Liquid: Surface Tension

Surface tension is the tendency of liquid surfaces to shrink to the smallest possible surface area when they are at rest. Surface tension is what allows items with a higher density than water, such as razor blades and insects (such as water striders), to float on a water surface without even becoming partially buried. Surface tension is increased by the higher attraction of liquid molecules to each other (due to cohesion) than air molecules at liquid–air contacts (due to adhesion). the two main mechanisms at work here. One increases  the liquid to compress due to an inward push on the surface molecules. The second is a tangential force parallel to the liquid’s surface. The name  surface tension is given to this tangential force. As a result, the liquid acts as if it were surrounded by a stretched elastic membrane. Although, this parallel must be used with caution because the tension in an elastic membrane is determined by the degree of deformation, whereas surface tension is a feature of the liquid–air or liquid–vapour interface.

Surface tension:

 An energy required to increase the liquid’s surface area by one unit is known as surface tension. A liquid’s surface tension is caused by an imbalance of intermolecular attractive interactions, or the forces that hold molecules together:

• In the bulk liquid, a molecule encounters cohesive forces from other molecules in all directions.
• A molecule at the liquid’s surface is solely subjected to net inward cohesive forces.

Adhesive Forces

An Adhesive force is the force of attraction between a solid surface and a liquid. The behaviour of a liquid in contact with a solid surface is determined by the strength differential between cohesive and adhesive forces.

• Because the cohesive forces within the droplets are stronger than the adhesive forces between the drops and the wax, water does not moisten waxed surfaces.
• Because the adhesive forces between the liquid and the glass are stronger than the cohesive forces within the water, water wets glass and spreads out across it.

Capillary Action

The rise of a liquid that wets the inside of a small diameter tube (i.e., a capillary) immersed in the liquid is known as capillary action.

• The liquid creeps up the inside of the tube (due to adhesive forces between the liquid and the tube’s inner walls) until the weight of the liquid balances the liquid’s adhesive and cohesive forces.
• The higher the liquid rises, the smaller the tube’s diameter.

Unit of surface tension: The Newton per Meter (N/m) is the SI unit for Surface Tension.

Surface tension in liquid: 

 Small items will “float” on the surface of a fluid due to surface tension as long as they do not break through and split the top layer of liquid molecules. If an object is placed on the fluid’s surface, the surface under stress acts as an elastic membrane. 

Methods of measurement:  The following are some examples of surface tension measurement methods:

• Spinning drop method
• Pendant drop method
• Du Noüy–Padday method
• Du Noüy ring method
• Wilhelmy plate method
• Pendant drop method
• Stalagmometric method
• Capillary rise method
• Bubble pressure method
• The vibrational frequency of levitated drops
• Sessile drop method

Factor affecting surface tension: 

Surface tension is a liquid phenomenon generated by an imbalance of molecular tensions near the liquid’s surface. Surface tension is affected by factors that cause changes in molecular forces. The degree of surface tension is determined by the nature of the liquid, its surroundings, and its purity.

1. Soluble Impurities: A surface tension may increase or decrease if the contaminants are soluble. When contaminants are less soluble, the surface tension of the liquid decreases.
2. Temperature: When we raise the temperature, the molecular attraction between liquid molecules weakens. As a result, as the temperature of the liquid rises, the surface tension of the liquid drops.

Application:

• Due to the surface tension of water, a needle placed in water can float.

• Because of the surface tension of water, mosquito eggs can float.
• Our toothpaste contains soap, which helps it spread readily in the mouth by reducing surface tension.
• Antiseptics, such as Dettol, have a low surface tension, allowing them to spread more quickly.

Conclusion:

Surface tension is the characteristics of a liquid surface that manifests itself as a stretched elastic membrane.The roughly spherical shape of little liquid drops and soap bubbles are examples of this phenomenon.Certain insects can stand on the surface of water due to this characteristic.The razor blade does not float; if pushed through the water’s surface, it sinks.The forces of attraction between the particles within a specific liquid, as well as the gas, solid, or liquid in contact with it, determine surface tension.The molecules in a drop of water, for example, have a modest attraction to one another.The water molecules deep inside the drop are assumed to be pulled in all directions equally by the surrounding molecules.Surface molecules, on the other hand, would be attracted back by neighbouring molecules if they could be moved slightly outward from the surface.The work or energy necessary to remove the surface layer of molecules in a unit area is roughly equivalent to the energy responsible for the phenomena of surface tension.As a result, surface tension can be measured in units of energy (joules) per unit area (square metres).At 20 °C (68 °F), water has a surface tension of 0.07275 joule per square metre.Water has a surface tension of 0.07275 joule per square metre at 20 °C (68 °F).As the temperature rises, the net force of attraction between molecules decreases, lowering surface tension.