In our industry, we often need to know if a material likes water (hydrophilic, easy to be wetted by water) or repels water (hydrophobic, dislike water). In some application areas such as used as acquisition layer, distribution layer, core wrapping tissue etc., we want the materials to be hydrophilic. But in other applications such as leg cuff etc., we want the materials to be hydrophobic. In the procedure to decide what kind of adhesives can be used to bond a specific material to a specific substrate, we also need to know if the adhesive of interesting can wet or spread over the surface of targeting materials or not.
Surface tension, surface energy and contact angle are key material surface properties which are closely related to wetting phenomenon, and which we often need to deal with in our daily works in products and materials development.
 
 
Let's first look at the definition of the terminologies.
 
  • Surface tension (of a liquid)
    Sometimes it is referred to as free energy per unit area, surface tension is the force required to increase unit surface area. This definition can be well illustrated by the figure below.

    A wire frame with soap film stretched across it, in which one side of the frame is movable. Assuming the wires do not have weight and there is no friction to remove it, the force needed to remove the movable side to increase one unit area of the wire frame is defined as the surface tension
 
  • Interfacial surface tension
    The force required to increase unit surface area of the interface (for example, gas phase and solid phase, liquid phase and solid phase, etc.).
  • Contact angle
    Contact angle is defined by Young's equation, which results from a balance of interfacial forces acting at the contact line (see the figure below)
 
 

Where γLV is the "surface tension" of the liquid which can be measured by a variety of methods.
γSV is the "surface tension" of the solid in air.
γSL is the "interfacial surface tension" of the solid and liquid.
gSV and gSL cannot be measured separately but their difference (= gLVcosq = the "wetting tension") can be measured directly.
 
  • Wetting

    Wetting of a solid by a liquid means that the liquid can spread over the solid surface. When the contact angle is zero or close to zero, it is called complete wetting, otherwise it is incomplete wetting. We are usually interested in incomplete wetting because it is close to the real world.

Contact angle of liquid on solid surface and wetting
 
Typical contact angle of water on various solid surfaces
 
Solid surface
θ, degree
　　Glass
　　0
　　PEG
　　18
　　Tooth enamel
　　36
　　Tooth dentin
　　39
　　Cellulose acetate
　　54
　　Nylon
　　70
　　PMMA
　　74
　　PET
　　77
　　PVC
　　85
　　Skin
　　55-120
　　Skin (ether clean)
　　80-100
　　Hair
　　89
　　Polyethylene
　　94
　　Polypropylene
　　99
　　Paraffin wax
　　110
　　PTFE
　　112
 
 
 
In the real world, the surfaces we deal with are often rough surfaces, not smooth surfaces. The roughness ( r ) of a surface is defined as:
For a rough surface, the contact angle is defined by Wenzel equation:
where r = "surface roughness"

According to this equation, the roughness of a surface further decreases the contact angle if the contact angle is < 90＜ , whereas the roughness further increases the contact angle if the contact angle is > 90＜ (see the illustration below).

 
According to Wenzel equation, surface roughness significantly changes the apparent contact angle. How about extremely rough surfaces, the so-called Fractal surfaces?
Fractal patterns (Fractal surface or Fractal line) are special patterns which do not have an integral value of dimension. Usually a straight line has a dimension of 1, a flat surface has dimension of 2, whereas a cube has a dimension of 3. However, the dimension of a Fractal line or surface has an infinite length or surface area of which the dimension is not 1, 2, or 3, but between 1 and 2, or 2 and 3. That is:

1< Dimension of Fractal line <2
2< Dimension of Fractal surface <3
The figure below illustrates a Fractal line which has an infinite length, and of which the dimension is neither 1 nor 2, but between 1 and 2.
In nature, there are many examples of Fractal patterns, such as
  • Coast line
  • Thunder flash
  • Feather: one of the few substances found in nature with a q close to 180 degree
The trend projected by Wenzel equation is also correct for Fractal patterns. That is, when the contact angle is less than 90＜ , creating a Fractal surface will make this material to be an ultra hydrophilic surface (q close to 0 degree). On the other hand, when the contact angle is larger than 90＜ , creating a Fractal surface will make this material to be an ultra hydrophobic surface (q close to 180 degree). There are a lot of attempts in recent years to make such ultra hydrophilic or ultra hydrophobic materials by using this Fractal pattern approach (see the AKD examples below).

In our industry, whenever we need to increase wet-ability (hydrophilicity) or repelling property (hydrophobicity) of nonwovens, films, laminates, etc., we can take such approaches such as increasing the surface roughness or creating Fractal surfaces to obtain ultra hydrophilic or hydrophobic surfaces.
 
Usually surfactants are adsorbed at interfaces so they reduce surface and interfacial tensions
If the liquid is water, adding surfactant to the water causes increase in cosq, i.e. reduction of q because of the adsorption of the surfactant molecules at LV and SL interfaces but not (at least initially) at the SV interface, which reduces gSL and gLV. In other words, surfactants usually promote wetting of solids by water.
 
  • Surface tension of a liquid is the force required to increase unit surface area.
  • Contact angle is defined by Young's equation, which results from a balance of interfacial forces acting at the contact line.
  • Partial wetting of solids by liquids is characterised by a contact angle.
  • Surface roughness decreases contact angle q if < 90＜, increases q if > 90＜.
  • Ultra "hydrophilic" or "hydrophobic" surfaces can be obtained by increasing surface roughness (Fractal surfaces) approach.
  • Surfactants usually promote wetting of hydrophobic solids by water.