In this study, the controlled formation, trapping, and self-oscillation of vapor microbubbles in ethanol was investigated using low-power continuous wave (CW) lasers. The formation of these microbubbles is achieved by evaporation of ethanol due to heating by light absorption (CW laser emitting at λ = 658 nm) in silver nanoparticles deposited at the distal end of a multimode optical fiber. A second low-power NIR laser (λ = 1,550 nm) coupled to a single-mode optical fiber is then used to trap the microbubbles. It has been shown that the bulk absorption of light at 1,550 nm in ethanol modulates the surface tension of the bubble wall, creating a three-dimensional potential well that efficiently traps the bubbles. Furthermore, it was observed that once the bubble is trapped, random variations in its radius create instabilities in the trap, resulting in microbubble oscillations. The trapped bubble tends to oscillate between two quasi-stationary equilibrium points along the propagation of light. These oscillations are the result of competition between several forces, such as the Marangoni, the upward of buoyancy, and the drag forces. The results presented in this work contribute significantly to the understanding of these phenomena and may have important applications in fields such as microfluidics and bubble manipulation.
We show the 3D quasi-steady-state trapping of ethanol vapor microbubbles and their real-time manipulation in liquids driven by the Marangoni force that is triggered by using a low-power continuous-wave laser. The light absorption phenomenon, caused by the silver nanoparticles photodeposited on the core of optical fibers, is employed for both the generation of vapor microbubbles and triggering the Marangoni effect. The thermal effects, activated by either the silver nanoparticles on the optical fiber´s core or the light bulk absorption, modulate the surface tension of the bubble wall, generating longitudinal and transversal forces, similar to the optical ones. The balance of the optothermal forces drives the quasi-steady-stated 3D manipulation of microbubbles. By numerical simulations, we acquire expressions for the temperature profiles and present analytical expressions for the Marangoni force. Moreover, using an array of three fibers with photodeposited nanoparticles is used to demonstrate the transfer of bubbles from one fiber to another by sequentially switching on and off the lasers.
We present 3D steady-state trapping and manipulation of vapor microbubbles in pure ethanol by optothermal effects induced by low power CW laser. Light absorption from silver nanoparticles photodeposited on the core of a multi-mode optical fiber is used as a heat source to induce microbubbles of several diameters. The optothermal effects generated by the light absorption of light bulk absorption modulates the microbubble wall's surface tension inducing both longitudinal and transversal forces just like optical forces, generating a 3D potential well. By numerical simulations, we find expressions to describe both the temperature profiles and the convective currents. Besides, using an array of three optical fibers with silver nanoparticles photodeposited on their cores we show the transmission of microbubbles from one fiber to another by switching the laser radiation from the CW lasers.
We present both the 3D trapping and manipulation of microbubbles by temperature gradients, induced by low power CW laser in absorbing liquid (ethanol). Two optical fibers were used: a multimode one for bubble generation and a single-mode one for both trapping and manipulating. One distal end of the multimode fiber was coupled to a Qswitched pulsed laser (λ=532 nm and pulse width τp=5 ns). The light propagates in the fiber and gets absorbed at silver nanoparticles, previously photodeposed at the other distal end, heating up the surrounding liquid and generating the microbubbles. On the other hand, a CW laser (λ = 1550 nm) was coupled to one distal end of the single-mode fiber, the other distal end was immersed in ethanol, inducing thermocapillary force, also called Marangoni force, that is the cornerstone in the trapping and manipulating of bubbles. The bubble generated on the multimode fiber travels towards the single-mode fiber by a careful switching of the temperature gradients. In addition to the Marangoni force, the microbubble immersed in ethanol suffers both drag force and buoyancy force. So, the equilibrium among these forces drives the 3D trapping and manipulation of the microbubble. To our best knowledge, this is the first time that 3D trapping and manipulation using low CW power es presented.
Manipulation and trapping of particles have taken a huge relevance in recent years thanks to many applications with revolutionary contributions to diverse fields. Several experiments have demonstrated that thermal effects can improve the current micromanipulation techniques such as DNA manipulation or assembly of colloidal crystals. In this work, we present the effect of laser-induced thermal effects, such as convection currents and thermophoresis, on the trap stiffness (spring constant) constant of an optical trap of 3-micrometer particles suspended in water. These effects are a consequence of light absorption in a thin layer of hydrogenated amorphous silicon (a-Si:H) deposited at the bottom of the chamber which generates a thermal gradient. Since these effects (and its correspondent forces) are symmetric around the beam focus, trapped particles, experience an increment in the trapping force. Around the beam focus, the drag force associated with convective currents is directed upwards and are compensated by optical scattering force. Depending on the laser power, the trap stiffness increases significantly, so a trapped particle can be dragged along the cell (by displacing the sample and leaving the beam fixed) at velocities around 90 μm/s without escaping the trap, whereas in the absence of the a-Si:H film, the escape velocity of the particle in the trap drops to velocities around 30 μm/s. This presents a simple, yet effective, option for optical manipulation at low powers (<5 mW) and its possible applications in the manipulation of a variety of biological micro samples.
We present the generation and 3D manipulation of microbubbles by thermal gradients, induced by low power nanosecond pulsed laser in non-absorbent liquids. Light absorption at photodeposited silver nanoparticles on the optical fiber tip heat up the surrounding liquid, which leads to optothermal effects. With each laser pulse a microbubble is detached from the optical fiber end, creating a microbubbles-stream. The microbubbles move away from the optical fiber end driven by non-spherical cavitation until they coalesce creating a main-bubble which is attracted towards the optical fiber end by Marangoni force. In addition, the main-bubbles are under the influence of buoyancy and gravity forces, which act upwards and downwards, respectively. The balance of these forces allows the 3D manipulation of the main-bubble. The main-bubble position can be controlled by careful control of the pulse energy. To our knowledge this is the first time that 3D manipulation of microbubbles using pulsed lasers is demonstrated.
Generation and 3D manipulation of microbubbles by means of temperature gradients induced by low power laser radiation is presented. Photodeposited silver nanoparticles on the distal end of two optical fibers act as thermal sources after light absorption. The temperature rises above liquid evaporation temperature generating a microbubble at the optical fibers end in non-absorbent liquids. Alternatively, switching the thermal gradients between the fibers, it is possible to generate forces in opposite directions, causing the migration of microbubbles from one fiber optic tip to another. Marangoni force induced by surface tension gradients in the bubble wall is the driving force behind the manipulation of microbubbles
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