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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.
Surface roughness has a strong effect on the contact angle and wettability of a surface. The effect of roughness depends on if the droplet will wet the surface grooves or if air pockets will be left between the droplet and the surface. [10] If the surface is wetted homogeneously, the droplet is in Wenzel state. [11]
The capillary length will vary for different liquids and different conditions. Here is a picture of a water droplet on a lotus leaf. If the temperature is 20 o then = 2.71mm . The capillary length or capillary constant is a length scaling factor that relates gravity and surface tension.
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.
When a liquid droplet interacts with a solid surface, its behaviour is governed by surface tension and energy. The liquid droplet could spread indefinitely or it could sit on the surface like a spherical cap at which point there exists a contact angle. Defining as the free energy change per unit area caused by a liquid spreading,
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: