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Abstract
We experimentally demonstrated a novel photocontrol scheme of a microbubble. The microbubble was confined in a fiber-based hollow microstructure and its movement was driven by the laser-induced photothermal Marangoni force. The position of the microbubble was controlled at a micrometer scale by simply adjusting the drive laser power. This scheme permitted the firsthand control of a microbubble with a divergent single laser beam. As a practical demonstration, we proposed a variable fiber all-optical attenuator by exploiting the total internal reflection on the surface of the photo-controlled microbubble to modulate the target light beam. The experimental results showed that such a compact fiber attenuator possessed a low insertion loss of 0.83 dB, a maximum extinction ratio of 28.7 dB, and had potential to be integrated into the lab-on-a-chip for the modulation of the light beam power.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Microbubbles have provided many significant functions, e. g. actuators [1,2], microvalves [3,4] and manipulation of micro-objects [5–7], which is of great importance in both scientific research and practical applications. However, the achievement of such functions is based on the profound microbubble control investigation. In particular, owing to the advantages of high flexibility, non-contact, no-destructive, and immunity to the electromagnetic interference, the photocontrol schemes for the manipulation of the microbubble have attracted increasing attentions in recent years.
Because the microbubble has a lower refractive index than the surrounding liquid medium, it will be repelled from the high intensity region of the tightly focused laser beam, which is essentially different from the microscopic particle with higher refractive index controlled by the conventional “optical tweezer” [8]. Nonetheless, there have been several alternative optical schemes proposed for controlling the microbubble, mainly employing the specially designed light beams [9–11] and the photothermal conversion induced fluid convection [12,13]. Although many progresses have been made, there still existed some drawbacks, such as bulky setups companied with auxiliary complex computer programs for the production of the special light beams, some precision deposited films or suspended noble metal or semiconductor nanoparticles for the light absorption and the following heat production, which inevitably increased the operation complexity of the proposed schemes. In addition, due to the inherent instability of the microbubble’s gas-liquid interface, the precise control of the microbubble remained a challenge. Along with the microbubble-based applications, the pressing requirement is the developing scheme for the photocontrol of the microbubble.
In this paper, we experimentally demonstrated a novel scheme of controlling the microbubble with a divergent single laser beam. The microbubble was introduced into the fiber-based hollow microstructure and its movement was driven by the laser-induced photothermal Marangoni force. The position of the microbubble can be controlled at a micrometer scale in the fiber microstructure by simply adjusting the drive laser power. With the advantages of easy operation and robustness, such a scheme broke the limitation of using the conventional light beams to control the microbubble. Based on such photocontrol scheme of the microbubble, a variable fiber all-optical attenuator was proposed to modulate the target light beam. The experimental results showed that the extinction ratio of the fiber attenuator can reach up to 28.7 dB. Such a compact fiber attenuator has potential to be integrated into the lab-on-a-chips for the power adjustment of the working light beams.
2. Experimental part
The process of introducing a microbubble into a fiber-based hollow microstructure was shown in Fig. 1. Firstly, a single mode fiber (Corning, SMF-28e) was spliced with a silica capillary tube1 (SCT1) (inner diameter of 80 µm/outer diameter of 140 µm), which was then cleaved at a length of 180 µm, as shown in Fig. 1(a). The cleaved SCT1 was spliced with another segment of SCT2 (10 µm/125 µm) to construct a cylindrical fiber-based hollow microstructure, as shown in Fig. 1(b). In this step, the SCT2 was connected to an ethanol-filled syringe in advance. Afterwards, the ethanol was injected to the constructed fiber microstructure slowly. During the injection process, a microbubble formed when the residual air was wrapped in the liquid because of the intermolecular cohesion force of the liquid. When the microbubble formed, the fiber microstructure was then translated by a distance of about 6 mm from the second splicing interface with a moving stage, and the SCT2 was stretched to crack under the action of the arc discharge, as shown in Fig. 1(c). Finally, the end of SCT2 was sealed off permanently by the collapsed silica wall and then the microbubble was kept stable in the fiber microstructure, as shown in Fig. 1(d). The microscope image of the fabricated fiber microstructure containing a microbubble was shown in Fig. 1(e).
Fig. 1. Schematic diagram of the processes of introducing a microbubble into a fiber-based hollow microstructure.
To exhibit the photocontrol scheme of the microbubble, the evolution of the microbubble in the fiber microstructure were captured by a CCD camera mounted on a horizontally placed microscope, as shown in Fig. 2. The measurement of the displacement distance was achieved by the aid of a 3D translation stage with a step precision of 0.1 µm. The fabricated fiber microstructure was firstly connected with a 1550 nm laser given that the hydroxyl group of the ethanol had a higher absorption coefficient at such wavelength [14], and then placed vertically along the z direction. When the laser was turned off, it can be seen from the x direction that the floated microbubble with a diameter of 76.1 µm initially kept stationary in the fiber microstructure, as shown in Fig. 2(a). When the laser was turned on and its power exceeded a threshold of 0.2 mW, the laser energy was converted to the mechanical work of the microbubble. The produced drive force overcame the buoyancy and the viscous force formed between the microbubble and the inwall of the fiber microstructure, which made the microbubble depart from the initial position, as shown in Fig. 2(b). As the laser power was increased with a step of 50 µW, the microbubble moved vertically downward along the inner wall of the fiber microstructure and finally stopped at separate balanced positions corresponding to the given power, as shown in Figs. 2(c)–2(k). However, when the laser power was increased beyond 1.8 mW, the microbubble will no longer move, meaning that it has arrived at the ultimate position, as shown in Figs. 2(k)–2(l). The displacement distance between the top of the microbubble and the upper inwall of the microstructure was denoted as d, as shown in Fig. 2(c). The dependence of d on the drive laser power was shown in Fig. 3(a). We can see that the shift variation of the microbubble decreased and the displacement distance gradually approached a maximum value of 60.8 µm with the increased drive laser power. Meanwhile, during the three times testing processes, the microbubble’s balanced positions had almost no change corresponding to the laser power, which indicated that such a photo-controlled scheme of the microbubble exhibited good repeatability. We inferred that the stable existence of the microbubble in the confined liquid environment was responsible for the good repeatability. To further investigate the photo-controlled scheme of the microbubble, the stability experiment was also conducted, as shown in Fig. 3(b). During each ten minutes monitoring process, we can see that the variations of the displacement distance corresponding to the different laser power was very small. The variations of a balanced position around 11.2 µm corresponding to a drive laser power of 0.25 mW was measured. The deviation of less than 1.4 um was obtained in the initial measuring process and the recovering process, which meant that the photocontrol scheme of the microbubble had a good repeatability. We also calculated the standard error (SE) of the varied displacement distances to evaluate the stability of the photo-controlled microbubble. The calculated values at 0.25 mW was less than 0.107 µm, meaning that the fluctuation of the displacement distance was very small during the ten minutes measurement process, which indicated that the microbubble can be kept stable by the drive laser beam and can be manipulated repeatedly by adjusting the drive laser power.
Fig. 2. (a) The initial position of the microbubble. (b)-(k) The position variation of the microbubble in the fiber microstructure controlled by adjusting the drive laser power. (l) The ultimate position of the microbubble.
Fig. 3. (a) The dependence of the displacement distance of the microbubble on the drive laser power. (b) The stable experiment of the photo-controlled microbubble corresponding to the different drive laser power.
The proposed photocontrol scheme of the microbubble has the following advantages: (i) easy operation and flexible control; (ii) highly effective and precise manipulation; (iii) no need of additional fabrication process or special structure in the constructed confined liquid environment; and (iv) allow the remote control without the inhibition of the performance by the aid of the optical fiber medium. With these advantages, the microbubble can be controlled precisely and robustly in the fiber microstructure by directly adjusting the drive laser power, which was greatly simplified compared with the reported photocontrol schemes of the microbubble [8–13].
In order to further demonstrate the feasibility of the photocontrol scheme of the microbubble in the fiber-based hollow microstructure, we introduced a smaller microbubble with a diameter of about 41 µm into a fiber microstructure filled with ethanol to go on with the experiment. The experiment was conducted using the same procedure. The experimental results were shown in Fig. 4. We can see that the smaller microbubble in the fiber microstructure can also be stably controlled by adjusting the drive laser power.
Fig. 4. The photocontrol process of a smaller microbubble in a silica capillary tube with an inner diameter of 60 µm and an outer diameter of 125 µm by adjusting the drive laser power. (a) 0.15 mW; (b) 0.2 mW; (c) 0.3 mW.
3. Discussion
The observed effects can be explained qualitatively as follows. The photocontrol scheme of the microbubble in a fiber-based hollow microstructure was illustrated in Fig. 5. When the fiber microstructure was placed vertically, the microbubble was floated against the upper inwall of the microstructure because of the buoyancy, as shown in Fig. 5(a). Once the laser was turned on, the liquid in the fiber microstructure was heated by the light. The increased temperature generated a temperature gradient on the microbubble surface [7]. The liquids around the microbubble were transferred from the hotter bottom to the colder regions situated on both sides of the bubble because of the Marangoni effect, which produced a shear stress to the microbubble. Such a stress is called Marangoni force and can be expressed as:
where ${{\partial \sigma } \mathord{\left/ {\vphantom {{\partial \sigma } {\partial T}}} \right.} {\partial T}}$ is the temperature derivative of the liquid surface tension $\sigma$, which is usually a negative constant for most liquids, R is the radius of the microbubble and $\nabla T$ is the temperature gradient on the microbubble [15,16]. Inferred from the equation, the produced Marangoni force (FMD) directed downward and drove the movement of the microbubble [17,18]. Meanwhile, the top of the microbubble was heated as the laser beam traveled across the microbubble. Similarly, the Marangoni force directed upward (FMU) was generated, which decreased the net Marangoni force exerted on the microbubble.Fig. 5. Schematic diagram of the photocontrol scheme of the microbubble in the fiber microstructure. (a) The initial state of the microbubble when the drive laser beam was turned off. (b) The mechanical equilibrium state of the microbubble controlled by a drive laser beam.
To better explain this process, we simulated the temperature distribution in the fiber-based hollow microstructure in the moment that the laser was turned on by using finite element analysis method. In order to simplify the algorithm, the simulation was based on two-dimension model, geometrical optics, and heat transfer theory in fluids. The position of the microbubble was fixed in the fiber microstructure with an interval of 10 µm and the boundary of the microstructure was set to follow the non-conducting boundary condition. In the simulation, the parameters of the microbubble diameter (76.1µm) and the inside dimension of the fiber microstructure (width of 80 µm/ height of 180 µm) were included in the vertical two-dimensional model. The drive laser beam with a set power of 5 mW was coupled into the simulation domain from the bottom. The initial temperature was set as 293.15 K. The refractive indices of air in the microbubble and ethanol in the fiber microstructure were set to be 1.00 and 1.36 respectively. When the drive laser beam transmitted by the optical fiber was incident into the liquid, a temperature field was formed near the end face of the optical fiber due to the photothermal effect, which caused the thermal convection in the liquid simultaneously. Based on the multi-physical field model coupled with hydrodynamics and heat transfer, the temperature inside the liquid was obtained by solving the equation governing the heat flow in the fluids, which can be expressed as:
where T is the temperature inside the liquid, $\rho$ is the density of the liquid, ${C_p}$ is the specific heat capacity of the liquid, u is the fluid velocity, $k$ is the thermal conductivity of the liquid, $\alpha$ is the absorption coefficient of the liquid, I is the resulted light intensity inside the liquid.The simulated results of the temperature field in the fiber microstructure was shown in the Fig. 6. We can see that the initial temperature value at 50 µm position along central axis direction is highest in the fiber microstructure. The varied position of the microbubble changed the temperature field, because the microbubble can act as a micro-lens to modulate the optical field and the light intensity distribution during the movement process. As a result, the temperature gradient on the microbubble also changes with the position the microbubble. However, the temperature gradient is difficult to be got given the small dimensions of the microbubble volume and time scales involved. A more precise analysis would require the determination of the temperature distribution around the bubble at a micrometer scale. Instead, we can estimate the temperature gradient according to the temperature variation at the bottom of the microbubble at different positions, as shown in the Fig. 6. After the data processing, the calculated temperature gradient in the fiber microstructure is about 2300 k/m. Therefore, the magnitude of the produced Marangoni force is 10−9 N, according to the Eq. (1).
Fig. 6. The simulation of the variation of the temperature distribution in the fiber microstructure corresponding to the different positions of the microbubble.
Besides, the moving microbubble was also subjected to buoyancy FB, gravity FG, optical force FO and drag forces FD. Among these acting forces, the optical force and the gravity force were negligible given the low laser power and the low bubble density [12]. The buoyancy FB and the drag forces FD can be expressed as:
where ${\rho _E} = \textrm{789} kg/{m^3}$ is the density of the ethanol, $g$ is the gravitational acceleration, $\mu = \textrm{1}\textrm{.071} \times \textrm{1}{\textrm{0}^{ - 3}}{P_a} \cdot s$ is the dynamic viscosity of ethanol, v is the velocity of the microbubble, respectively. The calculated magnitude of the buoyancy force is 1.4 nN for a microbubble with a diameter of 76.1 µm. The distance between the initial position and the ultimate position is 60.8 µm and the microbubble needed about three seconds to cover this distance, giving an average speed of 20 µm/s. According to the Eq. (4), the calculated magnitude of the FD is 0.015 nN, which is about two orders of magnitude smaller than the buoyancy and can also negligible during the movement of the photo-controlled microbubble. As the magnitude of the Marangoni force is about 10−9 N, we can deduce that the equilibrium position of the microbubble can be achieved by the synergistic effects between the buoyancy and the Marangoni force.Although the temperature gradient on the microbubble was difficult to be got, we can show the temperature gradient variation based on the measurement of the temperature difference between the top and the bottom of the microbubble versus distance, according to the simulated results shown in the Fig. 6. The variation of the temperature difference between the top and the bottom of the microbubble versus the displacement distance was shown in Fig. 7. The movement of the microbubble changed the optical field, which resulted in a fact that the temperature on the top side of the microbubble started to rise while that of the other side started to decrease. Because the temperature difference between the top and the bottom of the microbubble was directly related with the temperature gradient, the temperature gradient between the top and the bottom sides of the microbubble deceased subsequently, which caused the deceasing of the equivalent Marangoni force. When the microbubble initially arrived at the balanced position, it will not stop because of the inertia. When it passed the balanced position, the downward equivalent Marangoni force became smaller than the upward buoyancy. As a result, the resultant force became resistant to make the microbubble decelerate and dragged it back to the balanced position.
Fig. 7. The variation of the temperature difference between the top and the bottom of the microbubble versus the displacement distance.
Therefore, during the irradiation at a given laser power, the microbubble experienced an acceleration followed by a deceleration and finally stopped at an exact position to get a state of mechanical equilibrium, where the downward net equivalent Marangoni force was balanced with the upward buoyancy, as shown in Fig. 5(b). Since the temperature gradient $\nabla T$ was produced by the drive laser power [16,19], the initial net Marangoni force can be changed by adjusting the drive laser power, and subsequently modulated by the position of the microbubble to make the microbubble reach its equilibrium state. Provided that the volume of the moving microbubble was invariant, the buoyancy on it was considered to be constant. The equivalent upward and downward Marangoni forces acting on the microbubble became dominant with the increasing drive laser power, and the buoyancy was relatively small enough to be neglected. When reached the ultimate balanced position, the microbubble was trapped in an equivalent Marangoni force field with spatial symmetry. Even if the drive laser power continued to be further increased, the equivalent Marangoni forces increased synchronously and kept balance because the buoyancy had little effect on the temperature field, resulting that the microbubble will not move anymore and stay at the ultimate balanced position.
4. Application
The microbubble naturally possesses smooth surface of high relative refractive index difference, which makes it a high-quality optical modulation element [20,21]. As a demonstration in the practical application, we used the photo-controlled microbubble in the fiber microstructure to modulate the target beam. The experimental setup that was constructed to test the modulation performance of the fiber microstructure was shown in Fig. 8. The fiber microstructure was bonded onto a microscope slide and placed vertically along the z direction. A SMF (9 µm / 125 µm, NA=0.11) with a flat end was positioned precisely beside the fiber microstructure to deliver the target light beam with a wavelength of 632.8 nm, which was from a He–Ne laser source. The orthogonally placed fiber microstructure and the SMF were immersed in the glycerol that was mixed with the silica nanoparticles and covered with a coverslip. A multimode fiber (MMF) (62.5 µm/125 µm, NA=0.22) connected with an optical power meter (OPM) was aligned in the propagation direction of the target beam to record the modulated beam power. The target beams scattered by the silica nanoparticles in the modulation process were captured using a commercial CCD mounted on a horizontally placed microscope.
Fig. 8. Experimental setup for the optical modulation performance test of the fiber microstructure. Bottom left: the principle of the modulation process based on the total reflection at the gas-liquid interface of the photo-controlled microbubble.
The RIs of ethanol and air are 1.36 and 1.0 respectively, giving a critical angle of 47.3°. When the incidence angle was greater than the critical angle, the divergent target beam will be reflected by the moving microbubble surface via the total internal reflection (TIR), as shown in the insert of the Fig. 5, which made the fiber microstructure have ability to modulate the light beam. The modulation process was shown in Fig. 9. As shown in Fig. 9(a), when the laser was turned off, the incident target beam with a diameter of about 9 µm and a power of 546 µW was output from the SMF, which was displayed by the silica nanoparticles in the glycerol. When traveled across the fiber microstructure, the divergence of the transmitted light beam increased as a result of the multi-times reflection and transmission on the different interfaces together with the light scattering of the silica nanoparticles, which caused the diameter of the transmitted beam enlarged with the increase of the propagation distance. The beam divergence angle became about 4 degree after is passed the fiber attenuator. Once the drive laser was turned on and the power was increased, the microbubble moved downward in the fiber microstructure under the action of the drive Marangoni force, as shown in Fig. 9(b). Along with the movement of the microbubble, the divergent target beam was gradually reflected downward when it was incident on the microbubble surface, as shown in Figs. 9(c)–9(e). When the laser power was increased to 0.8 mW, the target beam was fully reflected (see Visualization 1), as shown in Fig. 9(f). The extinction ratio, a typical parameter to characterize the optical modulation efficiency of a photonic device, is defined as the ratio between the recorded modulated beam power and the unmodulated transmitted beam power [22]. With the increasing laser power, the modulated target beam power and the corresponding values of the extinction ratio (ER) in dB were shown in Fig. 10. The recorded power of the target beam changed to 450 µW after it crossed the fiber microstructure, meaning that the insertion loss of such a fiber microstructure was 0.83 dB. With the increased drive laser power, the recorded beam power decreased and the ER values of the fiber device increased, because more and more target beam was deflected by the downward moving microbubble from its initial transmission direction. The minimum beam power recorded by the MMF was only 0.61 µW, corresponding to a maximum ER of 28.7 dB.
Fig. 9. Microscope images of the process of the target beam modulated by adjusting the drive laser power. (a) and (b) The target beam without the disturbance of the microbubble. (c)-(f) The target beam fully reflected by the gas-liquid interface of the photo-controlled microbubble.
Fig. 10. Optical modulation performance of the fiber microstructure corresponding to the drive laser power.
The response time is an important property to evaluate the optical attenuator. We measured the signal from MMF to characterize the dynamic response of the fiber attenuator, which was switched between two separate routing states [corresponding to Figs. 9(d) and 9(f)], as shown in Fig. 11. The response time of the fiber attenuator was 1.3 s when the drive laser power was switched from 0.5 mW to 0.8 mW and that in the recovering process was 1.8 s. For the typical lab-on-a-chip applications, adjusting the working light power to the desired values within a few seconds was often acceptable, considering that the photo-controlled microbubble can be kept stable in the fiber microstructure and hence the fiber attenuator can provide stable output signals over seconds to minutes at a low power consumption.
Fig. 11. The target light beam power response to the drive laser power switched alternately between 0.5 mW and 0.8 mW.
It was worth noting that the modulated target beam power in the initial transmission direction and the ER values correlated with the position of the microbubble, which can be controlled continuously by simply adjusting the laser power. Namely, the target beam power can be fine modulated as desired, and thus, the controllable attenuation is achieved. Such a performance made the fabricated fiber microstructure function as a variable optical attenuator. In addition, compared with the conventional attenuators based on the microelectromechanical systems (MEMS) or planer lightwave circuits (PLC), no mechanical components, no electrical connections or no complex mechanisms were needed for the operation, which made such a fiber device have potential to be a new-type simple variable fiber all-optical attenuator.
5. Conclusions
In summary, we have experimentally demonstrated a novel scheme of optically controlling a microbubble. The microbubble was positioned at a micrometer scale in the fiber-based hollow microstructure by simply adjusting the laser power. With the advantages of easy operation, high integration, high controllability, low power consumption, and so on, such a scheme can be applied to the confined liquid environments, such as the networks in the lab-on-a-chip, for the microbubble manipulation by the aid of the fiber. As a practical application, we proposed a kind of new-type fiber optical attenuator using the photo-controlled microbubble. Such a compact fiber attenuator can operate efficiently in an all-optical way and has potential to be integrated to the lab-on-a-chips as a reusable and plug-in type device to modulate the power of the working light beams.
Funding
Natural Science Foundation of Shandong Province (ZR2019BA021, ZR2019MF057); National Natural Science Foundation of China (11504070, 61705097); High School Science & Technology Fund Planning Project of Shandong Province of China (J18KA222).
Disclosures
The authors declare no conflicts of interest.
References
1. R. Dijkink and C. D. Ohl, “Laser-induced cavitation based micropump,” Lab Chip 8(10), 1676–1681 (2008). [CrossRef]
2. A. Hashmi, G. Yu, M. Reilly-Collette, G. Heiman, and J. Xu, “Oscillating bubbles: a versatile tool for lab on a chip applications,” Lab Chip 12(21), 4216–4227 (2012). [CrossRef]
3. J. Ma, G. Y. Wang, L. Jin, K. O. H. and B, and O. Guan, “Photothermally generated bubble on fiber (BoF) for precise sensing and control of liquid flow along a microfluidic channel,” Opt. Express 27(14), 19768–19777 (2019). [CrossRef]
4. J. T. Li, F. S. Zhao, Y. Deng, D. Liu, C. H. Chen, and W. C. Shih, “Photothermal generation of programmable microbubble array on nanoporous gold disks,” Opt. Express 26(13), 16893–16902 (2018). [CrossRef]
5. W. Q. Hu, K. S. Ishii, and A. T. Ohta, “Micro-assembly using optically controlled bubble microrobots,” Appl. Phys. Lett. 99(9), 094103 (2011). [CrossRef]
6. S. J. Wu, Z. G. Zuo, H. A. Stone, and S. H. Liu, “Motion of a free-settling spherical particle driven by a laser-induced bubble,” Phys. Rev. Lett. 119(8), 084501 (2017). [CrossRef]
7. S. Ghosh, A. Biswas, B. Roy, and A. Banerjee, “Self-assembly and complex manipulation of colloidal mesoscopic particles by active thermocapillary stress,” Soft Matter 15(23), 4703–4713 (2019). [CrossRef]
8. K. Yang, Y. Zhou, Q. S. Ren, J. Y. Ye, and C. X. Deng, “Dynamics of microbubble generation and trapping by self-focused femtosecond laser pulses,” Appl. Phys. Lett. 95(5), 051107 (2009). [CrossRef]
9. P. A. Prentice, M. P. MacDonald, T. G. Frank, A. Cuschieri, G. C. Spalding, W. Sibbett, P. A. Campbell, and K. Dholakia, “Manipulation and filtration of low index particles with holographic Laguerre-Gaussian optical trap arrays,” Opt. Express 12(4), 593–600 (2004). [CrossRef]
10. P. H. Jones, E. Stride, and N. Saffari, “Trapping and manipulation of microscopic bubbles with a scanning optical tweezer,” Appl. Phys. Lett. 89(8), 081113 (2006). [CrossRef]
11. M. P. MacDonald, L. Paterson, W. Sibbett, K. Dholakia, and P. E. Bryant, “Trapping and manipulation of low-index particles in a two-dimensional interferometric optical trap,” Opt. Lett. 26(12), 863–865 (2001). [CrossRef]
12. A. Miniewicz, C. Quintard, H. Orlikowska, and S. Bartkiewicz, “On the origin of the driving force in the Marangoni propelled gas bubble trapping mechanism,” Phys. Chem. Chem. Phys. 19(28), 18695–18703 (2017). [CrossRef]
13. C. L. Zhao, Y. L. Xie, Z. M. Mao, Y. H. Zhao, J. Rufo, S. K. Yang, F. Guo, J. D. Mai, and T. J. Huang, “Theory and experiment on particle trapping and manipulation via optothermally generated bubbles,” Lab Chip 14(2), 384–391 (2014). [CrossRef]
14. Y. Zhang, Y. X. Zhang, Z. H. Liu, X. Y. Tang, X. H. Yang, J. Z. Zhang, J. Yang, and L. B. Yuan, “Laser-induced microsphere hammer-hit vibration in liquid,” Phys. Rev. Lett. 121(13), 133901 (2018). [CrossRef]
15. K. Namura, K. Nakajima, K. Kimura, and M. Suzuki, “Photothermally controlled Marangoni flow around a microbubble,” Appl. Phys. Lett. 106(4), 043101 (2015). [CrossRef]
16. F. Winterer, C. M. Maier, C. Pernpeintner, and T. Lohmüller, “Optofluidic transport and manipulation of plasmonic nanoparticles by thermocapillary convection,” Soft Matter 14(4), 628–634 (2018). [CrossRef]
17. A. Marcano O and L. Aranguren, “Laser-induced force for bubble-trapping in liquids,” Appl. Phys. B 56(6), 343–346 (1993). [CrossRef]
18. J. G. Ortega-Mendoza, J. A. Sarabia-Alonso, P. Zaca-Moran, A. Padilla-Vivanco, C. Toxqui-Quitl, I. Rivas-Cambero, J. Ramirez-Ramirez, S. A. Torres-Hurtado, and R. Ramos-Garcia, “Marangoni force-driven manipulation of photothermally-induced microbubbles,” Opt. Express 26(6), 6653–6662 (2018). [CrossRef]
19. C. L. Zhang, Y. Q. Wang, Y. Gong, Y. Wu, G. D. Peng, and Y. J. Rao, “The generation and assembly of laser-induced microbubbles,” J. Lightwave Technol. 36(12), 2492–2498 (2018). [CrossRef]
20. J. L. Jackel, J. J. Johnson, and W. J. Tomlinson, “Bistable optical switching using electrochemically generated bubbles,” Opt. Lett. 15(24), 1470–1472 (1990). [CrossRef]
21. V. Zagolla, E. Tremblay, and C. Moser, “Light induced fluidic waveguide coupling,” Opt. Express 20(S6), A924–A931 (2012). [CrossRef]
22. K. Campbell, A. Groisman, U. Levy, L. Pang, S. Mookherjea, D. Psaltis, and Y. Fainman, “A microfluidic 2 × 2 optical switch,” Appl. Phys. Lett. 85(25), 6119–6121 (2004). [CrossRef]
