(371g) Drag Force in Vibrating Nanowires Using Theory and Simulations

Nanoelectromechanical oscillators are very attractive as sensing devices because of their low power requirements as well as the high resolution they promise. Recent attempts to characterize the performance of these devices in fluid have led to disparate results and a standard calculation approach is lacking. In this article, we use our newly developed Bhatnagar-Gross-Krook based Low Mach number Direct Simulation Monte Carlo (BGK-LM-DSMC) method to study the non-continuum drag force acting on a cylinder oscillating normal to a wall. We explore both quasisteady flows in which ωτ_ff is the characteristic fluid relaxation time, as well as unsteady flows in which τ_f >> 1. The drag force is studied as a function of Knudsen number, defined in terms of the mean free path λ and the radius of the cylinder R as Kn = λ/R. For quasi-steady flows, we also present theoretical calculations for the slip regime, valid for the Kn > 1 regime. The quasi-steady simulations are in excellent agreement with our theories over a wide range of Kn and also approach continuum predictions as Kn → 0. Experiments indicate drag forces that are smaller and also show a much higher value of Kn at which the high Kn power-law scaling crosses over to the low Kn constant drag force behavior. Some possible reasons for this discrepancy are discussed. In the unsteady regime, we also explore the effect of varying the oscillation frequency on the drag force. The drag force attains constant values at low and high ω with crossover behavior at intermediate ω.