A theoretical introduction to contact angle measurement

1. Sample preparation

Samples with good surfaces should be prepared in advance and their surfaces should be properly cleaned before measuring the contact angle. It should be placed completely horizontal. Thorough cleaning of syringes and stainless steel needles is required. However, it increases the wettability of the steel by the liquid; therefore, the liquid (polarity) released from the needle may climb upwards on the outer surface of the needle during the contact angle measurement. To counteract this effect, the liquid repellency of the needle tip needs to be enhanced by rubbing the needle tip in Parafilm.

2. Ensure humidity/steam level conditions

The sample should be placed in a measuring cell partially filled with probe liquid and covered to saturate the phase. This should usually happen a few minutes before taking the contact angle measurement. A syringe filled with probe fluid should be installed at this stage with the needle pointed above the sample surface.

3. Measurement of static contact angle

A small droplet should be deployed on the sample surface with the needle on top of the deposited droplet. The diameter of the deposited droplet (or its base) should increase to at least 3-4 times (>5-7 mm) the diameter of the needle used - the supply of liquid should be continued until a clear spread of the droplet base is observed. The needle should be removed from the droplet and wait 1-3 minutes. The droplet is stabilized and then the static contact angle is measured.

4. Measurement of advancing contact angle

The needle should be brought back over the deposited drop (or close to it) and more liquid should be added until the bottom of the drop spreads out again. Remove the needle and immediately measure the advancing contact angle. Repeat this step 3-5 times and take the average value.

5. Measurement of receding contact angle

After the advancing contact angle measurement is complete, the needle is dipped into the droplet to draw out the liquid and reduce the droplet size. The droplet diameter should be observed during liquid extraction. The needle can be detached from the droplet and the receding contact angle can be measured after the diameter of the droplet base shrinks during liquid withdrawal. Repeat this step 3-5 times and take the average value.

6. Reproducibility

Contact angle measurements should usually be repeated at three to five different locations on the sample or on different samples of the same type and subjected to the same cleaning procedure.

Problems and Precautions in Contact Angle Measurement

• liquid seeps into the surface

• Liquid Dissolving Surface

• liquid contamination

• Droplet Size and Gravity Effects

• evaporation

• Usually limited to ambient temperature

• Usually limited to flat

• Surface roughness

• surface heterogeneity

• Surface group mobility and kinetics

• Advancing and receding contact angle

• Limited to relatively low energy solids (polymers)

• Personal eyeball "bias"