Contact angle
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The contact angle is the angle at which a liquid/vapor interface meets the solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat horizontal solid surface. The shape of the droplet is determined by the Young-Laplace equation. The contact angle plays the role of a boundary condition. Contact angle is measured using a contact angle goniometer. The contact angle is not limited to a liquid/vapour interface; it is equally applicable to the interface of two liquids or two vapours.
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[edit] Measuring methods
- The sessile drop method
- Sessile drop method is an optical contact angle method. This method is used to estimate wetting properties of a localized region on a solid surface. Angle between the baseline of the drop and the tangent at the drop boundary is measured. Ideal for curved samples or where one side of the sample has different properties than the other.
- 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 Wetting force on the solid is measured as the solid is immersed in or withdrawn from a liquid of known surface tension.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.
[edit] Typical contact angles
On extremely hydrophilic surfaces, a water droplet will completely spread (an effective contact angle of 0°). This occurs for surfaces that have a large affinity for water (including materials that absorb water). On many hydrophilic surfaces, water droplets will exhibit contact angles of 10° to 30°. On highly hydrophobic surfaces, which are incompatible with water, one observes a large contact angle (70° to 90°). Some surfaces have water contact angles as high as 150° or even nearly 180°. On these surfaces, water droplets simply rest on the surface, without actually wetting to any significant extent. These surfaces are termed superhydrophobic and can be obtained on fluorinated surfaces (Teflon-like coatings) that have been appropriately micropatterned. These new surfaces are based on lotus plants' surface (which has little protuberances) and would be superhydrophobic even to honey. The contact angle thus directly provides information on the interaction energy between the surface and the liquid.
[edit] Thermodynamics
The theoretical description of contact arises from the consideration of a thermodynamic equilibrium between the three phases: the liquid phase of the droplet (L), the solid phase of the substrate (S), and the gas/vapor phase of the ambient (V) (which will be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor). The V phase could also be another (immiscible) liquid phase. At equilibrium, the chemical potential in the three phases should be equal. It is convenient to frame the discussion in terms of the interfacial energies. We denote the solid-vapor interfacial energy as γSV, the solid-liquid interfacial energy as γSL and the liquid-vapor energy (i.e. the surface tension) as simply γ, we can write an equation that must be satisfied in equilibrium (known as the Young-Dupré equation):
where θ is the experimental contact angle. Thus the contact angle can be used to determine an interfacial energy (if other interfacial energies are known). This equation can be rewritten as
where ΔWSLV is the adhesion energy per unit area of the solid and liquid surfaces when in the medium V.
[edit] External links
- Theory of contact angle measurements
- Literature about surface chemistry and contact angle
- Glossary - Surface chemistry & contact angle
[edit] References
- P.G. de Gennes "Wetting: statics and dynamics" Reviews of Modern Physics, 57, 3 (part I), July 1985, p.827-863. DOI: 10.1103/RevModPhys.57.827
- Jacob Israelachvili, Intermolecular and Surface Forces, Academic Press (1985-2004)
