  Photoacoustic effect refers to the acoustic generation induced by laser
irradiation, where nanosecond pulsed laser source is normally used to provide
instantaneous heating and thermoelastic expansion of the sample. More
generally, photoacoustic generation requires active intensity modulation of
laser source to produce sharp temperature gradient, which is key to generate
acoustic wave. In this paper, we propose a novel photoacoustic effect from
moving droplet, which generates photoacoustic wave by a non-modulated
continuous-wave laser. When the droplet moves through the laser spot area, it
can be heated up and cooled down instantaneously, generating photoacoustic
waves. We name it passive photoacoustic effect. Theoretical analysis and
simulation study validated the existence of passive photoacoustic effect. This
phenomenon may find potential application in high-throughput photoacoustic
cytometry.
