Viscosity and Surface Tension

A fluid is a substance that has no fixed shape, Liquids and Gases can flow easily and are termed as fluids. In this topic, we will learn about two Mechanical Properties of Fluids which are Viscosity and Surface Tension. Both of them are dependent on molecular interactions. The viscosity of a fluid is a measure of how sticky the fluid is, on the other hand, Surface tension is the elastic property of a fluid which makes it acquire the least surface area possible. Now let’s discuss each of them.

What is Surface Tension?

Surface tension is defined as the tendency of a fluid to shrink into the minimum possible surface area. This property of liquid is based on the fact that the molecules of the liquid at the surface level are in a different situation than those in the interior of the liquid. The molecule lying inside the liquid surface is surrounded by other molecules and is attracted equally in all directions. So, the net force of attraction acting on the molecule is zero. Surface tension does not depend on the forces of attraction between the particles only but also depends on the forces of attraction of solid, liquid, or gas in close contact with it.

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The energy required for surface tension is equal to the work or energy required to remove the surface layer of the molecules in a unit area.  Surface tension is expressed in energy (joules) per unit area (square meters). It is often expressed as an amount of force exerted on the surface perpendicular to a line of unit length. Water has a surface tension of 0.07275 joules per square meter at 20 °C. While other organic liquids, such as benzene and alcohols have lower surface tensions, whereas mercury has higher surface tension. Increasing the temperature lowers the net force of attraction between the molecules and decreases surface tension. The most common example we can observe in our daily life is when we add soap to the water, its surface tension decreases, thereby the liquid with soap mixes easily with dirt on hand and cleans it. Another example, The drops of a liquid are spherical because, for a given volume, a sphere has a minimum surface area. (Surface tension tries to acquire the minimum surface area of the liquid). The driving forces behind the origin of surface tension are cohesive and adhesive forces.

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What is Viscosity?

Viscosity can be defined as the property of a fluid by virtue of which it opposes the relative motion between two surfaces of the fluid that are moving at different velocities or in simple terms, viscosity means friction between the molecules of fluid or opposition to flow. The liquids which flow fast, have lower internal resistance and vice versa. This is because of the strong intermolecular forces between the molecules. These liquids are less viscous and have low viscosity. The liquids which flow very slow have high internal resistance. This is because of the weak intermolecular forces between them. So, they are less viscous or have low viscosity.

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For Viscosity of fluids, the tangential, or shearing, stress that causes flow is directly proportional to the rate of shear strain, or rate of deformation, the shear stress divided by the rate of shear strain is constant for a given fluid at a given temperature. The constant is termed as the absolute, dynamic, or simply called viscosity, Fluids that behave in this way are called Newtonian fluids, The unit of viscosity, is newton-second per square meter, which is usually expressed as pascal-second in SI units.

The viscosity of liquids is decreased when the temperature is increased, and the viscosity of gases is increased when the temperature increases. Thus, upon heating, liquids will flow more easily, whereas gases flow more sluggishly. Eg: Oil has a high viscosity, but when we put it in the car and heat it up, it becomes less viscous and thus flows smoothly through an engine and other parts of the car. The higher the viscosity, the slower will be the liquid flow. 

FAQs (Frequently Asked Questions)

Question: Define Surface Tension?

Answer: Surface tension is defined as The ratio of the force (F) per unit length (L) along which the force acts. (Intermolecular forces  (Van der Waals force), draw the liquid particles together.)


Mathematically, T= F / L

F is the force 

L is the length in which force act

T is the surface tension

Question: Define Viscosity and Laminar Flow?

Answer: Viscosity of fluids is defined as the force of friction that one part of the liquid offers to another part of the liquid. The force of friction  between two layers each having cross-section area A sq cm, separated by a small distance dx cm, and having a velocity difference of dv cm/sec, is given by:

f ∝ A ( dv / dx )

f = η A ( dv/dx)

F is a force

A is area

dv/dx is called velocity gradient

η  is a constant (the coefficient of viscosity)

In Laminar flow or Streamline flow, the fluid travels smoothly or in regular paths. The pressure, velocity, and other flow properties remain constant at each point.

Question: Define Capillarity?

Answer: A tube with a uniform and fine bore throughout its length is termed a capillary tube. The phenomenon of rising or fall of liquid in a capillary tube is called capillarity. If one end of a capillary tube is put into a liquid, it is found that the liquid rises into the capillary tube to a certain height. This rise is because of the inward pull of surface tension acting on the surface that pushes the liquid inside the capillary tube.  On the other hand, in liquids that do not wet glass-like Mercury, the level inside the capillary falls below the level outside, whereas the upper surface of a liquid that wets glass is concave and that of mercury is convex.