MEASUREMENT OF SURFACE AND INTERFACIAL TENSION Wilhelmy Plate Methods: The DuNouy Ring Method.

1. IIIrd Semesester B. pharmacy
Physical Pharmaceutics-I
Unit-III
Surface and Interfacial tension [Part-3(b)]
(Measurement of Surface and Interfacial tension: 3.
Wilhelmy plate method 4. DuNouy ring method )
Miss. Pooja D. Bhandare
(Assistant professor)
Kandhar college of pharmacy

2.  MEASUREMENT OF SURFACE AND
INTERFACIAL TENSION
1. Capillary Rise Method.
2. Drop Count and Weight Method.
3. Wilhelmy Plate Methods
4. The DuNouy Ring Method.

3. 3. Wilhelmy plate method
• The apparatus consist of a thin mica, glass or platinum plate attached
to a suitable balance.
• When used as a detachment method, the plate is immersed in the
liquid, and the liquid container is gradually lowered.
• The reading on the balance immediately prior to detachment is noted.

4. • Immersion of the plate into the lower of two liquids in a container
and subsequent detachment will give the interfacial tension.

5. • The detachment force is equal to the surface tension multiplied by the
perimeter of the surface detached.
WL – W = 2 (L+T) γ
On rearrangement,
γ = (WL – W)/ 2 (L + T)
• Where,
WL= reading on the balance prior to detachment
W = weight of plate
L = length of plate
T= thickness of plate

6. 4. The DuNouy Ring Determination Method
• Suitable for measuring surface and interfacial tensions.
• Apparatus: DuNouy tensiometer or torsion balance.
• Principle: Force necessary to detach a platinum- iridium ring
immersed at the surface or interface is proportional to the surface or
interfacial tension.
• The force required to detach the ring in this manner is provided by a
torsion wire and is recorded in dynes on calibrated dial.

7. • Detachment force is equal to surface tension multiplied by perimeter of
liquid detached.
• Hence,
P = W = 2п (𝑟1+ 𝑟2) γ
On rearrangement,
γ =
𝑷
2п ( 𝒓 𝟏+ 𝒓 𝟐)
Where, P = pull exerted through torsion wire.
W = Force in terms of weight
𝑟1 and 𝑟2 = inner and outer radius of disk

9. Thank you!