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A drop striking a liquid surface; in this case, both the drop and the surface are water. In fluid dynamics, drop impact occurs when a drop of liquid strikes a solid or liquid surface. The resulting outcome depends on the properties of the drop, the surface, and the surrounding fluid, which is most commonly a gas.
Since mechanical systems try to find a state of minimum potential energy, a free droplet of liquid naturally assumes a spherical shape, which has the minimum surface area for a given volume. The equivalence of measurement of energy per unit area to force per unit length can be proven by dimensional analysis. [8]
'drop') is one of the most common methods for measuring surface tension. The principle is to measure the weight of drops of a fluid of interest falling from a capillary glass tube, and thereby calculate the surface tension of the fluid. We can determine the weight of the falling drops by counting them. From it we can determine the surface tension.
where r i is the radius of the inner ring of the liquid film pulled, and r a is the radius of the outer ring of the liquid film. [2] w ring is the weight of the ring minus the buoyant force due to the part of the ring below the liquid surface. [3] When the ring's thickness is much smaller than its diameter, this equation can be simplified to
The limit of this formula, as α goes to 90°, gives the maximum weight of a pendant drop for a liquid with a given surface tension, . m g = π d γ {\displaystyle \,mg=\pi d\gamma } This relationship is the basis of a convenient method of measuring surface tension, commonly used in the petroleum industry.
A drop of a less dense liquid or a gas bubble is placed inside the fluid. Since the rotation of the horizontal tube creates a centrifugal force towards the tube walls, the liquid drop will start to deform into an elongated shape; this elongation stops when the interfacial tension and centrifugal forces are balanced.
When the characteristic height of the liquid is sufficiently less than the capillary length, then the effect of hydrostatic pressure due to gravity can be neglected. [9] Using the same premises of capillary rise, one can find the capillary length as a function of the volume increase, and wetting perimeter of the capillary walls. [10]
In the top picture, the volume of the drop is being increased, and in the bottom it is being decreased. Each notated angle is an instance of a similar contact angle. The simplest way of measuring the contact angle of a sessile drop is with a contact angle goniometer, which allows the user to measure the contact angle visually. A droplet is ...