## Contact Angle Drop Profile Choices

The following choices are applicable both when the user provides the profile points by manually clicking and when these points are found automatically.

### Role of Yellow and Blue Curves

The yellow curve shows the points that are used by the regression algorithm that fits an equation to these points. The equation may then be used over a larger domain to find where the curve would go to, say, intersect the baseline.

The blue curve shows the baseline, or the
liquid-solid interface. The *ends* of the
baseline mark the three-phase points where the
drop profile curve is calculated to intersect
the baseline.

### Checkbox Spherical Fit

The profile fit will be a sphere (in three dimensions) which is the same as a circle in two dimensions, because of the assumption of axial symmetry. This is the most robust fit and can give good results with only a small fraction of the drop visible or clearly defined. It suffers, however, when the drop is distorted by gravity.

### Checkbox Non-spherical Fit

The profile fit will be separate polynomials
on the left and right sides. This will work
when only one side of the drop is visible or
well defined. The user can specifically select
only the left or only the right side. This is a
very general fit but is not robust. As you include
more or less of the overall drop, the answer will
invariably (it *must*) change. The algorithm
attemps to compensate for these variations. A
polynomial fit should be treated as never far wrong,
but also never quite as good as either a spherical or
Laplace-Young fit, if either of these can be used. Polynomials
are used as approximations, and the degree of the
polynomial affects the final answer. There is
no optimal degree, contrary to what you might guess.
More is not necessarily better.

### Checkbox Left Side Only and Right Side Only

These are available only for non-spherical fits. They allow you to ignore one of the sides if you can see it is corrupted or not present.

### Checkbox Laplace-Young Fit

This option uses the Laplace-Young equation and performs a regression to it. This approach is either very good or very bad. It assumes the drop is symmetric about a vertical central axis. This is a very strong assumption and requirement. It also assumes the drop is reasonably tall. In particular, it has real difficulty at low contact angles, say less than 30 or 40 degrees. Short, low angle drops may not converge to an answer.

This fit generates only one angle because of the assumption that the drop is perfectly symmetric about its central axis. It also presumes the drop is sitting on a level surface.

### Checkbox Auto Choice Fit

The algorithm will choose between spherical and non-spherical fits based on how tall the drop is, and therefore the likelihood that it is distorted by gravity.

### Checkbox Message on Fit Failure

Helpful hints will be given when a fit can not be obtained. Experienced users will find these a nuisance, because they will immediately recognize the problem anyway.

### Other References

See also

How to Make Manual and Automatic Contact Angle Measurements

Contact Angle Baseline Choices

Manual Contact Angles (includes example image and measurement)

FTA200 Measurement Capabilities (applies to all instruments)