# Contact angle

## Optical contact angle measurement and drop contour analysis

The contact angle is the angle measured through the liquid where a liquid-vapour interface meets a solid surface. From this the wettability of a solid surface can be calculated using the Young equation.  Wettability describes the attempt of a solid to form a common interface with a liquid which comes into contact with it. A given system of solid, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle.  The equilibrium contact angle reflects the relative strength of the liquid, solid, and vapour molecular interaction.

When a liquid comes into contact with a solid there is a relationship between the contact angle θ, the surface tension of the liquid σlg, the interfacial tension σsl between liquid and solid and the surface free energy σsg of the solid as shown in Young's equation:

σsg = σsl + σlg ⋅ cosθ

The equation is valid for three-phase systems in thermodynamic equilibrium for ideal (smooth and chemically homogenous) solids and pure liquids. It is the basis of all models for determining the surface free energy of solids by means of contact angle measurements.

Schematic diagram of the components of a three-phase system which are relevant for wetting

Contact angle measurements are useful in the development of surface coatings, composite materials, paints and varnishes or cleaning agents. Measuring the contact angle is necessary where control of wetting and adhesion properties is desired.

In house:

Data Physics contact angle system OCA20

Specifications:

Size of sample stage                       100 x 100 mm

Measurement range                       0 … 180° ; ± 0.1° measuring accuracy of the video system

Surface and interfacial tension       1x10-2 … 2x103 mN/m; resolution: min. ± 0.05 mN/m

Measuring methods                        Sessile and Captive Drop method

Tilting Plate method

Pendant Drop method

Optical Wilhelmy Plate and Rod/Thread method

Optics                                             Six-fold zoom lens (0.7 - 4.5-fold magnification)

CCD-camera (resolution max. 768 x 576 pixels)

Field of view (FOV): 1.75 x 1.4 … 11.7 x 9 mm

Optical distortion: < 0.05 %

Video system                                  High-performance image processing system

Up to 50 images/s

Temperature range                          2x Pt100 input for -60° … 450° C (Pt100 optional)

0.1 K resolution

1/3 DIN IEC 751 (± 0.03 %), class B Size