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Abstract
The measurement of airflow velocity is crucial in various fields, and several sensing approaches have been developed for detecting airflow, including optical fiber-based flowmeters. However, these sensors often require complex fabrication processes and precise optical alignment. In this paper, a simpler and more cost-effective approach has been used to measure air flow rate by utilizing the birefringence property of liquid crystals (LCs). LCs possess distinct optical characteristics, and their reorientation due to airflow can be detected by observing the intensity of the output light between crossed polarizers. The novelty of this study is the utilization of a textile grid to hold the LC layer, which simplifies the fabrication process. This LC-based gas flowmeter offers a simple, low-cost setup and provides rapid performance. This research presents what we believe to be a new approach to calculate airflow by exploiting the optical properties of LCs, which is a new frontier in gas flow measurement. The proposed airflow meter is capable of detecting airflow rates ranging from 0 l/min to 7.5 l/min with an accuracy of 0.5 l/min. It exhibits a stable response time in 75 seconds, and the sensor maintains acceptable stability over time.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The measurement of airflow velocity is crucial in various fields, including industrial environment monitoring, process control, navigation control, biomedical engineering, and aerodynamics [1–3]. Several sensing approaches have been developed for detecting airflow, utilizing principles such as piezoresistance [4], mechanoluminescence [5], electrical resistance [6], etc. The majority of reported airflow measurement systems depend on a thermal or mechanical change in a component of the system. On the other hand, the system's output may take the form of an electrical signal, optical light, mechanical movement, etc. In recent years, the combination of optical fiber-based airflow meters with traditional measurement techniques has garnered a lot of attention. These new flowmeters are light weight, highly sensitive, and fast [7]. To further improve the sensitivity of optical flowmeters, new structures has been utilized such as fiber Bragg gratings [8], long-period gratings [9], Fabry–Pérot interferometers [10], etc. It is worth noting that most of the reported optical fiber-based sensor systems involve complex fabrication processes and require precise optical alignment [11].
Liquid crystal (LC) is a state of matter that exists between a liquid and crystalline solid within a specific temperature range. Materials in this phase exhibit intriguing optical properties, such as birefringence. Due to their distinct optical characteristics, LCs have found widespread use in numerous optical devices [12–14]. Several types of flow meters based on LCs have been reported in the literature [15]. One example is the work by Kottapalli et al., who employed LC polymer (LCP) as a structural material [16]. The sensor in this study featured an LCP membrane with integrated thin film gold piezoresistors deposited on it as its primary components. LCP material exhibits a significantly higher sensitivity than silicon as a structural material for airflow sensing due to its much lower Young's modulus. This heightened sensitivity does not significantly compromise the sensor response. Although these sensors offer distinct advantages such as large operating range (0.1 to > 10 ms−1) and good sensitivity (3.695 mV (ms−1)−1), the manufacturing process can be quite challenging due to the complexity of the required devices, particularly the deposition of LC. Furthermore, the manufacturing process demands the expertise of a skilled operator. Another LC-based airflow sensor is featured in the research conducted by Adam A. S. Green et al. [17]. The researchers presented a non-invasive particle tracking velocimetry (PTV) technique for measuring air flow using a free film of suspended LC. They produced smectic films with a racetrack geometry that were freely suspended. The smectic LC, which was at room temperature, could be drawn into molecularly thin films that were suspended in the air. Initially, the microscopic image of the LC appeared dark. However, when the airflow passed through the channel, islands with a thickness greater than that of the LC film began to form. As the reflectance of thin, freely suspended films depends quadratically on the thickness [18], the island images were brighter than the background film. They could be easily visualized using video microscopy and the velocity of their movement was linearly coupled to the velocity of the airflow. Due to the designed structure, the air velocity varies at different heights within the channel. As a result, the average velocity of the air and the film may be slightly different. This sensor is designed with a simple, low-cost setup, and provides high-speed performance.
LCs possess phenyl molecular rings, which classify them as polar materials. Consequently, they tend to exhibit a planar orientation when in the vicinity of other polar materials, such as water. Conversely, when near non-polar materials like air, they are expected to assume a homeotropic arrangement [19]. Moreover, the reorientation of LCs can be detected by observing the intensity of the output light between crossed polarizers. Equation (1) expresses the intensity of light passing through an LC cell that is positioned between crossed polarizers. The variables in this equation include $\phi $, which denotes the azimuthal angle, d, which represents the thickness of the LC layer, $\lambda $, which represents the wavelength of the light, and $\mathrm{\Delta }{n_{eff}}$, which represents the effective birefringence of LC material. The effective birefringence is determined by calculating the difference between the ordinary refractive index (${n_o}$) and the effective refractive index [20,21].
Equation (2) demonstrates that the effective refractive index is dependent on the LC material and the inclination angle (θ). In this equation, ${n_e} $ represents the extraordinary refractive index of the LC material. Therefore, when air flows through the flow meter at different rates, the LC molecules undergo reorientation leading to changes in the output light intensity. As depicted in Fig. 1(a), in the initial state, with all LC molecules vertically oriented, the inclination angle is zero. This implies, according to Eq. (2), ${n_{eff}}$ equals ${n_o}$, resulting in $\mathrm{\Delta }{n_{eff}} = 0$. Considering this, the output light intensity is zero. As the flow rate increases (Fig. 1(b)), it induces a vertical orientation of LC molecules, leading to an increase in $\mathrm{\Delta }{n_{eff}}$. Consequently, the output light intensity rises with an increase in the flow rate, due to the sinusoidal relationship between that and $\mathrm{\Delta }{n_{eff}}$.
Fig. 1. Schematic depiction of LC molecule orientation within a single-pixel textile grid. (a) In the initial state, all LC molecules are vertically oriented. (b) As the flow rate increases, LC molecules undergo vertical orientation. Notably, at the grid boundaries, LC molecules exhibit a slight polar angle due to anchoring forces. (c) Actual image of the textile grid affixed atop a plastic substrate.
In this study, a simpler and more cost-effective approach has been used to measure the air flow rate. Notably, for the first time in this field, a textile grid is utilized to hold the LC layer [22]. LCs possess distinct optical characteristics, and their reorientation due to airflow can be detected by observing the intensity of the output light between crossed polarizers. The novelty of this study is the utilization of a textile grid to hold the LC layer, which simplifies the fabrication process. This research presents a new approach to calculate airflow by exploiting the optical properties of LCs, which is a new frontier in gas flow measurement. The proposed airflow meter is capable of detecting a range of airflow rates from 0 to 7.5 l/min, providing an accuracy of 0.5 l/min. It demonstrates complete reversibility, exhibits commendable stability, and offers a sensitivity resembling a linear response.
2. Experimental section
A. Materials
kit is designed as a gas box, with the following dimensions: 8 × 5.6 × 3 mm3 for the outer dimensions, and 6.5 × 5 × 2.4 mm3 for the internal dimensions. The polyester textile grids consist of pixels that are measured to be 300µm x 300µm x 40µm in size. The selection of the textile type was made based on factors such as cost-effectiveness and grid size optimization. Various grid sizes were experimented with, and it was observed that dimensions smaller than the chosen size did not permit effective reorientation of LC molecules due to the presence of strong anchoring forces at the boundaries. Conversely, larger grid holes were found unsuitable, as LC molecules did not adhere effectively. Therefore, the chosen textile type and grid size serve as the most practical options for mass production at this stage. To prepare the textiles for LC placement, they are first cleaned with a detergent solution, then washed with distilled water, and finally dried with high-pressure air. For air flow measurement, E7 was used as LC material in this study, along with a gas pump, regulator, and flowmeter (LZB, China). The LC material is meticulously placed onto the grid with precision using micropipettes. It is important to emphasize that air flowmeter performance can be influenced by factors like temperature, humidity, and other environmental conditions, in a manner similar to many sensors with specific operating requirements. Moreover, the choice of LC material and grid thickness can exert unique effects on the flowmeter's performance, necessitating calibration for each specific application.
B. Measurements
To control the amount and relative velocity of airflow, a plexiglass gas box and a regulator were used. Polarized optical microscope (POM) (Leitz, ANA-006, Germany) images and videos of the mesh textile within the box were recorded during and after airflow injection, using a CCD camera (Samwon, STCTC83USB, Korea) and evaluated for brightness using ImageJ. The LC molecules were confined within a textile with square holes resembling grids. The textile grid is tautly stretched from all sides and affixed to a plastic substrate to ensure a smooth surface without any irregularities. This substrate features a central aperture through which the textile grid's center is positioned, allowing for air contact from both sides (Fig. 1(c)). After fixing the mesh textile onto a relevant layer and placing it inside the box, the air pump was turned on, and the airflow was regulated and measured by a flowmeter. The reorientation of the LC molecules due to changes in airflow intensity was observed and recorded by the CCD camera under the microscope, as shown in Fig. 2.
3. Results and discussion
Figure 3 exhibits the initial outcomes attained using the proposed setup. Initially, the LC molecules’ homeotropic alignment results in a dark image. However, when the airflow enters the gas box, it causes a change in the LC molecules’ orientation, leading to an optical response visible even to the naked eye. The bright region observed at the boundaries of the textile grid is attributed to the orientation of LC molecules. As earlier demonstrated in Fig. 1, LC molecules at the boundaries tend to assume a slight polar angle due to the influence of anchoring forces. This orientation results in the bright frame at all grid boundaries. Conversely, the center of the grids displays darker regions where all LC molecules are oriented vertically due to their proximity to the air. Furthermore, an increase in the airflow and subsequent reorientation of the LC molecules alter the transmitted light's intensity. This change allows light to pass through the second polarizer, resulting in a bright image at the microscope's output. Modifying the airflow rate from 4 l/min to 7.5 l/min induces alterations in the light intensity. Additionally, based on the results obtained, it is evident that this process is reversible. It's important to note that fringes at 7.5 l/min result from phase shifts in light passing through the LC medium, varying with different flow conditions. The consistent thickness of the LC medium across all conditions allows us to confidently associate the observed fringes with changes in birefringence.
Fig. 3. The POM images of LC (E7) in textiles, along with the changes in light intensity caused by different airflows, are presented. Furthermore, the reversibility of the sensor is demonstrated. The black crossed arrows represent the directions of the polarizer (P) and the analyzer (A) optical axes, while the blue arrows indicate the airflow direction from right to left.
By utilizing IMAGEJ software, it is possible to measure changes in output light intensity based on various input air flows. The images captured by the camera are transferred to the software, which calculates the light intensity of each pixel in the image. Figure 4(a) shows that each specific airflow entering the gas box is associated with a specific light intensity. The horizontal axis of the graph represents the input airflow rate, while the vertical axis represents the average output light intensity of all the pixels. As shown, the airflow rate ranges from 0 l/min to 7.5 l/min with the steps of 0.5 l/min. As the airflow increases, the output light intensity also increases, indicating a direct relationship. Further increase in airflow causes significant changes in the LC molecules, resulting in saturated output light. It should be noted that this process is reversible. The black and red markers indicate an increase and decrease in airflow, respectively. Based on the results obtained, the sensor's repeatability has been verified. It is important to note that the sensing results are indeed influenced by the composition of the gas. Our calibration procedures, in accordance with standard practices for many commercial air flowmeters, are conducted with air as the reference gas. The presence of other gases (Such as CO2) can exert an influence on the alignment of LC molecules, and as a result of the differing physical properties of various gases, the reported air flow measurements may not be universally applicable or precise for gases other than air.
Fig. 4. (a) The average output light intensity of the pixels as a function of the input airflow rate. The black and red markers indicate an increase and decrease in airflow, respectively. (b) The stability of the proposed air flow meter using three distinct airflow input values.
Utilizing an exponential trendline equation, the relationship between airflow velocity and light intensity can be extracted using Eq. (3), where y represents light intensity, and x corresponds to airflow velocity.
The flowmeter responds to changes in a few hundred milliseconds, but its output stabilizes after 75 seconds. Furthermore, we evaluated the stability of the proposed air flow meter using three distinct airflow input values. As depicted in Fig. 4(b), the sensor demonstrates satisfactory stability over a duration of 200 seconds. The graph also reveals a noteworthy observation; regardless of the input values, the output light intensity experiences a slight decline within the initial 90 seconds before reaching a stable final value. This observation bears significance, particularly when employing the sensor for long-term applications. It is worth noting that the observed fluctuations primarily originate from ambient noise within our experimental setup. To alleviate the influence of these fluctuations, we can implement signal averaging over a defined time interval to achieve a more stable output.
Additionally, sensitivity analysis was conducted on the flowmeter to comprehend its responsiveness to changes in the measured airflow rate (Fig. 5). The change in light intensity for various airflow rates was examined to establish sensitivity. The resulting sensitivity values were subsequently graphed to visualize the response characteristics of the flowmeter. This graph offers a clear representation of how the output light intensity varies with different input airflow rates, providing valuable insights into its performance and suitability for applications involving airflow measurement.
4. Conclusion
In conclusion, the measurement of airflow velocity is a crucial aspect in various fields, and optical fiber-based airflow meters have shown great potential due to their lightweight, high sensitivity, and fast response. However, most of the reported optical fiber-based sensor systems involve complex fabrication processes and require precise optical alignment. LC-based airflow sensors offer an alternative approach that utilizes the unique optical properties of LC materials. LCs have been used to develop various flow meters, but their fabrication process can be quite challenging due to the complexity of the required devices. In this study, a simpler and more cost-effective approach has been used to measure the air flow rate by utilizing a textile grid to hold the LC layer. This approach demonstrates the potential for the use of LC materials in developing low-cost and easy-to-fabricate flow meters with high sensitivity. The results of this study pave the way for future research into the development of LC-based flow meters with improved performance for various applications.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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