Thermal molecular focusing: tunable cross effect of phoresis and light-driven hydrodynamic focusing

Visualization of Optical Vortex Forces Acting on Au Nanoparticles Transported in Nanofluidic Channels

The optical manipulation of nanoscale objects via structured light has attracted significant attention for its various applications, as well as for its fundamental physics. In such cases, the detailed behavior of nano-objects driven by optical forces must be precisely predicted and controlled, despite the thermal fluctuation of small particles in liquids. In this study, the optical forces of an optical vortex acting on gold nanoparticles (Au NPs) are visualized using dark-field microscopic observations in a nanofluidic channel with strictly suppressed forced convection. Manipulating Au NPs with an optical vortex allows the evaluation of the three optical force components, namely, gradient, scattering, and absorption forces, from the in-plane trajectory. We develop a Langevin dynamics simulation model coupled with Rayleigh scattering theory and compare the theoretical results with the experimental ones. Experimental results using Au NPs with diameters of 80-150 nm indicate that our experimental method can determine the radial trapping stiffness and tangential force with accuracies on the order of 0.1 fN/nm and 1 fN, respectively. Our experimental method will contribute to broadening not only applications of the optical-vortex manipulation of nano-objects, but also investigations of optical properties on unknown nanoscale materials via optical force analyses.

Synchronized resistive-pulse analysis with flow visualization for single micro- and nanoscale objects driven by optical vortex in double orifice

Resistive-pulse analysis is a powerful tool for identifying micro- and nanoscale objects. For low-concentration specimens, the pulse responses are rare, and it is difficult to obtain a sufficient number of electrical waveforms to clearly characterize the targets and reduce noise. In this study, we conducted a periodic resistive-pulse analysis using an optical vortex and a double orifice, which repetitively senses a single micro- or nanoscale target particle with a diameter ranging from 700 nm to 2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu$$\end{document}μm. The periodic motion results in the accumulation of a sufficient number of waveforms within a short period. Acquired pulses show periodic ionic-current drops associated with the translocation events through each orifice. Furthermore, a transparent fluidic device allows us to synchronously average the waveforms by the microscopic observation of the translocation events and improve the signal-to-noise ratio. By this method, we succeed in distinguishing single particle diameters. Additionally, the results of measured signals and the simultaneous high-speed observations are used to quantitatively and systematically discuss the effect of the complex fluid flow in the orifices on the amplitude of the resistive pulse. The synchronized resistive-pulse analysis by the optical vortex with the flow visualization improves the pulse-acquisition rate for a single specific particle and accuracy of the analysis, refining the micro- and nanoscale object identification.

Efficient microfluidic enrichment of nano-/submicroparticle in viscoelastic fluid

The enrichment and focusing of the nano-/submicroparticle (e.g., 150-1000 nm microvesicle shed from the plasma membrane) in the viscoelastic fluid has great potentials in the biomedical and clinical applications such as the disease diagnosis and the prognostic test for liquid biopsy. However, due to the small size and the resulting weak hydrodynamic force, the efficient manipulation of the nano-/submicroparticle by the passive viscoelastic microfluidic technology remains a major challenge. For instance, a typically long channel length is often required to achieve the focusing or the separation of the nano-/submicroparticle, which makes it difficult to be integrated in small chip area. In this work, a microchannel with gradually contracted cross-section and high aspect ratio (the ratio of the height to the average width of channel) is utilized to enhance the hydrodynamic force and change the force direction, eventually leading to the efficient enrichment of nano-/submicroparticles (500 and 860 nm) in a short channel length (2 cm). The influence of the flow rate, the particle size, the solid concentration, and the channel geometry on the enrichment of the nano-/submicroparticles are investigated. With simple structure, small footprint, easy operation, and good performance, the present device would be a promising platform for various lab-chip microvesicle-related biomedical research and disease diagnosis.

Opto-thermal diffusiophoresis of soft biological matter: from physical principle to molecular manipulation

Transport of ions and molecules under external field gradients is fundamental phenomena relevant to many biological systems including molecular motors in nature. As inspired from such biological transport, novel optical manipulation by using local solute gradient and the creation of self-propulsive particles are being developed using this technology. In this review article, we describe the basic principles behind those transport phenomena under a temperature and a solute concentration gradient and discuss novel manipulation tools for soft biological materials. The control of such micron-scale transport will bring new insight in design principles of functional materials showing autonomous motion as seen in molecular motors.

Scaling laws for external fluid flow induced by controlled periodic heating of a solid boundary

We demonstrate that considerable variation of mean Prandtl number (Pr_{0}) from unity brings in an additional length scale (called the viscous penetration depth, δ_{v}) into the dynamics of instantaneous as well as time-averaged (mean) flow induced by thermoviscous expansion along a periodically heated solid wall. We investigate the limiting cases of high and low Prandtl numbers (Pr_{0}≫1 and Pr_{0} ≪ 1) through detailed order-of-magnitude analysis. Our study reveals that the viscous penetration depth scales universally with Pr_{0} so long as such depth remains small compared to the wavelength of the applied thermal wave. While a high Pr_{0} is found to obstruct the mean flow, the converse is not necessarily true. Subsequent analysis clearly shows that a low-Pr_{0} flow can induce negative thermoviscous force within the thermal boundary layer and thus retard the mean motion, leading to a nontrivial reduction of net mass flow along the plate. Numerical prediction of friction factor variation with Pr_{0} agrees well with the scaling estimates for both high-Pr_{0} and low-Pr_{0} fluids. The findings may very well act as fundamental design basis for engineering devices that may potentially be developed for thermal molecular trapping and particle sorting and accumulation based on unsteady heating.