  In spite of the widespread use of optical tweezers as a quantitative tool to
measure small forces, there exists no unambiguous and simple experimental
method for either validating its theoretically predicted form or empirically
parameterizing it over the entire range. This problem is addressed by studying
the transition of a colloidal particle between two spatially separated optical
traps. The transition as a function of the relative intensity of the traps and
the separation between them reveals a formal resemblance to the `butterfly
catastrophe' which also maps onto to phase transitions observed, for example in
ferroelectrics, on a phenomenological level. The method has been used to
experimentally determine the force-displacement curve for an optical trap over
its entire range.
