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Optothermal generation, trapping, and manipulation of microbubbles

J. A. Sarabia-Alonso, J. G. Ortega-Mendoza, J. C. Ramírez-San-Juan, P. Zaca-Morán, J. Ramírez-Ramírez, A. Padilla-Vivanco, F. M. Muñoz-Pérez, and R. Ramos-García

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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.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Supplementary Material (2)

NameDescription
Visualization 1       Trapping and 3D manipulation of a microbubble by an optical fiber emitting in -z and +z direction
Visualization 2       The main-bubble moves in +z direction whereas the bubble stream does it in -z direction

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Figures (7)
Fig. 1.
Fig. 1. (a) Experimental setup for the generation and 3D manipulation of microbubbles. A pulsed laser is coupled to the multimode optical fiber using a microscope objective (MO). Bubbles dynamics are viewed with a fast Phantom camera. (b) Image of the distal end of the multimode optical fiber obtained with a SEM after 3.5 dB of attenuation was achieved. (c) Closer view of the optical fiber core showed on (b).

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Fig. 2.
Fig. 2. Growth of the main-bubble as a function of time. (a) Snapshots of the temporal evolution of the main-bubble radius recorded at 6,600 fps. (b) Blue dots correspond to the measured from the video main-bubble radius. The continuous blue line is fit to a double exponential function. The lower horizontal axis represents the number of coalesced bubbles and the upper one the corresponding elapsed time. The red solid line indicates the calculated radius of the main-bubble as a function of microbubbles generated at 2.1 µJ per pulse-energy at a repetition rate of 10 kHz.

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Fig. 3.
Fig. 3. (a) Profile of the bubble’s velocity as a function of the laser energy, extracted from recorded images at 43,000 fps. Continuous lines are fit to an exponential function. (b) Snapshot of the tracers and bubbles when a 2.6 µJ of laser energy was used. White circles and white squares indicate the tracer and bubble displacement, respectively. Both the bubble and the tracer start from the same position at t = 50 µs, however, the bubble moves faster as time goes on.

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Fig. 4.
Fig. 4. (a) Snapshot of the optothermal generation of microbubbles: (i) maximum bubble size, (ii) bubble collapse, (iii) bubble ejection, (iv-vi) bubble moves away from the optical fiber. (b) 4.2 µJ of laser-pulse energy. (i) Maximum cavitation bubble. (ii-iv) Temporal evolution of the remaining bubble. (v) Bright spots represent scatter laser-light due to AgNPs picked up by the video-camera. (vi) Bubble ejection due to the counterjet. The frame rate in all cases was 43,000 fps.

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Fig. 5.
Fig. 5. Temperature profile at the AgNPs-ethanol interface obtained by solving the heat diffusion equation coupled to the Navier-Stokes equations using COMSOL Multiphysics. The phase explosion is more likely to occur around Tc ∼243 °C (continuous red line). The temperature increase at the interface is a linear function of the laser energy. Color solid lines represent the temporal profile of the temperature at the AgNPs-ethanol interface due to light absorption. The blue broken line represents the temporal profile of one laser pulse. The pink double-dot line represents the pure ethanol boiling temperature Tb ∼ 78 °C.

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Fig. 6.
Fig. 6. (a) Free-body diagram of the forces involved in the main-bubble manipulation. (b) Total force over a main-bubble of R = 131 µm illuminated with pulses of 3.7 µJ of energy as a function of the propagation axis.

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Fig. 7.
Fig. 7. Spatial displacement of the main-bubble Visualization 2. (a) The main-bubble moves in + z direction whereas the bubbles-stream does it in -z direction. (b) Total force over a main-bubble around the quasi-steady-state trapping distance for bubbles of different radii. Total force over a bubble of 129.5 µm of radius (red triangles), 131 µm of radius (blue dots) and 132.5 µm of radius (green squares) obtaining quasi-steady-state trapping at -430.2 µm, -439.7 µm and -451.4µm, respectively.

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Equations (4)

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τ r ≈ τ c ( 1 + 0.205 γ ) ,
τ c ≈ 0.915 ρ l P ∞ − P v R m a x ,
F M → = − 2 π R 2 ∇ T d σ d T ,
F T → = F b → ± F M → ∓ F d → ∓ F i → ,
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