Contact Angle Measurements of Surface Wetting

Surface wetting occurs when a droplet spreads out over a surface, such that its contact angle is below 90°. When the droplet spreads out completely, this angle will be 0°, and 'complete wetting' will have occurred. Surface wetting is best measured using a Contact Angle Goniometer, because wetting and contact angle are directly related.

In this application note, the Ossila Contact Angle Goniometer is used to measure the reducing contact angle of deionised (DI) water as it spreads over a thin film of PEDOT:PSS. As the droplet flattens, we demonstrate that the Contact Angle Goniometer can measure angles less than 10°.

The measurements are achieved using a combination of edge detection and a polynomial fitting method, which are performed automatically in the Contact Angle software.

The Measurement Process

Setup

Level the tilt stage
Focus the camera
Prepare the substrate

Recording

Open the Contact Angle software
Choose recording settings
Begin recording
Dispense a droplet

Analysis

Click the 'Analysis' tab within the Contact Angle software
Load a recorded video
Choose a baseline
Take a contact angle measurement

Equipment and Materials Used in This Application Note

Experimental Method

Setup

Un-patterned glass/ITO substrates (S111) were cleaned in an ultrasonic bath for 5 minutes using Hellmanex, water, and isopropyl alcohol (IPA) consecutively. The camera focal point was confirmed by moving a pipette tip back and forth above the tilt stage (as shown in the video below). The substrate was then placed in this location in order to achieve the most accurate results.

Note: If the focal point of the camera is not above the stage, screw or unscrew the lens until the desired focal point is achieved.

Locating the camera focal point using a pipette tip

Recording

A thin film of PEDOT:PSS was spin cast onto a clean, un-patterned ITO substrate (S111). The spin speed was 6000 rpm and the pipette volume was 30 µl. The substrate was then placed on a hotplate at 90 °C to bake off any excess water.

The substrates were then placed on the tilt stage of the Contact Angle Goniometer. A deionised water droplet was dispensed using a micropipette, with a droplet volume of 10 µl. The figure below shows the substrate architecture used, with a PEDOT:PSS layer on top of a glass/ITO substrate.

Water droplet on PEDOT:PSS — Substrate architecture showing a water droplet on a glass/ITO Substrate with a spin-cast PEDOT:PSS layer.

The Ossila Contact Angle Goniometer was used to film the droplet as it landed on the substrate and spread over the PEDOT:PSS surface. Recording was performed at 30 frames per second (1 frame every 33 ms), and 19.7 seconds of surface wetting was captured in total.

Dispensing a water droplet onto a PEDOT:PSS thin film

Analysis

The Contact Angle software was then used to analyse the recorded footage. A region of interest (ROI) was defined by the user, with the bottom the ROI defining the measurement baseline. Edge detection of the droplet was combined with a polynomial fitting method to calculate the contact angle in each frame of the video. For more details of this method, please see our contact angle theory and measurement guide.

The video below shows the software measuring the contact angle of each frame as it plays through the video recording

Contact angle measurement using the Ossila Contact Angle software.

Results

The figure below shows the contact angle measurements of the droplet as it spreads over the PEDOT:PSS film. The green line indicates the angle on the left of the recorded image, and the blue line indicates the angle on the right.

Note: If the substrate is flat, the left and right angles should be similar. If they are different, check that the stage is level and that your chosen baseline is level with the droplet.

Plot showing contact angle of a water droplet on PEDOT:PSS — Contact angle vs time of a deionised water droplet on a PEDOT:PSS substrate.

As can be seen in the plot, the contact angle was above 50° when the droplet first landed, then began to rapidly decrease as the droplet spread. In the later stages of wetting, this angle decreased to 9±1°.

Conclusion

The Ossila Contact Angle Goniometer was used to record and analyse a droplet of DI water on a thin film of PEDOT:PSS. Measurements were taken in time intervals of 33 ms over a period of 19.7 seconds (592 measurements in total). The system was able to measure the contact angle as it reduced to below 10°.