Nathaniel Selden

Comparison of force balance calibration techniques for the nano-Newton range

With the rapid progress of micro- and nanoscale fabrication technology, devices are continually being created which produce extremely small forces. This creates a distinct need for a measurement instrument and adequate calibration techniques which can resolve forces below 1 μN. Two calibration methods for force balance measurements in the nano-Newton range are presented. These methods are based on a free molecule gas dynamic expansion through a thin-walled orifice and the electrostatic actuation of a miniature comb drive. Due to the advantages and disadvantages of every calibration technique, multiple techniques are often required to validate performance results for microscale devices. Because these calibration techniques typically rely on completely different physical processes and can be described by different sets of analytical equations, the comparison of one technique to another is necessary when high accuracy is required. The gas dynamic and electrostatic force calibration techniques have been compa...

Origins of radiometric forces on a circular vane with a temperature gradient

Radiometric force on a 0.12 m circular vane is studied experimentally and numerically over a wide range of pressures that cover the flow regimes from near free molecular to near continuum. In the experiment, the vane is resistively heated to about 419 K on one side and 394 K on the other side, and immersed in a rarefied argon gas. The radiometric force is then measured on a nano-Newton thrust stand in a 3 m vacuum chamber and compared with the present numerical predictions and analytical predictions proposed by various authors. The computational modelling is conducted with a kinetic approach based on the solution of ellipsoidal statistical Bhatnagar–Gross–Krook (ES-BGK) equation. Numerical modelling showed the importance of regions with elevated pressure observed near the edges of the vane for the radiometric force production. A simple empirical expression is proposed for the radiometric force as a function of pressure that is found to be in good agreement with the experimental data. The shear force on the lateral side of the vane was found to decrease the total radiometric force.

Analysis of accommodation coefficients of noble gases on aluminum surface with an experimental/computational method

A method that connects measurements of radiometric forces on a heated vane in the transitional flow regime with the kinetic modeling of the flow, and derives the accommodation coefficients through the successive analysis of measured and computed results, is proposed. The method utilizes the fact that radiometric forces exerted on heated objects immersed in rarefied gases are governed by the interaction of gas molecules with the surface. Experimental results on radiometric forces on a 0.11 m diameter circular vane are obtained on a nano-Newton thrust stand in a 3 m long vacuum chamber for pressures ranging from approximately 0.01 to 1 Pa. The vane was heated to 419 K on the hot side and 396 K on the cold side. The numerical modeling is conducted using a combined ellipsoidal statistical Bhatnagar–Gross–Krook/direct simulation Monte Carlo approach that allows accurate and time efficient analysis of radiometric forces on a vane in large vacuum chambers filled with rarefied gas. Accommodation coefficients for ...

Shear Force in Radiometric Flows

The impact of the radiometer vane thickness and edge geometry on the total radiometric force is examined numerically solving the ES BGK model kinetic equation. The flow of argon over a single vane and a multi‐vane configurations is considered in the range of Knudsen numbers from 0.02 to 1. The shear force is found to reduce the total radiometric force for most vane configurations. It is shown that a change in the vane shape may offset the losses due to the shear force in a multi‐vane geometry.

Analysis and Applications of Radiometeric Forces in Rarefied Gas Flows

An overview of recent numerical and computational studies of radiometric phenomena is presented. Several important factors affecting radiometric forces have been analyzed, including the contribution of area, edge, and shear forces for different pressures and gases, chamber size effect, gas‐surface accommodation, and multi‐vane geometries.

Experimental and Numerical Modeling of Rarefied Gas Flows Through Orifices and Short Tubes

Flow through circular orifices with thickness‐to‐diameter ratios varying from 0.015 to 1.2 is studied experimentally and numerically with kinetic and continuum approaches. Helium and nitrogen gases are used in the range of Reynolds numbers from 0.02 to over 700. Good agreement between experimental and numerical results is observed for mass flow and thrust corrected for the experimental facility background pressure. For thick‐to‐thin orifice ratios of mass flow and thrust vs pressure, a minimum is established. The thick orifice propulsion efficiency is much higher than that of a thin orifice. The effects of edge roundness and surface specularity on a thick orifice specific impulse were found to be relatively small.

Impact of vane size and separation on radiometric forces for microactuation

Abstract : A kinetic approach is used to study the feasibility of increasing the efficiency of microactuators that use radiometric force through etching holes in a single radiometer vane. It has been shown that a radiometer that consists of small vanes is capable of producing at least an order of magnitude larger force than a single vane radiometer that takes up the same area. The optimum gap between the vanes is found to be slightly smaller than the vane size, with the optimum Knudsen number of about 0.05 based on the vane height. Several competing processes have been suggested for the actuation of micromechanical devices. Electrostatic micro-actuators have been suggested(1), (2) where a potential difference between two sets of combs produces a force generated by the associated electric field. Generally, electrostatic micro-actuators are linear drive devices and are not well suited for rotational operation. Piezoelectric devices(3) have been considered where an electric field applied to a material causes mechanical deformation in the material. The piezoelectric effect typically produces very small material deflections; however, they can have a very fast actuation frequency from the kHz up to the MHz range. Electrically conducting polymers(4) can be used to change the volume of a material to provide more actuation distance than a typical piezoelectric material with lower applied voltages.

Effect of Chamber Wall Proximity on Radiometer Force Production

Abstract : The impact of chamber wall location on a radiometric device has been studied experimentally and numerically in chambers from 0.2m to 1.8m in diameter with background pressure ranging from 0.006 to 1Pa. The range of pressures investigated is broad enough to observe the impact of the chamber walls on a given radiometer configuration in both the free molecule and transitional regimes. The contribution of the chamber walls to both the flowfield structure and radiometric force production were examined for helium, argon, and nitrogen test gases. Various radiometer geometries were experimentally tested on a force balance, with nano- Newton resolution, placed in a stagnant gas. The computational results were obtained using the solution of the model kinetic equations and by the direct simulation Monte Carlo method. The simulations compliment the experimental work by varying the chamber dimensions to a degree not practically attainable, demonstrating both the decreasing temperature gradient of the flowfield and the reduced force production of the radiometer as the physical dimensions of the chamber increase. It is concluded that chamber wall location has a dramatic effect on the force production of a radiometer, especially at higher pressures, where increasing the distance from the vane to the wall reduces both temperature gradient and the total force produced.

Application of radiometric force to microactuation and energy transformation

The force that acts on a thin vane immersed in rarefied gas when a temperature gradient is imposed along or across the vane has historically been known as the Radiometric force. First observed by Fresnel in 1825, the radiometric force has regained its former popularity in recent decades due to the advent of micro-machines, where a transitional flow regime can occur at atmospheric pressures. Whether used for its force potential or simply viewed as a nuisance, this force cannot be ignored in micro-devices where thermal gradients exist. Potential applications of radiometric force now span from atomic force microscopy to astrophysics to high altitude flight. This paper describes an application of these forces to a conceptual micro-scale energy harvester, where two possible geometries of operation are described. It is shown that one configuration is significantly simpler to fabricate while the other geometry is more efficient at producing larger forces. The effect of pressure, feature separation, and feature-t...

Experimental and Computational Observation of Radiometric Forces on a Plate

.Reynolds also took part in the experiments conducted by Schuster 7 that turned up the first experimental evidence of gas forces being the dominant cause of the radiometric effect. In this experiment, the radiometer case was suspended by parallel fibers and light was dir ected onto the vanes. The radiometer case was pushed in the direction opposite the vanes, proving that the radiometric phenomenon is caused by the interaction between the heated side of the vane and the gas. The kinetic theory explanation given by Reynolds is in fact a free molecule approximation of the radiometric effect: the molecules leaving the hot side leave with an increased velocity relative to those leaving the cold side. This leads to a larger momentum change on the hot side, and results in the mot ion of the vanes with the hot side trailing. The situation is, however, different in transitional or near -continuum flow. The molecules with higher velocities leave the hot side of the vane and collide with incoming molecules. These collisions cut the sur face flux more efficiently than those reflected on the cold surface. Essentially, this means that these effects compensate each other, and pressures in the center of the vane are equal. This theory was first proposed by Reynolds 8