Uncertainty in contact angle measurements from the tangent method

The uncertainty in contact angles from sessile drops measured by the tangent method was estimated using a standard error propagation technique involving partial derivatives. If contact angles are<60 degrees, then uncertainty of the tangent method appears to be quite small,<= +/- 2 degrees. However, as theta values approach 90 degrees, uncertainty increases asymptotically and can exceed +/- 5 degrees.