Calorimetric Flow Research Articles

Polysaccharide solutions and gels: Isothermal dehydration study by dynamic calorimetric experiments with DSC

The isothermal dehydration of aqueous biosystems is a relevant topic in food, pharmaceutical and cosmetic industry and has been recently investigated for the assessment of a model calorimetric set-up and for the characterization of the parameters featuring the experimental calorimetric curve.In this study, the experimental Differential Scanning Calorimetry (DSC) data obtained under controlled conditions in isothermal mode have been collected on the dehydration of films consisting of solutions and gels of alginate, hydroxypropylmethylcellulose (HPMC), trehalose and mixtures thereof. Based on the proportionality between the calorimetric heat flow and water activity (aw) of solutions of known aw, the values calculated from calorimetry have been compared to those obtained with classic hygrometric measurements revealing a good consistency between the methods. Furthermore, the experimental data were mathematically turned into desorption isotherms, providing a continuous description of the water activity down to the low water activity limit. This experimental method represents an innovative approach to support other consolidated analytical techniques in the physico-chemical characterization of aqueous systems and, more importantly, a step forward in the determination of water activity as a continuous measurement in a timeframe far shorter than that necessary with other instruments (e.g., hygrometers).

Simulation of a membrane-based gas flow sensor

The output characteristics of a membrane-based heat-resistant calorimetric gas flow sensor are analyzed. The dependence of the output characteristics on the design parameters of the flow sensor is investigated. The change in heat distribution along the membrane depending on the flow rate is demonstrated and the effect of the distance between thermal resistors, as well as the thickness and material of the membrane, on the characteristics of the flow sensor is investigated. The simulation results are compared with the experimental data. The dependences found can be used for optimizing the design of flow sensors in order to increase their sensitivity and expand the dynamic range of a thermal mass flow meter.

A chip-integrated highly variable thermal flow rate sensor

In this work, we present an approach for seamless integration of a highly variable flow rate sensor in chip based microfluidic devices. This novel, optically readable microfluidic calorimetric flow rate sensor is realized by a combination of an inkjet printed heating element with a fluorescent sensor layer inside microfluidic channels. This enables to read out flow rate induced variances in the temperature profile along the channels, which results in an unsurpassed wide working range of the microfluidic anemometer from the lower nlmin−1 range up to several 100μlmin−1. The system was thoroughly investigated and revealed high flexibility, stability, repeatability and sensitivity.

Investigation of a zirconia co-fired ceramic calorimetric microsensor for high-temperature flow measurements

This paper describes the design, fabrication and characterization of a flow sensor for high-temperature, or otherwise aggressive, environments, like, e.g. the propulsion system of a small spacecraft. The sensor was fabricated using 8 mol% yttria stabilized zirconia (YSZ8) high-temperature co-fired ceramic (HTCC) tape and screen printed platinum paste. A calorimetric flow sensor design was used, with five 80 µm wide conductors, separated by 160 µm, in a 0.4 mm wide, 0.1 mm deep and 12.5 mm long flow channel. The central conductor was used as a heater for the sensor, and the two adjacent conductors were used to resistively measure the heat transferred from the heater by forced convection. The two outermost conductors were used to study the influence of an auxiliary heat source on the sensor. The resistances of the sensor conductors were measured using four-point connections, as the gas flow rate was slowly increased from 0 to 40 sccm, with different power supplied through the central heater, as well as with an upstream or downstream heater powered. In this study, the thermal and electrical integrability of microcomponents on the YSZ8 substrate was of particular interest and, hence, the influence of thermal and ionic conduction in the substrate was studied in detail. The effect of the ion conductivity of YSZ8 was studied by measuring the resistance of a platinum conductor and the resistance between two adjacent conductors on YSZ8, in a furnace at temperatures from 20 to 930 °C and by measuring the resistance with increasing current through a conductor. With this design, the influence of ion conductivity through the substrate became apparent above 700 °C. The sensitivity of the sensor was up to 1 mΩ sccm−1 in a range of 0–10 sccm. The results show that the signal from the sensor is influenced by the integrated auxiliary heating conductors and that these auxiliary heaters provide a way to balance disturbing heat sources, e.g. thrusters or other electronics, in conjunction with the flow sensor.

A high-temperature calorimetric flow sensor employing ion conduction in zirconia

This paper presents the use of the temperature-dependent ion conductivity of 8 mol % yttria-stabilized zirconia (YSZ8) in a miniature high-temperature calorimetric flow sensor. The sensor consists of 4 layers of high-temperature co-fired ceramic (HTCC) YSZ8 tape with a 400 μm wide, 100 μm deep, and 12 500 μm long internal flow channel. Across the center of the channel, four platinum conductors, each 80 μm wide with a spacing of 160 μm, were printed. The two center conductors were used as heaters, and the outer, up- and downstream conductors were used to probe the resistance through the zirconia substrate around the heaters. The thermal profile surrounding the two heaters could be made symmetrical by powering them independently, and hence, the temperature sensing elements could be balanced at zero flow. With nitrogen flowing through the channel, forced convection shifted the thermal profile downstream, and the resistance of the temperature sensing elements diverged. The sensor was characterized at nitrogen flows from 0 to 40 sccm, and resistances at zero-flow from 10 to 50 MΩ. A peak sensitivity of 3.1 MΩ/sccm was obtained. Moreover, the sensor response was found to be linear over the whole flow range, with R2 of around 0.999, and easy to tune with the individual temperature control of the heaters. The ability of the sensor to operate in high temperatures makes it promising for use in different harsh environments, e.g., for close integration with microthrusters.

Downregulation of microRNA-498 in colorectal cancers and its cellular effects

miR-498 is a non-coding RNA located intergenically in 19q13.41. Due to its predicted targeting of several genes involved in control of cellular growth, we examined the expression of miR-498 in colon cancer cell lines and a large cohort of patients with colorectal adenocarcinoma. Two colon cancer cancer cell lines (SW480 and SW48) and one normal colonic epithelial cell line (FHC) were recruited. The expression of miR-498 was tested in these cell lines by using quantitative real-time polymerase chain reaction (qRT-PCR). Tissues from 80 patients with surgical resection of colorectum (60 adenocarcinomas and 20 non-neoplastic tissues) were tested for miR-498 expression by qRT-PCR. In addition, an exogenous miR-498 (mimic) was used to detect the miRNA׳s effects on cell proliferation and cell cycle events in SW480 using MTT calorimetric assay and flow cytometry respectively. The colon cancer cell lines showed reduced expression of miR-498 compared to a normal colonic epithelial cell line. Mimic driven over expression of miR-498 in the SW480 cell line resulted in reduced cell proliferation and increased proportions of G2-M phase cells. In tissues, miR-498 expression was too low to be detected in all colorectal adenocarcinoma compared to non-neoplastic tissues. This suggests that the down regulation of miR-498 in colorectal cancer tissues and the direct suppressive cellular effect noted in cancer cell lines implies that miR-498 has some direct or indirect role in the pathogenesis of colorectal adenocarcinomas.

Regulation of microRNA‐1288 in colorectal cancer: Altered expression and its clinicopathological significance

We aim to examine the miR-1288 expression in cancer cell lines and a large cohort of patients with colorectal cancer. Two colon cancer cell lines (SW480 and SW48) and one normal colonic epithelial cell line (FHC) were recruited. The miRNA expressions of miR-1288 were tested on these cell lines by using quantitative real-time polymerase chain reaction (qRT-PCR). An exogenous miR-1288 (mimic) was used to detect cell proliferation and cell cycle changes in SW480 using MTT calorimetric assay and flow cytometry, respectively. In addition, tissues from 122 patients with surgical resection of colorectum (82 adenocarcinomas, 20 adenomas, and 20 non-neoplastic tissues) were tested for miR-1288 expression by qRT-PCR. The colon cancer cell lines showed reduced expression of miR-1288 compared to normal colonic epithelial cell line. Over expression of miR-1288 in SW480 cell line showed increased cell proliferation and increased G2-M phase cells. In tissues, reduced miR-1288 expression was noted in majority of colorectal adenocarcinoma compared to colorectal adenoma and non-neoplastic tissues. Reduced or absent expression of miR-1288 was noted in 76% (n = 62/82) of the cancers. The expression levels of miR-1288 were higher in distal colorectal adenocarcinomas (P = 0.013) and in cancers of lower T staging (P = 0.033). To conclude, alternation of miR-1288 expression is important in the progression of colorectal cancer. The differential regulation of miR-1288 was found to be related to cancer location and pathological staging in colorectal cancers.

Simultaneous flow and thermal conductivity measurement of gases utilizing a calorimetric flow sensor

This contribution presents a calorimetric flow sensor that is capable of determining the thermal conductivity of gases (k) under flow conditions. The measuring principle relies on using high frequency heat generation at 200Hz in order to confine the AC heat transfer into a thin region over the surface of the channel wall, where the flow velocity profile is always close to zero. The thermal conductivity of several common gases and their mixtures is measured for flow rates up to 750sccm. The sensor takes advantage of DC excitation to measure the flow rate (Q) provided that the volumetric heat capacity (ρcp) or the thermal diffusivity (α=k/(ρcp)) of the gas is known. This paper also presents two analytical models that qualitatively describe the measuring principles. Theoretically predicted functions fit well the experimental results.

Finite element analysis of the effect on employing thermal through vias and heat fingers to increase heat transfer to fluid in calorimetric flow sensors

Measurement results of a robust silicon calorimetric flow sensor with a 25μm thick silicon dioxide membrane with thermal silicon vias have been compared with results obtained from three-dimensional Finite Element Analysis (FEA). Based on the fabricated device, the sensor has been further developed to include heat-exchanging fingers extending down into the integrated flow channel for increased heat transfer. Using FEA, different designs of the fingers have been compared with respect to signal strength, sensitivity, power consumption and pressure loss in the channel at flow rates from 0 to about 650sccm. Using heat fingers, the sensor signal was improved by a factor of five. The sensor signal, i.e. the temperature difference between downstream and upstream elements, was more than 60°C when the central heater was heated 300°C above room temperature, which was comparable to a thin-membrane device modeled. The maximum sensitivity using the finger design was about 1.4°Csccm−1, and the maximum power consumption was almost 700mW, which is considerably higher than for thin-membrane sensors. A figure of merit used for evaluation, was the ratio of signal strength to power consumption. The results show that the device design is a promising concept that is suitable in systems requiring robust monolithically integratable flow sensors.

High-temperature zirconia microthruster with an integrated flow sensor

This paper describes the design, fabrication and characterization of a ceramic, heated cold-gas microthruster device made with silicon tools and high temperature co-fired ceramic processing. The device contains two opposing thrusters, each with an integrated calorimetric propellant flow sensor and a heater in the stagnation chamber of the nozzle. The exhaust from a thruster was photographed using schlieren imaging to study its behavior and search for leaks. The heater elements were tested under a cyclic thermal load and to the maximum power before failure. The nozzle heater was shown to improve the efficiency of the thruster by 6.9%, from a specific impulse of 66 to 71 s, as calculated from a decrease of the flow rate through the nozzle of 13%, from 44.9 to 39.2 sccm. The sensitivity of the integrated flow sensor was measured to 0.15 mΩ sccm−1 in the region of 0–15 sccm and to 0.04 mΩ sccm−1 above 20 sccm, with a zero-flow sensitivity of 0.27 mΩ sccm−1. The choice of yttria-stabilized zirconia as a material for the devices makes them robust and capable of surviving temperatures locally exceeding 1000 °C.

Membrane-based thermal flow sensors on flexible substrates

We investigated the fabrication, integration and resulting characteristics of a thermal flow sensor on flexible substrates with functional layers (heater, thermopiles for temperature measurement) being located on a membrane. The sensor was fabricated by realizing all high temperature deposition and structuring processes on a silicon wafer first followed by a transfer of the functional layers onto a flexible polyimide substrate. An integration of the sensor by flip chip soldering and conductive adhesives has successfully been demonstrated.The flexible flow sensor presented here is a further development of a standard membrane-based thermal flow sensor using silicon substrate with similar design of the functional layers. Thus, it has given us the chance to make a direct comparison between a flexible and non-flexible approach. The sensors’ characteristics have been experimentally evaluated in terms of response to fluid flow, response time and minimum detectable flow rate and compared with an existing similar silicon-based design. It has turned out that the membrane-based flexible flow sensors integrated by soldering showed expected characteristics of a calorimetric flow sensor. A second – not membrane-based – flexible flow sensor approach being integrated by conductive adhesives showed differences in the response to fluid flow which can be explained by the changed heat distribution. Sensitivities (at zero flow) being measured have values between 0.51mV/m/s (soldered) and 20.1mV/m/s (bonded by adhesives) compared to 30.4mV/m/s (silicon substrate).Finally, an application is presented showing the successful integration of the flexible flow sensor on an airfoil without restricting its structural health. A wind tunnel measurement has been done demonstrating capability of the flexible flow sensor concept for detection of boundary layer separation and reattachment on the overflowed surface.

Measurement and simulation of the frequency response of a thermal flow sensor at different flow speeds

This contribution presents the complete (amplitude and phase) frequency response of a calorimetric flow sensor in a free, laminar air stream at different flow speeds ranging from 0 to 5m/s. Given the geometry of the sensor, which features a central thermistor closely surrounded by a heater element, the complexity of the resulting frequency response is reduced by applying a simple transformation. This transformation involves taking the ratio between the temperature oscillations at the surrounding thermistors and the central thermistor. The resulting frequency response shows a second order transfer function of real poles. The parameters of this transfer function are used to analyze the effect of the flow speed on the frequency response. Results show that for air, the position of the first characteristic pole remains constant at the downstream thermistor for flow speeds up to 1m/s. Experimental and simulated results are consistent, which allows the further study of the sensor's response by means of numerical simulations in order to determine the effect of the thermal properties of the fluids on the frequency response. The ultimately goal is to achieve fluid independent flow measurement.

A highly integratable silicon thermal gas flow sensor

Thermal flow sensors have been designed, fabricated, and characterized. All bulk material in these devices is silicon so that they are integratable in silicon-based microsystems. To mitigate heat losses and to allow for use of corrosive gases, the heating and sensing thin film titanium/platinum elements, injecting and extracting heat, respectively, from the flow, are placed outside the channel on top of a membrane consisting of alternating layers of stress-balancing silicon dioxide and silicon nitride. For the fabrication, an unconventional bond surface protection method using sputter-deposited aluminum instead of thermal silicon dioxide is used in the process steps prior to silicon fusion bonding. A method for performing lift-off on top of the transparent membrane was also developed. The sensors, measuring 9.5 × 9.5 mm2, are characterized in calorimetric and time-of-flight modes with nitrogen flow rates between 0 sccm and 300 sccm. The maximum calorimetric sensor flow signal and sensitivity are 0.95 mV and 29 µV sccm−1, respectively, with power consumption less than 40 mW. The time-of-flight mode is found to have a wider detectable flow range compared with calorimetric mode, and the time of flight measured indicates a response time of the sensor in the millisecond range. The design and operation of a sensor with high sensitivity and large flow range are discussed. A key element of this discussion is the configuration of the array of heaters and gauges along the channel to obtain different sensitivities and extend the operational range. This means that the sensor can be tailored to different flow ranges.

Measurement technique for the thermal properties of thin-film diaphragms embedded in calorimetric flow sensors

In diaphragm-based micromachined calorimetric flow sensors, convective heat transfer through the test fluid competes with the spurious heat shunt induced by the thin-film diaphragm where heating and temperature sensing elements are embedded. Consequently, accurate knowledge of thermal conductivity, thermal diffusivity, and emissivity of the diaphragm is mandatory for design, simulation, optimization, and characterization of such devices. However, these parameters can differ considerably from those stated for bulk material and they typically depend on the production process. We developed a novel technique to extract the thermal thin-film properties directly from measurements carried out on calorimetric flow sensors. Here, the heat transfer frequency response from the heater to the spatially separated temperature sensors is measured and compared to a theoretically obtained relationship arising from an extensive two-dimensional analytical model. The model covers the heat generation by the resistive heater, the heat conduction within the diaphragm, the radiation loss at the diaphragm’s surface, and the heat sink caused by the supporting silicon frame. This contribution summarizes the analytical heat transfer analysis in the microstructure and its verification by a computer numerical model, the measurement setup, and the associated thermal parameter extraction procedure. Furthermore, we report on measurement results for the thermal conductivity, thermal diffusivity, and effective emissivity obtained from calorimetric flow sensor specimens featuring dielectric thin-film diaphragms made of plasma enhanced chemical vapor deposition silicon nitride.

Kinetics of Enzymatic High-Solid Hydrolysis of Lignocellulosic Biomass Studied by Calorimetry

Enzymatic hydrolysis of high-solid biomass (>10% w/w dry mass) has become increasingly important as a key step in the production of second-generation bioethanol. To this end, development of quantitative real-time assays is desirable both for empirical optimization and for detailed kinetic analysis. In the current work, we have investigated the application of isothermal calorimetry to study the kinetics of enzymatic hydrolysis of two substrates (pretreated corn stover and Avicel) at high-solid contents (up to 29% w/w). It was found that the calorimetric heat flow provided a true measure of the hydrolysis rate with a detection limit of about 500pmol glucose s(-1). Hence, calorimetry is shown to be a highly sensitive real-time method, applicable for high solids, and independent on the complexity of the substrate. Dose-response experiments with a typical cellulase cocktail enabled a multidimensional analysis of the interrelationships of enzyme load and the rate, time, and extent of the reaction. The results suggest that the hydrolysis rate of pretreated corn stover is limited initially by available attack points on the substrate surface (<10% conversion) but becomes proportional to enzyme dosage (excess of attack points) at later stages (>10% conversion). This kinetic profile is interpreted as an increase in polymer end concentration (substrate for CBH) as the hydrolysis progresses, probably due to EG activity in the enzyme cocktail. Finally, irreversible enzyme inactivation did not appear to be the source of reduced hydrolysis rate over time.

A Cross-Type Thermal Wind Sensor With Self-Testing Function

A 2-D smart flow sensor with self-testing function was designed, fabricated and tested in this paper. It is composed of cross-structure heaters and four symmetrically located sensing parts. When the flow angle changes in the clockwise direction, the temperature differences among the four parts, namely Mode 1 given by T13(T24) and Model 2 given by T12(T14) will be Sine(Cosine) functions of wind direction. Further study shows that the magnitude of mode 1 is 1.414 times that of Mode 2, and the phase of Mode 2 leads Mode 1 by 45°. By using the fixed phase gap between the two modes, the self-test function can be realized without additional components. In order to achieve a high sensitivity and robust sensor, thermal insulated substrate was introduced to fabricate calorimetric flow sensor instead of silicon wafers. The proposed sensor was fabricated on the Pyrex7740 substrate using single liftoff process. Finally, a wind tunnel test was carried out in constant power (CP) mode, and the test results show reasonable agreement with the simulation curves.

A computer mouse based on highly sensitive micromachined flow sensors

A PC mouse based on micromachined calorimetric flow sensors is presented. Two cylindrical openings in the bottom of the device are equipped with highly sensitive flow sensors (for the x- and y-velocity components). Due to fluid adherence to the boundary (mouse pad), the device motion induces an air-flow vortex in the cavities. A microcontroller processes the sensor signals and sends the velocity information via USB interface to the PC. In order to obtain the optimum cavity dimensions, comprehensive 2D and 3D models of the device were developed. A mouse prototype was build as a plug-and-play device and characterized utilizing a specially developed measurement setup. The measured characteristics are in good agreement with the simulated behavior. The device features very low power consumption and functions independently of the reflectivity and surface roughness of the mouse pad.

Direct Chip Attachment Packaging of a 2-D Thermal Flow Sensor

The design,fabrication and test of a direct chip attach packaged thermal flow sensor is given.The circular structure sensor measures the 2-D wind speed and direction by using calorimetric principle.The calorimetric flow sensor is fabricated by using two-step metal lift-off process on a ceramic slice.Then the KGD is glued to the backside of PCB board with package resin,and wire is bonded to the front side of PCB through a prefabricated hole.Finally,the chip is capsulated using thermal insulated package resin on the front side.The wind tunnel test of the packaged sensor chip is carried out.The sensor can detect 360°wind direction in a wind speed range of 6 m/s.

The Entropies of n-Heptane, n-Octane, n-Nonane, and n-Undecane over a Wide Range of the Liquid and Gaseous States

The entropies S of C7–C11 normal alkanes over the temperature and pressure ranges 300–620 K and 0.5–60 MPa were obtained by the integration of the experimental isobars of isobaric heat capacity Cp. The Cp data were obtained using a flow adiabatic calorimeter with calorimetric flow rate measurements. The entropy values were determined up to critical temperatures by extrapolation from the sides of liquid S′ and gas S″ phases. The p-T-S thermodynamic surfaces of the alkanes under consideration in the liquid and gas phases were constructed. A generalized temperature dependence of S′ and S″ was determined for the homologous series over the temperature range from the triple to critical point within the framework of similarity theory.

Investigation of kinetics of immobilized liver esterase by flow calorimetry

Flow calorimetry (FC) was shown to be a powerful tool for investigation of the kinetics of phenyl acetate hydrolysis catalyzed by pig liver carboxyl esterase. The enzyme was immobilized in alginate gel particles that were placed in a calorimetric flow column and the heat effect of enzyme reaction was followed in single flow and total recirculation conditions. It was shown that the registered temperature change was proportional to molar amount of substrate transformed in the column. A mathematical model describing the enzyme reaction, mass transfer, and heat effects in the calorimetric system was developed and used for the kinetic data evaluation. By combining data from single flow and recirculation modes true kinetic parameters were evaluated by the proposed mathematical procedure based on the model solution and successive approximations.The kinetic data for carboxyl esterase showed a slide substrate inhibition by phenyl acetate. The obtained kinetic parameters were as follows: Michaelis constant Km=2mmoldm−3 and substrate inhibition constant Ki=42mmoldm−3. The method can be applied to kinetic study of immobilized enzymes directly in the flow calorimeter without any requirement of an independent analytical technique.