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Abstract
A novel flow sensor based on dynamic fluid pressure changing in a packaged microbubble resonator without additional modification on its structure has been proposed and experimentally demonstrated. The results of sensing performance under both tunable laser source and broadband light source are presented. The flow rate sensitivity can reach up to 0.0196 pm / (µL/min). The fluid pressure variation caused by Bernoulli Effect is also analyzed theoretically.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the rapid development of microfluidic system in the past few decades, there have been a large number of applications based on microfluidic, such as chemical analysis, biological sensing, particle screening, and drug delivery, etc. These applications conversely contribute to the development of microfluidic technology. In this area, the liquid flow rate plays a significant role in the research of the microfluidic chips. Therefore, numerous flow rate sensors [1–13] have been developed based on different mechanisms. As reported in [14], nine types of sensing principles for flow rate have been given. Actually, more attentions are focusing on the two following types of detection schemes. One depends on drag-force [1–5] induced by the pressure difference during the fluidic flowing. The other one is based on the heat transfer [6–11] for the fluidic flow. For the first type of flow rate sensors, one needs to fabricate a beam, cantilever or include other pressure sensitive nanomaterials to detect the flow rate of the fluid. While for the second one, they usually need to work with high power light sources. However, the pressure of the fluid changes when the velocity of the fluid changes in the channel, which is known as the Bernoulli Effect. Therefore, a pressure sensor can be used to detect the flow rate of fluid. Such sensor needs neither a beam or its counterpart to sense the pressure change nor a high optical power light source.
Microbubble resonators (MBRs), as a type of high Q whispering gallery mode (WGM) microcavities, have attracted increasingly attentions for applications in nonlinear optics [15–19], optomechanics [20–25] and high sensitivity optical sensors [26–29]. A large number of sensors have been proposed and demonstrated, such as biosensors [26,27], liquid sound speed sensors [20], fluidic viscometers [24], surface mass sensors [23], stress sensors [10,25], flowing particles sensors [28] and pressure sensors [29–31]. Due to its unique hollow channel, tuning whispering gallery mode or sensing internal aerostatic pressure in MBRs or capillaries have been reported in many previous works [29–34]. All these works indicate that the MBRs exhibit a good performance of pressure sensing. Similarly, pressure sensing of liquid can be realized by using this type of cavity. According to the Bernoulli Effect of fluid, the pressure of the internal fluid is related to the flow rate. Hence, it is possible to use an MBR for flow rate sensing without any modification on the MBR structure itself. In this work, we report a novel type of flow rate sensor based on the packaged MBR (PMBR). The packaging process can not only ensure robustness of the MBR sensor but also greatly reduce the vibration noise of the external environment [27,35,36]. More importantly, it can stabilize coupling conditions and achieve sweep mode spectrum repeatedly. Therefore, the PMBR is the most promising microbubble cavity sensor to be implemented for practical applications. Here we realize a flow rate sensor by a PMBR using both tunable laser and broadband light source at a flow rate ranges from 10 µL/min to 200 µL/min. The theoretical analysis is also carried out by using COMSOL Multiphysics simulation software, and the simulation results agree well with our experimental results.
2. Principle of the flow rate sensors
As described in the Bernoulli Effect, the higher the velocity of the fluid, the lower the pressure in the fluid. It indicates that the internal pressure in the fluid is changed with the flow rate of the fluid. At the same time, the internal pressure of the fluid can be detected by the MBR, same as other aerostatic pressure sensors [29–32]. The pressure changes not only the radius of the cavity, but also the refractive index (RI) of the silica under the Elastic-Optic effect [31,33]. Hence, the resonant wavelength (λ) of the MBR will be shifted as
where, R is the outer radius of MBR and n is the RI of the MBR mode. In addition, the two terms on the right-hand side of Eq. (1) can be further expressed asFrom the above equations, when the external pressure is with a standard atmosphere, the internal pressure sensitivity of the MBR is
The pressure of the fluid in the micro-channel with different flow rates can be theoretical studied through the method of finite element analysis with COMSOL Multiphysics. The laminar flow interface is employed to study the pressure of the fluid with a velocity. As indicated by COMSOL Multiphysics, the governing equation for steady state study of the laminar flow is based on the Navier-Stokes equations,
3. Sample fabricated
The PMBRs are prepared using a fabricated MBR and a tapered fiber. The processing of this kind of PMBRs has been detailed in our previous work [35]. The MBRs are fabricated by fuse-and-blow technique with a fused silica capillary (Polymicro TSP075150) [15,36,37]. By flame taper method, tapered fibers with diameter around 2 µm can be obtained conveniently. With assistance of two 3-dimentional mechanical alignment stages, the coupled position of MBR and the tapered fiber are adjusted precisely. A charge-coupled device (CCD) camera is implemented to observe the entire adjustment process and the transmission spectrum of MBR is monitored by an oscilloscope. After position adjusting, the MY133 glue (MY Polymers, Israel) is glued in the coupling area. When the coupling area is fully covered, we can cure it with ultraviolet light at around 365 nm. Finally, as the MY133 glue need to be solidified with oxygen isolation, a cover glass slide is covered on the glue. Then the PMBR is removed from the alignment stages and is then attached onto a proper size glass dish.
The schematic and pictures of the PMBR are shown in Figs. 1(a) and 1(b) respectively. A one-yuan RMB coin is placed next to the sample for reference. The PMBR is robust after packaging as we have fabricated and characterized it in Shanghai and then carried out the sensing performance in Shenzhen with high stability. An empty PMBR and a PMBR filled with water photographs are shown in Figs. 1(c) and 1(d) respectively. When the MBR is filled with water, since the refractive index of the water and MY133 glue is nearly the same, there is no big contrast at the boundary of the two mediums as illustrated in Fig. 1(d). The outer diameter of the MBR sample is 210 µm and the wall thickness is around 3.5 µm. The outer diameter of the capillary is 128 µm and the wall thickness is around 4.5 µm.
Fig. 1. (a) The schematic of the PMBR and (b) Photographs of a packaged MBR with a one-yuan coin. The dotted square area indicates the location of the MBR. (c) An empty PMBR and (d) a PMBR filled with water.
4. Experimental setup and results
The optofluidic flow sensor is based on the PMBR without any other additional modifications on its own structure. The experimental setup to verify our proposed optofluidic flow sensor is illustrated in Fig. 2. Both a tunable laser source (TSL710, Santec) and an amplified spontaneous emission (ASE) source (ALS-CL-15-B-FA, Amonics) have been used for our sensing investigation. The tunable laser is connected to a PMBR by optical fibers. An optical fiber-based polarization controller is implemented to adjust the polarization of the light from the tunable laser. The output signal from the PMBR is detected by a low noise photoelectric detector and its transmission spectrum is observed through an oscilloscope. A signal generator generates signal to drive the tunable laser to sweep frequency and synchronizes with the oscilloscope, as shown in Fig. 2(a). On the other hand, an optical spectral analyzer (OSA, AQ6370D, Yokogawa) is used to monitor the transmission spectrum from the broadband light source after passing through the PMBR as illustrated in Fig. 2(b). This setup is relatively simple in optical path if compared to the previous one. For both cases, the fluid is injected into the PMBR by a syringe pump. The pump has a minimum controlled flow rate of 0.834 uL/min with a standard 10 mL syringe. In the experiment, all the flow rate settings are achieved with a standard 10 mL syringe and the syringe pump.
A WGM at 1552.985 nm with Q value around 1.8×105 for the PMBR is served as the sensing mode for the flow rate sensing, as shown in the inset of Fig. 3. The laser power is set at a relatively low power level of 0.2 mW to avoid thermal effects. We chose a shorter wavelength mode of the WGMs for sensing in our experiment. Deionized water is pumped into the PMBR as working liquid for detection. Before the syringe pump is set at a specified flow rate, the transmission spectrum is first recorded at the “pump off” state. After the pump is turned on for 90 seconds, the transmission spectrum is then recorded as the “pump on” state. By comparing the position of resonance peak of the transmission spectrum, we can observe a blue-shift phenomenon. For example, the wavelength is blue-shifted by 3.6 pm at a flow rate of 200 µL/min. It agrees well with the theoretical prediction that the flow rate will induce a decreasing of the internal pressure in the PMBR and the resonant wavelength of the cavity has a blue shift. The flow rate variation is adjusted from 10 µL/min to 200 µL/min. There is a linear relationship between resonant wavelength shift and the flow rate. The sensitivity of our proposed PMBR-based optofluidic flow rate sensor is measured to be 0.0196 pm/(µL/min). The correlation coefficient R2 of the linear result is 0.9797.
Fig. 3. Sensing performance of the PMBR flow rate sensor with the tunable laser. The insets show the transmission spectra at “pump off” and “pump on” state with a flow rate at 10 µL/min (upper left) and 200 µL/min (lower right) respectively.
We have repeated the sensing experiment by setting the laser power at 5 mW. The sensitivity is 0.0197 pm/(µL/min) which is close to the results in Fig. 3. This experimental result also indicates that the resonant wavelength shift of this sensor is independent of thermal effects at this pumped power. However, the flow rate has obvious influence on the shift resonant wavelength for the optofluidic MBR in the sensing experiment. It shows that unsteady flow rate will cause the shift of resonant wavelength during the optofluidic sensor experiment. Especially in a biosensor, different flow rates will bring a change of the inner pressure of the fluid. Then, the adsorption capacity of the surface of MBR will change, which will cause desorption effect [38].
For practical applications, the sensing scheme of broadband light source will be much simpler and cheaper. Therefore, the experimental results using an ASE source as the light source has also been shown in Fig. 4. The spectrum of the ASE source with 4 mW output power is shown in the inset of Fig. 4(a). It must be noted that the resolution of the spectrometer is 0.02 nm with a sampling interval set to be a minimum value of 0.001 nm. From the transmission spectrum after passing through the PMBR, we can observe a clearly free spectral range (FSR) of 2.4 nm, which matches the size of MBR with an outer diameter of 210 µm. After the deionized water is pumped into the PMBR, the transmission spectrum also shows a blue-shift when the pump state changes from “off” to “on”. A mode at around 1550.32 nm is taken as reference dip for the flow rate sensing, as shown in Fig. 4(b). Before the sensing experiment, we have tested the noise of this mode every 10 seconds during 110 seconds, which is 0.5 pm. Therefore, the resonant wavelength shift of the sensor in Fig. 4 is with great stability. The sensitivity is 0.0158 pm/(µL/min) from 10 µL/min to 200 µL/min. The correlation coefficient R2 of the linear result is as high as 0.9929. The sensitivities in the Figs. 3 and 4 are roughly on the same order of magnitude but with different value, which can be explained by Eq. (4). The order of the mode and the resonant wavelength of the mode will affect the sensitivity.
Fig. 4. (a) The transmission spectrum of the MBR with the ASE source. The FSR of the MBR is 2.4 nm. Insets show the MBR outer diameter is 210 µm and the ASE spectrum. (b) Sensing performance of the PMBR flow rate sensor with the ASE source. The insets show the transmission spectra at “pump off” and “pump on” state with a flow rate at 10 µL/min (upper left) and 200 µL/min (lower right)
5. Theoretical analysis
Fluid in a channel actually presents different states following the Reynolds number, which is defined as Re=ρuL/µ, where ρ and µ are the density (unit, kg/m3) and dynamic viscosity (unit, Pa·s) of the fluid. L is the characteristic linear dimension (unit, m). For a circle channel flow, the L is the diameter of the channel. u is the velocity of the fluid (unit, m/s). When Re < 2300, the flow exhibits a laminar flow property. Here in the PMBR, Re is 18.78, far away from 2300 for the water. Hence, in our simulation with COMSOL Multiphysics, the laminar flow interface is employed. The shape of the fluid is following the size of the PMBR, as shown in Fig. 5(a). D and d are the inner diameters of MBR and the capillary, respectively. L is the entire length of the capillary. In our simulation, the parameters are all set to be the measured size of the PMBR sample where D = 203 µm and d increased from 116 µm to 120 µm. The length of our capillary tube is approximately 4 cm. For the simulation, when d is 119 µm, the linear fitting slope is 0.0193 pm/(µL/min), which is consistent with the experimental results.
Fig. 5. (a) The simulation model of fluid in the PMBR. (b) The velocity distribution of the fluid in r-Z plane at 200 µL/min. The color bar in red is for high speed and in green is for low speed (unit, m/s). (c) The pressure distribution of the fluid in the equatorial plane at 200 µL/min. The color coordinate from red to blue is from -10000 Pa to -11000 Pa.
For the simulation, the fluid is selected to be water. The velocity field distribution and pressure distribution of the fluid obtained by simulation are shown as in Figs. 5(b) and 5(c). The velocity distribution in r-Z plane has been shown at 200 µL/min. The maximum velocity existed in the middle of the capillary area is around 0.6 m/s. While the minimum velocity existed near the wall, the velocity is zero. The results are consistent with the velocity field distribution of laminar flow. For the tapered fiber and microcavity are coupled at the center of the MBR, the resonant wavelength of MBR is mostly affected by the pressure in the central region. Here the pressure from the equatorial plane of the MBR has been studied when the flow rate is changed. As shown in Fig. 5(c), the pressure distribution of the fluid in the equatorial plane is shown. The pressure in the equatorial plane is highly uniform and the value is -10940 Pa. This pressure induces the resonant wavelength shift. Therefore, the pressure at the corresponding flow rate can be calculated according to Eq. (1). In Fig. 6, we show that the resonant wavelength shift of the fundamental mode varies with the flow rate for the different d of the MBR. When the d is 119 µm, the resonant wavelength shift and the sensitivity are consistent with the experimental results in Fig. 3.
Fig. 6. Simulation results of resonant wavelength variation caused by flow rate of different d. The linear fitting slopes are listed in order in the figure.
Following the value of the pressure in simulation and the resonant wavelength shift in experiment, we can further derive the pressure sensitivity to be 4.11 GHz/bar at 1550 nm for the sensing experiment in Fig. 3. Actually, the sensitivity of different microbubble sizes with different wall thickness have been demonstrated in our previous work [31]. In addition, the sensitivity can be improved by fabricating a thinner MBR as mentioned in [29] and choose a more sensitive mode. For example, the pressure sensitivity can be achieved by an order of magnitude higher as 19 GHz/bar at 1550 nm and 38 GHz/bar at the 780 nm. This will improve the sensitivity of the flow rate sensor in turn, and benefit various applications in practical. Combined with the method mentioned in [10], this type of flow rate sensor can achieve a larger sensing dynamic range.
6. Conclusions
We have fabricated a PMBR sample which is robust and compact. A PMBR flow rate sensor is further proposed based on the Bernoulli Effect of the fluid. The flow rate sensitivity can reach to 0.0196 pm/(µL/min) with the tunable laser operates at 0.2 mW and 0.0158 pm/(µL/min) when the ASE source operates at 4 mW. Theoretical investigation of the pressure changed induced by the flow rate changing has also been carried out. The simulation results are in good agreement with our experimental results. The flow rate sensing results also reminds us that it is important to maintain a constant flow rate in the optofluidic sensing experiments.
Funding
Shenzhen Science and Technology Innovation Commission (KQJSCX20170727163424873, JCYJ20170818094001391, JCYJ20180507183815699); Tsinghua-Berkeley Shenzhen Institute (TBSI) Faculty Start-up FundShenzhen Data Science and Information Technology Engineering LaboratoryNational Science and Technology Infrastructure Program (2016YFC0201401).
Disclosures
The authors declare no conflicts of interest.
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