Temperature elevation and fluid convection under optical trapping condition as revealed by fluorescence correlation spectroscopy

Kenji Setoura; Keisuke Fujita; Syoji Ito; Hiroshi Miyasaka

Temperature of matter increases under intense photoirradiation owing to photothermal conversion. The photothermal effect is sometimes a significant issue in optical manipulation usually requiring intense optical fields. Quantitative evaluation of local temperature under photoirradiation can, therefore, provide indispensable information for optical manipulation. In a previous work, we have applied fluorescence correlation spectroscopy (FCS) to monitor the temperature under the optical trapping condition in water, ethanol, and ethylene glycol. We pointed out that analyses of diffusion time of fluorescent dyes could provide information about temperature on the basis of temperature-dependent viscosities of the solvents. In the present work, the FCS thermometry was applied to seven solvents including primary aliphatic alcohols, to examine universal applicability of the method. To verify the experimental results, numerical simulations were performed on the basis of two-dimensional heat conduction at a stationary state. The numerical results on the temperature field satisfactorily reproduced the experimental data, proving that the FCS thermometry is applicable to ordinary solvents. In addition, we also performed numerical simulations on velocity fields in the solvent, to evaluate contribution of natural convection under typical optical trapping condition at light intensity of ∼MW cm − 2. It was revealed that the contribution of the natural convection is not negligible for mass transfer in the solvents.

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