The contact angle is the angle, conventionally measured through the liquid, where a liquid/vapor
interface meets a solid surface.
It quantifies the wettability of a solid surface by a liquid via the Young equation.
A given system of solid, liquid, and vapor at a given temperature and pressure has a
unique equilibrium contact angle.
However, in practice contact angle hysteresis is observed, ranging from the so-called advancing
contact angle to the receding contact angle.
The equilibrium contact is within those values, and can be calculated from them.
The equilibrium contact angle reflects the relative strength of the liquid, solid, and
vapor molecular interaction.
Thermodynamics
The shape of a liquid/vapor interface is determined by the Young–Laplace equation, with the
contact angle playing the role of a boundary condition via Young's Equation.
The theoretical description of contact arises from the consideration of a thermodynamic
equilibrium between the three phases: the liquid phase, the solid phase, and the gas/vapor
phase.
The “gaseous” phase could also be another liquid phase.
If the solid–vapor interfacial energy is denoted by , the solid–liquid interfacial
energy by , and the liquid–vapor interfacial energy by , then the equilibrium contact angle
is determined from these quantities by Young's Equation:
The contact angle can also be related to the work of adhesion via the Young-Dupré equation:
where is the solid - liquid adhesion energy per unit area when in the medium V.
Hysteresis If we add a small enough amount of liquid
to a drop, the contact line will still be pinned and the contact angle will increase;
similarly, if we remove a small enough amount of liquid from a drop, the contact line will
still be pinned and the contact angle will decrease.
Hence, a drop placed on a surface has a spectrum of contact angles ranging from the so-called
advancing contact angle, , to the so-called receding contact angle, . The Young equilibrium
contact angle is somewhere between those values, and the contact angle hysteresis is normally
defined as . The Young equation assumes a perfectly flat
surface.
Even in such a smooth surface a drop will assume contact angle hysteresis.
The equilibrium contact angle can be calculated from and as was shown theoretically by Tadmor
and confirmed experimentally by Chibowski as,
where
On a surface that is rough or contaminated, there will also be contact angle hysteresis,
but now the local equilibrium contact angle may vary from place to place on the surface.
According to the Young-Dupré equation, this means that the adhesion energy varies locally
– thus, the liquid has to overcome local energy barriers in order to wet the surface.
One consequence of these barriers is contact angle hysteresis: the extent of wetting, and
therefore the observed contact angle, depend on whether the liquid is advancing or receding
on the surface.
Since liquid advances over previously dry surface but recedes from previously wet surface,
contact angle hysteresis can also arise if the solid has been altered due to its previous
contact with the liquid.
Such alterations, if slow, can also produce measurably time-dependent contact angles.
Dynamic Contact Angles For liquid moving quickly over a surface,
the contact angle can be altered from its value at rest.
The advancing contact angle will increase with speed and the receding contact angle
will decrease.
Typical contact angles Contact angles are extremely sensitive to
contamination; values reproducible to better than a few degrees are generally only obtained
under laboratory conditions with purified liquids and very clean solid surfaces.
If the liquid molecules are strongly attracted to the solid molecules then the liquid drop
will completely spread out on the solid surface, corresponding to a contact angle of 0°.
This is often the case for water on bare metallic or ceramic surfaces, although the presence
of an oxide layer, or contaminants, on the solid surface can significantly increase the
contact angle.
Generally, if the water contact angle is smaller than 90°, the solid surface is considered
hydrophilic and if the water contact angle is larger than 90°, the solid surface is
considered hydrophobic.
Many polymers exhibit hydrophobic surfaces.
Highly hydrophobic surfaces made of low surface energy materials may have water contact angles
as high as ~120°.
Some materials with highly rough surfaces may have a water contact angle even greater
than 150°, due to the presence of air pockets under the liquid drop.
These are called superhydrophobic surfaces.
If the contact angle is measured through the gas instead of through the liquid, then it
should be replaced by 180° minus their given value.
Contact angles are equally applicable to the interface of two liquids, though they are
more commonly measured in solid products such as non-stick pans and waterproof fabrics.
Measuring methods
The static sessile drop method The sessile drop method is measured by a contact
angle goniometer using an optical subsystem to capture the profile of a pure liquid on
a solid substrate.
The angle formed between the liquid/solid interface and the liquid/vapor interface is
the contact angle.
Older systems used a microscope optical system with a back light.
Current-generation systems employ high resolution cameras and software to capture and analyze
the contact angle.
Angles measured in such a way are often quite close to advancing contact angles.
Equilibrium contact angles can be obtained through the application of well defined vibrations.
The dynamic sessile drop method The dynamic sessile drop is similar to the
static sessile drop but requires the drop to be modified.
A common type of dynamic sessile drop study determines the largest contact angle possible
without increasing its solid/liquid interfacial area by adding volume dynamically.
This maximum angle is the advancing angle.
Volume is removed to produce the smallest possible angle, the receding angle.
The difference between the advancing and receding angle is the contact angle hysteresis.
Dynamic Wilhelmy method A method for calculating average advancing
and receding contact angles on solids of uniform geometry.
Both sides of the solid must have the same properties.
Wetting force on the solid is measured as the solid is immersed in or withdrawn from
a liquid of known surface tension.
Also in that case it is possible to measure the equilibrium contact angle by applying
a very controlled vibration.
That methodology, called VIECA, can be implemented in a quite simple way on every Wilhelmy balance.
Single-fiber Wilhelmy method Dynamic Wilhelmy method applied to single
fibers to measure advancing and receding contact angles.
Washburn's equation capillary rise method Enables measurement of average contact angle
and sorption speed for powders and other porous materials.
Change of weight as a function of time is measured.
See also
References
Further reading Pierre-Gilles de Gennes, Françoise Brochard-Wyart,
David Quéré, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, Springer
Jacob Israelachvili, Intermolecular and Surface Forces, Academic Press
D.W. Van Krevelen, Properties of Polymers, 2nd revised edition, Elsevier Scientific Publishing
Company, Amsterdam-Oxford-New York