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Contact Angle Baseline Choices

The baseline location is influenced by choices on the contact angle tab whenever an automatic baseline is found.

Role of Previous Choices

Any time you check or uncheck a box, the choice is saved not only in the current Movie, but also in the Config File. When a new Movie is acquired, these most-recent settings are pre-loaded into it from the Config File.

Role of Checkbox Use Previous Baseline

This is the critical choice. It decides whether the baseline in each image is found from scratch or whether the baseline in a previous image in this Movie is used as guidance. The guidance is in the form of the equation for the baseline, not the exact endpoints.

If you provide manually clicked baseline points, these will be used irrespective of whether the checkbox is checked or not.

If you check the box, but there is no previous baseline to provide an equation, the baseline is found automatically as if the box were not checked.

Over-rides to Use Previous Baseline

If the Interfacial Tension checkbox Manually Fit Liquid-Tip Interface Line is checked, it will over-ride the first occurence of a contact angle baseline that would benefit from Use Previous Baseline. This is because both define the liquid-solid interface. This will occur only in the rare circumstance you analyze a pendant drop first (using this choice) and then expect to find a previous baseline waiting for the subsequent contact angle measurement.

If the checkbox Pre-touchoff Reflection Sets Baseline is checked, it completely over-rides any previous baselines. This is a logical requirement when a sequence of pendant drop images approaching the surface defines the liquid-solid interface. The complication arises when you check this box, but do not actually provide the sequence of pendant drop images. In this case, the algorithm does what it can, but the results are ill-defined. Basically you should not check Pre-touchoff Reflection Sets Baseline unless there will really be a sequence of pendant drop images at the beginning of the movie.

Checkbox Use Horizon

A horizon image occurs on small sample images where the front of the sample appears as a uniform dark region and its top surface is concurrent with the liquid-solid interfacial line in the image. The algorithm will search down the left and right edges of the image for the light-to-dark transition and draw a straight line between the two edge points. Checking this box will automatically uncheck any checked Reflection Image Present.

Checkbox Reflection Image Present

A reflection image occurs when the sample extends in front of the drop and a shadow of the drop appears below it in the image. The symmetry of this image is useful in defining the baseline. Checking this box will automatically uncheck any checked Use Horizon.

It is OK for both Use Horizon and Reflection Image Present to be unchecked. In this case, the algorithm finds the drop without either aid.

Checkbox Pre-touchoff Reflection Sets Baseline

This option allows the use of early images in the Movie to define the baseline before the drop actually touches the surface. This is a very powerful technique, but you should be sure to not check this box unless the early images have the reflection of the pendant drop present. It is only available when Reflection Image Present is checked.

When you click Contact Angle with this checkbox selected, the algorithm will automatically go back to the first images in the Movie in an attempt to find the reflection and the touch-off point.

Checkbox Dispense Tip in Drop

This option is used for expanding and contracting drop analysis for advancing and receding angles. It tells the algorithm the drop is centered about the needle and that the needle distorts the drop (to some degree). The algorithm then avoids the region about the needle.

You should never attempt to analyze a drop that has a needle in it without using this checkbox. It will think the drop is strangely distorted. Conversely, if there is no needle in the drop, but the needle is present, it may regard any liquid on the bottom of the needle as the drop.

Checkbox Tilting Baseline

This option allows the baseline to tilt. Otherwise, the baseline is assumed horizontal and is the best average of the left and right side contours.

Checkbox Keep Baseline Width Fixed

This option is only available when in Spherical Fit mode. It uses the baseline end points as specific points on the drop profile curve also, which pulls the curve in to the baseline ends. This is normally used only when Use Previous Baseline is checked.

Checkbox Use Previous Baseline

The equation (y = mx + b) for the previous baseline in the Movie is used for this baseline. See additional comments at the beginning.

Checkbox Store and Reuse this Baseline

Use this if you want to carry a baseline from one Movie to the next. Make sure the proper baseline is present on the image before checking the box. The current baseline is saved to disk at the time you check the box. It will be used as long as the box stays checked.

Checkbox Correct for Curved Baseline

If there is an equation present for a curved solid surface baseline, it will be used.

Checkbox Save this Curve as Baseline

This is how a curved baseline is initially saved. Analyze the total solid curvature in Spherical Fit mode as if it were a liquid drop. Use a manually fitted faux baseline in so doing. The baseline will do nothing other than allow this faux analysis to take place. The curvature will be used in place of a linear baseline as long as the previous checkbox is checked.

Tilting Plate Experiment

This option allows the algorithm to track the drop as it distorts and perhaps moves during a tilting plate experiment.

Other References

See also

How to Make Manual and Automatic Contact Angle Measurements

Contact Angle Drop Profile Choices

Manual Contact Angles (includes example image and measurement)

FTA200 Measurement Capabilities (applies to all instruments)

Curved Baselines