# The Detection of Water Flow in Rectangular Microchannels by Terahertz Time Domain Spectroscopy

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

_{H}= 80 μm:

_{H}is equal to 250, thus the entrance effects can be ignored [17]. The digital pressure gauges were connected to the microdevice by tubes in the experiments. All the connection tubes are 1.6 mm outer diameter and 0.6 mm inner diameter PTFE (Polytetrafluoroethylene) tubes. The working fluid was deionized water and it was introduced to the horizontal rectangular microchannel by digital injection pump. The flow velocity of the water was increased gradually at the range of 0–5.58 m/s. When setting a new flow-rate, the system ran for at least 5 min until the flow remained stable with very few fluctuations in the pressure drop readings. The density and viscosity of the water are 998 kg/m

^{3}and 1.01 mPa·s respectively.

## 3. Results and Discussion

_{sample}and E

_{reference}refers to the THz amplitude of sample and reference respectively [24].

_{P}changing along with Re. Therefore, the experimental results were further analyzed in the next section.

_{O}can be computed numerically as the following equation given by Hartnett and Kostic [27]:

_{exp}= 0.1816 × Re

^{0.16235}) is observed here, which is caused by the turbulent flow.

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Abbreviations

THz-TDS | Terahertz time domain spectroscopy |

Re | Reynolds number |

microPIV | Microscopic particle image velocimetry |

PDMS | Polydimethylsiloxane |

DOCT | Doppler optical coherence tomography |

PTFE | Polytetrafluoroethylene |

FFT | Fast Fourier Transform |

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**Figure 1.**Sketch of the experiment arrangement for detection of deionized water flowing in microchannel with transmission THz-TDS and setup of transmission THz-TDS systems (inset).

**Figure 2.**(

**a**) Time domain trace of the THz pulse through the flow at different flow-rates and the partial enlarged detail (inset); (

**b**) The absorbance curves against frequency for several flow rates; (

**c**) Output THz peak trough intensities versus Re for single water flow.

**Figure 4.**The schematic diagram for the interaction between THz wave and the flow. (

**a**) The sketch map of laminar and transition; (

**b**) Interaction between THz wave and the laminar and transition flow.

**Figure 5.**(

**a**) The curve for the pressure gradient as a function of Re; (

**b**) The flow resistance versus Re for the single flow in microchannel.

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Song, Y.; Zhao, K.; Zuo, J.; Wang, C.; Li, Y.; Miao, X.; Zhao, X.
The Detection of Water Flow in Rectangular Microchannels by Terahertz Time Domain Spectroscopy. *Sensors* **2017**, *17*, 2330.
https://doi.org/10.3390/s17102330

**AMA Style**

Song Y, Zhao K, Zuo J, Wang C, Li Y, Miao X, Zhao X.
The Detection of Water Flow in Rectangular Microchannels by Terahertz Time Domain Spectroscopy. *Sensors*. 2017; 17(10):2330.
https://doi.org/10.3390/s17102330

**Chicago/Turabian Style**

Song, Yan, Kun Zhao, Jian Zuo, Cuicui Wang, Yizhang Li, Xinyang Miao, and Xiaojing Zhao.
2017. "The Detection of Water Flow in Rectangular Microchannels by Terahertz Time Domain Spectroscopy" *Sensors* 17, no. 10: 2330.
https://doi.org/10.3390/s17102330