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A non-invasive sensor for the detection of process parameters for multiphase slug flows in microchannels Nicolai Antweiler, Sascha Gatberg, Günther Jestel, Joachim Franzke, and David W. Agar ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00420 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 2, 2016
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A non-invasive sensor for the detection of process parameters for multiphase slug flows in microchannels Nicolai Antweiler*,†, Sascha Gatberg†, Günther Jestel‡, Joachim Franzke‡, David W. Agar† †
Department of Biochemical and Chemical Engineering, Laboratory of Chemical Reaction Engineering, Technical University Dortmund ‡
Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V., Dortmund, Germany
Keywords: micro sensor, slug-flow, microchannel, flow characteristics, lab-on-a-chip,
ABSTRACT: The growing market for and increasing applications of lab-on-a-chip-technologies and microreaction technology has led to a need for appropriate microscale in-line- and on-line-analytics. A new non-invasive technique for the in-line measurement of process parameters in microchannels has been developed, which enables both the monitoring of flow characteristics, e.g. velocity, slug length, phase ratio and other microfluidic characteristics as well as temperature. A single metal ring electrode is attached to a non-conductive microchannel and detects the triboelectrical signal induced by the multiphase flow, which is amplified and analysed to obtain detailed information about the process parameters.
The growing utilisation of microprocess technology is increasingly influencing developments in the fields of reaction engineering, purification processes and lab-on-achip technologies. Microstructured processes operate under very well-defined conditions, which gives them clear advantages over existing macroscale processes 1–3. As in conventional processes though, reliable and accurate means for acquiring information about the process state are still required. The small channel dimensions and, in the case of multiphase slug flow, by the sensitivity of the flow structure to disturbances by probes, present major challenges to implementing such measurements. Consequently, there is an urgent demand for suitable noninvasive in-line monitoring techniques in microscale process technology.4 A survey of measurement techniques for ascertaining the flow structure of gas/liquid and liquid/liquid systems in microchannels has been given by Aubin et al.5. Most are imaging techniques requiring optical access to the system and often entailing high costs. Charnay et al. have demonstrated the detection of flow characteristics through a high speed and high definition camera.6 Li et al. tried to reduce the financial expenditure measuring the void fraction by laser diode and photodiode array sensors.7 Even the costs of diodes are low there is still optical access and place in the surrounding of the channel needed. A further possibility is the well-known contactless conductivity detection in capillary electrophoreses.8 Burlage et al. demonstrate the determination of multiphase flow parameters through a conductive technique by inserting gold electrodes into the flow9 and a contactless flow characterisation was developed by Liu et al.10 and
Cahill et al.11 The measurement of multiphase flow characteristics by a capacitance sensor is shown by different authors.12,13 The shortcoming of these techniques is the need for an externally modulated electrical field. A technique to resolve the temperature profiles in microchannels was developed by Haber et al. by using infrared imaging 14 and Ewinger et al. who used laser Raman spectroscopy15. The lack of cost effective analytical methods with no need for optical access was the motivation for the studies presented here. The new technique makes use of triboelectric charging to obtain information about the process. The fundamental mechanisms of contact electrification are still under discussion.16,17,18 Static electricity is already exploited for separation processes19 and many other applications as described by Matsusaka et al.20, such as electrophotography, electrostatic powder coating, electrostatic precipitation, electromechanical valves. The particle mass flow measurement based on static charging was introduced by Masuda et al.21 and patented for fluid media by Gangi et al.22. Two adjacent intrusive electrodes of different materials are employed to determine the mass flow inside a duct. For multiphase flow in microchannels this technique is difficult to implement at the prevailing small dimensions and any electrodes inside the channel would probably disturb the flow. A review of the electrical effects induced by liquid motion and an analysis of the flow electrification process is demonstrated by Touchard and Paillat.23,24 A review of triboelectric nanogenerators (TENGs) as self-powered active sensors is published by Wang et al..25 Ravelo et al. demonstrated the triboelectric effect by the flow of liquid water in insulating pipes with a diameter
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in the range of millimeters.26 The measured voltages of pure and ordinary drinking water are in a linear relationship with the flow rate. The technique presented here enabling the measurement of process parameters of multiphase slug flows with no direct contact to the fluids was patented in 2015.27 First of all, the principle and prototype of the sensor will be presented. The investigation of the factors that influence the measurement configuration and the analysis identifying the optimal arrangement will then be described. Finally, the interpretation of the signal obtained with respect to the flow parameters, temperature and different fluids being measured will be discussed.
PRINCIPLE A schematic of the prototype is shown in Figure 1. The slug flow inside a capillary with the peripheral electrodes is illustrated above and below a magnified view of the measuring arrangement and the electrode surface is depicted.
∆ qV , s
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∂Qel ∂φ ∂C ∂φ = I el = C ⋅ el − s + (φel − φs ) ⋅ ∂t ∂t ∂t ∂t
(2)
φ The time derivative of el is generated by the slug flow and the change in the electrical potential of the electrode environment. The time derivative of the electrical poten∂φs tial of the slugs ∂t is not present at equilibrium flow.
With the current-to-voltage converter and the amplification factor kamp a signal
U sig
results:
∂φ ∂φ U sig = k amp ⋅ I el = k amp ⋅ C ⋅ el − s ∂ t ∂t
∂C + (φ el − φ s ) ⋅ ∂t (3)
The critical factors are the varying electrical potential in the surroundings and the changing capacity due to the multiphase flow. To analyse the possible applications of these phenomena, several of the relevant parameters were investigated. To characterise the signal, the peak to peak amplitude and the signal amplitude ܵ is used:
r r E = −∇Φ(r )
ܵ=
ܷ௫ − ܷ 2
(4)
QEl
which gives information on the signal strength. The maximum and the minimum values of the signal represent the extremes observed. The signal width is defined as the time interval from the beginning to the end of the signal. Figure 1: Above: biphasic slug flow in a microchannel with an electrode attached to the capillary shown in black. Lower left-hand side: measurement set-up. The electrode and the amplifier are housed in a Faradays cage. Visualisation of the signal is obtained by a digital storage oscilloscope (DSO) and electronic data processing (EDP). Lower right-hand side: magnification of the spacing between electrode and dispersed phase. A triboelectrical charge is induced by the contact electricity arising from the multiphase flow inside the capillary, which causes a charge gradient between the materials. The variation in electrical charges lead to a polarisation of the surroundings and gives rise to a charge displacement on the electrode as a result of the changing capacity between the fluid phases and the electrode. The change in the potential difference between the electrode and the φ fluid inside the capillary el and the capacity C yields an electric current. Q el = C ⋅ (φ el − φ s )
Experimental set-up The experimental set-up is shown in Figure 2. The two immiscible liquids are introduced by syringe pumps (Landgraph Laborsysteme LA 100 HP) and combined in a specially constructed mixing element. This mixer enables the adjustment of the slug length under otherwise constant conditions. Water (double distilled) and toluene (technical quality) in PTFE-channels serve as the standard test system, which referred to unless otherwise indicated. Toluene is PTFE wetting and forms the continuous phase consequently water is the non-wetting dispersed phase. The flow characteristics are monitored optically by a CCD camera (Imagingsource DMK 23G618) for reference purposes.
(1)
For the time derivative:
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vide reliable and reproducible information about the liquid/liquid and gas/liquid segmented flows. In Figure 4, the signal for the liquid/liquid system toluene/water measured with a commercial oscilloscope is depicted. water
Figure 2: Experimental set-up
water
toluene
0.05
The distance between the phase mixer and the detection electrode, with a length of 25 cm, is designated as the inlet path. The electrode comprises a 50 micron thick copper wire attached to the PTFE-capillary with an inner diameter of 800 µm and an outer diameter of 1.6 mm. The electrode is housed within a Faraday cage together with the amplifier. A schematic diagram of the amplifier is presented in Figure 3.
Voltage / V
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Time / s Figure 3: Schematic diagram of the amplifier: two amplifying steps and a current to voltage converter. The electrical current is converted to a voltage and amplified in a first step by a factor k1. The first amplifier is connected to the second by capacitive coupling and amplified by a factor k2. The visualisation of the signal is realised by a digital storage oscilloscope (Voltcraft DSO 2250) connected to a computer.
Results The new technique for the measurement of the process parameters for multiphase flow in microchannels is based on the macroscopic monitoring of static electricity. To start with, the influence of the following measurement configuration parameters on the signal were investigated: • Channel dimensions • Channel material • Electrode material • Electrode width • Length of inlet path • Flow interruption interval Subsequently the dependency of the signal observed on the following microfluidic parameters and properties was studied: • Velocity • Slug length • Phase ratio • Temperature • Different Fluids Only relatively small charges are delocalised and exchanged due to contact electricity. One therefore has to query if the phenomenon is really intense enough to pro-
Figure 4: Signal of the system water (bidest) and toluene. do = 1/16“, di = 800 μm, capillary material: PTFE, width of the electrode 50 µm, phase ratio toluene/water φ = 5, u=1.65 cm/s, slug length Ls = 23 mm. When a slug passes through the capillary at the point where the electrode is positioned a positive and a negative peak are induced. The signal of the system toluene/water increases until the voltage reaches a maximum, then decreases until a minimum is achieved and increases again the baseline is restored. The reason for this characteristic signal sequence is probably the change of the electric potential from negative to positive when the slug reaches the electrode and from positive to negative when the slug leaves the electrode. A change in capacity between electrode and the phases is caused by the slug interface motion. The channel wall constitutes a dielectric barrier between electrode and multiphase flow and is also in contact with the fluids. The static electrical charging and the polarisability of the wall are decisive factors for the measurement. In Figure 5 the peak to peak amplitude for various wall materials is illustrated.
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Amplitude / V
4
2
0 PTFE
ETFE
PFA
FEP
PEEK
Figure 5: Amplitude for different channel materials. The use of different materials yields similar signals and signal-to-noise ratio, except for PEEK which exhibits an amplitude which is eight times higher. The dielectric constant of PEEK at 50 Hz is 3.2 and thus the highest of the materials used. The dielectric constant of ETFE is 2.6 and for PTFE, FEP and PFA it is 2.1. These results indicate that a higher permittivity of the channel material enables one to attain a higher signal to noise ratio. In addition to the wall material, the electrode material and width might also have an influence on the signal-tonoise ratio. The amplitude of copper electrodes of different widths are shown in Figure 6.
0.07
Amplitude Standard deviation 0.06
0.05
0.04
S/ V
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The amplitude increases with the width of the copper electrode until it reaches a maximum at 2.4 mm and then diminishes. It might be that the ratio of slug length to electrode width – which is about 2 at the maximum - is actually the crucial quantity for this behaviour. It is suspected that the capacity increases with the electrode width and hence the amplitude increases. The ratio of slug length to electrode width influences the distance between the rising and falling peaks. At a certain point the first and second peak are reducing each other. The standard deviations for the measurements indicate the reproducibility of the measurements. The slight variations observed can be primarily explained by deviations in the slug length. In Table 1 the amplitudes and conductivities of an electrode made of tin with a width of 1 mm and of a copper electrode with a width of 0.05 mm are listed. Table 1: Signal amplitude of copper electrode of 0.05 mm and a tin electrode of 1 mm width. do = 1/16“, di = 800 μm, capillary material: PTFE, u = 15 mm/s, φ = 5, Ls = 6 mm. Material
Amplitude / V
Conductivity / MS/m
Copper
0.12
60
Tin
0.15
9
The 0.15 V amplitude of the tin electrode is higher than that for the copper electrode (0.12 V). The higher conductivity has thus no significant effect on the signal-to-noise ratio. The explanation for the higher amplitude found with tin might be the superior contact to the PTFEcapillary and due to the greater width of the electrode. Whilst the copper electrode was attached to the capillary manually, the tin was poured into a mould surrounding the PTFE-capillary. Additionally, the role of the channel wall thickness was investigated by changing the inner diameter and keeping the outer diameter and all other parameters constant. The Amplitude S was found to increase with lower wall thickness b (Table 2). Table 2: Amplitude S for different inner diameters di. b is the wall thickness. (a) di = 500 μm, b = 544 μm; (b) di = 800 μm, b = 394 μm. do = 1/16“, capillary material: PTFE, u = 15 mm/s, φ = 5, Ls = 6 mm. s denotes the standard deviation of the amplitude value.
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0.01
0.00 0.2
0.4
1
1.6
2
2.2
2.4
2.6
2.8
3
Electrode / mm
Figure 6: Black: Amplitude for Copper electrodes of various widths. do = 1/16“, di = 800 μm, capillary material: PTFE, u = 27 mm/s, φ = 10, Ls = 5 mm. Gray: standard deviation
b
b / do
S
s
/ µm
/-
/ mV
/ mV
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0.25
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±2
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The triboelectric charging process is time-dependent and thus the signal amplitude might well change with the contact time. The contact time for the developed segmented flow was varied by changing the length of the inlet path. The results in Table 3 suggest that the inlet path length has no significant influence on the signal obtained. Table 3: Signal amplitude S as a function of the inlet path length LEinl.. do = 1/16“, di =800 μm, capillary material: PTFE, u = 15 mm/s, φ = 5, Ls = 7 mm. Inlet path / cm
9
25
37
225
Amplitude / V
0.09
0.01
0.01
0.086
If the flow is stopped for a while and then restarted, the amplitude is affected as shown as a function of the interruption interval in Figure 7.
10
Experimental Trend 8
S/ S0
are in contact with one another and giving rise to a different charge distribution. For the experimental set-up used the results show no significant effect of the width of the electrode and its material on the signal-to-noise ratio. The wall material, on the other hand, has a high impact. It might be that the dielectric constant of the wall material is responsible for the intensity of the electric field interactions. The higher the dielectric constant of the channel wall, the higher the signal-to-noise ratio. Furthermore, the thinner the channel wall, the higher the amplitude. The length of the inlet path apparently has no significant influence on the signal, whereas the interruption interval certainly does.
Flow structure The pertinent flow parameters for segmented biphasic systems include slug velocity, slug length and the phase ratio. The velocity and the phase ratio were varied by adjusting the individual flow rates at the syringe pumps. The slug length was manipulated by modifying the geometry in the mixing element while keeping all other parameters constant. The relationship of the signal amplitude to the overall velocity is shown in Figure 8. It exhibits linear behaviour with an excellent regression coefficient of R2 = 0,994.
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6
Experimental Linear Fit 4
150
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3000
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Figure 7: Amplitude normalised with the uninterrupted value. Relative amplitude as a function of the interruption time Δt. do = 1/16“, di = 800 μm, capillary material: PTFE, u = 15 mm/s, φ = 5, Ls = 4 mm. At first the amplitude increases linearly with respect to the flow interruption interval before it flattens off asymptotically. The explanation may well be due to the changing thickness of the organic phase wall film which is located between the dispersed aqueous phase slugs and the channel wall. When the flow is stopped, the interfacial tension squeezes the liquid wall film out of the space between the dispersed phase slugs and the channel wall. The dispersed aqueous phase thus comes into contact with the channel wall influencing the static electrical charge. Since the amplitude with interruption is much higher than without, this implies that different materials
0 0
5
10 15 20 25 30 35 40 45 50
u / mms-1 Figure 8: Amplitude S for different velocities u. do = 1/16“, di = 800 μm, capillary material: PTFE, φ = 5, Ls = 4 mm. For increasing velocity, the rate of charge variation is greater, which induces a higher electrical current in the electrode. In addition, the wall film becomes thicker. The distance between dispersed slugs and electrode and thus the capacity therefore rise. Nevertheless, the contribution of the rate of charge variation seems to dominate.
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Air / Water 1.6 1.2
Voltage / V
The amplitude also increases with increasing slug length linearly until it reaches an asymptotic value at a slug length of 15 mm (Figure 9). This is nineteen times the inner diameter of the channel and corresponds to the usual operating range of slug sizes. If the slug size changes, the volume specific interfacial area also varies and thus the triboelectrical potential with the surrounding is altered. The influence of charges diminishes with the square of the separation distance yielding the characteristics of the curve shown in Figure 9. Furthermore there might be an influence of a changing wall film thickness as a function of the slug length.
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500 -1.2
Experimental Trend
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6
8
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Time / s Figure 10: Measurement with for Air/Water. do = 1/16“, di = 800 μm, Capillary material: PTFE, u = 15 mm/s, Ls = 42 mm.
300
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100
0 0
10
20
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50
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Lslug / mm Figure 9: Amplitude as a function of the slug length do = 1/16“, di = 800 μm, capillary material: PTFE, φ = 5, u = 20 mm/s. It is conjectured that the zone of influence of the electrode attains its maximum for 15 mm slugs and that longer slugs thus result in no further increase. For larger slug lengths, the size can be ascertained from the signal period. A change in phase ratio causes a change in interfacial area for the segmented flow system. In the absence of time and interface dependent effects, the amplitude remained the same under the conditions chosen for phase ratios of 1, 2, 5, and 10.
A comparison with the liquid/liquid systems in Figure 12 demonstrates a higher signal-to-noise ratio for the gas/liquid slug flow. The reason is probably the more pronounced variation in the static charging and capacitance between the dispersed phase and the electrode. In the case of the gas/liquid systems there is contact between the PTFE-capillary wall and the dispersed aqueous phase. Thus a direct charge transfer between PTFE and water can occur. In addition, the electrical potential of the aqueous phase is not screened by the continuous organic phase. On the one hand, the organic wall film changes the static charging and thus the charge transfer, and on the other hand there is an additional material in the gap between the slug and the electrode. In Figure 12 the signals for various continuous phases are illustrated. The properties of the continuous liquid phases involved are listed in Table 4. Table 4: Material properties of toluene, cyclohexane and hexanol. Viscosity
Dielectric Interfacial Capillary constant tension number
η / Pa/s
ε/-
σ / mN/m Ca / -
0.6
2.4
36
0.32
Influence of Fluids
Toluene
The phase switch inside the microchannel induces an electric current in the electrode. Obviously a change in the system properties will result in a change of the signal generated. To investigate the influence of the material porperties, the systems toluene/water, cyclohexane/water, hexanol/water and air/water were compared. Figure 10 shows the signal of the gas/liquid system air/water.
Cyclohexane 0.89
2
51
0.33
Hexanol
13.3
6.4
12.8
4.3
The signal amplitude of the system water/cyclohexane is greater than the signal for water/hexanol, and the signal
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500 0 -500
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1000
55°C Voltage / mV
29°C Voltage / mV
Voltage / mV
1000
75°C
500 0 -500
-1000 0.4
Time / s
0.8
1.2
1.6
2.0
0.4
Time / s
0.8
1.2
1.6
2.0
Time / s
Figure 11: Signal for a temperature of 29 °C, 55 °C and 75 °C. do = 1/16“, di = 800 μm, capillary material: PTFE, u = 28 mm/s, Ls = 4 mm
for water/toluene is even weaker. The capacity between the dispersed phase and the electrode should be similar for the systems water/toluene and water/cyclohexane. The dielectric constant of toluene and cyclohexane are similar and the wall film thickness, which is a function of the capillary number, should also be similar. The reason for a different signal is probably due to a different electrical potential of the materials as a result of the charge transfer taking place. The dielectric constant of hexanol is the highest, but the wall film is also very thick due to the capillary number of 12.8. The results show that different combinations of media result in different signals even if the dielectric permittivity is nearly the same. 1.5
Hexanol Cyclohexane Toluene
1.0 0.5
Voltage / V
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0.2
0.4
0.6
0.8
1.0
Time / s Figure 12: Measurement with water/hexanol and water/cyclohexane. do = 1/16“, di = 800 μm, capillary material: PTFE, u = 19 mm/s, Ls = 5 mm.
Temperature In Figure 11 the signals at different temperatures of 29 °C, 55 °C and 75 °C are presented. The signal-to-noise ratio increases with temperature. The temperature modifies the triboelectric and dielectric properties of the materials. The charge transfer between the materials becomes more intense, but the permittivity decreases. In addition, the physical properties of the fluids, such as viscosity and interfacial tension, also vary and influence in turn the slug geometry and flow characteristics. Both the viscosity and the interfacial tension decrease with the temperature and, as a result, the wall film expands. The effect of the enhanced static charging seems to counteract the reduced capacitance of the system.
Conclusions and Outlook A novel method for the detection of process parameters in micro-channels has been developed. The technology enables the characterisation of biphasic gas/liquid and liquid/liquid flow structures and the monitoring of the fluid temperatures in microchannels. The influence of the measurement configuration has been examined. It has been demonstrated that the velocity, slug length and phase ratio are accessible from the signal amplitude, duration and form. In addition, the signal amplitude exhibits a linear relationship with the temperature. The static charging and dielectric properties of wall material between multiphase flow and the electrode also affect the signal measured. If these properties change with reaction or mass transfer, an on-line monitoring of such processes becomes feasible. To determine several parameters simultaneously, it might be necessary to install more than one electrode. The signal form changes in a different manner for the various process parameters. A measurement of the velocity is possible by observing the slew rate and the slug length can be obtained from the signal duration. After allowing for the influence of these flow parameters on the signal amplitude, the temperature can probably be de-
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termined from the residual signal. An essential aspect for maintaining the signal is the wettability and the thickness of the wall-film inside the microchannels. The chosen wall materials and experimental conditions cover a range of possible conditions. In further investigations the limitations due to these boundaries are one of the focuses. The optimisation of the prototype arrangement proved quite straightforward. The new technique could also be interesting for the measurement of three phase liquid/liquid/gas-systems, because the different phases exhibit significant differences in the signal amplitude. Another promising field of application might be as a diagnostic tool in single phase systems, in which a specially tailored functionalised dispersed droplet is introduced, that changes its properties dramatically in response to a particular process parameter. Various process parameters could thus be accessed “non-invasively” in microchannels with a single acquisition system and thus at less cost.
Acknowledgement The authors would like to acknowledge the financial support of the Federal Ministry of Education and Research Germany (AKZ: 03V0884) for the work presented.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] References 1 Togashi, S.; Miyamoto, T.; Asano, Y. and Endo, Y. Yield Improvement of Chemical Reactions by Using a Microreactor and Development of a Pilot Plant Using the Numbering-Up of Microreactors. JOURNAL OF CHEMICAL ENGINEERING OF JAPAN 2009, 42, 512–519. 2 Talimi, V.; Muzychka, Y. S.; Kocabiyik, S. A review on numerical studies of slug flow hydrodynamics and heat transfer in microtubes and microchannels. International Journal of Multiphase Flow 2012, 39, 88–104. 3 Burns, J. R.; Ramshaw, C. The intensification of rapid reactions in multiphase systems using slug flow in capillaries. Lab on a Chip 2001, 1, 10–15. 4 Hessel, V.; Löwe, H. Microchemical Engineering: Components, Plant Concepts, User Acceptance – Part Ш. Chemical Engineering & Technology 2003, 26, 531-544. 5 Aubin, J.; Ferrando, M.; Jiricny, V. Current methods for characterizing mixing and flow in microchannels. Chemical Engineering Science 2010, 65, 2065–2093. 6 Charnay, R.; Revellin, R.; Bonjour, J. Flow pattern characterization for R-245fa in minichannels. Optical measurment technique and experimental results. International Journal of Multiphase Flow 2013, 57, 169-181. 7 Li, H.; Ji, H.; Huang, Z.; Wang, B.; Li, H.; Wu, G. A New Void Fraction Measurement Method for Gas-Liquid Two-Phase Flow in Small Channels. Sensors 2016, 16(2), 159. 8 Kubáň, P.; Hauser, P. C. Contactless Conductivity Detection in Capillary Electrophoresis: A Review. Electroanalysis 2004, 16, 2009–2021. 9 Burlage, K.; Gerhardy, C.; Praefke, H.; Liauw, M. A.; Schomburg, W. K. Slug length monitoring in liquid-liquid Tay-
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lor-flow integrated in a novel PVDF micro-channel. Chemical Engineering Journal 2013, 227, 111–115. 10 Liu, S.; Gu, Y.; Le Roux, R. B.; Matthews S. M.; Bratton, D.; Yunus, K.; Fisher, A. C.; Huck, W. T. S. The electrochemical detection of droplets in microfluidic devices. Lab on a Chip 2008, 8, 1937–1942. 11 Cahill, B. P.; Land, R.; Nacke, T.; Min, M.; Beckmann D. Contactless sensing of the conductivity of aqueous droplets in segmented flow. Sensors and Actuators B 2011, 159, 286–293. 12 Ji, H,; Li, H.; Huang, Z.; Wang, B.; Li, H. Measurement of Gas-Liquid Two-Phase Flow in Micro-Pipes by Capacitance Sensor. Sensors 2014, 14, 22431-22446. 13 Mendorf, M.; Nachtrodt, H.; Mescher, A.; Ghaini, A.; Agar, D. W. Design and Control Techniques for the Numberingup of Capillary Microreactors with Uniform Multiphase Flow Distribution. Ind. Eng. Chem. Res. 2010, 49(21), 10908-10916. 14 Haber, J.; Kashid, M. N.; Borhani, N.; Thome, J.; Krtschil, U.; Renken, A.; Kiwi-Minsker, L. Infrared imaging of temperature profiles in microreactors for fast and exothermic reactions. Chemical Engineering Journal 2013, 214, 97–105. 15 Ewinger A., Rinke G., Urban, A.; Kerschbaum, S. In Situ measurement of the temperature of water in microchannels using laser Raman spectroscopy. Chemical Engineering Journal 2013, 223, 129-134. 16 McCarty, L. S.; Whitesides, George M. Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets. Angewandte Chemie International Edition 2008, 12, 2188-2207. 17 Baytekin, H. T.; Patashinski, A. Z.; Branicki, M.; Baytekin, B.; Soh, S.; Grzybowski, B. A. The Mosaic of Surface Charge in Contact Electrification. Science 2011, 6040, 308-312. 18 Lacks, D. J.; Sankaran, R. M. Contact electrification of insulating materials. Journal of Physics D: Applied Physics 2011, 45, 453001. 19 Kelly, E. G. and Spottiswood, D. J. The Theory of Electrostatic Sepertions: A Review. Minerals Engineering 1989, 2, 33– 46. 20 Matsusaka, S.; Maruyama, H.; Matsuyama, T.; Ghadiri, M. Triboelectric charging of powders: A review. Chemical Engineering Science 2010, 65, 5781–5807. 21 Masuda, H.; Matsusaka, S.; Shimomura, H Measurement of mass flow rate of polymer powder based on static electrification of particles. Advanced Powder Technology 1998, 9, 169–179. 22 Gangi, M.; Proepper, T.; Schulz, U. Device for triboelectric mass flow measurement in fluid media, EP1754957B1, 2009. 23 Touchard, G. Flow electrification of liquids. Electrostatics 2001: 9th International Conference on Electrostatics 2001, 440-447. 24 Paillat, T.; Touchard, G. Electrical charges and liquids motion. 11th International Conference on Electrostatics11th International Conference on Electrostatics 2009, 2-3, 326-334. 25 Wang, S.; Lin, L.; Wang, Z. L. Triboelectric nanogenerators as self-powered active sensors. Nano Energy 2015, 436-462. 26 Ravelo, B.; Duval, F.; Kane, S.; Nsom, B. Demonstration of the triboelectricity effect by the flow of liquid water in the insulating pipe. Journal of Electrostatics 2011, 6, 473-478. 27 Franzke, J.; Jestel, G.; Antweiler, N.; Agar, D. W., Verfahren und vorrichtung zur nicht invasiven bestimmung von processparametern bei mehrphasenströmungen. WO2015150298 A1, Okt 8, 2015.
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