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Thermocavitation, the creation of a vapor bubble by heating a liquid with acontinuous-wave laser, has been studied for a wide range of applications.Examples include the development of an actuator for needle-free jet injectors,as the pumping mechanism in microfluidic channels and crystallization ornanoparticle synthesis. Optimal use in these applications require control overthe dynamics of the laser-generated bubble through the laser power and beamradius. In contrast to pulsed lasers, for continuous-wave lasers the influenceof the laser beam radius on the bubble characteristics is not fully understood.Here, we present a novel way to control the size of the beam from an opticalfiber by changing the distance from the glass-liquid interface. We show thatthe increase in beam size results in a longer nucleation time. Numericalsimulations of the experiment show that the maximum temperature at the momentof nucleation is 237$\pm$5{\deg}C and independent of laser parameters. Due todelayed nucleation for larger beam sizes, more energy is absorbed by the liquidat the nucleation instant. Consequently, a larger beam size results in a fastergrowing bubble, producing the same effect as reducing the laser power. Weconclude that the total bubble energy only depends on the amount of absorbedoptical energy and it is independent of the beam radius and laser power for anyamount of absorbed energy. This effect contrasts with pulsed lasers, where anincrease in beam radius results in a reduction of bubble energy. Our resultsare of relevance for the use of continuous-wave laser-actuated cavitation inneedle-free jet injectors as well as other applications of thermocavitation inmicrofluidic confinement.

Laser beam properties and microfluidic confinement control thermocavitation