RESUMEN
Gas bubbles can be trapped and then manipulated with laser light. In this report, we propose the detailed optical trapping mechanism of gas bubbles confined inside a thin light-absorbing liquid layer between two glass plates. The necessary condition of bubble trapping in this case is the direct absorption of light by the solution containing a dye. Due to heat release, fluid whirls propelled by the surface Marangoni effect at the liquid/gas interface emerge and extend to large distances. We report the experimental microscopic observation of the origin of whirls at an initially flat liquid/air interface as well as at the curved interface of a liquid/gas bubble and support this finding with advanced numerical simulations using the finite element method within the COMSOL Multiphysics platform. The simulation results were in good agreement with the observations, which allowed us to propose a simple physical model for this particular trapping mechanism, to establish the origin of forces attracting bubbles toward a laser beam and to predict other phenomena related to this effect.
RESUMEN
In the report we demonstrate how, using laser light, effectively trap gas bubbles and transport them through a liquid phase to a desired destination by shifting the laser beam position. The physics underlying the effect is complex but quite general as it comes from the limited to two-dimension, well-known, Marangoni effect. The experimental microscope-based system consists of a thin layer of liquid placed between two glass plates containing a dye dissolved in a solvent and a laser light beam that is strongly absorbed by the dye. This point-like heat source locally changes surface tension of nearby liquid-air interface. Because of temperature gradients a photo-triggered Marangoni flows are induced leading to self-amplification of the effect and formation of large-scale whirls. The interface is bending toward beam position allowing formation of a gas bubble upon suitable beam steering. Using various techniques (employing luminescent particles or liquid crystals), we visualize liquid flows propelled by the tangential to interface forces. This helped us to understand the physics of the phenomenon and analyze accompanying effects leading to gas bubble trapping. The manipulation of sessile droplets moving on the glass surface induced via controlled with laser light interface bending (i.e. "droplet catapult") is demonstrated as well.
RESUMEN
Using the direct coupling mechanism of light with a liquid via molecular absorption, i.e. the opto-thermal effect, we demonstrate the formation of well-controlled three-dimensional circular flows, i.e. a toroidal vortex, inside the liquid crystal (LC) droplet placed on a glass plate in its isotropic phase. We investigated the behavior of a droplet formed of a phototropic liquid crystal and composed of a mesogenic azobenzene derivative under the Gaussian beam light illumination in four different geometries. The light-induced liquid flows in the isotropic phase of the LC were visualized by dispersing carbon micro-particles in the volume of the LC. Movements of the particles could be observed under an optical microscope from the top and side views, respectively. The formation of the stable in time toroidal vortex (the photonic vortex) is dependent on laser light illumination geometry, properties of the liquid and substrate but does not depend on gravitational forces being similar for droplets situated either above or below the glass plate. The main mechanism of the indirect conversion of light into mechanical work is related to the temperature induced gradient of surface tension known as the Marangoni effect.
RESUMEN
Tightly focused laser beams can trap micro- and nanoparticles suspended in liquids in their focal spots enabling different functionalities including 3D manipulations and assembling. Here, we report on remarkably strong liquid-liquid phase separation and crystallization experiments in para-nitroaniline dissolved in 1,4-dioxane. For optical trapping of para-nitroaniline we used low-power, weakly focused light beam from continuous-wave laser partially absorbed by the solute. The experiments were performed in solution deposited on glass with an upper free-surface and solution contained between two glass plates. The usual gradient force field and scattering force solely are insufficient to properly describe the observed particle gathering effects extending far beyond the optical trap potential. The concept of whirl-enhanced and temperature assisted optical trapping is postulated. The relative simplicity of the used geometry for trapping will broaden the understanding of the light-matter interaction and promises the widespread application of the observed effect in optically controlled crystallization.
RESUMEN
In this paper we present results of experiments designed to increase our understanding of the photorefractive effect occurring during processes of dynamic hologram generation in Hybrid Photorefractive Liquid Crystal Structures (HPLCS). We also propose equivalent mathematical model which can be used to optimize those structures in order to obtain the highest diffraction efficiency in possibly shortest time.
Asunto(s)
Cristales Líquidos/química , Modelos Químicos , Refractometría/instrumentación , Simulación por Computador , Diseño Asistido por Computadora , Diseño de Equipo , Análisis de Falla de EquipoRESUMEN
A joint Fourier-transform optical correlator for image recognition based on a novel hybrid photoconducting polymer-nematic liquid-crystal structure is described. The optically addressed active element that we have designed is capable of performing real-time image processing 20 times/s, at light-intensity levels of 10mW/cm(2) with dc operating voltage of the order of 10 V. We present the results of correlation of simple objects as well as complicated photographs. The sensitivity, durability, and reliability of the presented system open possibilities for many applications.
RESUMEN
A novel, to our knowledge, liquid-crystal panel suitable for real-time holographic purposes has been prepared. A nematic liquid-crystal layer sandwiched between photoconducting polymeric layers, when exposed to a sinusoidal light-intensity pattern, shows efficient formation of refractive-index gratings. The unique feature of the presented panel is its ability to switch energy from beam to beam in a manner similar to the charge-diffusion-controlled photorefractive effect. In a two-wave-mixing experiment multiple orders of diffraction are present, and a very high two-beam coupling-gain ratio (2.5) and a net exponential gain coefficient of ? = 931 cm(-1) have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam-incidence bisector at 45 degrees . The time constants for grating formation and erasure in the studied system are functions of the applied voltage and can be made as short as a few milliseconds under favorable conditions. The mechanism of beam coupling is linked with an electric-field-driven reorientation of the nematic director as a result of a spatially modulated space-charge field created by light in a photoconducting poly(3-octyl)thiophene polymeric layer.
RESUMEN
Methylene blue sensitized poly(methyl methacrylate) is shown to be an efficient medium for recording three-dimensional holographic gratings. Phase and/or amplitude holograms can be written in the methylene blue sensitized films of poly(methyl methacrylate) with a conventional source of light, a He-Ne laser operating at 632.8 nm. Diffraction efficiencies of 60% were found for thick holograms. Multiple holograms were recorded in the described system, and an optical erasing of holograms was achieved. Hologram recording speed was found to increase with temperature.