Manual and Automatic Contact Angle Measurements
You have a choice of all-manual, all-automatic, and mixed manual-automatic measurements. The most common mixed mode measurement is to manually set the baseline (the liquid-solid interfacial line) and then have the software automatically find the drop profile.
All measurements have options that are controlled from the Contact Angle tab. These must be set before any manual points are set or the measurement made. See further description at the end of this article.
If a measurement is unsuccessful and you change modes, the safe thing is to use the Clear tool to remove any residual data from the previous attempt before you start the new attempt.
Simply click the Contact Angle tool button and the measurement will be made if the drop is clear enough. If the measurement is successful, new Results are displayed. If the measurement is not successful, consider a mixed manual-automatic mode.
The most useful mode is to manually set the baseline and then use the automatic drop profile:
Make sure you are in the correct mode to receive manual mouse clicks. In 2.0, click the Setup Pts tool button on the Movie form. In 1.96, click the Man Contact Angle tool button.
Right mouse clicks define end points of the baseline. This line will be drawn in light blue.
Click the Contact Angle (Auto button in 1.96) tool button to complete the measurement. If the measurement is successful, new Results are displayed. If the automatic portion can not find the profile, no new Results are displayed and no new graphics drawn. If the drop profile looks clear, check your setup options on the Contact Angle tab carefully. For example, you may have the Upside Down Drop box checked, or something else that precludes a successful measurement.
Check your Contact Angle setup options first, because if these must be changed you will have to re-enter all points.
Set the baseline with right mouse clicks first. This will draw a blue line. The software uses the last two points clicked.
Set the drop profile points next. These draw yellow lines to show the points clicked. Spread the points out as much as possible.
When you have the right number of points set, click the Contact Angle tool to calculate the actual angle. The graphics will be re-drawn to show where the net baseline and profile fall. New Results will be displayed. On some systems, once you have entered the necessary number of points, the angle is automatically calculated. If the measurement is not calculated after the last mouse click, you must click the Contact Angle tool yourself; this is the usual mode.
For a further discussion of manual contact angles, see the paper Manual Contact Angles.
Not all systems have all options available, so the following is a general discussion. These are set on the Contact Angle tab. When live measurements are available, they use the last choices you made on a Movie's Contact Angle tab.
Choose the type of contact angle measurement: sessile drop (the usual choice), capilliary rise, or protractor modes.
Choose spherical or non-spherical fitting to the drop profile. The number of user points varies with the choice. Three points are required for spherical fits. Non-spherical fits require separate points on the left and right sides. Non-spherical fits generally accept three to five points. If you do not enter enough points, you receive an error message.
Choose a straight or curved baseline. Straight baselines are the default and should be used unless you know the curvature of the specimen is causing real errors.
Choose the orientation of the drop, normal or upside down. This is important when air bubbles are imaged.
Decide if the reported angle needs to be complemented (subtracted from 180 degrees). This occurs in inverted measurements. The true contact angle is always measured through the liquid phase. For most measurements, you will not complement the result.