Tilting Angle Measurements

Tilting angle measurements provide information about the adhesion conditions of a droplet on a surface. They are performed by using either a tilting cradle or a tilting stage to physically rotate a substrate and droplet. As the drop rotation increases, gravity will pull the droplet downward until it eventually rolls off the substrate, and the tilting angle is measured at the point where the drop starts moving. The leading edge of the drop is called the advancing contact angle, while the trailing edge is called the receding contact angle. The difference between the two angles is the hysteresis. Highly uniform samples will have a small contact angle hysteresis and low roll-off angle. Even surfaces that are hydrophilic can be manufactured to have low hysteresis and low roll-off angles¹.

Figure 1 – Schematic of a tilting contact angle. Image courtesy of Biolin Scientific.

The tilting cradle accessory is a device that rotates the entire instrument to perform the tilting angle measurement. The optical tensiometer is mounted onto the tilting platform and secured using the screw-in platform legs. After a sessile drop is placed onto the sample, the tilting cradle rotates the entire instrument and measures the tilting angle and left and right contact angles dynamically. A stop trigger can be placed in the camera frame to stop recording automatically once the drop starts moving to reduce the number of unusable frames.

The tilting cradle has the following features:

• It is integrated with the software so that tilting angle is automatically recorded in each frame
• The camera position is constant relative to the sample stage so the baseline does not move during the measurement

A vacuum stage and pump can be added to the tilting cradle to prevent the sample from sliding off the stage when the tilting angle is large.

Figure 2 – Sample pictures of a drop measured with the tilting cradle.

The tilting stage accessory provides a small platform coupled with a rotating dial that allows the user manually rotate the stage and sample. Using this setup the stage is rotating relative to the camera and the automatic baseline is best for tracking the change in contact angle vs. tilting angle. The angle at which the sample starts rolling can be read from a dial on the stage and recorded manually into the file. The sample in this case should be secured with an adhesive to ensure that it does not fall off the stage during the rotation.

Figure 3 – Example of a tilting angle drop measured with the tilting stage.

One potential point of confusion is that the terms advancing, receding, and hysteresis are also commonly used for a technique where the needle is placed into the droplet and the volume is changed dynamically while the contact angle is recorded. While the results obtained for the two measurements are related, they are not the same. One main difference between these two measurements is the fact that the results obtained for the tilting angle measurement depend on the droplet volume. As the drop volume used for the measurement increases, the advancing and receding contact angles tend to converge. Conversely, differences between the advancing and receding contact angles tend to be larger at smaller drop volumes².

Video 1 – A video demonstrating operation of the tilting cradle with the Attension Theta Flow.