Assembly for and method of forming localized surface wrinkles

Abstract

An assembly including and method of forming arbitrary localized wrinkles upon a surface utilizing a shape memory polymer substrate and rigid overlay, wherein the geometrical distribution of the wrinkles is produced by recovering a lateral strain history within the substrate and buckling the overlay, and the localized wrinkles are used to create, among other things, optically three-dimensional engaging surfaces, structural colors, modified surface texturing, and haptic alerts.

1 . A method of forming localized wrinkles upon a surface, said method comprising:
 a. indenting a substrate at least partly formed of shape memory polymer, so that the substrate defines a first area experiencing purely compressive strain and a second area adjacent the first area and experiencing compressive and tensile strains;   b. attaching a relaxed overlay to the substrate, so as to overlay the first and second areas; and define the surface;   c. activating the polymer, so as to recover the strains;   d. causing wrinkles to form in the overlay, as a result of recovering the tensile strain in the second area; and   e. deactivating the polymer.   
     
     
         2 . The method as claimed in  claim 1 , wherein steps a) and b) further comprises heating the polymer to a first temperature, and applying a load to the substrate after the substrate achieves the first temperature, and cooling the polymer to a second temperature below the first temperature, while continuing to apply the load, so as to lock in the strains. 
     
     
         3 . The method as claimed in  claim 1 , wherein step a) further comprises the steps of applying a press to the substrate, wherein the press defines an engaging relief. 
     
     
         4 . The method as claimed in  claim 3 , wherein the relief presents and the first or second area defines, indicia, a logo, an engaging surface, or a shape. 
     
     
         5 . The method as claimed in  claim 1 , wherein the overlay, substrate, and strains are cooperatively configured to produce wrinkles having a wavelength within the visible spectrum. 
     
     
         6 . The method as claimed in  claim 1 , wherein the polymer consists essentially of an aromatic diepoxide, an aliphatic diepoxide, and an aliphatic diamine curing agent. 
     
     
         7 . The method as claimed in  claim 1 , wherein the overlay is a metallic thin film. 
     
     
         8 . The method as claimed in  claim 7 , wherein the overlay is formed of white gold. 
     
     
         9 . The method as claimed in  claim 1 , wherein step c) further includes the steps of depositing the overlay. 
     
     
         10 . The method as claimed in  claim 1 , wherein the substrate presents a first modulus, the overlay presents a second modulus greater than the first, and the first and second modulus present a ratio greater than a predetermined threshold ratio. 
     
     
         11 . The method as claimed in  claim 10 , wherein the overlay presents a critical bucking strain (ε c ), and steps a) and c) further includes the steps of defining and recovering a tensile strain greater than the critical bucking strain. 
     
     
         12 . The method as claimed in  claim 11 , wherein the critical bucking strain is determined in accordance with the formula:
   ε c   =[E   s   2   /E   f   2 ] 1/3  
   E s  is the modulus of the substrate, and   E f  is the modulus of the film.   
     
     
         13 . The method as claimed in  claim 1 , wherein the second area circumscribes the first area. 
     
     
         14 . The method as claimed in  claim 1 , wherein the overlay presents a thickness approximately equal to 10 nm. 
     
     
         15 . A method of measuring the cracking strain/stress of a nanoscopically thin overlay, said method comprising the steps of:
 a. indenting a substrate at least partly formed of shape memory polymer presenting a glass transition temperature, so that the substrate defines a first area experiencing a purely compressive strain, and a second area adjacent the first area and experiencing compressive and tensile strains;   b. attaching the overlay to the substrate, so as to cover the first and second areas, and define the surface;   c. activating the polymer, so as to recover the strains;   d. determining the formation of a crack within the overlay, as a result of recovering the tensile strain in the second area; and   e. observing and measuring the crack relative to the strains, when the crack is formed.   
     
     
         16 . An assembly for forming localized wrinkles within a continuous surface, said assembly comprising:
 a substrate at least partially formed of a shape memory polymer presenting a glass transition temperature and first elastic modulus when activated, being plastically indented so as to define a first area experiencing a purely compressive strain, and a second area adjacent the first area and experiencing compressive and tensile strains, when the polymer is deactivated, and operable to recover a the strains when the polymer is activated; and   a relaxed overlay fixedly attached to and configured to cover the first and second areas, and defining the surface, a second elastic modulus, and a height,   wherein the second elastic modulus is greater than the first modulus, and the moduli, height, and strains are cooperatively configured to cause buckling in the overlay, when the polymer is activated and recovers the strains.   
     
     
         17 . The method as claimed in  claim 16 , wherein the polymer consists essentially of an aromatic diepoxide, an aliphatic diepoxide, and an aliphatic diamine curing agent. 
     
     
         18 . The method as claimed in  claim 16 , wherein the overlay is a metallic thin film. 
     
     
         19 . The method as claimed in  claim 18 , wherein the overlay is formed of white gold.