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Research Papers

Particle Migration by Optical Scattering Force in Microfluidic System With Light-Absorbing Liquid

[+] Author and Article Information
Masahiro Motosuke1

Jun Shimakawa, Dai Akutsu, Shinji Honami

 Department of Mechanical Engineering, Tokyo University of Science, 1-14-6, Kudankita, Chiyoda-ku, Tokyo, Japan

1

Corresponding author.

J. Heat Transfer 134(5), 051025 (Apr 13, 2012) (6 pages) doi:10.1115/1.4005714 History: Received September 20, 2010; Revised September 02, 2011; Published April 13, 2012; Online April 13, 2012

Optical force offers a promise of being applied as a noninvasive manipulation tool for microscopic objects without physical contact. Particle control in a microfluidic system is achieved by optics showing advantages over electric or the other methods. With optics, the fluid need not to be contamination free and there is no need for electrode fabrication. Particles can experience different forces depending on the optical configuration. The scattering force is predominant under parallel or gently focused irradiation, while the gradient force is predominant in tightly focused irradiation. This paper reports the experimental and theoretical investigations of the potential of optical scattering force for particle control technique in a microfluidic system with a light-absorbing liquid. The light-absorption of the incident laser beam in the liquid causes a temperature rise and induces the corresponding property changes of liquid and particles. The experiments were presented for particle migration using the scattering force exerted by a compact diode laser with a wavelength of 635 nm. The absorption of the light in the liquid was controlled by the concentration of dye substance added in a buffer solution. The velocities of polystyrene particles with a diameter of 1.9 μm and the temperature distributions of the liquid under laser irradiation were measured by tracking their movement and by temperature-sensitive fluorophore, respectively. When there is no light absorption in the liquid, the migration velocity of particles under the laser beam is linearly increased with the increase of the laser power, in agreement with the calculations based on ray optics theory. In the case of light-absorbing liquid, the migration speed of particles experiencing the optical force indicates a nonlinear increase as the laser power increases. This enhancement mainly attributes to the temperature-sensitive change of liquid viscosity resulting in a reduction of viscous drag for migrating particles. An appropriate arrangement of light absorption leads to an enhancement in the photophoretic velocity of particles, and eventual performance promotion of particle separation and/or sorting using the optical force.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Diagram of ray-optics model of optical radiation pressure to a spherical particle irradiated by a single ray. (b) Qs and Qg profiles under parallel irradiation of light with uniform intensity to a spherical particle. Considering the net force on the sphere, the gradient force is cancelled due to the symmetry of Qg , and the particle experiences only the scattering force.

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Figure 2

Schematic of the experimental system. Laser beam exerting an optical force on the particles and heating the liquid is focused and irradiated horizontally from the side via a prism. Measurements of particle velocities and liquid temperatures are performed by the same optics, from the lower side of the microchannel, based on an inverted microscope.

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Figure 3

(a) Top view of microchannel and (b) CCD image of light path of the incident beam in the channel from which horizontal irradiation with gently focused beam is confirmed. Dotted region is an observation area to evaluate the particle movement under light irradiation.

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Figure 4

Absorption and emission spectra for Brilliant Blue FCF aqueous solution (BB) and diode laser used in our experiments (LD). Absorption for BB and emission for LD are shown. Working fluid can be efficiently heated by irradiation of the LD beam.

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Figure 5

(a) Particle velocity field under laser irradiation perpendicular to the flow direction. The position of laser irradiation is indicated by an arrow. (b) Spanwise velocity of particles by the scattering force at different laser powers. Calculated velocity is obtained from Eq. 6 adding a constant which adjusts the energy intensity representing the optical loss due to reflection or scattering.

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Figure 6

Temperature dependence of fluorescent intensity of the sample buffer solution with 0.1 mM fluorescein and Brilliand Blue FCF with the same molar concentration as fluorescein. This relationship is used to determine the liquid temperature in micro-LIF measurement.

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Figure 7

(a) Temperature distribution of light-absorbing liquid in microchannel under laser irradiation. (b) Spanwise particle movement under laser irradiation.

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Figure 8

Temperature increase in the liquid by laser heating measured by micro-LIF and corresponding viscosity reduction. Circles and line in the figure mean the temperature values and estimated viscosities, respectively (arrows indicate corresponding vertical axes). The liquid viscosity is assumed to be the same as that of water, including the temperature dependence [26].

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Figure 9

Enhancement of particle velocity by the optical scattering force in light-absorbing liquid. The no absorption case is also shown for comparison. This nonlinear increase in migration velocity agrees well with the calculation taking the temperature-sensitive viscosity reduction into account (dotted line).

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