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
The aim of this study was to compare the effectiveness of Rose Bengal (RB) and Methylene Blue (MB) as photosensitizers (PS) in Photodynamic Inactivation (PDI) on planktonic cultures of Candida albicans, a well-known opportunistic pathogen. RB and MB at concentrations ranging from 0.5 to 60 μM and fluences of 10, 30, 45 and 60 J/cm2 were tested. The light sources consist of an array of 12 led diodes with 30 mW of optical power each; 490-540 nm (green light) to activate RB and 600 -650 nm (red light) to activate MB. We first optimize the in vitro PDI technique using a single light dose and the optimum PS concentration. The novelty of our approach consist in reducing further the PS concentration than the optimum obtained with a single light exposure and using smaller light fluence doses by using repetitive light exposures (two to three times). MB and RB were tested for repetitive exposures at concentrations ranging from 0.1 to 10 μM, with fluences of 3 to 20 J/cm2, doses well below than those reported previously. All experiments were done in triplicate with the corresponding controls; cells without treatment, light control and dark toxicity control. RB-PDI and MB-PDI significantly reduced the number of CFU/mL when compared to the control groups. The results showed that RB was more effective than MB for C. albicans inactivation. Thus, we show that is possible to reduce significantly the amount of PS and light fluence requirements using repetitive light doses of PDI in vitro.
In this work we demonstrate the increasing of the trap stiffness (spring constant) constant of an optical trap of particles suspended in water by laser-induced convection currents. These currents are the result of thermal gradients created by a light absorption in a thin layer of hydrogenated amorphous silicon (a:Si-H) deposited at the bottom of cell. Since convection currents (and therefore drag forces) are symmetric around the beam focus particles trapped by the beam are further contained. Around the focus the drag force is directed upwards and partially compensated by radiation pressure depending on the laser power increasing the stiffness of the optical trapping increases significatively so a particle trapped could dragged (by moving the translation stage leaving the beam fixed) at velocities as high as 90μm/s without escaping the trap, whereas with no a:Si-H film, the particle escapes from the trap at lower velocities (30μm/s).
We show that trapping and manipulation of microparticles can be achieved by Rayleigh convection currents using a low power lasers. Light absorbed by thin film of amorphous silicon (a:Si) creates the convection currents. In contrast to previous works, we show that multiple trapping can be achieved using solid microparticles without the creation of vapor bubbles. For low power (~1 mW), particles are trapped at the center of the beam, however at higher powers (~3 mW) particles are trapped on a ring around the beam due to two competing forces: Stokes and thermophoretic forces. Numerical simulations confirm that thermal gradients are responsible for the trapping mechanism.
Trapping with evanescent fields has become an important tool in many research fields. Evanescent fields allow trapping of particles in close proximity to a surface. However, excitation of these waves may be cumbersome. Recently, trapping with photorefractive electric fields has been demonstrated using dielectric and metallic nano and microparticles. Excitation of these fields is straight forward and, in principle, can be excited with microwatt power level. In this work, we give a comparison of photorefractive and plasmonic trapping emphasizing its advantages and disadvantages. We show that single beam and holographic photorefractive photovoltaic trapping in LiNbO3 of microparticle in water is possible.
Novel results are presented on thermocavitation in highly absorbing solutions using CW low power
laser (λ=975 nm). Due to the large absorption coefficient (135 cm-1) at the laser wavelength,
penetration length is only ~74μm inside the liquid and asymmetric bubbles are generated near the
beam's entrance wall. We report the temporal dynamic of the cavitation bubble, which is much shorter
than previously reported. We found that the amplitude of the shock wave decreases exponentially with
the beam power. As shown in this work, thermocavitation is a phenomenon that has a great application
potential in areas such as ultrasonic waves generation and controlled tissue ablation for use in