MEMS Techniques Research Articles

MEMS quadrupole mass spectrometer

This paper presents the development results of MEMS-based miniature Quadrupole Mass Spectrometer (mQMS). The core components required for the analysis – the ion source, mass analyzer, and ion detector – are fabricated in MEMS technology using monocrystalline silicone and borosilicate glass. These components are housed within a 3D-printed package, ensuring mechanical stability and proper alignment of elements. The utilized MEMS Quadrupole mass analyzer, measuring 70 mm in length with internal radius of 500 µm, provides an analysis resolution of 50 and a mass range between 1 and 200 u when operating at 6 MHz. Our work demonstrates that devices miniaturized using MEMS techniques, along with novel 3D printing fabrication methods, present an emerging coupling, enabling the opportunity to develop portable yet powerful instrumentation for a range of applications. This opens new possibilities for on-site, real-time analysis across various fields.

Experimental investigation on the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe

This study experimentally investigates the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe (PHP). Rectangular grooves are engraved on the sidewalls of the channel in the evaporator section in a direction transverse to the oscillating flow, and these grooves extend from the top surface to the bottom surface of the sidewalls. Two PHPs are fabricated for experimental evaluation using MEMS techniques: one with a grooved channel and another with a smooth channel. Both PHPs have overall dimensions of 29 × 29 × 1.5 mm3. A rectangular meandering channel is etched on a silicon wafer, and this channel possesses dual hydraulic diameters with alternating widths of 1 mm and 0.4 mm and a height of 0.5 mm. In the PHP with the grooved channel, the dimensions of the grooves are analytically determined to maximize the film evaporation heat transfer rate: the depth, the width, and the spacing are determined to be 50 μm, 20 μm, and 20 μm, respectively. R-236fa is used as the working fluid, and the filling ratio is approximately 55 % by volume. Thermal performances of the PHP with the grooved channel and the PHP with the smooth channel are evaluated and compared through thermometry. Experimental results reveal that engraving transverse grooves on sidewalls of the channel effectively enhances the thermal performance of the PHP: the thermal resistance is decreased by an average of 34.6 % across the entire range of input heat fluxes, from the heat flux required for startup to the maximum allowable heat flux. High-speed photography is used, and it is observed that the liquid film in the grooved channel exhibits an 69.2 % thinner average film thickness than that in the smooth channel. This decrease in the average film thickness contributes to the increased evaporation heat transfer rate.

The Cutting Edge of Vibration Energy Harvesting Technology

Energy harvesting has been around for more than a decade, with continual research tackling the issues of charging and powering up electronic gadgets. Because of its multiple advantages, such as greater mobility and a longer lifespan, the notion of energy harvesting has acquired broad popularity. Researchers are investigating methods to harness the energy created by vibrations from various materials and transducers as part of the energy conservation movement. This paper examines major advancements in vibration energy collecting during the last 15 years. It focuses on the many processes used to collect vibration energy, such as piezoelectric, electromagnetic, electrostatic generators, and MEMs techniques, as well as power management circuits, to enhance various elements of vibration energy harvesting devices from diverse sources. While the research on vibration energy harvesting has grown significantly, this work summarises significant achievements in the subject over the last 15 years and updates prior review publications.

Plotter Cut Stencil Masks for the Deposition of Organic and Inorganic Materials and a New Rapid, Cost Effective Technique for Antimicrobial Evaluations.

Plotter cutters in stencil mask prototyping are underutilized but have several advantages over traditional MEMS techniques. In this paper we investigate the use of a conventional plotter cutter as a highly effective benchtop tool for the rapid prototyping of stencil masks in the sub-250 μm range and characterize patterned layers of organic/inorganic materials. Furthermore, we show a new diagnostic monitoring application for use in healthcare, and a potential replacement of the Standard Kirby-Bauer Diffusion Antibiotic Resistance tests was developed and tested on both Escherichia coli and Xanthomonas alfalfae as pathogens with Oxytetracycline, Streptomycin and Kanamycin. We show that the reduction in area required for the minimum inhibitory concentration tests; allow for three times the number of tests to be performed within the same nutrient agar Petri dish, demonstrated both theoretically and experimentally resulting in correlations of R ≈ 0.96 and 0.985, respectively for both pathogens.

Effects of the micro-post array on the maximum allowable input power for a micro pulsating heat pipe

This study is performed to experimentally investigate the effects of the micro-post array (MPA) and its arrangements on the maximum allowable input power for a micro pulsating heat pipe (MPHP). The maximum allowable input powers for MPHPs with the MPAs in three different arrangements are measured and compared to that for the conventional MPHP (without the MPA). To do this, ten-turn MPHPs with and without the MPA are fabricated using MEMS techniques. The overall dimensions of the MPHPs are 53 × 38 × 1.5 mm3, and the width and the height of the meandering rectangular channels are 1500 µm and 660 µm, respectively. The MPA in each case is placed on the bottom surfaces of the channels. The dimensions of the MPA are analytically determined in such a way to maximize the rate of the capillarity-induced flow by solving the Brinkman-extended Darcy's equation: the diameter, the pitch and the height are determined to be 33 µm, 97 µm, and 80 µm, respectively. The MPA is placed in three different arrangements: in alternating pairs of channels, in alternating channels, or in all channels. Ethanol is selected as the working fluid. The experiments are conducted in a horizontal orientation, and the flow is visualized with high-speed photography. The experimental results show that the MPA can have either a favorable or an adverse effect depending on its arrangement. If the MPA is placed in alternating channels, the maximum allowable input power is enhanced by 44%, compared to that for the conventional MPHP, and if it is placed in alternating pairs of channels, the maximum allowable input power is enhanced by 36%. On the other hand, the MPHP does not operate at all at any input power if the MPA is placed in all channels. Therefore, in case of the MPHP where a vapor plug oscillates in its own channel, the MPA should be placed in alternating pairs of channels or in alternating channels to enhance the maximum allowable input power for an MPHP.

Free chlorine ultra-sensitive detection in tap water via an enrichment-sensing process by an interdigitated microelectrode sensor

Electrochemical free chlorine sensor is considered to be a feasible device for on-line monitoring of free chlorine in tap water due to their reagent-free and short-response-time etc. However, the low sensitivity and the difficulty of to mass-production of the sensor always are the stubborn issues for reliable detection and application in ultra-low concentration range. In this paper, we design an interdigitated microelectrode structured electrochemical sensor chip for free chlorine sensing in tap water and fabricate by MEMS. The batch-fabricated sensor demonstrates an acceptable limit of detection (<0.05 mg/L), a desirable linearity (R2 = 0.991). What's more, a novel detection method (TSAM) is proposed/applied to improve the sensitivity for free chlorine sensing though an enrichment-sensing process. A small and positive potential (0.1 V) is applied to the interdigital electrode to form a ClO−enrichment region near WE before the operating bias voltage (-1.65 V). Fortunately, about 30 times increase (46.334 μA/mgL−1) in sensitivity of sensor by TSAM is witnessed, comparing with the amperometric method. Far greater than the sensitivity of other researches on free chlorine sensor. Beside, by employing the MEMS technique, favorable response consistency and high reproducibility (RSD<3.85%) is witnessed, offering the opportunity of mass-producing the proposed sensor in future. It is anticipated that the proposed sensor and sensing method will provide a competitive strategy to design high sensitivity, reliable and mini sensor for water contaminants detection.

A batch microfabrication of a self-cleaning, ultradurable electrochemical sensor employing a BDD film for the online monitoring of free chlorine in tap water

Free chlorine is one of the key water quality parameters in tap water. However, a free chlorine sensor with the characteristics of batch processing, durability, antibiofouling/antiorganic passivation and in situ monitoring of free chlorine in tap water continues to be a challenging issue. In this paper, a novel silicon-based electrochemical sensor for free chlorine that can self-clean and be mass produced via microfabrication technique/MEMS (Micro-Electro-Mechanical System) is proposed. A liquid-conjugated Ag/AgCl reference electrode is fabricated, and electrochemically stable BDD/Pt is employed as the working/counter electrode to verify the effectiveness of the as-fabricated sensor for free chlorine detection. The sensor demonstrates an acceptable limit of detection (0.056 mg/L) and desirable linearity (R2 = 0.998). Particularly, at a potential of +2.5 V, hydroxyl radicals are generated on the BBD electrode by electrolyzing water, which then remove the organic matter attached to the surface of the sensor though an electrochemical digestion process. The performance of the fouled sensor recovers from 50.2 to 94.1% compared with the initial state after self-cleaning for 30 min. In addition, by employing the MEMS technique, favorable response consistency and high reproducibility (RSD < 4.05%) are observed, offering the opportunity to mass produce the proposed sensor in the future. A desirable linear dependency between the pH, temperature, and flow rate and the detection of free chlorine is observed, ensuring the accuracy of the sensor with any hydrologic parameter. The interesting sensing and self-cleaning behavior of the as-proposed sensor indicate that this study of the mass production of free chlorine sensors by MEMS is successful in developing a competitive device for the online monitoring of free chlorine in tap water.

Chip-Based MEMS Platform for Thermogravimetric/Differential Thermal Analysis (TG/DTA) Joint Characterization of Materials

Combined use of thermal analysis techniques can realize complementarity of different characterization methods. Comprehensive thermal analysis with both thermogravimetric analysis and differential thermal analysis (TG/DTA) can measure not only mass change of a sample but also its temperature change during programmed heating-induced reaction or phase transition processes, thereby obtaining multiaspect thermal information of the material such as dehydration, structural decomposition, phase change and thermal stability. This study proposes and develops a MEMS chip-based TG/DTA microsystem that integrates both programmed heating and detecting elements into a TG chip and a DTA chip to enable the microinstrument performing TG/DTA joint characterization under microscope observation. The TG chip contains a self-heating resonant microcantilever to measure heating-induced mass change of a sample and the DTA chip is with a microheater and a temperature-detecting thermopile integrated on a suspended thermal-insulating diaphragm. Only nanogram and microgram-level samples are needed for the TG and DTA chips, thereby achieving safe measurement to energetic materials such as strong oxidants. The chip-based microinstrument surpasses the state-of-the-art commercial TG/DTA instruments that have, in the long term, suffered from large sample-amount (milligram level) requirements and have been unable to measure energetic materials. Compared with commercial instruments, the chip-based microinstrument is advantageous given its more accurate analysis, much higher heating rate, much smaller instrument volume and much lower power consumption, etc. The microinstrument has been fabricated by using wafer-level MEMS techniques. Testing results show that the mass-detection sensitivity of the TG-chip is as high as 0.45 Hz/pg in air and the temperature sensitivity of the DTA chip achieves 2.9 mV/K under the high heating rate of 25 °C/s. The strong oxidant of KMnO4 is analyzed with the TG/DTA joint characterization under microscopic observation. At the same time as microscope observation of the thermal decomposition phenomena, two-step thermal decomposition process of KMnO4 is identified and the thermal decomposition temperatures are obtained. The TG/DTA microinstrument is promising to be applied for study of various materials.

An LC Wireless Passive Pressure Sensor Based on Single-Crystal MgO MEMS Processing Technique for High Temperature Applications.

An LC wireless passive pressure sensor based on a single-crystalline magnesium oxide (MgO) MEMS processing technique is proposed and experimentally demonstrated for applications in environmental conditions of 900 °C. Compared to other high-temperature resistant materials, MgO was selected as the sensor substrate material for the first time in the field of wireless passive sensing because of its ultra-high melting point (2800 °C) and excellent mechanical properties at elevated temperatures. The sensor mainly consists of inductance coils and an embedded sealed cavity. The cavity length decreases with the applied pressure, leading to a monotonic variation in the resonant frequency of the sensor, which can be retrieved wirelessly via a readout antenna. The capacitor cavity was fabricated using a MgO MEMS technique. This MEMS processing technique, including the wet chemical etching and direct bonding process, can improve the operating temperature of the sensor. The experimental results indicate that the proposed sensor can stably operate at an ambient environment of 22–900 °C and 0–700 kPa, and the pressure sensitivity of this sensor at room temperature is 14.52 kHz/kPa. In addition, the sensor with a simple fabrication process shows high potential for practical engineering applications in harsh environments.

Tunable Terahertz Metamaterial with Electromagnetically Induced Transparency Characteristic for Sensing Application.

We present and demonstrate a MEMS-based tunable terahertz metamaterial (TTM) composed of inner triadius and outer electric split-ring resonator (eSRR) structures. With the aim to explore the electromagnetic responses of TTM device, different geometrical parameters are compared and discussed to optimize the suitable TTM design, including the length, radius, and height of TTM device. The height of triadius structure could be changed by using MEMS technique to perform active tunability. TTM shows the polarization-dependent and electromagnetic induced transparency (EIT) characteristics owing to the eSRR configuration. The electromagnetic responses of TTM exhibit tunable characteristics in resonance, polarization-dependent, and electromagnetically induced transparency (EIT). By properly tailoring the length and height of the inner triadius structure and the radius of the outer eSRR structure, the corresponding resonance tuning range reaches 0.32 THz. In addition to the above optical characteristics of TTM, we further investigate its potential application in a refraction index sensor. TTM is exposed on the surrounding ambient with different refraction indexes. The corresponding key sensing performances, such as figure of merit (FOM), sensitivity (S), and quality factor (Q-factor) values, are calculated and discussed, respectively. The calculated sensitivity of TTM is 0.379 THz/RIU, while the average values of Q-factor and FOM are 66.01 and 63.83, respectively. These characteristics indicate that the presented MEMS-based TTM device could be widely used in tunable filters, perfect absorbers, high-efficient environmental sensors, and optical switches applications for THz-wave optoelectronics.

Al/Ti-film-resistor based thermal heater with Zig-Zag configuration for MEMS application

In this paper, we proposed a micro-heater based on Al/Ti-film resistors for MEMS application. The film heater, designed with unique Zig-Zag configuration, has a high heating efficiency as generated Joule heat could be confined in Zig-Zag area. Consequently, much higher maximum temperature could be achieved under the lower applied voltage, compared with conventional strategy. Based on MEMS technique, thin film based micro-heater was fabricated, and experimental characterization was carried out in room temperature. Since the micro-heaters are all in the form of thin film-resistors, the structure design is simple and fabrication process compatibility is good, which is particularly suitable in the fields of MEMS integration and actuation application.

Investigation of structural electron density distribution and enhanced electrochemical properties of spinel structured MnCo2O4/polyaniline nanocomposite prepared by facile and economical method

A nanocomposite of AB2O4 structured binary metal oxide and conducting polymer was synthesized by cost effective procedure. The procedure for synthesis of chosen MnCo2O4 was solution combustion method and the composite MnCo2O4/polyaniline was obtained by physical blending. The procedure involved was very simple, speedy and inexpensive. The derived results from Scherrer and size strain plot methods showed good concurrence. The Strain value of the composite from Scherrer was 0.3406 and from SSP was 0.3332. Lattice parameter revealed in Rietveld method also matched well with Scherrer. The observations of functional groups through FT-IR confirmed the purity of the samples and presence of polyaniline and metal oxide. The morphological changes examined through High Resolution Scanning Electron Microscopy, favored enhanced conductivity in the composite showed that the changes occurred have facilitated more conductivity in the MnCo2O4/polyaniline composite. The size of the nanoparticle revealed by High Resolution Transmission Electron Microscopy (HR-TEM) was 8–50 nm, showing good concurrence with the size observed in XRD which was 10–65 nm and shape in SEM. Selected Area Electron Diffraction patterns were also in confirmation of crystallinity of the samples and planes revealed by XRD. The variations in bond length and electron density distribution investigated by MEM technique, a unique report in this work also assured the improved energy storage potential of the nanocomposite material. The electro chemical analyses viz., cyclic voltammetry, chronopotentiometry and Electrochemical Impedance Spectroscopy by a three electrode assemblage with 6 M KOH as electrolyte have supported the above findings by confirming higher specific capacitance and low resistance. In the nanocomposite material synthesized. The maximum specific capacitance of the MnCo2O4/polyaniline nanocomposite was 875 F/g and, the pure MnCo2O4 was 713 F/g at the scan rate 5 mV/S. The ESR value for pure was 4.07 Ω and that of the composite electrode was 2.75 Ω.

Cryogen adaptive and integrated differential pressure sensor for level sensing based on an optical Fabry-Perot interferometer.

In this paper, we introduce a cryogen-adaptive sensor based on a micro-electromechanical system (MEMS) for level measurement of cryogenic fluids. The sensor is fabricated by an optical fiber inserted in a glass ferrule and an integrated Fabry-Perot (FP) chip using the MEMS technique. We carried a liquid nitrogen level measurement experiment to verify the performance of the sensor and a low coherent interference system is used to transform the liquid level to absolute phase. The measuring range is 24 cm and can be expanded more widely. The experimental results show that the sensor has a good monotonic linear response (coefficient determination ${{\rm R}^2} \gt {0.998}$R2>0.998), and the measurement error is less than ${ \pm 5}\;{\rm mm}$±5mm in liquid nitrogen. The excellent cryogenic temperature performance from $ - {260}^\circ {\rm C}$-260∘C to $ - {100}^\circ {\rm C}$-100∘C also is demonstrated, which shows the potential application in level measurement of various cryogenic liquids.

Micro thermal conductivity detector based on SOI substrate with low detection limit

In this paper, a micro thermal conductivity detector (TCD) with suspended thermistors based on a Silicon-On-Insulator (SOI) substrate was successfully fabricated by using a MEMS technique. The top layer of silicon of the SOI substrate was utilized as the supporting layer of the thermistor, which possessed excellent mechanical strength. The buried SiO2 was utilized as the etch stop layer of deep reactive ion etching in the thermistor release process, which precisely defined the channels with a high depth-to-width ratio and acquired an ultra-small dead volume (∼200 nL). The micro TCD showed sensitive detection at levels as low as 5 ppm of n-alkanes C14-C16 mixtures. Besides, the micro TCD demonstrated excellent universal detection to gas mixtures, which detected all thirteen analytes in the standard sample of n-alkanes C8-C20 mixtures and all eight analytes in the standard sample of VOCs (volatile organic compounds).

Effect of a flow behavior on the thermal performance of closed-loop and closed-end pulsating heat pipes

In this study, the effect of a flow behavior is taken into account to determine which type of a pulsating heat pipe (PHP) performs better between a closed-loop pulsating heat pipe (CLPHP) and a closed-end pulsating heat pipe (CEPHP). The thermal performances of a CLPHP and a CEPHP are experimentally evaluated and compared. For this, MEMS techniques are used to fabricate silicon-based PHPs with ten turns. Pyrex glass covers the etched silicon wafer to visualize the flow behavior inside the PHPs. The PHPs have rectangular channels with a hydraulic diameter of 923μm. Ethanol and R-134a are used as the working fluids. Based on the experimental data, a criterion of a diameter ratio (Dh/Dh, crit) is proposed to quantify the effect of a flow behavior on the thermal performance of two types of PHPs. A CEPHP performs better than a CLPHP when Dh/Dh, crit < 0.5. In the case of PHPs filled with ethanol (Dh/Dh, crit = 0.29), the CEPHP shows 33% lower thermal resistance and 20% higher maximum allowable heat flux than the CLPHP. In this case, an oscillation mode is observed inside both types of PHPs. In contrast, a CLPHP performs better than a CEPHP when Dh/Dh, crit > 0.5. In the case of PHPs filled with R-134a (Dh/Dh, crit = 0.69), the CLPHP shows 72% lower thermal resistance and 117% higher maximum allowable heat flux than the CEPHP. In this case, a circulation mode is observed inside the CLPHP, while an oscillation mode is observed inside the CEPHP. To explain why one type of PHP performs better than the other, an average volumetric fraction is introduced. An average volumetric fraction in the condenser section is experimentally shown to be a major contributor to the thermal performance; an increase of the average volumetric fraction in the condenser section leads to an improvement of the thermal performance due to enhanced latent heat transfer in the condenser section.

Finite element analysis of fiber-optic Fabry-Perot pressure sensors based on silicon diaphragms.

Fiber-optic Fabry-Perot pressure sensors based on silicon diaphragms of different thicknesses were fabricated using surface and bulk MEMS techniques in this study. The multi-beam interference resulting from multiple reflecting mirrors with the elastic deformation of the Fabry-Perot sensor was simulated by finite element analysis. The pressure sensitivities of the sensors with different diaphragm thicknesses and the relationship between the pressure and the wavelength shift were simulated. The simulation results were in good agreement with the test results. This study provides guidance for future sensor models and parameter design.

Commercializing Piezo-MEMS electroacoustic sensors

Microphones are ubiquitous, appearing in consumer electronic devices like mobile phones, smart speakers, and voice activated appliances. In this talk, one path taken to transitioning a particular University-discovered technology, a piezoelectric micro-electro-mechanical-systems microphone, from a research project to a venture backed company (Vesper Technologies) will be described. The low dielectric loss of aluminum nitride (AlN) combined with its elastic and piezoelectric properties proved to be the enabling material for the design of electroacoustic sensors with low input referred noise for in-air acoustic sensing. The availability of industrial quality AlN deposition tools has made the manufacture of these devices at scale for the consumer electronics industry possible. Designs that mitigate the deleterious effects of variable residual stresses and allow for consistent sensitivity over the wafer (from device to device) are described. The flexibility afforded by MEMS technique has enabled the fabrication of various other sensors and experimental examples of function along with modeling results will be shown for both in-air and underwater devices. The potential advantages of enhanced piezoelectric materials will be presented. Mechanisms for leveraging University, Governmental (Local, State, and Federal), and Equity based funding sources are described.

Halbach undulators using right triangular magnets

Undulators and wigglers based on Halbach arrays have been in widespread use since their description in the 1980s, due to the high fields they generate from permanent magnets. The most common implementation of a pure permanent magnet Halbach undulator consists of four magnetic segment widths per undulator period. In typical undulators with periods of order $\ensuremath{\sim}\mathrm{cm}$ it is straightforward to use sufficiently small magnetic segments to achieve the desired undulator period. However, in the context of new microundulators with periods of order $\ensuremath{\sim}\mathrm{mm}$ or shorter, there is a lower bound on the feature sizes achievable by existing fabrication techniques. This motivates the use of modified Halbach arrays comprised of right triangle magnet segments with only two magnet segment widths per undulator period. These configurations are demonstrated to have superior field strengths to up-down arrays with no increase to the undulator period. Additional considerations arising from the broken symmetries in such configurations, such as the presence of previously forbidden harmonic components, are also considered. Such right triangle Halbach arrays may be fabricated using MEMS techniques for use in the next generation of compact light sources.

Characterization of Graphene based Capacitive Microphone

This research focuses on the design, fabrication and characterization of the graphene based capacitive microphone. Finite element analysis (FEA) is first simulated in order to design and study the proposed graphene based capacitive microphone. While the fabrication introduced MEMS technique in order to reduce the physical size, volume and cost without neglecting the performance. This study discusses on physical characteristics of graphene diaphragm for capacitive microphone. The fabrication of 200 nm air gap and the free-standing suspended graphene with the contribution of the van der Waals force between the graphene layer as a diaphragm and the substrate are presented in this study. The first stage involved in this study was the photolithography process of patterning electrodes with 4 different dimensions of diaphragm. The characterization was performed by using surface profilometer, optical microscopy, Raman spectroscopy and FESEM to evaluate the physical characteristics of the diaphragm. In the last stage, LCR meter was used to measure the capacitive change with different diameter of graphene diaphragm within frequency range of 20 Hz to 20 kHz. FEA analysis showed the good sensitivity against the frequency response for the largest proposed diameter of diaphragm.

Experimental study on a criterion for normal operation of pulsating heat pipes in a horizontal orientation

In this study, a criterion for normal operation of micro pulsating heat pipes (MPHPs) even in a horizontal orientation is developed through experimental and theoretical investigations. Using MEMS techniques, a ten-turn closed-loop MPHP with overall dimensions of 50 × 33.5 × 1.7 mm3 is fabricated. The rectangular micro-channels engraved on a silicon wafer have a width of 1 mm and a depth of 0.5 mm. Ethanol is used as the working fluid and the filling ratio is 47% on the average. To allow flow visualization, a glass wafer is bonded to the silicon wafer. The temporal variations in the temperature and pressure of two neighboring vapor plugs in the MPHP are measured using micro-thermocouples and pressure transducers. The flow behavior in the MPHP is recorded using a high-speed camera and fully synchronized in real time with the measured temperature and pressure data. To find a criterion for normal operation of PHPs, the MPHP is treated as a mass-spring-damper system, and it is postulated that the transition from (to) a stopover to (from) a large-amplitude oscillation in the MPHP occurs with negative (positive) damping. This postulate is experimentally confirmed with the MPHP operating in a horizontal orientation. Finally, a figure of merit for normal operation of PHPs is proposed. The ratio of the minimum thermal resistance in a vertical orientation to that in a horizontal orientation (Rth, V, min/Rth, H, min) is proportional to (Dh/Leff)2, the square of the ratio of the hydraulic diameter to the effective length of PHPs. For PHPs filled with ethanol, it is experimentally confirmed that Dh/Leff has to be greater than approximately 0.03 for normal operation without any stopovers in a horizontal orientation.