Article pubs.acs.org/EF
Microfluidic Bubble Point Measurement Using Thermal Nucleation Matthew T. Sullivan* and Dan E. Angelescu† Schlumberger-Doll Research, 1 Hampshire Street, Cambridge, Massachusetts 02139, United States ABSTRACT: We present a highly miniaturized sensor for performing accurate on-chip detection of gas equilibration (bubble point) pressures in liquids, which is manufactured using low-cost batch microelectromechanical system technology. The measurement of gas equilibration (bubble point) pressures in liquids has a diverse spectrum of applications ranging from the oil industry, where the bubble point of a crude oil is important for making informed decisions on production and exploration, to the fisheries industry, where gas saturation needs to be controlled to ensure animal health. Oilfield measurements of bubble point pressures often require acquisition of large sample volumes that are subsequently investigated off-line in laboratory facilities, with important consequences on both cost and operational response times. The present work demonstrates the potential of microelectromechanical system technology to respond to such diverse industry needs by providing an economical solution able to perform in situ real-time monitoring of gas equilibration pressures with significantly reduced equipment size and analysis time. By implementing rapid on-chip local heating using an integrated platinum electrode and subsequently observing the fluid behavioreither microscopically or with embedded thermal conductivity sensorswe show that we can effectively overcome the nucleation barrier and perform highly accurate, repeatable measurements of the bubble point pressure.
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primarily been focused on incremental reduction of conventional technology,5 where size reduction does not introduce significant changes in the flow regime. As analysis systems are pushed down to the microfluidic scale, achieving thermodynamic equilibrium through mixing becomes increasingly difficult. Recent work has taken advantage of the rapid mixing in two-phase flow to ensure equilibrium during microfluidic bubble point measurements. Mostowfi, Molla, and Tabeling6 took advantage of the rapid mixing possible in two-phase flow to make a flow-through microfluidic bubble point measurement. Pinho et al.7 built on this concept and introduced a dynamic stop-flow method to allow precise control of pressure. Flow equilibration methods require the presence of two phases to ensure effective equilibration and do not directly overcome the nucleation barrier. For simple mixtures, measurements can be made by starting with a twophase mixture and increasing pressure until only one phase is present. In crude oils, however, depressurization below the bubble point can induce changes in the sample that can be exceedingly slow to reverse.8 For rapid measurement of crude oil samples, a novel method of overcoming the nucleation barrier without the need for subsequent mixing is needed. While there has been little previous work on microfluidic nucleation for bubble point measurements, there has been considerably more effort on the related problem of boiling in microfluidic chips. In microfluidic boiling, an integrated electrode is used to heat a fluid until the liquid boils, either by gradual heating9 or by rapidly injecting heat to produce explosive boiling.10,11 The rapid heat injection technique has found use in a variety of applications where the fluid motion and pressure surge upon boiling can be used to pump fluids12 or to instantly expel droplets in inkjet printing.13−15 In those
nderstanding the phase behavior of crude oils is important for managing production and understanding fluid variations in the reservoir. Crude oil is a complex mixture of components and can have significant variation of its physical and chemical properties even within a single well,1 generally requiring experimental measurement to understand a given reservoir fluid. Analysis is typically undertaken by collecting a representative sample from the formation and transporting the sample to the surface for analysis in a laboratory.2 This is a time-consuming and costly process and provides stringent limitations on the number and volume of fluid samples available for analysis. Chip-based microfluidics for crude oil analysis offers the possibility of reducing the amount of sample consumed while decreasing the amount of time required for the analysis. In laboratory analysis, measurement is typically made by decompressing a single-phase sample until bubbles are observed or a sample compressibility change indicates the presence of gas. A nucleation barrier to the formation of bubbles can significantly impede measurement. There is a competition between the bulk free energy of gas formation which encourages bubble formation and the surface energy required to separate two phases. At a given supersaturation, there will be a critical radius above which the volume contribution dominates and the bubble will grow and below which the surface tension dominates and the bubble will shrink. While the critical radius is finite for any pressure below the saturation pressure, for small supersaturations, it is generally so large that spontaneous nucleation of a second phase does not occur on a reasonable laboratory time frame.3 In conventional instruments, the nucleation barrier can be overcome using an acoustic transducer4 or mechanical agitation to help nucleate bubbles and ensure their equilibrium once formed. Even with active mixing, equilibration is typically a slow process and measurements can take many hours. Miniaturization of bubble point measurement systems has © 2016 American Chemical Society
Received: December 7, 2015 Revised: February 8, 2016 Published: February 17, 2016 2655
DOI: 10.1021/acs.energyfuels.5b02862 Energy Fuels 2016, 30, 2655−2661
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to be able to monitor and control the heater resistance, and hence its temperature during, and subsequent to, the heating phase. Fluidic Setup. Unlike many other fluid properties, there is no readily available standard for bubble point measurement, so test fluids must be prepared in the laboratory. To prepare a sample with a known bubble point, we saturate hexadecane with carbon dioxide by placing it in a high-pressure qualified metal cylinder (equilibration bottle) and contacting it with carbon dioxide gas regulated at a fixed pressure. This system is agitated using a magnetic stir bar and allowed to equilibrate for periods of the order of weeks. The bubble point pressure, after equilibration, is equal to the CO2 pressure with which the sample is equilibrated. The gas-saturated hexadecane is removed from the metal cylinder by siphoning using a capillary tube. Samples used in this work are presented in Table 1.
applications, the boiled volume is significant compared to the total enclosed volume of the system and large pressure changes can be effected. Instead, if we heat a relatively small volume as compared to the total system volume, the pressure will remain approximately constant. By combining the methodology of conventional bubble point measurements with the nucleation provided by rapid microfluidic boiling, a microfluidic bubble point instrument is fabricated. Bubbles are formed by rapidly heating a fluid sample with a heat pulse from an integrated electrode. The heating does not directly nucleate bubbles, but instead brings the heated fluid to a highly supersaturated state such that spontaneous nucleation will occur rapidly. The small size and large thermal conductivity of a microfluidic chip allow rapid dissipation of the injected heat, and the system quickly returns to its ambient conditions before the nucleated bubbles can dissolve. The bubble point can be measured by the subsequent behavior of the bubbles: if the system is above the bubble point pressure, the bubbles will shrink and disappear; if the pressure is below the bubble point pressure, bubbles will grow. By decompressing the sampleeither in discrete steps or continuouslyand monitoring the behavior of nucleated bubbles at different pressures, the bubble point pressure is measured. For laboratory measurements, visual observation of the bubble behavior can be used. For applications where direct optical observation is not feasible, bubbles are detected by measuring the thermal conductivity of the fluid, which is significantly reduced when bubbles are present.
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Table 1. Sample Formulationsa sample
pressure/kPa
temperature/°C
1 2 3
755 ± 5 530 ± 5 530 ± 5
21 ± 1 21 ± 1 21 ± 1
a Samples are prepared by equilibrating n-hexadecane with carbon dioxide at the listed temperature and pressure.
The sample pressure during experiments is controlled in two different ways: by controlling the pressure of a gas reservoir in communication with the system using a regulator or by isolating the sample in a closed volume and changing the sample volume using a custom syringe pump with a maximum expanded volume of 60 μL. The hydraulic configuration depends on the type of pressure control used (Figure 2). For regulator control, there is a line connecting the
EXPERIMENTAL SETUP
Device Design and Fabrication. The device (Figure 1) consists of a three-wafer sandwich: a silicon wafer between two glass wafers. A
Figure 1. A schematic drawing of the microfluidic bubble point chip. The device consists of a three-wafer sandwich: a 75 μm silicon wafer (dark gray) between two glass wafers (light gray). The silicon wafer is etched through to define the microfluidic channel in the chip. Inlet and outlet ports are etched through the bottom layer of glass. Platinum electrodes are patterned onto one of the glass wafers in etched troughs that keep the platinum from interfering with anodic bonding. The inset shows a schematic of the electrodes as seen through the glass. A thin heater (center) connects larger platinum electrodes.
Figure 2. (a) Schematic of the hydraulic configuration for static measurement experiments. Saturated liquid is siphoned from the bottom of the equilibration vessel at pressures slightly lower than the gas equilibration pressure point, and loaded into the measurement chip. A nitrogen regulator fixed to the outlet of the chip is used to control the sample pressure for experiments. (b) In continuous decompression experiments, the sample is loaded as above but then isolated using two valves. A computer-controlled syringe pump is used to regulate the fluid pressure.
fluidic channel is fully etched across the 75 μm thick silicon wafer, and fluidic access holes as well as platinum heater electrodes are fabricated on the bottom glass wafer. Access holes for contacting the electrodes are etched through the silicon and top glass wafers. Silicon machining is performed using deep reactive ion etching, glass etching using a concentrated hydrofluoric acid solution, whereas the heater is manufactured by patterning a sputtered platinum thin film (100 nm thick, including the 10 nm titanium adhesion layer) using a lift-off process. To facilitate subsequent anodic bonding to the silicon substrate, the platinum film is deposited within 100 nm deep trenches pre-etched on the glass using buffered hydrofluoric acid etch. The heater design is chosen to create an approximately uniform square heating area with a lateral dimension of 100 μm and a total resistance of about 100 Ω, connected by electrodes with a large cross-section that do not generate significant Ohmic heating. A four-probe setup is used
chip to the sample and a second line connecting the chip to the pressure regulator with a valve allowing the sample to be isolated from the chip. For syringe-pump control, there is an additional “T” junction connecting the main capillary line to the pump, another “T” junction connecting to a pressure gauge, and an additional valve between the pressure regulator and chip. Connection between the chip and the capillary line is made using a NanoPort connector (N-121S, Idex Health and Science), which consists of a plastic fitting that is centered on a hole in the chip and attached using epoxy and an elastomer gasket for sealing. 2656
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Energy & Fuels To load fluid into the chip, the nitrogen bottle outlet pressure is set slightly below the equilibration bottle pressure and all valves are opened. The pressure difference causes fluid to flow from the sample bottle, through the chip, to the nitrogen buffer. The volume of the regulator and associated gas connections is considerably larger than any other volume in the system, so the small volume of sample that flows can safely collect without contaminating the regulator. The fluid is observed during this loading process to ensure that the lowered pressure does not cause spontaneous nucleation in the flow path. Once sufficient sample has been flowed through, the valve connecting the chip to the sample is closed. The pressure can then be modified by the nitrogen regulator. For continuous depressurization experiments, the valve connecting the chip to the regulator is closed, which allows the syringe pump to control the total sample volume and thus the pressure. Data Acquisition. Measurement of the bubble point requires controlled depressurization of the sample, which is accomplished either in discrete pressure steps or through continuous decompression. For pressure-step experiments, the sample pressure is set externally using a pressure regulator, allowed to equilibrate, and then nucleation experiments are performed. Depending on the number of measurements made, each step takes between 1 and 10 min. Once pressure becomes lower than the bubble point, nucleated bubbles continue to grow until the system reaches equilibrium. There is no mechanism for recombining these two phases in the microfluidic chip, so once stable bubbles are formed, the sample has to be discarded. Continuous depressurization is achieved by isolating the sample and then slowly expanding its volume by moving a computer-controlled syringe pump. The depressurization rate is set by adjusting the volumetric rate of the syringe. A miniaturized pressure gauge (microsapphire gauge, Schlumberger) is used to continuously monitor the pressure of the system. Before bulk phase separation, the compressibility is nearly constant and the depressurization rate, typically between 5 and 50 kPa/s, is approximately uniform. Once phase separation has occurred, however, the compressibility increases dramatically and the depressurization rate decreases accordingly. As this region is below the sample bubble point, this change in rate has no practical effect on measurement. In both cases described above, bubbles are nucleated using a shortduration high-current electrical pulse. Typical durations are 10−100 ms and the typical current is 100 mA, resulting a total energy of 10− 100 mJ. The pulse duration is set by looking for an energy that will reliably nucleate bubbles at the highest pressure of interest; in general, the higher the pressure, the more heat is needed to create a locally superheated fluid that will spontaneously form bubbles. For discrete pressure steps, a single pulse is generated at each pressure step and the subsequent behavior is observed. For continuous depressurization, pulses are generated at a constant frequency by a pulse generator. A pulse rate of 1−10 Hz is used as a compromise between pressure resolution, which improves with a higher pulse rate, and the requirement of thermal dissipation on the nucleation electrode, which limits the pulse rate. Bubble behavior is monitored using a microscope (Leica DM IRM) connected to a high-speed digital camera (Basler A504k). Customdeveloped software (using LabWindows from National Instruments) integrates the pulse generation and image recording to synchronize detection and acquisition. The movies are saved to a disk during experiments for later, more detailed analysis. For discrete experiments, the behavior above and below the bubble point is easy to recognize by eye and the bubble point can be determined without external analysis. Continuous depressurization experiments are too fast (lasting from a few seconds to a minute) to accurately gauge the bubble point without analyzing the movies and synchronizing with pressure measurements. A direct electrical method for measuring bubble behavior is also implemented, relying on the thermal conductivity differences between liquid and vapor. This thermal conductivity detector operates on the principle of a hot-wire anemometer. An electrode heater, either the same as is used for thermal nucleation or an independent heater located nearby, is maintained, subsequent to the bubble-generation pulse, at a temperature slightly above ambient (∼1 °C) using a
dedicated feedback circuit. The voltage needed to maintain the wire temperature is monitored and indicates the thermal conductivity of the fluid surrounding the wire. In the presence of a bubble, the thermal conductivity decreases and so does the voltage required to maintain the thermal offset, allowing bubble detection.
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RESULTS AND DISCUSSION Bubble Growth. We observe that bubbles nucleated at pressures above the bubble point shrink and disappear in a few hundred milliseconds; bubbles nucleated below this pressure grow quickly and eventually fill the field of view. In the absence of thermal nucleation, bubbles are not observed to form until about 300 kPa below the bubble point, showing that the thermal nucleation technique is necessary to overcome the bubble nucleation barrier and provide accurate bubble point measurements. Image analysis software is used to measure bubble size from videos recorded during experiments, which are made on a slightly modified heater substrate that gives a better view of the bubbles as they grow (Figure 3). Even in this modified cell,
Figure 3. Example of the bubble-finding algorithm used to detect a bubble shown in (a). A gradient edge-detection algorithm is used to find the edges of the bubble (b). Next, a Hough transform is created by drawing lines normal to the detected gradient edge and collecting all of the lines generated in this manner (c). The peak of the Hough transform is identified as the center of the bubble. The radius is calculated using the center and the detected edges (d). For bubbles larger than the fluidic channel height (75 μm), the bubble is vertically confined and two radii are detected as shown in (e). Bubbles of intermediate size appear to be spheroidal in shape until the liquid dewets the top and bottom of the channel.
bubbles are often partially obscured, so an algorithm to find bubbles based on a portion of the entire bubble is used. First, a gradient edge-detection is performed and edges are found using a threshold value. Next, a Hough transform is implemented by drawing lines in parameter space normal to the gradient edge; for circular objects, this leads to a brightness peak near the object center. A peak finding algorithm identifies the bubble 2657
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Figure 4. (a) Image analysis software allows measurement of bubble radius versus time showing growth for pressures below the saturation pressure and rapid dissolution above the saturation pressure. This growth approaches a 1/2 power law with time, characteristic of diffusive growth. (b) A growth constant β can be extracted from the radius R and mass diffusivity D using R = β Dt . This constant increases with increasing supersaturation, defined as the saturation pressure minus the system pressure.
center and uses the edge points to refine this estimate and measure the radius of the bubble with a precision of about 1 micron. Bubbles larger than the channel height of 75 μm are confined by the chip and visually show an inner and outer radius; measurements are taken with respect to the outer radius. After the initial bubble formation, there is a slight decrease in the diameter, even well below the bubble point pressure. For system pressures above the bubble point, this decrease continues steadily until the bubble has dissolved completely. After the initial relaxation of bubble size, bubble growth can be observed for systems below the bubble point pressure. This behavior has its roots in the interplay between heat and mass transport. In single-component fluids, growth of vapor bubbles is limited by the need to transport heat to the interface, allowing the growth process to be entirely described by heat transport in the system;16 other effects such as surface tension, fluid inertia, and pressure gradients are generally small and can be neglected.17 In multicomponent fluids, on the other hand, the liquid and gas phases do not have identical stoichiometry, and the component that is dominant in the bubble must diffuse through the liquid phase to continue growth. In such multicomponent systems, therefore, both heat and mass transfer to the interface control the growth dynamics.18 As the microfluidic system under study is confined on top and bottom by glass plates, which are effective at transporting heat but do not allow diffusive transport, long-term bubble dynamics in our system are dominated by mass transport. In general, bubble growth is influenced by fluid inertia, fluid viscosity, surface tension, and heat and mass transport to the bubble interface. Similar to earlier work,18 all but heat and mass transport are small enough that they can be safely neglected on the timeframes of interest. Early in the bubble evolution, the contributions of temperature and mass transport are expected to be comparable, but diffusive equilibrium is expected to take significantly longer than temperature equilibrium for two reasons. First, the thermal diffusivity of n-hexadecane,19 75 × 10−9 m2/s, is significantly larger than the diffusivity of carbon dioxide in n-hexadecane20 of 2.3 × 10−9 m2/s. Second, the chip substrate is effective at thermal transport and will quickly relax any temperature gradients in the fluid but is impervious to mass diffusion. Faster temperature relaxation can explain why there is often a slight decrease in the bubble radius nucleation within the first 100 ms, even for bubbles nucleated well below the bubble point pressure. Once the temperature has relaxed, the bubble behavior is controlled by mass transport (diffusion) of the volatile
component through the liquid to the bubble interface. In the limit that behavior is dominated by diffusion, the bubble dynamics will be described by the diffusion equation for ∂C concentration C, given by ∂t = D∇2 C , where D is the diffusivity and t is time. It is possible to show21 that, in an infinite diffusion-limited bubble growth regime, bubble dynamics can be described by a solution of the form r = β Dt , where r is the spatial variable and β is a unitless numerical constant. While this solution is not expected to be valid early on in the bubble evolution process when temperature and other effects are still important, the bubble growth behavior should approach this diffusive limit late in the evolution. Looking at the observed bubble radius as a function of time (Figure 4), a 1/2 power law is seen for bubble radius growth after about 1 s. The behavior of bubbles at different pressures can be quantified by extracting the dimensionless growth constant β from the asymptotic bubble radius as a function of time. This growth constant increases with increasing supersaturation, defined here as the difference between the bubble point pressure and the system pressure (Figure 4b). Electronic Detection. Two different systems are used for the electronic detection of bubbles, one where the same electrode is used to both nucleate and detect bubbles and a second where two different electrodes are used. The detection electronics operate on the principle of a hot-wire anemometer. An electronic controller uses a small current to slightly heat the electrode to a fixed temperature above ambient (typically about 1 °C) and changes the heater voltage to keep a constant electrode resistance. While typically used to measure changes in flow rate, changes in the thermal capacity of the overlying fluid produce a significant change in the power required to maintain an increased temperature, allowing the heater voltage to indicate the presence of a bubble above the electrode. In single-electrode sensing, bubbles are nucleated by applying a large voltage to the electrode, increasing the local temperature by approximately 100 °C. This interrupts the anemometric function of the electrode for a brief period corresponding to the thermal response time of the system, typically about 200 ms. Once the nucleation pulse heat has dissipated, the heater voltage is used to indicate the presence of a bubble on the electrode. Microscopy of the electrode is used to confirm the performance of the detection electrode. Measurements are taken using fixed-step depressurization: the pressure is set, the heater is pulsed, and the existence of bubbles is determined 2658
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detection capacity of the electrodes. Increased sensitivity could, in principle, be gained by thermally isolating the electrode by suspending it in the cell, but the surface tension of generated bubbles makes keeping bubbles over suspended electrodes difficult. The limits of the detection method can best be understood by looking at the voltage behavior at pressures in the proximity of the bubble point. The maximum voltage drop is achieved only once the bubble can completely cover the electrodes. Consequently, at 480 kPa (just below the bubble point), the reduction is smaller than that observed at lower pressures, where growth is faster. This is due to the relatively small size of bubbles at this pressure and the sensitivity of detection to the region immediately surrounding the electrode. Conversely, at 550 kPa (just above the bubble point), the bubble is nearly stable and takes noticeably longer to dissolve. The above drawbacks can be eliminated by the second electronic detection system that we developed, which implements the spatial separation of the nucleation and detection electrodes. This method can be used in the continuous decompression setup described above, thus eliminating the imprecision due to discretization of the pressure steps. In this configuration, the heater generates heat pulses at a fixed frequency, from 1 to 10 Hz, and a second separate electrode is used to detect the presence of bubbles (Figure 6). This detection requires some
from the heater voltage. If no bubbles are found, the pressure is lowered and the experiment is repeated. If bubbles are detected, then pressure is increased slightly, a small amount of new sample is circulated through the chip, and the pressure is lowered to the next step. Measurements from a sample with a bubble point pressure of 530 ± 5 kPa are presented in Figure 5.
Figure 5. Micrographs of the heater electrode 10 ms before (a) and 10 ms after (b) the current pulse is applied show the nucleation of a bubble on the electrode. The heater electrode voltage before and after the short pulse indicates the amount of power needed to keep the electrode at a fixed temperature slightly above ambient and can be used to detect bubbles without the risk of inadvertently nucleating them. The normalized heater voltage vs time for pulses applied at varying sample pressures is shown in (c). A large external voltage pulse is applied to the electrodes to nucleate bubbles, showing as a large drop in the heater voltage from the feedback circuit. After a few hundred milliseconds, the heat from this pulse dissipates and the heater voltage can again be used to determine the properties of the fluid above the electrode.
Figure 6. With continuous decompression, sufficient flow is present to advect bubbles formed on the heater electrode to a nearby detection electrode placed downstream. This is demonstrated in (a), where the bubbles are nucleated on the left electrode and advected by flow to the detection electrode on the right. Without a large current or heat pulse passing through the detection electrode, more sensitive electronics can be implemented, greatly increasing the detection sensitivity. Starting from high pressure, the sample is smoothly depressurized while pulsing the heater electrode at 10 Hz. The resulting detector voltage is shown in (b) as a function of pressure. Above the bubble point, the voltage increases only slightly as pressure is decreased. At the bubble point (530 kPa), there is a sharp increase in the detector voltage corresponding to a bubble arriving at the detection electrode. A closer look at the heater voltage near the bubble point (c) shows the large fluctuations in the heater voltage with bubbles arriving, growing, and departing from the detection electrode.
Above the bubble point, the voltage recovers to its prepulse voltage; below the bubble point, there is a persistent decrease in the voltage as the bubble covers the heater, reducing the thermal conductivity. The technique described above has a number of drawbacks. First, the precision of the bubble point detection is limited by the magnitude of the discrete pressure steps (10 psi or 70 kPa in the case of the experiment presented in Figure 5). Second, the reduction in heater voltage depends on the size of the bubble, with smaller bubbles that do not completely cover the electrode region giving smaller decreases in the required heater voltage. For bubbles that completely cover the heater (e.g., Figure 5b), the voltage decreases by only about 3%. While this change is well within the detection limits of the anemometry circuit, its low level makes detection less robust than desired. The low sensitivity is due to the high thermal conductivity of the glass substrate as compared to the fluid, which effectively short-circuits most of the thermal
method for ensuring that bubbles nucleated at the heater are advected to the detection electrode. We make use of the syringe pump in the system that is used for decompression, placing the pump such that fluid flow is directed from heater to detector. The detector electronics uses an anemometric circuit, similar to that used in the single-electrode configuration. Since the isolated detector does not experience the large temperature 2659
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increase associated with thermal pulses, the electronics can be made more sensitive and the signal is not temporarily distorted by the heat pulse. Above the bubble point, the bubbles shrink quickly and disappear before reaching the detection electrode. At and below the bubble point, the bubbles grow and make it to the detection electrode before dissolving. Thus, detection of bubbles at the detection electrode indicates that the system has reached the bubble point pressure. Results of a typical experiment are presented in Figure 5b, which shows the detector voltage as a function of pressure. Above the bubble point, the detector voltage is stable and there is no indication of bubbles or residual effects from fluid heating. As pressure is decreased to reach the bubble point, there is an initial rapid increase in detector voltage, followed by a large decrease. This phenomenon continues as pressure further decreases, resulting in highly recognizable oscillations of the detector signal. On the basis of six repeated measurements of different aliquots of the same sample, the measured bubble point is 530 ± 10 kPa, in excellent agreement with the formulated bubble point of 530 ± 5 kPa. Analysis of the video images can help explain the observed behavior below the bubble point. Bubbles nucleate on the heater electrode and then are advected by the flow to the detector electrode. Bubbles are observed to stick to the detection electrodes, caused by thermocapillary (Marangoni) stresses on the bubble surface which confine it to the slightly elevated temperatures surrounding the detection electrode.22 There is an increase in the detector voltage corresponding to the bubble arrival time as the bubble must be heated to reach the slightly higher anemometer temperature. The bubble then grows due to a combination of gas diffusion from the fluid into the bubble and the arrival of additional bubbles generated at the heater. The low thermal capacity of the bubble causes a decrease in the detector signal, just like in the combined heater/detector implementation. Eventually, the bubble becomes too large for the thermocapillary stresses to trap it and it is advected downstream. This is accompanied by a large increase in the heater voltage as a new bubble is trapped and heated and the signal fluctuation repeats. With lower system pressure, these subsequent bubbles grow faster and leave the detector sooner, decreasing the time between capture events. Eventually, the bubbles grow so large before reaching the electrode that they can no longer be trapped; in which case, the bubble simply moves past the electrode, giving a rapid fluctuation in heater voltage.
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Fluidion SAS, 231 rue St. Honoré, 75001 Paris, France.
Author Contributions
The manuscript was written through contributions of both authors. Both authors have given approval. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Christopher Harrison for a helpful review of this manuscript.
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ABBREVIATIONS MEMS = microelectromechanical systems REFERENCES
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CONCLUSIONS We have demonstrated a MEMS-based microfluidic device capable of measuring the bubble point of an unknown fluid using short thermal pulses. These thermal pulses are capable of overcoming the nucleation barrier to the formation of bubbles while leaving the bulk system at the ambient temperature and pressure. Observation of bubble behavior after nucleation performed while varying the system pressure allows the bubble point to be determined. Above the bubble point pressure, bubbles shrink; below the bubble point pressure, they grow. Bubble measurements are demonstrated by visual observation using video microscopy, as well as by thermal conductivity contrast using electrodes integrated on-chip. Physical separation of the electrode used for bubble nucleation and the electrode used for bubble detection increases the speed and sensitivity of detection and allows for rapid measurement of the bubble point with a precision of approximately 0.5 kPa. 2660
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