Separation of Nano- and Microparticle Flows Using Thermophoresis in Branched Microfluidic Channels

Abstract

Particle flow separation is a useful technique in lab-on-a-chip applications to selectively transport dispersed phases to a desired branch in microfluidic devices. The present study aims to demonstrate both nano- and microparticle flow separation using microscale thermophoresis at a Y-shaped branch in microfluidic channels. Microscale thermophoresis is the transport of tiny particles induced by a temperature gradient in fluids where the temperature variation is localized in the region of micrometer order. Localized temperature increases near the branch are achieved using the Joule heat from a thin-film micro electrode embedded in the bottom wall of the microfluidic channel. The inlet flow of the particle dispersion is divided into two outlet flows which are controlled to possess the same flow rates at the symmetric branches. The particle flow into one of the outlets is blocked by microscale thermophoresis since the particles are repelled from the hot region in the experimental conditions used here. As a result, only the solvent at one of outlets and the residual particle dispersion at the other outlet are obtained, i.e., the separation of particles flows is achieved. A simple model to explain the dynamic behavior of the nanoparticle distribution near the electrode is proposed, and a qualitative agreement with the experimental results is obtained. The proposed method can be easily combined with standard microfluidic devices and is expected to facilitate the development of novel particle separation and filtration technologies.

Keywords: micro-electro-mechanical-systems (MEMS) technologies; microfluidic channels; microscale thermophoresis; multiphase flow; nano/microparticle separation.

Figure 1

(a) Schematic of the test section. The branched microfluidic channel has a rectangular cross-section in the $yz$ plane with a height $h=17.2$ $\mathsf{\mu }$m and a width $w=450$ $\mathsf{\mu }$m. The inlet flow is divided into two outlet flows $\alpha$ and $\beta$. A thin-film electrode heater is fabricated at the entrance of the outlet flow $\alpha$. Flow profiles of the inlet and outlets are schematically drawn based on the analytical solution of the Poiseuille flow in a rectangular channel [58]. (b) Overview of the experimental setup. EF: emission filter. AF: absorption filter. DM: dichroic mirror. OL: objective lens. PC: personal computer.

Figure 2

Flow field at the test section without Joule heating for $\Delta P=1\times {10}^{-2}$ Pa. The inlet flow is equally separated into two outlet flows. (a) Experimental result obtained by the particle image velocimetry (PIV) analysis. (b) Numerical result at $z=h/2$ obtained by the simulation using a finite element method.

Figure 3

(a–g) Time series of the particle flow separation induced by microscale thermophoresis for the case with a particle diameter $d=1$ $\mathsf{\mu }$m and $\Delta P=1.0$ Pa. At $t=0$ s, the heating by electrode is initiated. Particle flow from the inlet is separated at the Y-shaped branch. Because the thermophoresis is directed to the colder region, the PS particles cannot enter the outlet $\alpha$.

Figure 4

Time-development for fluorescence intensity of microparticles in the regions A, B, C, and D indicated in Figure 3a. The pressure difference $\Delta P$ is set to 1.0 Pa.

Figure 5

(a) Schematic figure of the test section and the positions of region A, B, C, and D analyzed in Figure 6; (b–g) Time series of the nanoparticle fluorescence. The particle flow separation is induced by microscale thermophoresis for the case with a particle diameter $d=100$ nm and $\Delta P=1.0$ Pa. At $t=0$ s, the heating by the electrode is initiated. Particle flow from the inlet is separated at the Y-shaped branch. Because the thermophoresis is directed to the colder region, the PS particles cannot enter the outlet $\alpha$. (h) Magnified figures of (b–g) for a rectangular region indicated in (b).

Figure 6

Time-development for fluorescence intensity of nanoparticles in the regions A, B, C, and D indicated in Figure 5a. The pressure difference $\Delta P$ is set to 1.0 Pa.

Figure 7

(a) Schematic figure of the test section and the positions of region A, B, C, and D analyzed in Figure 8; (b–h) Time series of the nanoparticle fluorescence. The particle flow separation is induced by microscale thermophoresis for the case with a particle diameter $d=100$ nm and $\Delta P=0.5$ Pa. At $t=0$ s, the heating by electrode is initiated. Particle flow from the inlet is separated at the Y-shaped branch. Because the thermophoresis is directed to the colder region, the PS particles hardly enter the outlet $\alpha$.

Figure 8

Time-development for fluorescence intensity of nanoparticles in regions A, B, C, and D indicated in Figure 7a. The pressure difference $\Delta P$ is set to 0.5 Pa.

Figure 9

(a) Schematic of the numerical model on the concentration increase at the branch. (b) Numerical results regarding the time-development for fluorescence intensity of nanoparticles in the regions A ($Y\approx 0$ $\mathsf{\mu }$m), B ($Y\approx -72$ $\mathsf{\mu }$m), C ($Y\approx -144$ $\mathsf{\mu }$m), and D ($Y\approx -198$ $\mathsf{\mu }$m), shown in panel (a) and corresponding to Figure 6, where $Y=0$ $\mathsf{\mu }$m is placed at the center of the outlet $\alpha$ in the Y direction.