  Photothermal effects have been pointed out as prominent sources of forces in
optomechanical systems, competing with the standard radiation pressure
interactions. In this Article, we derive a novel and accurate model for the
prediction of photothermal forces and establish how some previous proposals can
be complemented to yield precise results. As a proof-of-concept, we perform
numerical and experimental tests on GaAs microdisks cavities and obtain
striking agreement with our framework, revealing the importance of considering
surface photothermal forces and the effects of multiple thermal modes in
microphotonic devices.
