Tiêu đề 1.4 Surface tension.pdf ✅

Unit 1 1.4 Surface Tension
SURFACE TENSION • is defined as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids such that the contact surface behaves like a membrane under tension. • The magnitude of this force per unit length of the free surface will have the same value as the surface energy per unit area. • denoted by Greek letter σ (called sigma). • SI units - N/m.

Surface Tension on Liquid Droplet • Consider a small spherical droplet of a liquid of radius ‘r’ . On the entire surface of the droplet, the tensile force due to surface tension will be acting σ = Surface tension of the liquid P = Pressure intensity inside the droplet (in excess of the outside pressure intensity)) d = Dia. of droplet. Let the droplet is cut into two halves. The forces acting on one half (say left half) will be (i) tensile force due to surface tension acting around the circumference of the cut portion and this is equal to = σ x Circumference = σ x πd (ii) Pressure force on the area π 4 d 2 = p x π 4 d 2 These two forces will be equal and opposite under equilibrium conditions σ x πd = p x π 4 d 2 p = 4σ d