Ultrafast vapourization dynamics of laser-activated polymeric microcapsules

Abstract

Precision control of vapourization, both in space and time, has many potential applications; however, the physical mechanisms underlying controlled boiling are not well understood. The reason is the combined microscopic length scales and ultrashort timescales associated with the initiation and subsequent dynamical behaviour of the vapour bubbles formed. Here we study the nanoseconds vapour bubble dynamics of laser-heated single oil-filled microcapsules using coupled optical and acoustic detection. Pulsed laser excitation leads to vapour formation and collapse, and a simple physical model captures the observed radial dynamics and resulting acoustic pressures. Continuous wave laser excitation leads to a sequence of vapourization/condensation cycles, the result of absorbing microcapsule fragments moving in and out of the laser beam. A model incorporating thermal diffusion from the capsule shell into the oil core and surrounding water reveals the mechanisms behind the onset of vapourization. Excellent agreement is observed between the modelled dynamics and experiment.

Characterization of periodic cavitation in optical tweezers

“…These transient nanobubbles have shown great potential for medical treatments [15]. Optical tweezers or highly focused continuous (CW) lasers have been used to create micron sized cavitation bubbles in liquids that are transparent to the laser wavelength [16,17]. Dholakia and coworkers [16] trapped a polystyrene nanoparticle that was later irradiated with a nanosecond laser pulse creating optical breakdown at the trapped nanoparticle.…”

“…Dholakia and coworkers [16] trapped a polystyrene nanoparticle that was later irradiated with a nanosecond laser pulse creating optical breakdown at the trapped nanoparticle. Another approach used custom made microparticles filled with absorbing dye that was rapidly vaporized creating multiple cycles of bubble expansion and collapse in a timescale of a few microseconds [17].…”

Ultrasound contrast agents: bubbles drops and particles

“…Upon light irradiation of sufficient intensity, such particles can generate bubbles that radiate strong acoustic emissions enabling them to be detected 123 . The majority of the research on light-activated bubbles however has made use of plasmonic nanoparticles such as gold nanospheres (AuNS) or nanorods (AuNR).…”

“…Alternatively, bubbles may be produced through thermally induced vaporization of an injected, initially liquid agent such as perfluorocarbon droplets 129 , typically at lower optical intensities 128 . In most cases, the laser is directly coupled through an observation microscope using a dichroic filter 112,123,130 , allowing for an exact spatial localization of the beam and for a selective excitation of the agents in the sample.…”

“…These are well established as contrast agents for ultrasound imaging on account of their excellent acoustic scattering cross-section and nonlinear behavior 315,316 . Phase change microdroplets that turn into bubbles upon laser irradiation have shown a ten-fold increase in acoustic emissions as compared to nanoparticles 118,123 . However this technology is inherently limited by the amount of energy required to vaporize the surrounding liquid.…”