Dynamics of a liquid droplet excited by infrared multi-photon absorption

A micrometer-sized liquid droplet is excited by multi-photon absorption of a mid-infrared laser pulse. The dynamical processes following the excitation are probed by a time-resolved imaging experiment. When the wavelength of the laser is resonant with the vibrational frequency of the constituent molecules, multiphoton absorption initiates superheating of the surface region of the droplet, followed by a shock wave propagating across the droplet and by disintegration into many small fragments (shattering). On the other hand, when the wavelength is slightly detuned from the resonance, the droplet exhibits completely different dynamics because the interior molecules are excited more intensely than those in the vicinity of the illuminated surface. The dynamics depend on the pulse energy of excitation as well. In particular, a critical energy was found for shock-wave generation. A simple molecular-dynamics simulation performed for a van der Waals cluster provides further insight into the disintegration dynamics and the critical energy. These studies clarify how the molecules cooperate with each other in the energy dissipation processes leading to disintegration of a droplet.