The present work shows the oscillation of a microbubble using temperature gradients. This gradient is caused by the absorption of laser light by silver nanoparticles (AgNPs) immobilized on the tip of a single-mode optical fiber FO (9/125 μm). The immobilization of these nanoparticles was performed using the technique known as photodeposition. Subsequently, the tip with the nanoparticles was immersed in ethanol. We used a infrared (λ=1550 nm) laser with fiber optic output which was controlled (modulate) with a waveform generator. When the laser pulse is at its high level, a radial temperature gradient is generated and the liquid near the tip of the optical fiber evaporates creating a microbubble. This microbubble remains attached to the face of the optical fiber due to the Marangoni force (FM) that brings it to the point of highest temperature. When the laser pulse changes to its low level, the temperature gradient disappears and the Marangoni force becomes zero. This causes the buoyancy force (FB) to become predominant driving the microbubble to the surface. However, for a new laser pulse the cycle repeats itself, keeping the microbubble oscillating within a region. As the laser modulation frequency increases the oscillation distance of the microbubble decreases.
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.