Friday, 24 June 2016

Surface Tension

Surface Tension
The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area

Surface Tension [: cohesive forces among liquid molecules are responsible for the phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero]

The molecules at the surface do not have the same molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area, which produces spheres of rain water (beading) on a waxy surface, such as a leaf. Water adheres weakly to wax and strongly to itself, so water clusters into drops. Surface tension gives them their near-spherical shape, because a sphere has the smallest possible surface area to volume ratio. Surface tension is responsible for the shape of liquid droplets or spheres. 

Although easily deformed, droplets of water tend to be pulled into a spherical shape by the imbalance in cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be approximately spherical. The spherical shape minimizes the necessary "wall tension" of the surface layer according to the Young–Laplace equation, describing pressure difference over an interface in fluid mechanics

Surface energy [: is the elastic tendency of liquids which makes them acquire the least surface area possible] 

It has the dimension of force per unit length, or of energy per unit area. Surface energy and surface tension are equivalents, but when referring to energy per unit of area, people use the term surface energy, which is a more general term in the sense that it applies also to solids and not just liquids. In order to successfully form a continuous coating, the liquid should be able to wet the surface of the material. 

Wettability depends on one specific property of the surface: surface energy. The surface energy of the solid substrate directly affects how well a liquid wets the surface. To achieve good wettability, the surface energy of the substrate needs to exceed that of the surface tension of the liquid by around 2 - 10 mN/m (Dynes). The Dyne level reflects the surface wettability - the higher the Dyne level, the better the wettability/adhesion.

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