ethanol, methanol isopropanol, butanol, tert-butanol, tetrahydrofuran, hexane, heptane, acetone, acetonitrile, cyclohexane, or diethyl ether. 
     
     
         12 . The method of  claim 1  wherein the metallic or metallic alloy material is comprising at least one metal including aluminum, zinc, iron, copper and titanium. 
     
     
         13 . The method of  claim 1  wherein the metal etching step has a time duration from 1 minute to 48 hours. 
     
     
         14 . The method of  claim 1  wherein the metal etching step is at a temperature from −10° C. to 100° C. 
     
     
         15 . The method of  claim 6  wherein the adding of external energy is operable to vary the at least one salt rate of dissociation and effect of promoter or catalyst activity by varying at least one parameter from electrical current, from temperature of the etching solution, from temperature of the surface, from agitation speed of mechanical agitation in the etching solution, from introducing external pressure and introducing a gas including nitrogen, air, hydrogen or argon in the etching solution. 
     
     
         16 . The method of  claim 1  wherein a rate of etching the metal surface is operable by varying the rate of salt dissociation by further comprising the etching solution with the promoter or catalyst and at least one of the solvent or a buffer solution in the etching solution. 
     
     
         17 . The method of  claim 1  wherein the optional or the catalyst in the etching solution is a solid inorganic salt, solid organic salt, or liquid solution. 
     
     
         18 . The method of  claim 1  further comprised of a chemical functionalization step of the etched surface operable to generate a low energy surface on the etched surface with hydrophobic, oleophobic or omniphobic properties by a tethering processes including silanization, phosphatization, parkerization, carboxylation, amination, sulfonation, carbonation, esterification, hydroxylation, thiolation amidation, azolation, silylation, halogenation including fluorination, alkylation or alkoxylation. 
     
     
         19 . The method of  claim 19  wherein the chemical functionalization step possesses a re-entrant surface texture. 
     
     
         20 . The method of  claim 19  wherein re-entrant texture is a hierarchical texture having re-entrant nano- and micro-textures by further comprising the step of adding the solvent into the etching solution and the step of boiling the etching solution at a temperature from 40° C. to 200° C. 
     
     
         21 . The method of  claim 20  wherein the solvent is water or steam. 
     
     
         22 . The method of  claim 1  further comprised of a chemical functionalization step of the etched surface operable to generate a low energy surface with hydrophobic, oleophobic or omniphobic properties and washing step with the solvent including a polar, apolar or neutral solvent comprised of at least solvent selected from ethanol, methanol isopropanol, butanol, tert-butanol, tetrahydrofuran, hexane, heptane, acetone, acetonitrile, cyclohexane, or diethyl ether. 
     
     
         23 . The method of  claim 1  wherein the surface is superhydrophobic or superoleophobic, or superomniphobic. 
     
     
         24 . The method of  claim 1  further comprised of a step applying a low surface energy and thin layer coated on the etched surface operable to obtain at least one of superhydrophobic or superoleophobic property having a contact angle from 150° to 180° with at least one of water or oil. 
     
     
         25 . The method of  claim 1  further comprised of a step applying a low surface energy and thin layer coated on the etched surface operable to obtain at least one of omniphobic property having a contact angle from 100° to 179° with water and oil. 
     
     
         26 . The method of  claim 1  further comprised of at least one additional step including etching by sand blasting or abrasion using sand paper or file to generate nano- or micro-patterned rough surface, followed by an additional step of boiling in the solvent, or after the etching step in a salt solution followed by an additional step of spraying, layer-brushing, painting, spin coating, dip coating, or lithography on the etched surface operable to develop a low surface energy thin layer coating. 
     
     
         27 . The method of  claim 1  wherein the etched surface has a predetermined wettability selected to be hydrophobic or oleophobic, has an initial non-reacted chemical functional groups, having a first apparent advancing dynamic contact angle of greater than or equal to 150° for water, and having a second apparent advancing dynamic contact angle of greater than or equal to 150° for a preselected oil, and an additional step of applying a low surface energy coating of a fluoroalkyl silane comprising heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane, heptadecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane, or combinations thereof, and an additional subsequent step of reacting the low surface energy fluoroalkyl silane with oxygen groups, carbon groups, carboxyl groups, nitride groups, carbide groups, and hydroxyl groups on the etched surface for a step duration of time until greater than or equal to 60% of the initial non-reacted chemical functional groups are reacted or physisorbed by the fluoroalkyl silane. 
     
     
         28 . The method of  claim 1  wherein the solvent used for at least one step of etching, rinsing or chemical functionalization is comprised of a liquid or vapor phase water, tetrahydrofuran, hexafluorobenzene, cyclohexane, or a combination of 3,3-Dichloro-1,1, 1,2,2-pentafluoropropane and 1,3-Dichloro-1,1,2,2,3-pentafluoropropane, toluene, chloroform, methylene chloride, and other polar or non-polar organic solvents.