Zhengdong Wang

Measurement of temperature-dependent diffusion coefficients using a confocal Raman microscope with microfluidic chips considering laser-induced heating effect.

Conventional methods for measuring diffusion coefficients (D) are complex and time consuming. This study presents a method for the continuous measurement of temperature-dependent diffusion coefficients using a confocal Raman microscope with microfluidic chips. Concentration information was collected by a Raman microscope to extract D values. An isothermal diffusion process at various temperatures was ensured by coupling the silicon-based microfluidic chip with an isothermal plate. In the simple silicon/glass chip, the heating effect induced by a Raman laser was observed to contribute to abnormally high D values. To eliminate the heating effect, a 200nm-thick aluminum (Al) reflection film was used to coat the channel bottom. The Al film substantially reduced absorption of laser power, thus ensuring precise D values in excellent agreement with literature data. Other potential methods to eliminate the heating effect were also evaluated by computational fluid dynamics (CFD) simulations and were found impractical for implementation. Consequently, this method for the continuous measurement of temperature-dependent diffusion coefficients is proven to be accurate, efficient, and reliable.

Effect of Laser-Induced Heating on Raman Measurement within a Silicon Microfluidic Channel

When Raman microscopy is adopted to detect the chemical and biological processes in the silicon microfluidic channel, the laser-induced heating effect will cause a temperature rise in the sample liquid. This undesired temperature rise will mislead the Raman measurement during the temperature-influencing processes. In this paper, computational fluid dynamics simulations were conducted to evaluate the maximum local temperature-rise (MLT). Through the orthogonal analysis, the sensitivity of potential influencing parameters to the MLT was determined. In addition, it was found from transient simulations that it is reasonable to assume the actual measurement to be steady-state. Simulation results were qualitatively validated by experimental data from the Raman measurement of diffusion, a temperature-dependent process. A correlation was proposed for the first time to estimate the MLT. Simple in form and convenient for calculation, this correlation can be efficiently applied to Raman measurement in a silicon microfluidic channel.

Electron and ion kinetics in three-dimensional confined microwave-induced microplasmas at low gas pressures

The effects of the gas pressure (pg), microcavity height (t), Au vapor addition, and microwave frequency on the properties of three-dimensional confined microwave-induced microplasmas were discussed in light of simulation results of a glow microdischarge in a three-dimensional microcavity (diameter dh = 1000 μm) driven at constant voltage loading on the drive electrode (Vrf) of 180 V. The simulation was performed using the PIC/MCC method, whose results were experimentally verified. In all the cases we investigated in this study, the microplasmas were in the γ-mode. When pg increased, the maximum electron (ne) or ion density (nAr+) distributions turned narrow and close to the discharge gap due to the decrease in the mean free path of the secondary electron emission (SEE) electrons (λSEE-e). The peak ne and nAr+ were not a monotonic function of pg, resulting from the two conflicting effects of pg on ne and nAr+. The impact of ions on the electrode was enhanced when pg increased. This was determined after comparing the results of ion energy distribution function (IEDFs) at various pg. The effects of t on the peaks and distributions of ne and nAr+ were negligible in the range of t from 1.0 to 3.0 mm. The minimum t of 0.6 mm for a steady glow discharge was predicted for pg of 800 Pa and Vrf of 180 V. The Au vapor addition increased the peaks of ne and nAr+, due to the lower ionization voltage of Au atom. The acceleration of ions in the sheaths was intensified with the addition of Au vapor because of the increased potential difference in the sheath at the drive electrode.

Operational Limitation and Instability of a Microwave-Induced Microplasma Enclosed in a Microcavity at Low Gas Pressures

In this paper, operational limitations (the extinguishing of the stable glow discharge) and instabilities [glow-to-arc transition (GAT)] of microwave-induced microplasmas enclosed in microcavities operated at low gas pressures were investigated by experiments, in comparison with unenclosed microplasmas. For enclosed microplasmas, when gas pressure decreased, GAT occasionally occurred, whereas GAT was never detected for unenclosed microplasmas, because the gas temperatures of enclosed microplasmas were higher than those of unenclosed ones. For enclosed microplasmas operated at low gas pressures, an increase in the microcavity dimension is a valid method to avoid GAT. Extinguishing pressure of stable glow-discharge microplasma (pext) for microwave-induced microplasma enclosed in a microcavity microplasma was lower than that for microwave-induced microplasma generator without the PDMS cavity (UEC) microplasmas. The increase in input power decreased pext for UEC microplasmas but showed a slight influence on pext for EC microplasmas. This paper shed some light on understanding of the enclosed microplasmas operated at low gas pressures.

The glass-silicon-glass sandwich structured microplasma chip as the electron source of a micro mass spectrometer

Micro mass spectrometer is one of the most powerful porta-ble analytical instruments. And the micro electron impact (EI) ionization source, which has crucial effects on micro mass spectrometers performance, has been a research focus. In this study, a microplasma source as the electron source of the EI ionization source was developed to solve the difficulty in miniaturization on filament structure for conventional elec-tron source. The microplasma source was fabricated as a three wafer glass-silicon-glass sandwich with all the struc-tures realized in a highly doped silicon wafer via a deep reac-tive ion etch (DRIE) process. The microplasma source is 33 mm wide, 44 mm long, and 1.3 mm high.