Temperature determination of single micrometre-sized particles from forced/free convection and photophoresis

A novel technique for determination of the temperature of spherical micrometre-sized particles from forced/free convection and photophoretic forces has been developed. The three forces were measured for suspended charged particles in an electrodynamic chamber. The forces were applied to the particle in three-dimensions, normally to each other, and measured directly in terms of the three-dimensional electric field required to maintain the particle at a fixed position. The suspended particle was irradiated horizontally, by a focused laser beam, from one side (to generate a photophoretic force) and forced convection was applied horizontally and normally to the laser radiation. Free convection was generated upwards (naturally). Glassy carbon particles in the size range of 60-150 μm in diameter were used, varying their temperature in the range 300-1200 K. The particles were subjected to nitrogen How at velocities in the range 0-0.1 m s^-1, providing very low Reynolds numbers, that is in the Stokes regime. According to theoretical considerations the ratio of hot to cold forced-convection forces should depend linearly on the ratio of the viscosity at the film temperature to the surroundings' viscosity. This was verified experimentally and the particle temperature was readily attained from the viscosity. Using this method, laser power was converted into temperature and the other forces (free convection and photophoretic) were presented as functions of the temperature. A theoretical correlation for the photophoretic force that depends on temperature was consequently verified experimentally, providing a perfect fit between theoretical results and experimental ones. The photophoretic force was found to depend very strongly on temperature and hence can be a sensitive means of determining the particle temperature. An empirical correlation between the free-convection force and a particle's temperature and diameter was found. This empirical correlation can be used as an additional means for determining the particle temperature. The current calibration in the forced-and free-convection cases is valid, regardless of the particle material, whereas the calibration of the photophoretic case is limited to the examined material only.