Optothermal generation, trapping, and manipulation of microbubbles

Abstract

The most common approach to optically generate and manipulate bubbles in liquids involves temperature gradients induced by CW lasers. In this work, we present a method to accomplish both the generation of microbubbles and their 3D manipulation in ethanol through optothermal forces. These forces are triggered by light absorption from a nanosecond pulsed laser (λ = 532 nm) at silver nanoparticles photodeposited at the distal end of a multimode optical fiber. Light absorbed from each laser pulse quickly heats up the silver-ethanol interface beyond the ethanol critical-point (∼ 243 °C) before the heat diffuses through the liquid. Therefore, the liquid achieves a metastable state and owing to spontaneous nucleation converted to a vapor bubble attached to the optical fiber. The bubble grows with semi-spherical shape producing a counterjet in the final stage of the collapse. This jet reaches the hot nanoparticles vaporizing almost immediately and ejecting a microbubble. This microbubble-generation mechanism takes place with every laser pulse (10 kHz repetition rate) leading to the generation of a microbubbles stream. The microbubbles' velocities decrease as they move away from the optical fiber and eventually coalesce forming a larger bubble. The larger bubble is attracted to the optical fiber by the Marangoni force once it reaches a critical size while being continuously fed with each bubble of the microbubbles stream. The balance of the optothermal forces owing to the laser-pulse drives the 3D manipulation of the main bubble. A complete characterization of the trapping conditions is provided in this paper.

Steady-State 3D Trapping and Manipulation of Microbubbles Using Thermocapillary

“…Experimental results show the trapping and manipulation of microbubbles, previously generated, using optically-induced temperature gradients caused by light absorption in ethanol [20]. Microbubbles are generated in ethanol by thermocavitation, i.e., the explosive phase transition from liquid to vapor around its critical-point (243°C) [25] after light from a pulsed laser is absorbed at AgNPs deposited at the end of an optical fiber.…”

“…By heat transfer, the surrounding liquid is heated up well beyond its boiling temperature and eventually, evaporates explosively creating a microbubble that is expelled from the fiber end [20]. The longer the pulsed laser is on, the larger the bubble's diameter [20]. In particular, the radius reached by the microbubble was approximately R ∼ 42 µm for all the cases shown in Figure 2.…”

“…The optical power loss caused by AgNPs absorption was approximately 2 dB. When light impinges on the AgNPs, they are heated up well beyond the ethanol's boiling temperature leading to the creation of thermocavitation bubbles [20]. A second laser, continuous wave (CW) laser (λ 1,550 nm, Thorlabs model SFL1550S, and current controller model CDL1015) with single-mode optical fiber output (OF T , 9/125 µm) was used for trapping and manipulation of the microbubbles generated at fiber OF G .…”

“…The AgNPs strongly absorb light from the laser at 532 nm increasing its temperature. By heat transfer, the surrounding liquid is heated up well beyond its boiling temperature and eventually, evaporates explosively creating a microbubble that is expelled from the fiber end [20]. The longer the pulsed laser is on, the larger the bubble's diameter [20].…”

“…However, they require rather large optical powers >100 mW; besides, they did neither show steady-state trapping nor stably manipulation. Recently, our research group has demonstrated both the generation and quasi-steadystate trapping and manipulation of single microbubbles in optical fibers using the Marangoni effect [19,20].…”