The optical force offers the promise of being applied as noninvasive manipulation tool for microscopic objects without physical contact. The optical scattering force is predominant in the light field with parallel or gently focused beam, while the gradient force in the tightly focused one. The optical approach using the scattering force for the microfluidic system has advantages over the electric or the other methods of particle control, such as no physical contact or no need for electrode fabrication. This paper reports experimental and theoretical investigations of the potential of optical scattering force for particle control technique in a microfluidic system with light-absorbing liquid. Light-absorption of liquid induces the change of liquid properties mainly in the viscosity, which means the decrease of viscous drag for suspended particles. In the experimental system, the light source to exert the optical force was a compact diode laser with the wavelength of 635 nm. The absorption of the beam was controlled by the concentration of dye substance in a buffer solution. Polystyrene particles with the diameter of 1.9 μm were suspended in the liquid flowing in the microchannel. The particle velocity field and fluid temperature distribution were measured by the image processing with the individual particle tracking and by the temperature-sensitive fluorescent dye dissolved in the fluid, respectively. When there is no absorption of the light in the liquid, the particle velocity is linearly increased with the increase of the laser power. The calculated optical force based on the ray optics agrees with this tendency. In the case of the light-absorbing liquid, the migration speed of the particle indicates the nonlinear increase as the laser power increases. This nonlinearity is attributed to temperature dependence of the viscosity of the liquid.

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