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Abstract
The recent development of liquid-phase chemical analyses, drug delivery, and flow cytometry requires precise sensing and control of the liquid flow in a microfluidic chip environment. The channel in microfluidic chips is getting narrower to cope with complex liquid controls on a single chip, where small-footprint sensors and actuators are in urgent demand for accurate flow management. In this study, a unique microscopic bubble-on-fiber (BoF) device that can be readily integrated to current microfluidic chips was proposed and demonstrated for in situ sensing and control of microfluidic flow rate. The single microbubble was optically generated on the gold-deposited facet of an optical fiber by the local heating due to optical absorption. The BoF is a microscopic Fabry-Perot cavity, which serves as a thermal flow sensor precisely detecting the flow-induced temperature changes in the optical frequency domain. Experimentally we achieved the minimum detectable flow rate of ~0.06 mm/s in a single microfluidic channel, which is equivalent to a volume flow rate of 22 nL/s, and a response time of ~6 s. We also demonstrated that the BoF functioned as a microfluidic valve to regulate the flow rate in a Y-shape microfluidic chip by optically varying the bubble diameter. In addition to advantages of highly integrated functionalities and microscopic form factor, the proposed BoF can obviate the usage of chemical tracer such as dyes and can provide a high sensitivity over repeated flow cycles in a highly consistent manner. The BoF is promising for the timely development of high-density lab-on-a-chip devices using its efficient liquid flow management capability.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years lab-on-a-chip (LOC) technologies have drawn substantial attention in chemical, biological and medical applications due to their conspicuous benefits in contrast to bulk counterparts in terms of low liquid consumption, fast response, compact size and mass producibility [1–3]. In order to cope with the ever-increasing demands for solutions to complex flow controls within LOCs, key microfluidic components such as pumps, valves, mixers, reactors, and sensors are being integrated into a single chip [4–9]. These components are based on technological development in recent decades achieving accurate control and sensing of diverse liquid properties such as flow rate, pressure, temperature, and pH value, to name a few [10,11]. Among these properties, especially the flow rate is gaining greater importance in the liquid phase chemical analysis, drug preparation/delivery, cell sorting, and flow cytometry. Various types of mass flow sensors have been developed using thermo-resistive, thermo-electric, and mechanically resonant effects in recent years achieving impressive performances in terms of high sensitivity, short response time, and wide dynamical range [12]. However, their multi-step fabrication process, as well as the complex architecture, have been hindering microscopic integration into a single microfluidic chip. As an alternative, fiber-optic flow sensor has provided a unique advantage in the integration issue by directly embedding the fiber optic sensing heads within microfluidic channels [13–16]. For instance, an optical fiber tip vertically aligned to a top-opened microfluidic channel has enabled direct measurements of the flow-induced changes in the optical reflection at the air-water interface [13]. Another fiber-optic configuration relied on light intensity changes in a microfiber-cantilever [14]. However, these prior fiber-optic sensors have suffered from either the inherent operation instability at the air-liquid interface in an open channel or a millimeter-scale form factor requiring even larger channel widths in LOCs.
Bubbles have been widely investigated for applications in microfluidic valves, mixers, pumps, and actuators because of their unique advantages such as easy fabrication, high controllability, and mechanical flexibility [17–22]. Moreover, multifunctional bubbles have provided a solution to build compact LOC devices for complex fluidic management in bioassay, chemical analysis applications without using multiple individual components [23]. Despite numerous prior studies on bubbles, the bubbles have mostly served as an active component to control flow and lacked the important capability to sense the flow rate, which significantly limited their roles in prior LOC development.
Here, we proposed and experimentally demonstrated a new scheme to utilize a single bubble on fiber (BoF) as an efficient microfluidic sensor and a flow controller simultaneously, for the first time to the best knowledge of the authors. The single microbubble was optically generated on the facet of an optical fiber, where a gold-thin film was deposited and locally heated by the optical absorption at the fiber core. The BoF is a microscopic Fabry-Perot cavity, which serves as a thermal flow sensor precisely detecting flow-induced temperature changes in the optical frequency domain. Compared with previous flow rate sensors [24,25], the proposed BoF can obviate the usage of chemical tracer such as fluorescently tagged polystyrene particles and conductive liquids, and can provide a high sensitivity over repeated flow cycles in a highly consistent manner. We also experimentally demonstrated that the BoF could function as a microfluidic valve to regulate the flow rate in a Y-shape microfluidic chip by optically controlling the bubble diameter. In the following, we described the principle and the BoF sensor integration, and explained how we achieved both the sensing and the control of microflow successfully.
2. Principle
The proposed BoF device is schematically shown in Fig. 1, where the BoF is embedded within a microfluidic channel made of polydimethylsiloxane (PDMS). The gold (Au) thin film is deposited on the cleaved facet of conventional single mode fiber (SMF). When heating light is injected, the Au thin film is locally heated near the core of SMF to generate a single bubble attached to the fiber facet. A similar BoF has been used as an acousto-optic sensor immersed in a tank filled with water exposed to ambient environment by the authors [26]. In contrast, we investigated the BoF potentials as both a flow sensor and a controller in a microfluidic channel where the liquid flowed continuously in this study. When the liquid flows through the microfluidic channel, the heat is taken away from the bubble surface to cause a local temperature drop, which consequently reduces the bubble diameter. As the BoF is a Fabry-Perot cavity, any changes in the bubble diameter are directly measured in real-time via the high-resolution interferometry. Once the reduction in the bubble diameter is detected, the heating laser power is raised accordingly to maintain the bubble diameter. The flow rates in microfluidic channels can thus be acquired based on the relationship between the flow rate and the heating laser power.
The BoF was fabricated in the following process: a cleaved facet of a commercial optical fiber (Corning, SMF28) was coated with a 4 nm-thick gold (Au) film using a thermal evaporator. The Au film serves as an optical absorber to enhance the heating effect. The fiber tip was inserted into a microfluidic channel filled with distilled water. The heating laser at λ = 980 nm was then delivered to the fiber tip (see Fig. 1). The Au film absorbed the heating laser with the power of ~20 mW near the core to vaporize the water near the fiber core initializing the bubble generation process. The gas in the water diffused into the bubble enlarging its diameter. The Au-coated fiber tip and the bubble air-water interface act as two reflectors of the BoF-based Fabry-Perot cavity. To maximize the fringe contrast of the cavity reflection spectrum, the two reflectors should have the same reflectivity if not considering the cavity transmission loss [27]. Since the cleaved fiber tip before Au coating already has higher reflectivity than that of the air-water interface, the reflection-enhancing Au film deposited on the fiber tip should be thin enough to reduce the mismatch between the reflectivity of the two reflectors. Here, 4 nm is chosen as the optimum Au film thickness because further reducing the Au film thickness will greatly increase the heating power for bubble generation. Figure 2 shows the microscope images of the growth of the single bubble on a fiber tip in a microfluidic channel.
Fig. 2 Microscope images of the bubble generation on the gold-coated fiber facet by injecting the heating laser at λ = 980 nm with the power of 20 mW.
Once the bubble reached the designated diameter, the heating power was dynamically servo-controlled by a PID-algorithm-based LabView program to stabilize the bubble size. The response time to complete the whole servo-control process is ~100 ms, which is the time needed for the program to read feedback signals from the photodetector and adjust the heating power using an electronically variable optical attenuator. The detailed process of the bubble stabilization can be found in a previous report [26]. The bubble diameter fluctuation was greatly reduced by using the servo-control process such that the root mean square (RMS) variation for the bubble with a diameter of 30 μm was ~1 nm, three orders of magnitude smaller than the probe laser wavelength near 1550 nm. BoFs with the diameters of 13 and 80 μm showed a larger fluctuation of ~2.2 and ~5.6 nm, respectively, as shown in Fig. 3(a). The 30 μm-diameter BoF is most stable, which is attributed to the high fringe contrast of its reflection spectrum as shown in Fig. 3(b). Higher fringe contrast provides larger feedback signal to the PID servo-control unit, and thus enables more efficient stabilization of small fluctuations. The 80 μm-diameter BoF has a similar fringe contrast compared with the 13 μm-diameter BoF but shows a much larger diameter fluctuation, which might result from its higher sensitivity to environmental pressure and temperature fluctuations [26].
Fig. 3 (a) Temporal fluctuations in the bubble diameter of the fabricated BoFs before and after PID-algorithm-based servo-control. (b) Reflection spectrum of BoFs with the diameters of 13 μm, 30 μm and 80 μm.
As the BoF served as an optical thermometer measuring the temperature changes induced by liquid flows, its temperature response was investigated by changing the temperature of the distilled water in the microfluidic channel. Upon the temperature variations, the initially stabilized BoF expanded or shrank due to the unbalanced gas diffusion at the bubble air-water interface [28,29]. This immediately triggered the servo-control of the heating laser power to compensate the temperature induced variations keeping the bubble diameter stabilized. Figure 4 plots the relationship between the water temperature and the heating laser power for the BoFs with various bubble diameters. The data were well-fitted to linear curves with their slopes corresponding to the temperature sensitivity of the BoF in the unit of mW/°C. The temperature sensitivity of the BoF was found to increase with its bubble diameter, which may result from the higher heat dissipation rates for larger bubbles with increased surface areas.
3. Simulation of the heat transfer in fluid flow
With knowledge of the BoF temperature sensitivity, the dependence of the BoF temperature on the flow rate was studied by using the finite element method (FEM) software Comsol Multiphysics 5.2. A three-dimensional model containing the heat transfer (ht) and laminar flow (spf) modules was adopted. The radii of the fiber core and cladding were set to be 4.2 μm and 62.5 μm, respectively. The microfluidic channel had a square cross-section in the dimension of 600 μm × 600 μm. The physical parameters of the materials including the fluid (water), gold and silica were set according to the built-in material library of Comsol. The temperature dependence of the material properties was considered. The initial temperatures at the channel ports and the distal fiber end were set to be 20 °C. Flow rates ranging from 0 to 10 mm/s were applied to the channel inlet and ‘Outflow’ boundary condition was set for the channel outlet. The thickness of the gold film used in the simulation was 4 nm, and the optical power delivered to the fiber tip was 20 mW. Figures 5(a) and 5(b) show the temperature distribution around the BoF for the flow rate of 0 and 5 mm/s, respectively. The microfluidic flow passing the BoF took away the heat and resulted in a temperature drop. At the flow rate of 5 mm/s, the average temperature over the surface of a 40 μm-diameter BoF decreased by ~4.5 °C. This reduction in temperature corresponded to an increase of the heating laser power by ~36 mW, according to the measured temperature sensitivity of 7.9 mW/°C shown in Fig. 4. As a result, by monitoring the heating laser power that was used to keep the bubble diameter constant against the flow-induced temperature drop, the BoF can measure the flow rate in a so-called constant-temperature mode featuring a fast response and high sensitivity [12].
Fig. 5 Simulated temperature distribution around the BoF in a microfluidic channel with the water flow rate of (a) 0 mm/s and (b) 5mm/s; (c) Average temperature over the BoF surface as a function of the flow rate in the channel.
4. Experimental results
To experimentally prove the proposed concept, a BoF with the bubble diameter of 40 μm was embedded in one of the arms of a Y-branch microfluidic channel where water flow rate was controlled using syringe pumps as schematically shown in Fig. 6. A microscope was used to monitor the shape of the bubble in the BoF. To diminish the influence of dissolved air on the heating power for bubble generation/stabilization, we waited the water to reach air-equilibrated and then injected it into the channel.
Fig. 6 Schematic of the experimental setup for measuring the flow rate using a BoF in a microfluidic channel.
The flow rates were changed in repeated cycles from 0, 1, 2, 3, and 4 mm/s and the corresponding heating laser power was measured as shown in Fig. 7(a). The higher the flow rate was, the more heat dissipated and thus the larger heating laser power was required to stabilize the BoF. Based on the RMS value of the temporal change in heating laser power when no flow was applied, the minimum detectable flow rate was estimated as ~0.06 mm/s, corresponding to an equivalent flow volume rate of ~22 nL/s for the microfluidic channel with a 600 μm × 600 μm cross section. The response time of the sensor was measured to be ~6 s. Figure 7(b) shows the dependence of the heating laser power on the flow rate, exhibiting a nonlinear relationship consistent with our simulation results in Fig. 5(c).
Fig. 7 (a) Dynamic response of the BoF with a bubble diameter of 40 μm to the flow rates in repeated cycles; (b) Heating laser power as a function of the flow rate.
We further investigated the role of the bubble diameter in the BoF based flow sensing and the results are summarized in Fig. 8(a). The response curves show vertical offsets in the heating laser power since the heating the power needed for the bubble generation differs with its diameter. BoFs with larger bubble diameters were found to be more sensitive to the flow rate change due to their higher temperature sensitivities (see Fig. 4). On the other hand, the maximum measurable flow decreased as the bubble diameter increased. As the flow rate increased beyond the threshold value, a larger BoF experienced a stronger impinging force exerted by the liquid. This force would deviate the bubble from the fiber core, which significantly reduced the fringe contrast of the reflection spectrum. These thresholds would limit the sensing range of the BoF based flow rate sensing. Figure 8(b) summarized the flow rate sensitivity (0-2 mm/s) and the threshold flow rate as a function of the bubble diameter.
Fig. 8 (a) Heating laser power as a function of the flow rate for BoFs with various bubble diameters. (b) Dependence of the sensitivity and the threshold flow rates on the bubble diameter.
As the flow is pressure-driven into the microfluidic chip, the pressure drops continuously across the channel [30]. This means that the pressure at the location of BoF will be larger than that at the microchannel outlet, which is open to atmospheric environment. As the flow rate increases, the pressure experienced by the BoF rises accordingly. The pressure rise will then increase the heating power needed for the BoF stabilization. To experimentally measure the pressure dependence of the heating power for the BoF stabilization, the BoF was fixed in a water tank and a valve was used to control the height of the liquid level. When the valve was opened, the height of the liquid level gradually reduced and the pressure nearby the BoF decreased accordingly. During this process, the heating power for BoF stabilization was continuously monitored until the valve was closed. By linearly fitting the measured data as plotted in Fig. 9, the pressure sensitivity for the BoF with a diameter of 40 μm is estimated to be ~0.016 mW/Pa. For the microchannel with the cross-section of 600 μm × 600 μm as shown in Fig. 2, the BoF with a distance of 20 cm from the channel outlet will experience a pressure rise of 20 Pa for every 1 mm/s flow rate increase [31]. As a result, for a 40 μm diameter BoF with a flow rate sensitivity of ~5.5 mW/(mm/s), the pressure rise presents ~6% contribution to the BoF sensitivity, which is negligible. For narrower channels in which the flow induced pressure change becomes greater, the pressure effect plays a more important role and needs to be considered.
Fig. 9 The relationship between the heating power for the BoF stabilization and the liquid pressure. Inset: temporal evolution of the heating power during a continuous pressure drop.
It needs to point out that we trigger the PID servo-control process to stabilize the bubble once its diameter reaches the designated value during the tests. However, the high bubble growth rate (~10 μm/s for the diameter change) and relatively long initialization time of the PID servo-control, prevent precise control over the bubble diameter after being stabilized. Based on the previous study [26], the bubble decay much slower after the heating light is switched off. This indicates that the bubble diameter can be more precisely controlled if we firstly reduce the heating light power to slowly shrink the bubble to the designated diameter and then trigger the PID program, which is currently under investigation.
5. Flow regulation
In addition to flow rate sensing, a single-shot microfluidic valve using a BoF has also been demonstrated to prove its potential as a multi-functional microfluidic device. As shown in Fig. 10(a), a BoF was inserted into one branch of a Y shaped-channel in a PDMS/silica microfluidic chip. The channels have a cross-section dimension of 150 × 150 μm. Distilled water and red ink aqueous solution were injected into the individual channels at the same flow rate of 0.5 mm/s by using syringe pumps. The two fluids merged at the main channel connected to the outlet. The relative flow rates were estimated by measuring the width of the fluids occupying the channel as shown in Figs. 10(c)–10(f). When there was no bubble at the BoF as in Fig. 10(c), the ratio of the water/red ink flow rate was nearly equal to 1. With the increase of the bubble diameter, the water channel flow rate decreased substantially and the channel was completely blocked by the bubble. Therefore, we realized both a shutting valve and flow regulator in a single BoF device, as summarized in Fig. 10(g). It was also noted that effective flow regulation only occurred after the bubble touched and conformed to the channel inner surface (Fig. 10(f)). For smaller bubbles, even though the bubble diameter was very close to the channel width (Fig. 10(e)), the water could pass around the bubble. In the future, microfluidic chips with narrower channel could be explored to regulate the flow more efficiently [32].
Fig. 10 (a) Schematic and (b) microscopic image of a microfluidic chip incorporating a BoF; (c-f) The flow patterns in the channel at different bubble diameters; (g) Ratio of the water/red ink flow rate as a function of the bubble diameter.
6. Summary
We developed a novel BoF for in situ sensing of flow rate in microfluidic chips achieving the minimum detectable flow rate of ~0.06 mm/s and a response time of ~6 s. We also demonstrated the capability of the BoF as a microfluidic valve to regulate the flow rate in a Y-shape microfluidic chip by optically varying the bubble diameter. With the advantages such as easy generation, small form factor and no need of chemical tracer, the multifunctional BoF is promising to develop high-density lab-on-a-chip devices using its efficient liquid flow management capability.
Funding
National Natural Science Foundation of China (61705082, U1701268); Natural Science Foundation of Guangdong Province (2017A030313361).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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