Abstract:
A fluidic device includes a porous substrate, a non-wetting region extending through a first portion of the porous substrate from a first side of the substrate, in which the non-wetting region is impermeable to fluid transport, and a wetting region extending through a second portion of the porous substrate from a second side of the substrate, in which the wetting region is permeable to fluid transport.

Description:
BACKGROUND 
       [0001]    This disclosure relates to a combined wetting/non-wetting element for low and high surface tension liquids. 
         [0002]    Microfluidic systems are important tools for research and development in many application areas including industrial engineering, bio/pharmaceuticals, food service, power and energy storage. In many cases, it is desirable to incorporate structures in the microfluidic systems that exhibit both non-wetting and wetting properties in order to facilitate control of fluid flow and reactions. Typically, such structures are developed using two separate and independent devices, in which one of the devices provides the non-wetting properties and the other device provides the wetting properties. The two devices then are incorporated into a single structure based on a desired functionality of the final microfluidic system. 
       SUMMARY 
       [0003]    The details of one or more implementations of the invention are set forth in the description below, the accompanying drawings and the claims. 
         [0004]    For example, in one aspect, a fluidic device includes a porous substrate, a non-wetting region extending through a first portion of the porous substrate from a first side of the substrate, in which the non-wetting region is impermeable to fluid transport, and a wetting region extending through a second portion of the porous substrate from a second side of the substrate, in which the wetting region is permeable to fluid transport. 
         [0005]    Some implementations include one or more of the following features. 
         [0006]    In some implementations, the porous substrate includes fibers. The fibers can be woven. 
         [0007]    In some cases, the porous substrate includes filaments. 
         [0008]    In certain examples, the substrate includes a textile. 
         [0009]    In some implementations, the substrate includes a filter. 
         [0010]    In certain cases, the porous substrate includes micro-pores. In some cases, the substrate includes nano-pores. 
         [0011]    In some examples, the non-wetting region includes a non-wetting coating. The non-wetting coating can include a self-assembled monolayer. Alternatively, or in addition, the non-wetting coating can include a fluoropolymer. 
         [0012]    In certain implementations, the porous substrate includes at least one of a fiber, filament, pore, cavity or crevice and the non-wetting coating covers the surface of the fiber, filament, pore, cavity or crevice in the non-wetting region. 
         [0013]    In some cases, the porous substrate is flexible. 
         [0014]    In certain examples, the non-wetting region is hydrophobic or super-hydrophobic. In some cases, the non-wetting region is super-lyophobic. 
         [0015]    In some implementations, the porous substrate includes a first porous material fixed to a second porous material. 
         [0016]    In some cases, the wetting region is planar. 
         [0017]    In another aspect, a fluidic device includes non-wetting regions extending along a thickness direction of the fluidic device, in which each non-wetting region is impermeable to fluid transport. The device further includes wetting regions extending along a thickness direction of the fluidic device, in which each wetting region is permeable to fluid transport. 
         [0018]    In some implementations, the non-wetting regions and wetting regions are arranged in an alternating pattern. 
         [0019]    In some cases, each of the non-wetting regions and wetting regions includes a porous substrate. 
         [0020]    In certain examples, the thickness of each of the non-wetting regions is different from the thickness of each of the wetting regions. 
         [0021]    In some examples, the non-wetting region includes a substrate selected from the group consisting of a hydrophobic substrate and a super-lyophobic substrate. In some cases, the wetting region includes a substrate selected from the group consisting of a hydrophobic substrate and a super-lyophobic substrate. 
         [0022]    In another aspect, a fluidic device includes non-wetting regions, in which each non-wetting region has a different degree of fluid permeability. 
         [0023]    In some implementations, the degree of fluid permeability is a minimum in a first non-wetting region on one side of the device and a maximum in a second non-wetting region on a second opposite side of the device, in which the fluid permeability increases in each of the regions from the first non-wetting region to the second non-wetting region. 
         [0024]    In another aspect, a method of fabricating a fluidic device includes applying a non-wetting coating to a porous substrate and removing the non-wetting coating from the porous substrate to form a wetting region and a non-wetting region, in which the non-wetting region extends from a first side of the porous substrate through a first portion of the substrate and wherein the wetting region extends from a second side of the porous substrate through a second portion of the substrate. 
         [0025]    In some cases, applying the non-wetting coating includes dip-coating the substrate in a non-wetting coating material. 
         [0026]    In certain examples, applying the non-wetting coating includes chemical vapor deposition of the non-wetting coating on the porous substrate. 
         [0027]    In certain implementations, applying the non-wetting coating includes self-assembly of the non-wetting coating on the porous substrate. 
         [0028]    In some examples, removing the non-wetting coating includes exposing the porous substrate to ozone. 
         [0029]    In some cases, removing the non-wetting coating includes exposing the porous substrate to ultraviolet light. 
         [0030]    In some implementations, removing the non-wetting coating includes exposing the porous substrate to plasma. 
         [0031]    In another aspect, a method of fabricating a fluidic device includes fixing a first porous substrate to a second porous substrate, in which each of the first and second porous substrates having a wetting region and a non-wetting region extending along a thickness direction of the fluidic device. 
         [0032]    Other features will be readily apparent from the detailed description, drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIGS. 1A-1B  illustrate examples of a fluidic device that includes a non-wetting region and a wetting region. 
           [0034]      FIGS. 2A-2C  illustrate a method of fabricating a fluidic device that include a non-wetting region and a wetting region. 
           [0035]      FIGS. 3A-3C  illustrate examples of a fluidic device that includes a non-wetting region and a wetting region. 
           [0036]      FIG. 3D  shows an example of fluidic device that includes a non-wetting region and a patterned wetting region. 
           [0037]      FIG. 3E  shows a top view of a porous non-wetting substrate that includes a planar wetting region. 
           [0038]      FIGS. 4A-4C  show example SEM images of glass fiber filters. 
           [0039]      FIGS. 4D-4E  show example SEM images of PVDF. 
           [0040]      FIG. 5  shows an example image of a water droplet on a non-wetting region of a fluidic device. 
           [0041]      FIG. 6  shows an example process of fixing a first polymeric filter to a second polymeric filter. 
           [0042]      FIG. 7  illustrates an example of a fluidic device that includes several wetting substrates  26  and non-wetting substrates  28  fixed together in an alternating pattern. 
           [0043]      FIG. 8  illustrates an example of a fluidic device that includes wetting regions and non-wetting regions having different thicknesses. 
           [0044]      FIG. 9  shows an example of a fluidic device that includes super-hydrophobic filters stacked and arranged in an alternating pattern with super-lyophobic filters. 
           [0045]      FIG. 10  shows an example of a fluidic device that includes multiple filters stacked together in which each filter has a different non-wetting coating. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]      FIGS. 1A and 1B  show an example of a discrete substrate  1  that exhibits both non-wetting and wetting properties. The substrate  1  can be used to manipulate low and high surface tension liquids including organic and aqueous liquids. As illustrated in  FIGS. 1A and 1B , the substrate  1  includes two regions: a non-wetting first region  5  and an adjacent wetting second region  7 . Due to the non-wetting nature of the first region  5 , a liquid  9  placed on the surface of region  5  displays minimal affinity for spreading. For example, upon contact with the surface, the liquid  9  forms a spherically shaped droplet having a contact angle  11  greater than or equal to 90 degrees. The contact angle  11  of the droplet corresponds to the angle between the liquid-vapor and the liquid-solid interfaces when the substrate  1  and liquid  9  are in a vapor environment. In contrast, when the liquid  9  is placed on the wetting second region  7 , the liquid  9  spreads out and is partially or completely absorbed by the second region  7 , as shown in  FIG. 1B . However, the presence of the adjacent non-wetting region  5  prevents the liquid from passing completely through the substrate  1  from the wetting region  7  to the opposite side of the substrate  1 . 
         [0047]    The non-wetting nature of region  5  can be characterized as hydrophobic, super-hydrophobic, super-lyophobic or as a combined hydrophobic/super-lyophobic region. A hydrophobic surface has minimal affinity for water, aqueous solutions and other high surface tension liquids. Accordingly, those liquids do not readily wet objects with hydrophobic properties. In some cases, the region  5  can be considered to be superhydrophobic, with the resulting liquid contact angle well above 90 degrees. An advantage of hydrophobic and superhydrophobic surfaces is that liquids placed on such surfaces can be manipulated and transported easily. 
         [0048]    On the other hand, low surface tension liquids, which include, but are not limited to, kerosene, oils, hexane and various alcohols, tend to quickly spread and wet hydrophobic and superhydrophobic surfaces such that liquid handling is difficult. Instead, those liquids exhibit non-wetting properties on surfaces characterized as super-lyophobic. Super-lyophobic surfaces have minimal affinity for low surface tension liquids such that the liquids do not spread easily and can be relatively simple to manipulate. 
         [0049]      FIGS. 2A-2C  show a method of fabricating the structure illustrated in  FIGS. 1A-1B . Preferably, the substrate  1  is formed from a single porous and absorptive material that allows liquid to pass through it. For example, the substrate  1  can include a uniform composition of woven or non-woven materials, such as glass fiber filters, textiles and polymeric filters, that are composed of a network of natural or artificial filaments, e.g. fibers. In some cases, the substrate can be formed of a material having ordered or disordered micro-pores including, for example, polyvinylidene fluoride (PVDF). Porous microstructures can be fabricated by means of common processing techniques that include chemical etching and plasma etching or purchased from commercial vendors. In some cases, the substrate material is also flexible to provide enhanced durability. 
         [0050]    As shown in  FIG. 2B , the substrate  1  is covered with a non-wetting coating  13 . Preferably, although not required, the coating  13  covers the entire substrate  1  including the surfaces of any fibers, filaments, crevices and pores. Examples of non-wetting coatings include polytetrafluoroethylene, fluoropolymers, CYTOP® material, and self-assembled monolayers (SAM). Depending on the non-wetting coating used, the level of hydrophobicity exhibited by the substrate  1  will differ. For example, a substrate coated with a SAM having fluorinated functional groups may appear to aqueous liquids as more non-wetting, i.e., hydrophobic, than a substrate coated with a SAM having a methyl functional group. The non-wetting coating  13  can be applied using, for example, dip-coating, spin-coating, chemical vapor deposition, spraying or self-assembly techniques. Other non-wetting coatings and methods for applying the coatings can be used as well. 
         [0051]    In some implementations, the physical structure of the substrate material enhances the non-wetting features. For example, the fibers or pores of the substrate  1  can provide a micro and nano-scale surface roughness that, when can be combined with a non-wetting coating, exhibits super non-wetting properties. A material that exhibits super non-wetting properties is extremely difficult to wet. In many cases, the contact angle of a liquid on the surface of a super non-wetting material exceeds 120 degrees. 
         [0052]    After the non-wetting coating  13  has been applied, the coating  13  is partially removed from the substrate  1  (see  FIG. 2C ) to form a first non-wetting region  5  and a second wetting region  7 . Removal of the non-wetting coating  13  can be accomplished by exposing a side  14  of the substrate  1  to ozone, ultraviolet light and plasmas. Other coating removal methods may be applied as well. In the region of the substrate  1  where the non-wetting coating is removed, the coating  13  is eliminated from the surface of any fibers, filaments, crevices or pores to which it is attached. In some cases, the removal process also oxidizes the surface of the substrate material such that it exhibits wetting properties, i.e., liquids will tend to wet the surface. Depending on the process conditions under which the non-wetting coating is removed, the depth to which the wetting properties extend in the substrate  1  can be varied. For example, as shown in  FIG. 3A , exposing the side  14  of substrate  1  to an oxygen plasma for a few seconds at low power may create a shallow wetting region  7  in the substrate  1 . The remaining non-wetting region  5  of the substrate  1  is unaffected. Alternatively, the side  14  can be exposed to a high power plasma for several minutes such that the wetting region  7  extends beyond half the thickness of the substrate  1 , as illustrated in  FIG. 3B . Accordingly, it is possible to fabricate a bi-layered wetting/non-wetting material in which the thickness of the wetting and non-wetting regions can be controlled. 
         [0053]    As a result of the non-uniformity of some coating removal methods, the depth of a boundary  15  between the wetting and non-wetting regions can be uneven or circuitous along the length and width of the substrate  1  as shown in  FIG. 3C . 
         [0054]    In some implementations, a mask can be applied to the side  14  of substrate  1  prior to exposing the device to a plasma. During subsequent application of the plasma, the regions of side  14  covered by the mask will retain the non-wetting coating  13 . In contrast, the regions of side  14  that are exposed to the plasma through the mask will have the coating  13  removed. As a result, a variety of non-wetting/wetting patterns can be formed in the substrate  1  based on the design of the mask. For example,  FIG. 3D  shows a porous substrate  1  having a non-wetting coating  13  in which the bottom side  14  of the substrate  1  was exposed to a plasma through a shadow mask. As evident in the figure, portions  17 , which were covered by the shadow mask, retain a non-wetting coating. In contrast, portions  19  that were exposed to the plasma through the shadow mask have had the coating  13  removed. Accordingly, the plasma exposed portions  19  have a higher affinity for liquids and exhibit preferentially wetting properties. In addition to shadow masks, other masks, such as photosensitive resists, can be used. 
         [0055]    As explained above, the depth to which the non-wetting coating is removed can be controlled based on the total amount of time the substrate is exposed to a plasma. In some cases, the plasma exposure is so brief that only a thin layer of the non-wetting coating  13  is removed. For example,  FIG. 3E  shows a top view of a substrate  1  having a non-wetting coating  13  in which the substrate is exposed to a plasma through a shadow mask for a very brief period, on the order of half a second. The mask is designed to have a single hole in the center. As a result of the brief plasma exposure, only a very thin amount of the non-wetting coating  13  is removed from a region  21  of the substrate  1  that is underneath the mask hole. A liquid subsequently placed on the substrate  1  spreads out in the wetting region  21  but is confined at boundaries where the non-wetting coating  13  remains. Given that the coating  13  has only been removed in a very thin layer during the brief plasma exposure, the liquid will not be absorbed by the substrate  1 . Accordingly, the wetting region is confined to a plane of the substrate. The plasma exposure time and power required to remove a thin layer of the non-wetting coating  13  can vary depending on the type of coating used. 
         [0056]      FIGS. 4A-4C  show example scanning electron microscope (SEM) images corresponding to glass fiber filter substrate material APFA, APFC and APFD, respectively. The substrates shown in  FIGS. 4A-4C  are manufactured by Millipore Corporation of Billerica, Mass.  FIGS. 4D-4E  show example SEM images of PVDF substrate material taken at different magnifications. 
         [0057]    A bi-layer hydrophilic/hydrophobic structure was successfully prepared with the APFC glass fiber filter used as the core substrate material. The substrate was coated with a self-assembled monolayer that included chlorinated silanes. One side of the substrate was exposed to an oxygen plasma at 200 W for 30 seconds, such that the coating was removed and the surface of the substrate readily absorbed liquids. The opposite side of the substrate, in contrast, retained the super-hydrophobic properties. An example of the bi-layer structure including a water droplet  23  on the hydrophobic surface  25  is shown in  FIG. 5 . 
         [0058]    In some implementations, the substrate is formed by fixing together two separate and discrete porous materials as opposed to using a single substrate material. In the example shown in  FIG. 6 , a first polymeric filter  20  is covered with a conformal non-wetting coating. The coating can be applied to the filter  20  in a manner similar to the process described with reference to  FIG. 2B . As indicated by the arrows in  FIG. 6 , the first polymeric filter  20  then is fixed to a second polymeric filter  22  that does not include a non-wetting coating. Various methods of adhesion may be used to fix the substrates together. For example, in some implementations, the first substrate can bond with the second substrate by means of Van der Waals forces. If there is a large contact area between the two substrates, the total Van der Waals force can be high, providing significant adhesion strength. In another example, a liquid can be applied between the substrates such that, as the liquid dries, capillary forces pull the substrates closer together and increase the contact area where Van der Waals bonding can occur. Alternatively, or in addition, fibers from the first filter  20  and second filter  22  can interlock to hold the materials together, adhering them in a manner that is similar to the use of VELCRO® tape. 
         [0059]    In contrast to the first polymeric filter  20 , the second polymeric filter  22  is not covered with a non-wetting coating  13 . Rather, the filter  22  is kept free of contamination and coating layers so as to maintain hydrophilic wetting properties. Accordingly, when the first and second filters  20 ,  22  are fixed together, a liquid droplet  13  placed on the surface of the first filter  20  is precluded from penetrating into the second filter  22  as a result of the non-wetting characteristics of the first filter  20 . In some implementations, the first filter  20 , the second filter  22  or both filters are replaced with substrates having micro-pores or nano-pores, in which the average diameter of a pore is in the range of several nanometers to several thousand microns. 
         [0060]    In some implementations, multiple wetting and non-wetting regions can be arranged through the thickness of the device. For example, as shown in  FIG. 7 , a fluidic device  24  is composed of several wetting substrates  26  and non-wetting substrates  28  that have been fixed together in an alternating pattern. Accordingly, it is possible to trap liquids in the wetting regions of the device  24  and between the non-wetting substrates  28 . Alternatively, or in addition, multiple non-wetting substrates  28  can be fixed in series to create thicker non-wetting regions in the device  24 . Similarly, multiple wetting substrates  26  can be fixed together to create thicker wetting regions. Extending the wetting region in this manner allows, for example, greater amounts of liquid to be stored or trapped in the device  24 . In some cases, the device  24  can include multiple substrates  30  fixed together in which each substrate  30  is modified to include both a non-wetting region  32  as well as a wetting region  34  having predefined thicknesses as shown in  FIG. 8 . As a result, the thickness of the wetting region or non-wetting region is not limited to the thickness of the substrate. 
         [0061]    It also is possible to fabricate a non-wetting structure such that it includes both hydrophobic and super-lyophobic properties. For example,  FIG. 9  shows a device  36  that includes filters  38  with super-hydrophobic properties stacked and arranged in an alternating pattern with filters  40  having super-lyophobic properties. Liquids having low surface tension, such as 1-butanol (surface tension equal to 26.2 mN/m) or 1-octanol (surface tension equal to 27.6 mN/m), would pass directly through the super-hydrophobic top filter  38  of the stack. Thus, the top filter  38  appears to low-surface tension liquids as a wetting region, even though it is super-hydrophobic. Upon reaching the super-lyophobic filter  40  located beneath the top stage, the low surface tension liquids would stop spreading and would be contained by the super-lyophobic filter. On the other hand, high-surface tension liquids, such as water (surface tension equal to 72.0 mN/m), would not pass through the top super-hydrophobic filter  38 . Similarly, the super-lyophobic filter  40  can appear to some liquids as a wetting region. 
         [0062]    By varying the level of non-wetting characteristics in each stage of the stack (e.g., by increasing or decreasing the level of hydrophobicity), it is possible to fabricate a structure that separates liquids based on surface tension. For example,  FIG. 10  shows multiple filters ( 42 ,  44 ,  46 ,  48 ) stacked together in which each filter includes a different non-wetting coating. The filters are arranged based on an increasing level of hydrophobicity exhibited by the filter coating, such that low-surface tension liquids would pass through the top filter  42  (having a low level of hydrophobicity) but not through the bottom filter  48  (having a high level of hydrophobicity). Filters with super-lyophobic properties can be used as well to increase the liquid selectively of the filter stack. 
         [0063]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Other implementations also are within the scope of the claims.