  Acoustic radiation force is a net force experienced by an object under the
action of an acoustic wave. Most theoretical models require the acoustic wave
to be periodic, if not purely monofrequency, and are therefore irrelevant for
the study of acoustic radiation force due to acoustic pulses. Here, we
introduce the concept of finite-duration pulses, which is the most general
condition to derive the acoustic radiation force. In the case of small spheres,
we extend the Gor'kov to formula to unsteady acoustic fields such as traveling
pulses and interfering wave packets. In the latter case, our study suggests
that the concept of acoustic contrast is also relevant to express the acoustic
radiation force. For negative acoustic contrast particles, the acoustic
trapping region narrows with shorter pulses, whereas positive contrast
particles (such as biological cells) can fall in secondary traps when the pulse
width deviates from an optimal value. This theoretical insight may help to
improve the selectivity of pulsed acoustic tweezers.
