Quantitative Evaluation of Optical Forces by Single Particle Tracking in Slit-like Microfluidic Channels

Abstract

Optical trapping and manipulation techniques have attracted significant attention in various research fields. Optical forces divided into two terms, such as a scattering force and gradient one, work to push forward and attract objects, respectively. This is a typical property of optical forces. In particular, a tool known as optical tweezers can be created when a laser beam is converged at a focal point, causing strong forces to be generated so as to trap and manipulate small objects. In this study, we propose a novel method to build up assembled structures of polystyrene particles by using optical trapping techniques. Recording trajectories of single particles, the optical forces are quantitatively evaluated using particle tracking velocimetry. Herein, we treat various particle sizes whose diameters range from 1 to 4 μm and expose them to a converged laser beam of 1064 nm wavelength. As a result, both experimental and theoretical results are in good agreement. The behavior of particles is understood in the framework of Ashkin’s ray optics. This finding clarifies optical force fields of microparticles distributed in a slit-like microfluidic channel and will be applicable for effectively forming ordered structures in liquids.

Flow with nanoparticle clustering controlled by optical forces in quartz glass nanoslits

“…The spot size is determined as the diameter at which the laser intensity becomes 1∕e 2 = 0.135 of the maximum. The temperature increase due to the laser irradiation with a similar optical setup used in our previous study (Nito et al 2018) was confirmed to be 1 K, and we consider the effect of temperature rise to be negligible. An excitation light is generated from a mercury lamp (U-HGLGPS, Olympus, Tokyo, Japan) thorough an excitation filter and irradiates the sample solution in the nanoslit device.…”

“…Therefore, we expect that the scattering force in the z direction does not play a significant role in the particle motion for the case of d = 490 nm, and only the gradient forces should be taken into account. On the other hand, for the case of d = 190 nm, scattering force pushes the particles in the z direction, that is, the optically trapped particles stay close to the lid of the channel, as in the previous study (Nito et al 2018).…”

“…The d = 490 nm particles are expected to yield a two-dimensional cluster in the nanoslit. In our previous study (Nito et al 2018), where the channel height was H = 7.0 µm and d = 2.93 µm, particles formed a twodimensional cluster with a hexagonal packing state at the solid-liquid interface, that is, the roof of the channel, due to the gradient forces. In Nito et al (2018), only two particles could stack in the z direction because H∕d ≈ 2.39 .…”

“…The theoretical model of optical forces varies according to the ratio between the particle diameter d and the wavelength of light L : we use ray optics for d ≫ L (Ashkin et al 1986), the Lorentz-Mie theory for d ∼ L , and Rayleigh theory for d ≪ L (Harada and Asakura 1996). Since the laser with the wavelength of L = 1064 nm is frequently used due to its low absorption to aqueous solutions, microparticles with diameter d > L are usually described by ray optics (Nito et al 2018) and nanoparticles with diameter d < L are by the Lorentz-Mie or Rayleigh theory (Harada and Asakura 1996;Nagura et al 2019).…”

“…In these days, the optical trapping of microparticles is well established technique and even commercialized. However, as the size of the targets shrinks, the optical force acting on particles decreases rapidly (Harada and Asakura 1996;Nito et al 2018) and the Brownian motion becomes more prominent. These matters make the nanoparticle optical trapping difficult.…”