Patent Publication Number: US-2007110982-A1

Title: Device having a hydrophobic and/or lipophobic surface and method of producing one such device

Description:
TECHNICAL FIELD  
      The invention relates to a device with a hydrophobic surface, i.e., which repels water, does not absorb it or is not dissolved therein, and/or “lipophobic” surface, i.e., by analogy, which repels fatty substances, does not absorb them or is not dissolved therein, and a method for making such a device  
     STATE OF THE PRIOR ART  
      Making super-hydrophobic surfaces is increasingly of interest because such surfaces find many fields of application.  
      Such surfaces may be obtained by changing their roughness and their surface energy.  
      Practically, geometrical patterns may be engraved on such surfaces by using photolithography or machining methods. It is then necessary to make these surfaces hydrophobic by grafting or depositing hydrophobic compounds. They may also be obtained by dispersing micrometric particles in a gel or a resin applied onto this surface. In this case, the particles are intrinsically hydrophobic.  
      Such surfaces may also be made hydrophobic by depositing nanofibers, i.e., fibers of nanometric size, on these surfaces, followed by a chemical reaction on these nanofibers.  
      An article “Super-Amphiphobic aligned carbon nanotube films” of Huanjun Li, Xianbao Wang, Yanlin Song, Yungi Liu, Qianshu Li, Lei Jiang, and Daoben Zhu (Angew. Chem. Int., Ed. 2001, 40, No. 9, pages 1743-1746) thus describes the growth of films consisting of aligned carbon nanofibers (NTC) positioned perpendicularly to the surface of a substrate, and packed closely, with a uniform length and diameter, and then the immersion of these nanofibers in a methanol solution of hydrolyzed fluoroalkylsilane.  
       FIG. 1  illustrates an exemplary device obtained from such carbon nanofibers  10  made hydrophobic by chemical reaction. As illustrated in this figure:  
      Each carbon nanofiber  10  is laid on the surface  11  and does not adhere to the latter.  
      Only the upper portion  12  of each carbon nanofiber  10  is made hydrophobic.  
      There is no continuity of treatment: 
          over the whole surface of each carbon nanofiber,     over the surface between the carbon nanofibers.        

      Such treatment non-continuity is mainly due to the means used for making the carbon nanofibers hydrophobic. The liquid reagent used cannot attain the whole surface of each carbon nanofibers  10  because of capillarity phenomena. Moreover, this liquid reagent does not react with carbon and not with the underlying surface.  
      In the presence of a steam condensation phenomenon, this steam is formed in priority on the surface  13  between the carbon nanofibers, which is not hydrophobic. This surface  13  is therefore automatically polluted by this condensation and the impurities conveyed by the latter.  
      The object of the invention is to improve hydrophobicity of such a device by using another method for depositing polymer film.  
     DISCUSSION OF THE INVENTION  
      The invention relates to a device with a hydrophobic and/or lipophobic surface comprising a carpet of nanofibers, for example carbon nanofibers, characterized in that these carbon nanofibers are totally cladded with a hydrophobic and/or lipophobic continuous polymer film, for example polysiloxane, or a carbofluorinated polymer, and in that the surface between these nanofibers is covered with a layer of this same polymer.  
      The invention also relates to a method for making such a device with a hydrophobic and/or lipophobic surface which comprises a step for depositing nanofibers on a surface of said device, characterized in that it subsequently includes a step for cladding these nanofibers with a hydrophobic and/or lipophobic polymer achieved by a technique for dry physical deposition, or by an electro-grafting technique.  
      In an exemplary embodiment, the method of the invention includes the following steps:  
      a step for depositing carbon nanofibers on a surface of a part, which successively comprises: 
          depositing a catalyst by a PVD (Physical Vapor Deposition) method, the catalyst being deposited in vacuo at a pressure of a few 10 −3  mbars, a target consisting of catalytic material being bombarded by a flux of ionized argon, the thereby ejected atoms from the target covering this surface,     introducing the thereby covered part into the chamber of a CVD oven in vacuo in order to achieve the deposition of carbon nanofibers, the catalyst being first of all transformed into drops under the effect of the rise in temperature of the part, a hydrocarbon precursor being subsequently introduced into this chamber, the growth of carbon nanofibers being performed at the location where the catalyst is transformed into drops.        

      a step for cladding the nanofibers with a hydrophobic polymer with a PECVD (Plasma Enhanced Chemical Vapor Deposition) technique, or by an electro-grafting technique.  
      With the present invention, it is possible to make hydrophobic nanofibers, with which very large contact angles of a liquid on a solid may be obtained: for example, larger than 160°.  
      The fields of applications of the invention are very wide. For example, these are the making of:  
      electrochemical electrodes for analytic analysis,  
      ink injection systems for printing on paper,  
      channels for distributing or retaining liquid in biological analysis Microsystems,  
      surfaces of pistons for injecting liquid food,  
      textured plates of heat exchangers,  
      biological sensors or microcavities in which fluids flow, requiring the presence of a hydrophobic surface.  
      Such a technology is also applicable to the requirements of self-cleaning and/or anti-condensation surfaces. 
    
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a device from the prior art provided with a hydrophobic surface.  
       FIG. 2  illustrates a device with a hydrophobic surface according to the invention.  
       FIG. 3  illustrates the shape of a drop of water deposited on the surface formed by the upper end of the nanofibers of the device of the invention. 
    
    
     DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS  
      The device of the invention, as illustrated in  FIG. 2 , is a device with a hydrophobic and/or lipophobic surface comprising a carpet of nanofibers  20 , which are totally cladded with a hydrophobic and/or lipophobic, continuous polymer film  21 . The surface  22  existing between these nanofibers is itself covered with a layer of this same polymer.  
      Continuity of the polymer film allows the nanofibers to be bonded or firmly attached onto the surface  23 .  
      The cladding may be achieved by a dry physical deposition technique or by an electrografting technique.  
      The following characteristics may thereby be obtained:  
      exemplary nanofibers used: carbon nanofibers  
      exemplary polymer film used: polysiloxane or carbofluorinated polymer  
      diameter of a nanofiber  20 : about 20 to 30 nm  
      length of a nanofiber: about 3 μm  
      thickness of the hydrophobic polymer film: about 50 nm.  
      The method for making such a device with a hydrophobic and/or lipophobic surface thus comprises a step for depositing nanofibers on a surface of said device, and then a step for cladding these nanofibers with a hydrophobic and/or lipophobic polymer by means of a dry physical deposition technique or an electrografting technique.  
       FIG. 3  illustrates the shape of a drop of water  30  of about 1.5 mm in diameter deposited on the carpet of thereby treated carbon nanofibers  20  forming a hydrophobic surface. This drop  31  is slightly deformed by its weight, the actual contact angle θ for an undeformed drop therefore being larger than 175°.  
      In an exemplary embodiment of a super-hydrophobic layer, the following steps are performed:  
      a step for depositing a carpet of carbon nanofibers on a surface of a part, successively comprising: 
          depositing a catalyst by a PVD method, this catalyst being deposited in vacuo at a pressure of a few 10 −3  mbars, a target consisting of a catalytic material being bombarded by a flux of ionized argon, the thereby ejected atoms of the target covering this surface,     introducing this thereby covered part into a CVD (Chemical Vapor Deposition) oven in vacuo in order to perform deposition of carbon nanofibers, the limiting vacuum being of a few 10 −3  mbars, the catalyst being first of all transformed into drops under the effect of the rise in temperature of the part, a hydrocarbon precursor being then introduced into the chamber, the growth of carbon nanofibers being performed at the location where this catalyst is transformed into drops,        

      a step for cladding the nanofibers with a hydrophobic polymer by a PECVD technique or an electrografting technique.  
      During the deposition step, the pressure is located between 0.1 and 3 mbars. A polysiloxane precursor (hexamethyl disiloxane, octamethyl cyclotetrasiloxane, hexamethyldisilane, diphenyl methylsilane, . . . ) or a carbofluorinated precursor is introduced into the chamber and diluted with carrier gas (Ar, He, H 2 , . . . ). The thickness of the deposited nanofiber carpet is of the order of a hundred nanometers.  
      Further, it is worthwhile to note that this hydrophobic material, although intrinsically an electrical insulator, has not insignificant electric conduction properties when it is deposited as a thin layer on nanotubes.  
      The use of nanotubes covered with a hydrophobic polymer as electrodes may then be contemplated. In particular, the carpet of nanotubes before depositing the hydrophobic material may be structured as blocks isolated from each other and then each of these blocks may be covered with the hydrophobic polymer so as to reform a matrix of electrodes.