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Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to float on a water surface without becoming even partly submerged.
Drop of water bouncing on a water surface subject to vibrations Surface tension prevents water droplet from being cut by a hydrophobic knife. Liquid forms drops because it exhibits surface tension. [1] A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter.
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. The surface of the liquid has the shape of a spherical cap, due to Laplace pressure
The surface tension can be calculated provided the radius of the tube (r) and mass of the fluid droplet (m) are known. Alternatively, since the surface tension is proportional to the weight of the drop, the fluid of interest may be compared to a reference fluid of known surface tension (typically water):
Three examples of droplet detachment for different fluids: (left) water, (center) glycerol, (right) a solution of PEG in water. In fluid dynamics, the Plateau–Rayleigh instability, often just called the Rayleigh instability, explains why and how a falling stream of fluid breaks up into smaller packets with the same total volume but less surface area per droplet.
The angle of a drop of the liquid on the solid as seen in Figure 1 degrees or radians 1-cos(θ SL) The y-axis of the Zisman Plot representing wetting unitless γ L: The surface tension of the respective liquid dyne / cm γ C: The critical surface tension of the liquid needed to effectively wet the solid substrate dyne / cm
On contact of liquid with a surface, adhesion forces result in wetting of the surface. Either complete or incomplete wetting may occur depending on the structure of the surface and the fluid tension of the droplet. [12] The cause of self-cleaning properties is the hydrophobic water-repellent double structure of the surface. [13]
The Tolman length (also known as Tolman's delta) measures the extent by which the surface tension of a small liquid drop deviates from its planar value. It is conveniently defined in terms of an expansion in /, with = the equimolar radius (defined below) of the liquid drop, of the pressure difference across the droplet's surface: