# Fields at rest; Surface tension in water

Stationary liquids usually take the shape of containers. If the volume of the container is greater than the volume of the liquid placed in it, there will be a free surface at the top of the liquid. The difference between a gas and a liquid is that the gas has neither size nor shape. The surface tension in water is how much it passes through an area in a given time.

### CONTENT

WHAT IS SURFACE TENSION

EFFECTS OF SURFACE TENSION

REDUCING SURFACE TENSION

CAPILLARITY

DIFFUSION OF GASES

OSMOSIS

VISCOSITY

#### WHAT IS SURFACE TENSION

Surface tension can be defined as the force per unit length normal or perpendicular to the line on the surface of the liquid. Surface tension exists because of the molecular attraction between liquid molecules. It is the tendency of the liquid surface to shrink to a minimum surface area when stationary. Surface tension is the reason that objects with a density higher than water (such as razor blades and insects (such as water columns) are allowed to float on the water without being partially submerged.

### EFFECTS OF SURFACE TENSION IN WATER

These effects are seen with ordinary water

Insect walking on water surface

1. Beaded rainwater on waxy surfaces(such as leaves). The adhesion of water to the wax is very weak, and the adhesion to itself is very strong, so the water gathers into water droplets. Surface tension gives them a shape close to spherical, because the sphere has the smallest possible surface area to volume ratio.
2. When a mass of liquid is stretched, the formation of droplets occurs. If a stream of water flows out of the faucet, the water will decompose into water droplets in the autumn. Gravity stretches the flow, and then surface tension pinches it into a sphere.
3. Flotation of objects denser than water occurs when the object is not wettable and its weight is small enough to be borne by the force generated by the surface tension. The surface of the water behaves like an elastic film: the insect’s feet cause indentation on the water surface, increasing its surface area, and the trend of minimizing the surface curvature of the water (so area) pushes the insect’s feet upward.
4. The separation of oil and water (in this case, water and liquid wax)is caused by the surface tension between different liquids. This type of surface tension is called “interfacial tension”, but its chemical properties are the same.
5. The tears of wine form water droplets and streams on the side of the glass containing alcoholic beverages. The reason for it is the complex interaction between the different surface tension of water and ethanol; it is caused by a modified combination of the surface tension of ethanol and water whose evaporation rate is faster than that of water.

#### REDUCING SURFACE TENSION

1. Increasing the temperature reduces the surface tension in water
2. Adding soap or detergent to the liquid
5. Some liquids such as oil and kerosene can destroy surface tension in water.

#### CAPILLARITY

The tendency of liquids in capillaries or absorbent materials to rise or fall due to surface tension. Capillary action, the rise or depression of the liquid in a small channel such as a tube with a small cross-sectional area, such as a space between the towel fibers or an opening in a porous material.

Capillary action is not limited to the vertical direction. Regardless of the orientation of the towel, the water will be sucked into the fibers of the towel. The liquid that rises in the small hole tube where the liquid is inserted is called a wetting tube, while the liquid that is recessed in the thin tube below the surface of the surrounding liquid does not wetting the tube. Water is a liquid that moistens the glass capillary; mercury is a liquid that is not wetted. When wetting does not occur, capillary action does not occur.

Capillary action is the result of surface or interfacial forces. The rise of water in the thin tube inserted into the water is caused by the attraction between the molecules of water and the glass wall, as well as between the molecules of water itself. These forces of attraction are just the water column that balances gravity and has risen to a characteristic height. The narrower the hole of the capillary, the higher the water rise. On the contrary, the greater the degree of mercury is pressed down, the narrower the hole.

Cohesion, in physics, the attraction between molecules acts between two adjacent parts of a substance, especially a solid or liquid. It is this power that binds a piece of material together. Intermolecular forces also act between two different substances in contact, a phenomenon called adhesion.

These forces are mainly derived from coulomb (electric) forces, such as Van der Waals forces. When two molecules are close together, they are repelled; when they are farther apart, they are attracted; when they are at an intermediate distance, their potential energy is at a minimum, and it takes work to approximate or separate them. Therefore, work is required to pull apart two objects in close contact, whether they are the same or different materials.

The attraction effect of cohesion and adhesion is within a very short range, and varies according to the different substances concerned. If a piece of glass is submerged in water and then pulled out, it will be wet—that is, the water will cling to it, indicating that the adhesion between the water and the glass molecules is greater than the cohesion between the water molecules.

#### GAS DIFFUSION

Diffusion, a process generated by the random movement of molecules, through which the substance flows from the high concentration area to the low concentration area. A familiar example is the perfume of flowers, which quickly penetrates into the still air of the room. Heat conduction in a fluid involves transporting or diffusing heat energy from a higher temperature to a lower temperature. The operation of a nuclear reactor involves the diffusion of neutrons through a medium that leads to frequent scattering, but only rare neutron absorption.

#### OSMOSIS

This is the spontaneous passage or diffusion of water or other solvents through a semi—permeable membrane (a membrane that prevents the passage of dissolved substances-i.e. solutes). This process is important in biology and was first thoroughly studied by the German plant physiologist Wilhelm Pfeffer in 1877.

The general term osmose (now osmosis) was introduced by the British chemist Thomas Graham in 1854. It can also be used to describe a physical process in which any solvent passes through a selective permeable membrane (permeable solvent, but not permeable solute) to separate two solutions of different concentrations.

Osmotic pressure is defined as the applied external pressure, so that the solvent does not have a net movement through the membrane. Osmotic pressure is a synergistic property, which means that osmotic pressure depends on the molar concentration of the solute, but does not depend on its identity. Osmosis is a vital process in biological systems because biofilms are semi-permeable. In general, these membranes are impermeable to large polar molecules (such as ions, proteins, and polysaccharides), while non-polar or hydrophobic molecules (such as lipids) and small molecules (such as oxygen, carbon dioxide, nitrogen, and nitric oxide) are impermeable to large polar molecules (such as ions, proteins, and polysaccharides).

#### VISCOSITY

The viscosity of a fluid is a measure of its ability to resist deformation at a given rate. For liquids, it corresponds to the informal concept of “thickness”: for example, the viscosity of syrup is higher than water. Viscosity can be conceptualized to quantify the internal friction generated between adjacent fluid layers in relative motion. For example, when the viscous fluid is forced through the tube, its flow rate near the tube shaft is faster than the flow rate near the tube wall. In this case, the experiment shows that some stress (such as the pressure difference between the two ends of the tube) is required to maintain the flow through the tube. This is because a force is needed to overcome the friction between the relatively moving fluid layers. Therefore, for a tube having a constant flow rate,the strength of the compensating force is proportional to the viscosity of the fluid.

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