Young's Equation

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Definition - What does Young's Equation mean?

Young’s equation is a formula developed by the English physicist Thomas Young, which is used to define the relationship between the contact angle, the surface tension, the interfacial tension between a liquid and a solid surface, and the surface free energy of the solid. In simpler terms, the equation helps describe the shape and behavior of a liquid where the liquid-vapor interface meets a solid surface.

Young’s equation is also known as the Young–Dupré equation.

Corrosionpedia explains Young's Equation

Young’s equation is defined as:

σsg = σsl + σlg ⋅ cosθ

Where:

σsg = the surface free energy of the solid

σsl = the interfacial tension between the liquid and the solid

σlg = the surface tension of the liquid

cosθ = the contact angle between the liquid-vapor interface and the solid surface

Young’s equation takes into consideration the thermodynamic equilibrium between the three phases of matter: solid, liquid and gas. As the three elements come into contact with each other, such as when a droplet of water lands on a solid surface, the water droplet assumes a shape where all three phases are balanced.

This equation, therefore, plays a crucial role in describing the wettability of a surface. If variables, such as σsg = σsl + σlg are known, then the contact angle can be calculated. The higher the contact angle formed between the liquid and the surface, the lower the surface’s wettability.

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