Andrew Strongrich

Microstructure actuation and gas sensing by the Knudsen thermal force

The generation of forces and moments on structures immersed in rarefied non-isothermal gas flows has received limited practical implementation since first being discovered over a century ago. The formation of significant thermal stresses requires both large thermal gradients and characteristic dimensions which are comparable to the gas molecular mean free path. For macroscopic geometries, this necessitates impractically high temperatures and very low pressures. At the microscale, however, these conditions are easily achieved, allowing the effects to be exploited, namely, for gas-property sensing and microstructure actuation. In this letter, we introduce and experimentally evaluate performance of a microelectromechanical in-plane Knudsen radiometric actuator, a self-contained device having Knudsen thermal force generation, sensing, and tuning mechanisms integrated onto the same platform. Sensitivity to ambient pressure, temperature gradient, as well as gas composition is demonstrated. Results are presented in terms of a non-dimensional force coefficient, allowing measurements to be directly compared to the previous experimental and computational data on out-of-plane cantilevered configurations.

Low-pressure gas sensor exploiting the Knudsen thermal force: DSMC modeling and experimental validation

We present performance modeling and experimental validation of a novel MEMS vacuum gas sensor based on the Knudsen thermal force. Direct simulation Monte Carlo (DSMC) modeling of thermally-driven gas flow and force measurements show a non-monotonic dependence on ambient pressure, peaking at a Knudsen number on the order of unity. Combining force dependence on pressure with the monotonically varying heat transfer rate allows both ambient pressure and species concentration to be determined if the constituents are known. The DSMC modeling also shows that thermal gradients between the shuttle and heater induce complex vortical flow structures that could be applied to control mixing/separation in gas-phase microfluidic devices.

DSMC simulation of microstructure actuation by Knudsen thermal forces including binary mixtures

Complex and non-intuitive flow structures controlled only by thermal gradients can be observed in rarefied gas flows. A force of thermophoretic nature, often referred to as Knudsen or radiometric force, can become dominant in microflow applications. A Microelectromechanical In-plane Knudsen Radiometric Actuator (MIKRA) that exploits these forces has been developed and tested at Purdue. Previous efforts used DSMC to understand the MIKRA flow structure and validate numerical modeling for simple gases. This work investigated more realistic boundary conditions as well as species separation around the MIKRA beams for Xe-He and N2-H2O mixtures. The main goal of this work was to run DSMC simulations of the MIKRA sensor to not only understand how it can be accurately modeled, but to look at future applications for gas mixture sensing as well.

Amplification and reversal of Knudsen force by thermoelectric heating

We show that the Knudsen thermal force generated by a thermally-induced flow over a heated beam near a colder wall could be amplified significantly by thermoelectric heating. Bidirectional actuation is achieved by switching the polarity of the thermoelectric device bias voltage. The measurements of the resulting thermal forces at different rarefaction regimes, realized by changing geometry and gas pressure, are done using torsional microbalance. The repulsive or attractive forces between a thermoelectrically heated or cooled plate and a substrate are shown to be up to an order of magnitude larger than for previously studied configurations and heating methods due to favorable coupling of two thermal gradients. The amplification and reversal of the Knudsen force is confirmed by numerical solution of the Boltzmann-ESBGK kinetic model equation. Because of the favorable scaling with decreasing system size, the Knudsen force with thermoelectric heating offers a novel actuation and sensing mechanism for nano/microsystems.

In-Situ Molecular Vapor Composition Measurements During Lyophilization

PurposeMonitoring process conditions during lyophilization is essential to ensuring product quality for lyophilized pharmaceutical products. Residual gas analysis has been applied previously in lyophilization applications for leak detection, determination of endpoint in primary and secondary drying, monitoring sterilization processes, and measuring complex solvents. The purpose of this study is to investigate the temporal evolution of the process gas for various formulations during lyophilization to better understand the relative extraction rates of various molecular compounds over the course of primary drying.MethodsIn this study, residual gas analysis is used to monitor molecular composition of gases in the product chamber during lyophilization of aqueous formulations typical for pharmaceuticals. Residual gas analysis is also used in the determination of the primary drying endpoint and compared to the results obtained using the comparative pressure measurement technique.ResultsThe dynamics of solvent vapors, those species dissolved therein, and the ballast gas (the gas supplied to maintain a set-point pressure in the product chamber) are observed throughout the course of lyophilization. In addition to water vapor and nitrogen, the two most abundant gases for all considered aqueous formulations are oxygen and carbon dioxide. In particular, it is observed that the relative concentrations of carbon dioxide and oxygen vary depending on the formulation, an observation which stems from the varying solubility of these species. This result has implications on product shelf life and stability during the lyophilization process.ConclusionsChamber process gas composition during lyophilization is quantified for several representative formulations using residual gas analysis. The advantages of the technique lie in its ability to measure the relative concentration of various species during the lyophilization process. This feature gives residual gas analysis utility in a host of applications from endpoint determination to quality assurance. In contrast to other methods, residual gas analysis is able to determine oxygen and water vapor content in the process gas. These compounds have been shown to directly influence product shelf life. With these results, residual gas analysis technique presents a potential new method for real-time lyophilization process control and improved understanding of formulation and processing effects for lyophilized pharmaceutical products.

Microscale In-Plane Knudsen Radiometric Actuator: Design, Characterization, and Performance Modeling

Thermally driven gas flow is exploited in a microscale device for the purpose of gas pressure dependent actuation in rarefied environments. The device relies on the in-plane motion of a shuttle mass in response to Knudsen thermal forces. The shuttle response is measured capacitively and the magnitude is used to estimate ambient pressure. Combined with heating element resistance, these measurements, in principle, enable the simultaneous determination of gas composition if the constituents are known. Numerical simulations using direct simulation Monte Carlo are carried out to elucidate the mechanisms of force production. Results reveal highly vortical flow structures forming high pressure regions at the shuttle surface. The experimental and numerical results are expressed in terms of a non-dimensional Knudsen force coefficient, allowing force magnitude to be estimated for dynamically similar geometries. [2016-0228]

Experimental measurements and numerical modeling of a thermostress convection-based actuator

The capability of gas actuation using thermal gradients in a rarefied gas flow is demonstrated for arrays of interdigitated hot and cold vanes. The force acting on the cold vane array is quantified using a microNewton torsional balance over a range of pressures between 3.8 mTorr to 900 mTorr. Simulations are carried out using the SPARTA direct simulation monte carlo software to elucidate the mechanisms governing force production. Both experimental and numerical results are non-dimensionalized and fitted using the linear least-squares method. These correlations facilitate prediction of performance for dynamically similar geometries.

Experimental measurements and modeling of convective heat transfer in the transitional rarefied regime

We present experimental measurements and numerical simulations of convective heat transfer performance in the transitional rarefied regime for an isolated rectangular beam geometry. Experiments were performed using single crystalline silicon beam elements having width-to-thickness aspect ratios of 8.5 and 17.4. Devices were enclosed in a vacuum chamber and heated resistively using a DC power supply. A range of pressures corresponding to Knudsen numbers between 0.096 and 43.2 in terms of device thickness were swept, adjusting applied power to maintain a constant temperature of 50 K above the ambient temperature. Both parasitic electrical resistance associated with the hardware and radiative exchange with the environment were removed from measured data, allowing purely convective heat flux to be extracted. Numerical simulations were carried out deterministically through solution of the Ellipsoidal Statistical Bhatnagar-Gross-Krook collision model of the Boltzmann equation. Results agree with experimental data, revealing a strong coupling between dissipated heat flux and thermal stresses within the flowfield as well as a nonlinear transition between the free-molecule and continuum regimes.

Knudsen Thermal Force Generation at the Microscale

The paper reports on the experimental studies targeting Knudsen forces on silicon microcantilevers. The devices were fabricated on a Silicon-On-Insulator wafer with one end anchored to the substrate. Measurements were performed through direct external heating of the underlying substrate. Device deflection was quantified using confocal microscopy, showing an increase in force magnitude for decreasing Knudsen number. Measurements were carried out at a constant substrate temperature with Knudsen number being controlled directly via ambient pressure.Copyright

Convective Cooling in the Transitional Rarefied Flow Regime

Convective heat transfer in the transitional rarefied flow regime is evaluated both numerically using the ES-BGK collision model and experimentally for freely suspended silicon beams. Results are compared to both empirically derived continuum correlations as well as analytical free-molecule solutions, demonstrating a monotonic nonlinear transition between the two regimes. Both horizontal and vertical orientations were explored in an effort to evaluate the potential influence of buoyancy on heat transfer performance. Results demonstrate that such effects are largely insignificant for transitional rarefied flows and convective thermal dissipation is instead driven by thermal creep phenomena.Copyright