What is Ring Detachment Method ( Du Nouy Tensiometer ) it principle

Ring Detachment Method - ( Du Nouy Tensiometer )

Ring detachment method is also suitable for measuring both surface and interfacial tension. 

The apparatus used | for the determination is called DU Nouy tensiometer or Torsion balance or Du Nouy balance and is shown in Fig.

Principle 

The apparatus works on the principle that the force necessary to detach a Platinum / Tridium ring from the surface or interface of two immiscible liquids is proportional to the surface or | interfacial tension , respectively.

Aplatinum / Iridium wire ring of about four centimetres in circumference is suspended from the loop attached to a scale through a torsion wire . The liquid , whose surface tension is to be determined, 

is taken in a pan and the position of the pan is adjusted so that the ring just touches the surface of the liquid . The torsion on the wire is increased gradually so that the ring just detaches from the surface of the liquid (Fig) . The force required to detach the ring is read in dynes from the graduated scale . This detachment force is equal to the surface tension multiplied by the perimeter of the liquid detached . Hence ,

P = w = 2π(r1+r2) γ
Or

γ = P/2π(r1+r2)

where Pis the pull exerted through the torsion wire on thering and is read on the scale , wis the force | in terms of weight and r1 and r2 are the inner and outer radii of the disc .

A correction factor is required before accurate results can be obtained since the above equation does not take into account variables such as the radius of the ring . the radius of the wire used to form thering and the shape of the liquid supported by the ring during detachment . Introducing a correction factor ( β ) , the equation becomes :

γ = P/2π(r1+r2) × β

Or

γ = dial reading in dynes/2 × ring circumference × Correction factor

If the radius of the wire is small and that r1 = r2

γ = P/4πr  × β