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In physics, the Young–Laplace equation (/ l ə ˈ p l ɑː s /) is an algebraic equation that describes the capillary pressure difference sustained across the interface between two static fluids, such as water and air, due to the phenomenon of surface tension or wall tension, although use of the latter is only applicable if assuming that the wall is very thin.
In contrast, the equilibrium contact angle described by the Young-Laplace equation is measured from a static state. Static measurements yield values in-between the advancing and receding contact angle depending on deposition parameters (e.g. velocity, angle, and drop size) and drop history (e.g. evaporation from time of deposition).
The Young–Laplace equation is the force up description of capillary pressure, and the most commonly used variation of the capillary pressure equation: [2] [1] = where: is the interfacial tension is the effective radius of the interface is the wetting angle of the liquid on the surface of the capillary
The contact angle is defined as the angle formed by the intersection of the liquid-solid interface and the liquid–vapour interface. [2] The size of the angle quantifies the wettability of liquid, i.e., the interaction between the liquid and solid surface. A contact angle of = can be considered, perfect wetting.
The Laplace pressure is the pressure difference between the inside and the outside of a curved surface that forms the boundary between two fluid regions. [1] The pressure difference is caused by the surface tension of the interface between liquid and gas, or between two immiscible liquids.
The opposite process—spreading of a liquid on a substrate—is called wetting. The factor determining the spontaneous spreading and dewetting for a drop of liquid placed on a solid substrate with ambient gas, is the so-called spreading coefficient S: Surface tension diagram of a liquid droplet on a solid substrate.
Diagram illustrating the image method for Laplace's equation for a sphere of radius R. The green point is a charge q lying inside the sphere at a distance p from the origin, the red point is the image of that point, having charge −qR/p, lying outside the sphere at a distance of R 2 /p from the origin. The potential produced by the two charges ...
For this reason, contact angle is coupled inherently to the curvature term of the Kelvin equation. As the contact angle increases, the radius of curvature will increase as well. This is to say that a system with perfect wetting will exhibit a larger amount of liquid in its pores than a system with non-perfect wetting ( > 0 o).