Surface tension is a physical effect, which makes the surface of liquid always try to get the smallest smooth area, just as if it is an elastic film. The reason is that the surface of liquid always tries to reach the lowest energy state.
affect
Some insects, such as water strider, can crawl on the water by surface tension. Very flat objects, such as aluminum or nickel coins, shaving blades or aluminum films, can also float on the water by surface tension.
In the case of high surface tension, water is not easy to wet the object and will bounce back from the surface of the object. One of the functions of washing powder is to reduce the surface tension of water.
Other examples of surface tension in life:
Water drops form a ball.
Black bean bugs and water strider can walk on water. ?
Will the needle float on the water?
Are the water drops on the lotus leaf spherical?
definition
To enlarge the surface area of a certain volume of liquid, it is necessary to do work on the liquid. Surface tension is defined as the work done to expand the surface of a liquid divided by the area to be expanded. Therefore, surface tension can also be regarded as the density of surface energy.
Thermodynamic definition
Thermodynamics broadly defines surface tension as:
Surface tension σ is the partial derivative of Gibbs free energy g with respect to area a at constant temperature t and pressure p:
The unit of Gibbs free energy is energy unit, so the unit of surface tension is energy/area.
cause
Graphic surface tension Surface tension is composed of the attractive force between molecules and atoms that constitute the state of matter. The interface between the surfaces or states of a substance can be approximately regarded as a tangent plane, while the surface tension can be regarded as the energy with unsatisfied chemical valence per unit area. For macroscopic systems, the surface tension has nothing to do with the surface shape.
This explanation is a simplification and may also cause misunderstanding.
If the model is correct
Surface tension is caused by the attractive force inside the substance. Taking liquid as an example, the attraction between molecules in liquid is generally greater than that between molecules in gas or between molecules in gas and liquid. ?
The reason of surface tension is actually the asymmetry caused by the interface. ?
Beware of misunderstanding
Surface tension is the force on the surface, not the force exerted on the surface. The surface tension is not necessarily perpendicular to the surface.
Generally speaking, atoms or molecules in a material state attract and repel each other in a stable state. The two forces are in balance. In this state, the average distance between atoms or molecules is roughly the same. For simplicity, repulsive force is not mentioned in the model, but if repulsive force is lacking, atoms or molecules will be accelerated and approached by gravity. Because the repulsive force of atoms or molecules on the surface is relatively small, the distance between atoms or molecules on the interface is greater than that between atoms or molecules inside. The density of atoms or molecules here is relatively small, and the energy of atoms or molecules is relatively high relative to the energy inside the state of matter, which is the reason for the surface tension.
Surface tension is an internal force, which exists even in equilibrium. For example, the gaseous state and the liquid state of a substance exist in balance at the same time, and the boundary between the two states remains unchanged, that is, there is no force perpendicular to the interface at the interface.
result
The surface tension urges the liquid to reduce its surface area to reduce the unsatisfactory valence. Because the sphere is the smallest object with the same volume, the liquid is always spherical in equilibrium without external force (such as weightlessness).
In a droplet (such as a water drop) or a bubble in a liquid, the pressure at the interface is higher than the pressure inside the liquid due to surface tension. Similarly, the pressure inside the soap bubble is higher than that outside. The formula to describe this pressure difference is the Yang-Laplace formula.
measure
Mercury can measure surface tension by ring, plate, tensiometer or capillary phenomenon.
People can also determine the surface tension of liquid by optical analysis and measurement of suspended droplets.
Here are some measurement methods:
Capillary rising method: simple, it can be measured by inserting the capillary into the liquid, although the accuracy may not be high. ?
Hanging ring method: this is a classic method to measure surface tension, and it can even be used under the condition that it is difficult to get wet. Pull out a layer of liquid film (similar to soap bubbles) from the liquid with the ring initially immersed in the liquid, and measure the force required to raise the height of the ring. ?
Wilhelmj plate method: this is a general measurement method, especially suitable for measuring surface tension for a long time. The measured quantity is the force exerted on the flat plate perpendicular to the liquid surface during wetting. ?
Rotating drop method: used to measure interfacial tension, especially in the low or very low tension range. The measured value is the diameter of the rotating droplet in the relatively dense state of the substance. ?
Hanging drop method: suitable for measuring interfacial tension and surface tension. It can also be measured at very high pressure and temperature. Measure the geometry of the droplet. ?
Maximum bubble method: it is very suitable for measuring the change of surface tension with time. Measure the maximum pressure of bubbles. ?
Droplet volume method: very suitable for dynamic measurement of interfacial tension. The measured value is the number of droplets divided into a certain volume of liquid. ?
numerical value
The surface tension dimension of water at 20℃ is 73 Mn/m. The following data were also measured at 20℃:
Liquid? Surface tension?
mN/m？
Acetone? 23.3?
Benzene? 28.9?
Ethanol? 22.55?
N-hexane? 18.4?
Methanol? 22.60?
N-pentane? 16.0?
Polyethylene? 36. 1?
Polyether ketone? 46.0?
Teflon? 22.5?
Mercury? 476?
Water? 72.75?
Relatively speaking, the surface tension of water is quite high, and only the surface tension of mercury is much higher. The empirical approximate equation for the change of surface tension of water with temperature t is:
Relationship with temperature and composition
Surfactants reduce surface tension. This effect can be described as a parallel pressure π which is opposite to the surface tension. However, π is not real pressure, and its unit is the same as surface tension.
The vapor pressure of liquid in the air near the liquid surface has reached saturation. If there are other vapor permeation, the surface tension will change greatly.
Generally speaking, the surface tension decreases with the increase of temperature. At the critical point, its value drops to 0. This relationship is described by Choteforth formula.
history
The concept of surface tension first appeared in 1629. Thomas Young made great contributions to the theory of surface tension in 1805, Pierre-Simon Laplace in 1806, Simeon Dennis Poisson in 1830 and Joseph Prato from 1842 to 1868.