  Acoustic levitation is frequently used for non-contact manipulation of
objects and to study the impact of microgravity on physical and biological
processes. While the force field produced by sound pressure lifts particles
against gravity (primary acoustic force), multiple levitating objects in the
same acoustic cavity interact via forces that arise from scattered sound
(secondary acoustic forces). Current experimental techniques for obtaining
these force fields are not well-suited for mapping the primary force field at
high spatial resolution and cannot directly measure the secondary scattering
force. Here we introduce a method that can measure both acoustic forces in
situ, including secondary forces in the near-field limit between arbitrarily
shaped, closely spaced objects. Operating similarly to an atomic force
microscope, the method inserts into the acoustic cavity a suitably shaped probe
tip at the end of a long, flexible cantilever and optically detects its
deflection. This makes it possible to measure forces with a resolution better
than 50 nN, and also to apply stress or strain in a controlled manner to
manipulate levitated objects. We demonstrate this by extracting the acoustic
potential present in a levitation cavity, directly measuring the acoustic
scattering force between two objects, and applying tension to a levitated
granular raft of acoustically-bound particles in order to obtain the
force-displacement curve for its deformation.
