Optical Contact Angle/Surface tension measurement

(Measure contact angle with CAM instruments)

Definition:

Image based instrument based on axisymmetrical drop shape analysis (ADSA) of liquid droplets or air bubbles in liquid for determining contact angles and surface / interfacial tensions of liquids.

Summary/Description:

Analysis of the shape of a sessile drop of test liquid or a air bubble in liquid placed on a solid is the basis for optical contact angle (CA) / surface tension measurement or goniometry. The basic elements of a measuring device include a light source, sample stage, liquid/air bubble dispensing device, lens and image capture. Contact angle can be assessed directly by measuring the angle formed between the solid and the tangent to the drop surface.

The shape of a drop of liquid hanging from a capillary or a raising bubble in a liquid is determined from the balance of forces, which include the surface tension of that liquid. The surface or interfacial tension at the liquid interface can be related to the drop shape through the following equation:

= θρgR₀2 /β ,

where γ = surface tension

Δρ = difference in density between fluids at interfaces

g = gravitational constant

R₀ = radius of drop curvature at apex

β = shape factor

The shape factor, β, can be defined through the Young-Laplace equation expressed as 3 dimensionless first order equations.

Modern computational methods using iterative approximations allow solution of the Young-Laplace equation for to be performed. Thus for any pendant drop where the densities of the two fluids in contact are known, the surface tension may be measured based upon the Young-Laplace equation.

The optical method has advantages in that it is able to use very small volumes of liquid for contact angle and surface / interfacial tension measurements, measure very low interfacial tensions and can measure contact angles and surface tension of molten materials easily.