Washburn Method

The Washburn method measures the contact angle and surface free energy of porous materials like powders and textiles by observing how a liquid is drawn up into the material via capillary action.

A glass tube filled with powder is dipped in a test liquid, and the liquid’s mass uptake over time is recorded. Treating the powder as many tiny capillaries, the relationship between mass squared (m2m^2m2) and time (ttt) is linear and described by the Washburn equation:

m2=cσcos⁡θηρ×tm^2 = \frac{c \sigma \cos \theta}{\eta \rho} \times tm2=ηρcσcosθ​×t

• ccc is a capillary constant related to pore size and packing,
• θ\thetaθ is the contact angle,
• σ,η,ρ\sigma, \eta, \rhoσ,η,ρ are the liquid’s surface tension, viscosity, and density.

To find ccc, measure with a fully wetting liquid (contact angle 0∘0^\circ0∘), then use this to calculate θ\thetaθ for other liquids.

Notes:

• Powder packing must be consistent since ccc depends on bulk density.
• Angles above 90° cannot be measured (no wetting). For these, drop shape analysis is used instead.