Contact angle
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 (maximal) contact angle to the receding (minimal) 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.
Contents
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 (L), the solid phase (S), and the gas/vapor phase (G) (which could be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor). The “gaseous” phase could also be another (immiscible) liquid phase. If the solid–vapor interfacial energy is denoted by \(\gamma_{SG}\), the solid–liquid interfacial energy by \(\gamma_{SL}\), and the liquid–vapor interfacial energy (i.e. the surface tension) by \(\gamma_{LG}\), then the equilibrium contact angle \(\theta_\mathrm{C}\) is determined from these quantities by Young's Equation:
\[0=\gamma_\mathrm{SG} - \gamma_\mathrm{SL} - \gamma_\mathrm{LG} \cos \theta_\mathrm{C} \,\]
The contact angle can also be related to the work of adhesion via the Young-Dupré equation:
\[\gamma (1 + \cos \theta_\mathrm{C} )= \Delta W_\mathrm{SLV} \,\]
where \(\Delta W_\mathrm{SLV}\) is the solid - liquid adhesion energy per unit area when in the medium V.
Hysteresis
On a surface that is rough or contaminated, Young's equation is still locally valid, but the equilibrium contact angle may vary from place to place on the surface.[1] 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 (averaged along the contact line), 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 (e.g., by a chemical reaction, or absorption). Such alterations, if slow, can also produce measurably time-dependent contact angles.
The highest observed contact angle is the advancing angle \(\theta_\mathrm{A}\), and the lowest observed contact angle is the receding angle \(\theta_\mathrm{R}\). The contact angle hysteresis is \(\theta_\mathrm{A} - \theta_\mathrm{R}\).
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 (\(\theta_\mathrm{c}\)) can be calculated from \(\theta_\mathrm{A}\) and \(\theta_\mathrm{R}\) as was shown theoretically by Tadmor [2] and confirmed experimentally by Chibowski [3] as,
\[ \theta_\mathrm{c}=\arccos\left(\frac{r_\mathrm{A}\cos{\theta_\mathrm{A}}+r_\mathrm{R}\cos{\theta_\mathrm{R}}}{r_\mathrm{A}+r_\mathrm{R}}\right) \] where \[ r_\mathrm{A}=\left(\frac{\sin^3{\theta_\mathrm{A}}}{2-3\cos{\theta_\mathrm{A}}+\cos^3{\theta_\mathrm{A}}}\right)^{1/3} ~;~~ r_\mathrm{R}=\left(\frac{\sin^3{\theta_\mathrm{R}}}{2-3\cos{\theta_\mathrm{R}}+\cos^3{\theta_\mathrm{R}}}\right)^{1/3} \]
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,[4] 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.[5] 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 (e.g. fluorinated) materials may have water contact angles as high as ~120°.[4] 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.
 Image from a video contact angle device. Water drop on glass, with reflection below. |
File:A water droplet DWR-coated surface2 edit1.jpg Fluorine-containing durable water repellent makes a nearly spherical water bead on a fabric. |
File:DropConnectionAngel.jpg A water drop on a lotus leaf surface showing contact angles of approximately 147°. |
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.
- 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.
- Single-fiber Wilhelmy method
- Dynamic Wilhelmy method applied to single fibers to measure advancing and receding contact angles.
- Powder contact angle 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 (2004)
- Jacob Israelachvili, Intermolecular and Surface Forces, Academic Press (1985–2004)
- D.W. Van Krevelen, Properties of Polymers, 2nd revised edition, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York (1976)
