Cylindrical tubes with large diameters and thin walls for application in microfluidic density and mass flow sensors
Mahdieh Yariesbouei is a PhD student in the department Integrated Devices and Systems. Supervisors are dr.ir. R.J. Wiegerink and prof.dr.ir. J.C Lötters from the Faculty of Electrical Engineering, Mathematics and Computer Science.
A Coriolis mass flow sensor detects the mass flow by utilizing a secondary vibration induced by the Coriolis forces resulting from fluid flow inside a vibrating tube. The amplitude of this secondary vibration is directly proportional to the mass flow, regardless of the fluid properties. Twenty-eight years ago, the first microfluidic density and micro-Coriolis mass flow sensors were introduced. Since then, to improve the performance of the microfluidic sensor, various designs and fabrication methods were proposed by different groups. Most micro-Coriolis mass flow sensors are fabricated based on silicon micromachining techniques, e.g., anisotropic wet and dry etching, polymer photolithography, and surface channel technology. All these fabrication methods lead to a freely suspended channel with a non-circular cross-sectional shape, such as hexagonal, rectangular, or semicircular, and a limited range of wall thicknesses and cross-sectional areas, which leads to a limited flow range and pressure dependency of the sensor. Besides, the ratio of the diameter to wall thickness should be increased to obtain a higher sensitivity to mass flow and fluid density. To overcome these drawbacks, this research investigates new fabrication methods that allow the realization of freely suspended tubes with a circular cross-section and a wide range of diameters, and a relatively thin, chemically inert channel wall. The research specifically focuses on fabrication methods that do not require the frequently used silicon micromachining techniques.
The initial phase of the research focused on developing a novel fabrication method for a freely suspended tube with a circular cross-sectional shape and high diameter-to-wall thickness ratio. Three potential fabrication methods, namely sol-gel deposition, 3D printing, and electroplating, were explored. The sol-gel deposition method, based on the ESCARGOT technique, was able to produce channels with suitable diameters ranging from 100 μm to 1 mm. However, issues were encountered in creating a free-standing tube, including non-uniform channel preparation, cracks in the coating, and damage to the PDMS mold. Despite optimization efforts, this method did not result in a free suspended channel. The second fabrication method involved 3D printing using a Mini 4K Phrozen printer and various types of resin, such as clear resin, Ivory 4K, 4K, 8K grey, and castable wax. Freely suspended tubes were successfully printed with the Mini 4K Phrozen printer. The smallest dimensions that could be achieved were around 500 μm diameter with a minimum wall thickness of 200 μm, for a straight structure. Printing tubes with larger diameters or thicker walls posed fewer challenges. The minimum diameter and wall thickness depend on the structure shape. The third fabrication method was electroplating, which involved the use of a pre-shaped circular cross-section wire of acrylonitrile butadiene styrene (ABS) as a seed layer for the electroplating process. This method achieved a wide range of diameters, from 120 µm to 1 mm, and wall thicknesses, from 8 µm to 60 µm.
The second part of the research focuses on realizing a demonstrator for application in density and mass flow sensing with the most promising fabrication methods: 3D printing and electroplating. The modeling, fabrication, and test of a fully 3D-printed Coriolis mass flow sensor are reported. A mass flow sensor was designed based on a 3D-printed U-shaped tube with a total length of 42 mm. The tube had a circular cross-section with an inner diameter of about 900 μm, and a wall thickness of approximately 230 μm, which were the smallest dimensions for a U-shaped tube that could be reliably printed using 8K Phrozen resin. The final realized mass flow sensor was actuated by Lorentz force and read out by a laser Doppler vibrometer. This sensor was tested with the flow of water, IPA, and nitrogen. The measured mass flow rate as a function of the applied mass flow rate showed a linear response for all three fluids. Furthermore, apart from a constant factor, the sensitivity of the sensor corresponded very well with the predicted sensitivity based on a lumped element model.
Two demonstrator sensors were designed and fabricated based on electroplated tubes with 600 µm and 280 µm diameter, and 20 µm and 60 µm wall thickness, respectively. The measurement results for fluid density based on the tube with 600 µm diameter and 20 µm wall thickness show an accuracy of 0.5% for the density of liquids such as DI water, IPA, and their mixtures, and an accuracy of 5% for the density of gases like nitrogen, argon, and helium. Initial mass flow measurements show that the devices also work as Coriolis mass flow sensors. However, this demonstrator setup did not allow accurate control of the actuation amplitude, especially when the resonance frequency of the tube changes due to a change in the density of the liquid. Furthermore, a good symmetry of the U-shaped tube appeared to be very important, and reliable mass flow measurements could only be obtained with the relatively insensitive device with a tube diameter of 280 µm and wall thickness of 60 µm. To improve the performance of the sensor and to allow for control of the actuation amplitude a demonstrator device was designed with an integrated optical readout. In this regard, a nickel-plated micro-Coriolis mass flow sensor with a tube diameter of 580 μm, a wall thickness of 8 μm, and an integrated optical read-out were presented. The integrated optical readout consists of two emitters and detectors in both corners of the tube that measures the phase shift between the twist and swing mode. The phase shift shows a linear response as a function of flow rate with similar sensitivity for water and isopropyl alcohol. The sensitivity for water is equal to degree/(g/h). Thanks to the large diameter and circular cross-section the maximum pressure drop in the flow range of 0-125 g/h is only 0.19 bar.