Near-infrared Laser-induced Temperature Elevation in Optically-trapped Aqueous Droplets in Air

Abstract

Near-infrared laser-induced temperature elevation in single aqueous ammonium sulfate droplets levitated in air were evaluated by means of laser trapping and Raman spectroscopy. Since the vapor pressure in an aqueous solution droplet should be thermodynamically in equilibrium with that of water in air, the equilibrium size of the droplet varies sensitively through evaporation/condensation of water in accordance with the temperature change of the droplet. In this study, we demonstrated that the changes in the size of an optically levitated aqueous ammonium sulfate droplet were induced by irradiation of a 1064-nm laser beam as a heat source under an optical microscope. Temperature elevation in the droplet was evaluated successfully by means of Raman spectroscopy, and the values determined were shown to be in good agreement with those by the theoretical calculations based on the absorption coefficient of water at 1064-nm and the thermal conductivity of air. To the best of our knowledge, this is the first experimental demonstration showing that the absorption coefficient evaluated from changes in the size of optically-trapped aqueous droplets is consistent with that of pure water.

Raman Spectroscopy of Single Light-Absorbing Carbonaceous Particles Levitated in Air Using an Annular Laser Beam

“…A laser trapping technique is a fundamental basis for studying aerosol droplets, since a metastable liquid state such as supercooled or supersaturated water droplets can be stably observed without contact with solid substrates. ,− Therefore, the laser trapping technique coupled with Raman spectroscopy is a promising experimental approach for evaluating the reaction dynamics and water uptake behavior of soot particles in the atmosphere. However, a conventional laser trapping technique based on radiation pressure is not applicable to the optical trapping of light-absorbing carbonaceous particles because the repulsive force (photophoretic force) caused by heat is orders of magnitude larger than the attractive force of radiation pressure. − To date, several methods for the optical trapping of light-absorbing particles in air using the photophoretic force have been developed, in which counter-propagating laser beams, ,, a speckled coherent beam, , an optical bottle beam, , a hollow laser beam, , or a slowly diverging vortex beam were used as trapping light sources.…”

Spherical Spontaneous Capillary-Wave Resonance on Optically Trapped Aerosol Droplet

“…The apparatus used for generation and trapping of aerosol droplets was similar to previously reported ones (Figure ). , Briefly, droplets of the sample aqueous solution were generated by means of an ultrasonic nebulizer. The droplets were fed into a sample chamber set on an inverted microscope.…”

“…In addition, whispering gallery modes in the −OH stretching mode were used to precisely determine the radius of the droplet as required. 30,32,33 Operation Procedure. First, the air flow and the ultrasonic nebulizer were turned on, and numbers of aerosol droplets having radius variety were introduced into the chamber.…”

Optical Trapping–Polarized Raman Microspectroscopy of Single Ethanol Aerosol Microdroplets: Droplet Size Effects on Rotational Relaxation Time and Viscosity

“…Optical trapping–microspectroscopy is the indispensable basis to elucidate the chemical and physical properties of individual microparticles in colloidal and aerosol systems, not studied by ensemble measurements of the particles: size and shape effects on the properties as examples. Focusing on aerosol microparticles, an optical trapping technique − has been combined successfully with absorption, , luminescence, − light scattering, ,− and photoacoustic spectroscopies. , These studies have played essential roles to elucidate the chemical and physical characteristics of aerosol microparticles: size, ,,,,, shape, composition, ,,,,− temperature, − ,,, viscosity, phase transition/separation, ,, and light resonance. ,,,,,,, Thus, an optical trapping–microspectroscopy technique has been devoted for the development of aerosol chemistry in the past decades. In this spectroscopy, nevertheless, linearly polarized optical anisotropy measurements have been rarely explored, although they can provide direct information on the rotational/diffusional motions of a molecule in an aerosol particle.…”

“…Focusing on aerosol microparticles, an optical trapping technique 1−4 has been combined successfully with absorption, 5,6 luminescence, 6−12 light scattering, 6,12−38 and photoacoustic spectroscopies. 39,40 These studies have played essential roles to elucidate the chemical and physical characteristics of aerosol microparticles: size, 19,21,23,24,28,37 shape, 37 composition, 15,17,21,27,30−32 temperature, [14][15][16]18,26,29 viscosity, 9 phase transition/separation, 33,34,36 and light resonance. 8,10,13,14,19,20,23,24 Thus, an optical trapping−microspectroscopy technique has been devoted for the development of aerosol chemistry in the past decades.…”