Mass-manufacturable polymer microfluidic device for dual fiber optical trapping

Abstract

We present a microfluidic chip in Polymethyl methacrylate (PMMA) for optical trapping of particles in an 80µm wide microchannel using two counterpropagating single-mode beams. The trapping fibers are separated from the sample fluid by 70µm thick polymer walls. We calculate the optical forces that act on particles flowing in the microchannel using wave optics in combination with non-sequential ray-tracing and further mathematical processing. Our results are compared with a theoretical model and the Mie theory. We use a novel fabrication process that consists of a premilling step and ultraprecision diamond tooling for the manufacturing of the molds and double-sided hot embossing for replication, resulting in a robust microfluidic chip for optical trapping. In a proof-of-concept demonstration, we show the trapping capabilities of the hot embossed chip by trapping spherical beads with a diameter of 6µm, 8µm and 10µm and use the power spectrum analysis of the trapped particle displacements to characterize the trap strength.

Dual fiber optical trapping in a polymer-based microfluidic chip

“…The average transverse stiffness (κ x ) of the trap is calculated from the corner frequencies fc of the power spectra. These are plotted for the three bead sizes together with the 50% confidence interval error bars in Figure 8 [10]. We clearly see a dependency of the trapping stiffness on the trapping power, the bead size and the fiber separation.…”

“…When the trap stiffness increases, higher frequency components will start dominating the particle movement. The calculated power spectrum is fitted with a Lorentzian spectrum [10].…”

“…This comparison showed a good agreement between the simulated and numerically integrated trapping force, but both calculations were based on a ray-tracing approach. To our knowledge, the model developed in [10] for the calculation of optical forces acting on a particle in an optical trap, has more functionalities than those proposed in literature. The calculation of optical torque was demonstrated by Aspnes et al [11].…”

“…A particle is trapped in the center of the channel by two divergent light beams emitted by the trapping fibers. The channel is sealed at the top by a second PMMA layer [10].…”

Reflective liquid crystal hybrid beam-steerer

“…The blazed grating has a blaze angle of 10 • , a periodicity of Λ = 50 µm, and is made of poly(methyl methacrylate) (PMMA), an isotropic medium with a refractive index of n PMMA = 1.489 at a wavelength of λ = 633 nm. The PMMA grating is obtained by hot-embossing a PMMA sheet with a brass mold that was manufactured using ultraprecision diamond milling with a cylindrical end-mill [14]. Because of the tool geometry, the two slopes of the grating make a right angle.…”