![]() The support metal substrate is Gas microflows measurements in rarefied conditions The translucid parallelepipeds structures are the TPP polymerized matter which contain the micro-tubes (Fig. Thanks to the focus-variation 3D optical measurement, it is possible to appreciate the 3D nature of the structure. The characterization of the four samples was realized through a focus-variation 3D optical measurement microscope (Alicona Infinite Focus SL), a digital microscope (VHX 7100, Keyence) and an EasyTom XL tomograph (RX Solutions). ![]() The dimensions were especially chosen to test the Optical characterization of the 3D-TPP structures In this study, four micro-tube samples with nominal diameters of 50 and 20 μm and lengths of 300 and 150 μm were manufactured (Fig. It is also noted for its ability to work without the need of supporting structures, which allows for the manufacture of especially complex 3D-structures with complex internal details. In comparison with other additive 3D-printing methods, two-photon-polymerization offers an outstanding precision and resolution. Section snippets Two-photon-polymerization manufacturing of micro-tubes The results of this study proof that TPP is an excellent candidate for fabrication of microfluidic devices with precisely controlled geometry for rarefied gas flows applications. The experimental results obtained have been compared to kinetic theory based numerical results. The microtubes were tested by means of the constant volume technique in order to measure global parameters of the flow, such as the mass flow rate and the conductance. In this work, a series of microtubes was manufactured by means of the two-photon-polymerization technique. The constant volume technique has been widely used in order to test microchannels manufactured via micro-machining, ,, ] and microtubes fabricated with standard fabrication procedures and standard materials, such as metals, , ] and glass. Mass flow rates in single microchannels are typically smaller than 10 -10 kg/s, hence, alternative experimental techniques, such as the constant volume technique, have been developed. Obtaining with conventional flow sensors high precision measurements of gas mass flow rate in microchannels is, however, challenging. In the case of isothermal flows, the gas/surface accommodation in terms of momentum can be extracted indirectly via experimental measurements of mass flow rates generated by pressure gradients in microchannels with simple geometries. That is why a precise model of gas/surface interactions is needed in order to accurately compute mass and heat transports in rarefied gas flows. In slightly rarefied flows, these effects prevail in the proximities of the solid boundaries that restrict the flow, giving rise to velocity slip, thermal transpiration, and temperature jump at the wall. The main characteristics of rarefied gas flows consists in thermodynamic non-equilibrium effects resulting from the low rate of intermolecular collisions in the flow. The level of flow rarefaction is often characterized by the Knudsen number K n = λ f p / L, with the slip flow regime in the 0.01 − 0.1 range (slight rarefaction), the transition regime in the 0.1 − 10 range (moderate rarefaction) and the free molecular regime in the 10 − range (high rarefaction). Micro gas flows are rarefied when the gas molecular mean free path, λ f p, has the same order of magnitude as the characteristic length, L, of the system. In this work, the technique has been modified so as to achieve larger writing fields with high precision for manufacturing leak proof microfluidic devices working with rarefied gas flows. Recently, it has been proposed for the development of new bio-medical applications at the micro-scale and biomimetic surfaces. This technique has been originally developed for the rapid, cheap and customizable fabrication of nanostructures, , ]. TPP is an additive manufacturing technology based on the two-photon polymerization of a photoresist with ultra-short laser pulses. In this work, we have tested the fabrication capabilities of two-photon-polymerization (TPP), also called 3D direct laser writing (3D-DLW), in order to manufacture a micro-device suitable for handling gas flows in rarefied conditions. That is why new three-dimensional (3D) fabrication techniques are needed, in order to further expand the design potential of fluidic MEMS. Nevertheless, these fabrication techniques are often limited to manufacturing two-dimensional (2D) structures. The conventional fabrication techniques employed in order to manufacture these devices are usually micro-machining and photo-lithography. Gas flows in micro-electro-mechanical systems (MEMS) are of great interest for a wide variety of applications, such as flow meters, sensors, actuators for flow control, pumps and gas separators, just to name a few, , ].
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