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Microfluidics at High Pressures: Understanding, Sensing, and Control
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Microsystems Technology.ORCID iD: 0000-0002-3966-0220
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis explores understanding, sensing, and control in high-pressure microfluidics. The high-pressure regime allows fluids to be forced through narrow channels at substantial speed and creates conditions for fluids of high density and low viscosity—features desired in flow-based chemical analyses. With changes to pressure and temperature, fluid properties vary, and for miniaturized flow systems, sensing and control are needed.

For miniaturized chemical analytics to utilize high-pressure fluids, like supercritical CO2, sensors are required for flow characterization. In this thesis, high-pressure tolerant sensors in glass chips have been developed and investigated. By the use of chip-integrated temperature, flow, and relative permittivity sensors, the variable behavior of supercritical CO2 or binary component CO2-alcohol mixtures have been investigated. To be able to change flow rates, a heat-based actuator chip has been developed. By a flow control system, which combines a relative permittivity sensor and heat actuated flow regulators on a modular system, the composition of binary component CO2-alcohol mixtures can be tuned and controlled with feedback.

Flows of multiphase CO2-H2O hold promise for miniaturized extraction systems. In this thesis, parallel multiphase CO2-H2O flow has been studied. To achieve control, methods have been investigated where channels have been modified by the introduction of a guiding ridge and altered by a chemical coating. Flow is a dynamic process, where pressure and temperature can vary with time and place. As the properties of fluids containing CO2 may change with pressure and temperature, properties will also change with time and place. Because of this, instruments with spatial and temporal resolution are needed to better understand dynamic chemical effects at flow. In this thesis, a tool is presented to study the dynamic acidification of aqueous solutions that come in contact with flowing CO2.

By a study performed to understand the strength and pressure tolerance of glass chips, it has been found that the fracture is not only determined by the applied pressure, but also on time and environment.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 60
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1687
Keywords [en]
High-pressure microfluidics, supercritical CO2, compressible flow, relative permittivity, integrated electrodes
National Category
Chemical Engineering Materials Engineering
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
URN: urn:nbn:se:uu:diva-353964ISBN: 978-91-513-0372-7 (print)OAI: oai:DiVA.org:uu-353964DiVA, id: diva2:1221121
Public defence
2018-09-14, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2018-08-21 Created: 2018-06-19 Last updated: 2018-08-27
List of papers
1. Fracture strength of glass chips for high-pressure microfluidics
Open this publication in new window or tab >>Fracture strength of glass chips for high-pressure microfluidics
2016 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 26, no 9, article id 095009Article in journal (Refereed) Published
Abstract [en]

High-pressure microfluidics exposes new areas in chemistry. In this paper, the reliability of transparent borosilicate glass chips is investigated. Two designs of circular cavities are used for fracture strength tests, either 1.6 mm wide with rounded corners to the fluid inlets, or 2.0 mm wide with sharp inlet corners. Two kinds of tests are done, either short-term,e.g. pressurization to fracture at room temperature, or long-term, with fracture at constant pressurization for up to one week, in the temperature region 11–125 °C. The speed of crack fronts is measured using a high-speed camera. Results show fracture stresses in the range of 129 and 254 MPa for short-term measurements. Long-term measurements conclude the presences of a temperature and stress dependent delayed fracture. For a reliability ofone week at 11–38 °C, a pressure limit is found at the lower end of the short-term measurements, or 15% lower than the average. At 80 °C, this pressure limit is 45% lower. Crack speeds are measured to be 10−5 m s-1 during short-term fracture. These measurements are comparable with estimations based on slow crack growth and show that the growth affects the reliability of glass chips. This effect is strongly affected by high temperatures, thus lowers the operating window of high-pressure glass microfluidic devices.

Keywords
glass, fracture strength, high pressure microfluidics, crack growth, temperature dependence
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-309983 (URN)10.1088/0960-1317/26/9/095009 (DOI)000402408400009 ()
Funder
Swedish Research Council, 2011:5037Knut and Alice Wallenberg Foundation
Available from: 2016-12-08 Created: 2016-12-08 Last updated: 2018-06-19Bibliographically approved
2. Influence of flow rate, temperature and pressure on multiphase flows of supercritical carbon dioxide and water using multivariate partial least square regression
Open this publication in new window or tab >>Influence of flow rate, temperature and pressure on multiphase flows of supercritical carbon dioxide and water using multivariate partial least square regression
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2015 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 25, no 10, article id 105001Article in journal (Refereed) Published
Abstract [en]

Supercritical carbon dioxide (scCO2) is often used to replace harmful solvents and can dissolve a wide range of organic compounds. With a favorable critical point at 31 °C and 7.4 MPa, reaching above the critical point for scCO2 is fairly accessible. Because of the compressible nature of scCO2 and the large changes of viscosity and density with temperature and pressure, there is a need to determine the behavior of scCO2 in microfluidic systems. Here, the influence of how parameters such as flow rate, temperature, pressure, and flow ratio affects the length of parallel flow of water and scCO2 and the length of the created CO2 segments are investigated and modeled using multivariate data analysis for a 10 mm long double-y channel. The parallel length and segment size were observed in the laminar regime around and above the critical point of CO2. The flow ratio between the two fluids together with the flow rate influenced both the parallel length and the segment sizes, and a higher pressure resulted in shorter parallel lengths. Regarding the segment length of CO2, longer segments were a result of a higher Weber number for H2O together with a higher temperature in the channel. 

Keywords
Supercritical fluids, microfluidics, carbon dioxide, partial least square regression, principal component analysis, fluid dynamics, multiphase flow
National Category
Engineering and Technology Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-253552 (URN)10.1088/0960-1317/25/10/105001 (DOI)000366827400017 ()
Funder
Swedish Research Council, 2011-5037Knut and Alice Wallenberg Foundation
Available from: 2015-05-29 Created: 2015-05-29 Last updated: 2018-06-19Bibliographically approved
3. Influence of surface modifications and channel structure for microflows of supercritical carbon dioxide and water
Open this publication in new window or tab >>Influence of surface modifications and channel structure for microflows of supercritical carbon dioxide and water
2016 (English)In: Journal of Supercritical Fluids, ISSN 0896-8446, E-ISSN 1872-8162, Vol. 107, p. 649-656Article in journal (Refereed) Published
Abstract [en]

Miniaturization offers a possibility to increase the performance and decrease the time scales of systems. Existing microsystems using supercritical CO2 mainly utilizes multiphase segmented flows. To allow for a broader toolbox for future systems, also parallel flows are useful which eases the separation of the different phases. Here, the effect of different surface coatings are studied for multiphase flows of scCO2 and H2O in flat microchannels, with and without a 4 μm high ridge guide, which allows for pinning of the fluid interface inside the 190 μm wide and 35 μm high channel. Three different surfaces with different wettings towards scCO2 and H2O are studied, where a surface terminated with a hydrocarbon-based silane was observed to be neutral in the H2O/scCO2 system, a surface terminated with a fluorocarbon-based silane was hydrophobic, and an uncoated glass surface was hydrophilic.

Using two flow rates of 5:5 μl/min (CO2:H2O) and 6.5:3.5 μl/min (CO2:H2O), a parallel flow between scCO2 and H2O was observed for uncoated and flat channels where the H2O flow pushed the CO2 to the side, before the flows eventually breaks up into segments. With a ridge guide in the middle of the channel, the interface was pinned at half the channel width, although still breaking up into segments. The neutral hydrocarbon-based surface coating with approximately 90° contact angles resulted in evenly created segments without a ridge guide. Including a guide in the middle of the channel, a parallel flow was observed throughout the channel, although occasionally small CO2 segments entered the H2O outlet. Using the fluorocarbon-based silane resulted in an unstable segmented system with pressure fluctuations.

Using surface modifications, an increased control can be achieved for either segmentation or parallel flow where a neutral surface is favored for a stable flow behavior. Together with a ridge guide, the fluid interface was pinned at the center. 

Keywords
Microfluidics, Supercritical CO2, Silane coating, Parallel flow, Segmented flow, Surface modification
National Category
Engineering and Technology Chemical Engineering
Identifiers
urn:nbn:se:uu:diva-253554 (URN)10.1016/j.supflu.2015.07.027 (DOI)000366077100077 ()
Funder
Swedish Research Council, 2011-5037Knut and Alice Wallenberg Foundation
Available from: 2015-05-29 Created: 2015-05-29 Last updated: 2018-06-19Bibliographically approved
4. Thin film metal sensors in fusion bonded glass chips for high-pressure microfluidics
Open this publication in new window or tab >>Thin film metal sensors in fusion bonded glass chips for high-pressure microfluidics
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2017 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 27, no 1, article id 015018Article in journal (Refereed) Published
Abstract [en]

High-pressure microfluidics offers fast analyses of thermodynamic parameters for compressed process solvents. However, microfluidic platforms handling highly compressible supercritical CO2 are difficult to control, and on-chip sensing would offer added control of the devices. Therefore, there is a need to integrate sensors into highly pressure tolerant glass chips. In this paper, thin film Pt sensors were embedded in shallow etched trenches in a glass wafer that was bonded with another glass wafer having microfluidic channels. The devices having sensors integrated into the flow channels sustained pressures up to 220 bar, typical for the operation of supercritical CO2. No leakage from the devices could be found. Integrated temperature sensors were capable of measuring local decompression cooling effects and integrated calorimetric sensors measured flow velocities over the range 0.5-13.8 mm/s. By this, a better control of high-pressure microfluidic platforms has been achieved.

Keywords
supercritical carbon dioxide, high pressure microfluidics, integrated electrodes, temperature sensing, flow sensing, glass
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-310063 (URN)10.1088/0960-1317/27/1/015018 (DOI)000388703300003 ()
Funder
Swedish Research Council, 2011-5037VINNOVAKnut and Alice Wallenberg Foundation
Note

Part financed through Swedish Agency for the Innovation System, Vinnova, through the Centre for Natural Disaster Science (CNDS)

Available from: 2016-12-09 Created: 2016-12-09 Last updated: 2018-06-19Bibliographically approved
5. A microfluidic relative permittivity sensor for feedback control of carbon dioxide expanded liquid flows
Open this publication in new window or tab >>A microfluidic relative permittivity sensor for feedback control of carbon dioxide expanded liquid flows
2019 (English)In: Sensors and Actuators A-Physical, ISSN 0924-4247, E-ISSN 1873-3069, Vol. 285, p. 165-172Article in journal (Refereed) Published
Abstract [en]

Binary CO2-alcohol mixtures, such as CO2-expanded liquids (CXLs), are promising green solvents for reaching higher performance in flow chemistry and separation processing. However, their compressibility and high working pressure makes handling challenging. These mixtures allow for a tuneable polarity but, to do so, requires precise flow control. Here, a high-pressure tolerant microfluidic system containing a relative permittivity sensor and a mixing chip is used to actively regulate the relative permittivity of these fluids and indirectly—composition. The sensor is a fluid-filled plate capacitor created using embedded 3D-structured thin films and has a linearity of 0.9999, a sensitivity of 4.88 pF per unit of relative permittivity, and a precision within 0.6% for a sampling volume of 0.3 μL. Composition and relative permittivity of CO2-ethanol mixtures were measured at 82 bar and 21 °C during flow. By flow and dielectric models, this relationship was found to be described by the pure components and a quadratic mixing rule with an interaction parameter, kij, of -0.63 ± 0.02. Microflows with a relative permittivity of 1.7–21.4 were generated, and using the models, this was found to correspond to compositions of 6–90 mol % ethanol in CO2. With the sensor, a closed loop control system was realised and CO2-ethanol flows were tuned to setpoints of the relative permittivity in 30 s.

Keywords
Relative permittivity, Process control, CO2-expanded liquids, Binary fluid mixtures, High-pressure microfluidics
National Category
Chemical Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-353945 (URN)10.1016/j.sna.2018.11.015 (DOI)000456902600021 ()
Funder
Knut and Alice Wallenberg Foundation
Available from: 2018-06-19 Created: 2018-06-19 Last updated: 2019-02-25Bibliographically approved
6. A microfluidic control board for high-pressure flow, composition, and relative permittivity
Open this publication in new window or tab >>A microfluidic control board for high-pressure flow, composition, and relative permittivity
2018 (English)In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 90, no 21, p. 12601-12608Article in journal (Refereed) Published
Abstract [en]

Flow control is central to microfluidics and chromatography. With decreasing dimensions and high pressures, precise fluid flows are often needed. In this paper, a high-pressure flow control system is presented, allowing for the miniaturization of chromatographic systems and the increased performance of microfluidic setups by controlling flow, composition and relative permittivity of two-component flows with CO2. The system consists of four chips: two flow actuator chips, one mixing chip and one relative permittivity sensor. The actuator chips, throttling the flow, required no moving parts as they instead relied on internal heaters to change the fluid resistance. This allows for flow control using miniaturized fluid delivery systems containing only a single pump or pressure source. Mobile phase gradients between 49% to 74% methanol in CO2 were demonstrated. Depending on how the actuator chips were dimensioned, the position of this range could be set for different method-specific needs. With the microfluidic control board, both flow and composition could be controlled from constant pressure sources, drift could be removed, and variations in composition could be lowered by 84%, resulting in microflows of CO2 and methanol with a variation in the composition of 0.30%.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:uu:diva-353953 (URN)10.1021/acs.analchem.8b02758 (DOI)000449722500039 ()30269500 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation
Available from: 2018-06-19 Created: 2018-06-19 Last updated: 2018-12-21Bibliographically approved
7. Dynamic pH determination at high pressure of aqueous additive mixtures in contact with dense CO2
Open this publication in new window or tab >>Dynamic pH determination at high pressure of aqueous additive mixtures in contact with dense CO2
Show others...
2018 (English)In: Journal of Supercritical Fluids, ISSN 0896-8446, E-ISSN 1872-8162, Vol. 136, p. 95-101Article in journal (Refereed) Published
Abstract [en]

A system consisting of a high-pressure tolerant microfluidic glass chip, high-speed absorbance imaging, and image processing has been developed to study rapid dynamic events like pH change in a multiphase flow. The system gives both kinetic and quantitative equilibrated information. By tracking the interactions of aqueous additive mixtures and liquid CO2, at 80 bar and 24 °C, under flow, measurement at a given P, T condition is done in 0.25 s. The acidification rate to steady state was found to be mass transport limited, occurring in less than 1 s. For 30 mM of the additives ammonium acetate and ammonium formate, equilibrium pH of 4.5 and 4.1, respectively, was seen. These additives are of key importance in common mobile phases used in SFC.

Keywords
Supercritical fluid chromatography, High-pressure microfluidics, Additive salts, Dense CO, Multiphase flow, Image analysis
National Category
Chemical Engineering Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-353940 (URN)10.1016/j.supflu.2018.02.012 (DOI)000430767400011 ()
Funder
Knut and Alice Wallenberg Foundation
Available from: 2018-06-18 Created: 2018-06-18 Last updated: 2018-08-02Bibliographically approved

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