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 show that colloidal crystals can be assembled by means of temperature gradients produced by light absorption (λ=1070 nm) in a 21 nm titanium thin film deposited on one of the cell´s walls. Depending on the position of a 100x microscope objective focus within the 20 μm thick cell, three different regimes of crystal formation can be identified: 1) convective currents regime; 2) convective-thermophoresis regime, and 3) thermophoresis regime. We show that defects on the crystal can be modified dynamically by switching on and off the laser beam. In addition, the crystal can be 2D manipulated along the substrate. This technique could lead to the formation of large area colloidal crystals for photonics applications.
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
Here we report the creation and manipulation of colloidal crystals by inducing temperature gradients in a colloidal suspension of silica microparticles. A colloidal crystal is an ordered array of colloid particles analogous to their atomic or molecular counterparts with proper scaling considerations. The generation and properties of colloidal crystals have been of great interest for diverse science applications such as photonic crystals, chemical sensors among others. We report a technique that utilizes particles of silica of different diameters to form colloidal crystals by temperature gradients produced by light absorption at a metallic thin film deposited on one of the substrates. Moreover, we study the behavior of the particles by having different number of hot zones.
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.