06.Properties Of Fluids And Solids

PRELIMS

1. Cohesion & Adhesion

• Cohesion: Force of attraction between molecules of the same substance. (e.g., mercury beads up on glass due to strong cohesion).
• Adhesion: Force of attraction between molecules of different substances. (e.g., water wets glass due to strong adhesion with glass molecules).

2. Surface Tension

• Definition: Property by which the free surface of a liquid at rest behaves like a stretched membrane and tends to acquire the minimum surface area.
• Cause: Net inward cohesive force on surface molecules (molecules below the surface are pulled equally in all directions, but surface molecules experience a net downward/inward pull).
• Unit: (force per unit length).
• Temperature Effect: Surface tension decreases with increase in temperature (becomes zero at the critical temperature).
• Examples:
  • Small drops of liquid and soap bubbles are spherical (minimum surface area for a given volume).
  • Insects/water striders can walk on water.
  • A needle can float on water despite being denser.
  • Rain drops are spherical (except when distorted by air resistance into a slightly flattened shape while falling).
• Detergents/Soaps: Reduce the surface tension of water, helping it penetrate fabric and remove dirt.

3. Capillarity

• Definition: Phenomenon of rise or fall of a liquid in a narrow tube (capillary tube) due to the combined effect of cohesion and adhesion.
• Rise: Occurs when adhesion > cohesion (e.g., water in a glass capillary; kerosene rising in a wick; ink rising in a blotting paper; water rising from roots to leaves in plants; oil rising in a lamp wick).
• Fall (Depression): Occurs when cohesion > adhesion (e.g., mercury in a glass capillary tube).
• Relation: Height of rise/fall is inversely proportional to the radius of the capillary tube (narrower tube → greater rise).

4. Viscosity

• Definition: Property of a fluid by virtue of which it opposes the relative motion between its adjacent layers (internal friction of a fluid in motion).
• Cause:
  1. Liquids: Due to cohesive forces between molecules.
  2. Gases: Due to diffusion of molecules.
  • Temperature Effects: Viscosity decreases with temperature in liquids; increases in gases.
  • Ideal Fluid: Viscosity is zero.
• Units: Decapoise ( ) or Pascal-second.
• Terminal Velocity: Constant velocity reached by a body falling through a viscous medium (Net force = 0).
  • Proportional to the square of the radius of a spherical body.

5. Streamline (Laminar) vs. Turbulent Flow

• Streamline/Laminar Flow: Fluid particles follow smooth, well-defined paths (streamlines) that do not cross each other; occurs at low velocity, below a critical velocity.
• Turbulent Flow: Irregular, chaotic flow with eddies and vortices; occurs when flow velocity exceeds the critical velocity.
• Reynolds Number: Dimensionless number used to predict whether flow will be streamline or turbulent (low value → streamline; high value → turbulent).

Bernoulli's Theorem

• Statement: In a streamline flow of an ideal fluid, the total energy (Pressure + Potential + Kinetic) per unit volume remains constant at all points.
• Application: Venturimeter (used to measure the rate of flow of a fluid).
• Real-world Note: Volcanic lava flows swiftly despite high viscosity because it stays below its critical velocity (where flow depends on density rather than viscosity).

Elasticity and Hooke's Law

• Elasticity: Property of a body to regain its original shape/size after removal of deforming force.
• Stress: Restoring force per unit area.
• Strain: Relative change in dimension (Length or Volume).
• Hooke's Law: Under the elastic limit, Stress is proportional to Strain ( ).
  • Young's Modulus (Y): Longitudinal Stress / Longitudinal Strain.
  • Bulk Modulus (K): Volume Stress / Volume Strain.
  • Rigidity Modulus ( ): Shear Stress / Shear Strain.
• Elastic Limit: Maximum stress up to which a body regains its original shape after removal of the deforming force; beyond this limit, the body becomes permanently deformed (plastic deformation).
• Perfectly Elastic Body: Regains its original shape completely (e.g., quartz, phosphor bronze — idealised examples).
• Perfectly Plastic Body: Does not regain its shape at all (e.g., putty, wet clay).

Commonly Confused Pairs

• Streamline vs. Turbulent Flow: Streamline flow is smooth and orderly at low velocity; turbulent flow is chaotic and occurs above the critical velocity.
• Cohesion vs. Adhesion: Cohesion is attraction between like molecules (holds a liquid together, causes surface tension); adhesion is attraction between unlike molecules (causes wetting/capillary rise).
• Elasticity vs. Plasticity: Elastic bodies regain shape after the deforming force is removed (within elastic limit); plastic bodies retain the deformed shape.

UPSC Relevance

The mechanical properties of matter are common in GS Paper I (Science).

• Capillarity: Examples of oil wicks and plant roots are classic "Everyday Science" questions.
• Surface Tension: Larvae sinking vs. hot soup being tasty—these conceptual applications are recurring themes.
• Viscosity & Terminal Velocity: The relationship and the temperature dependency (especially the gas/liquid inverse behavior) are key for MCQ precision.
• Elastic Limit: Distinguishing elastic vs. plastic behavior of materials, relevant to why steel is preferred over other metals for bridges/machine parts (high Young's modulus).