Effect of Support Compliance on the Resonant Behavior of Microcantilever-Based Sensors in Viscous Fluids

Resonant microcantilevers are often considered for use in chemical sensing and biosensing applications. However, when excited in the conventional transverse flexural mode, their performance in liquids is severely compromised. Theoretical and experimental studies have shown that the detrimental effects of the liquid may be mitigated by operating the microcantilever in lateral flexure, especially for microbeams having smaller length-to-width (L/b) ratios. However, for these most promising geometries the predictions of existing models tend to diverge from experimental data for resonant frequency (fres) and quality factor (Q). A likely reason for these discrepancies is support compliance, which has been neglected in existing models. Thus, the derivation of an analytical model for the lateral-mode dynamic response of a microcantilever in a viscous fluid, including the effects of support compliance, is warranted and is the focus of this dissertation. Analytical solutions for natural frequency and Q are first obtained for the free-vibration case, followed by solutions for the forced-vibration response when the cantilever is excited by an imposed harmonic relative rotation near the support (simulating electrothermal actuation). Values of fres and Q are extracted from the response spectra for the tip deflection and the bending strain near the support. The support compliance (required as model input) is analytically related to device dimensions by employing dimensional analysis and 3-D FEA. The analytical results for the resonant characteristics are also related to sensor performance metrics (sensitivity and limit of detection), thus permitting one to exploit the potential of lateral-mode microcantilever-based liquid-phase sensors. The impact of support compliance, fluid resistance, and beam dimensions on the free- and forced-vibration response are explored, as are the differences associated with the two output signals. Comparisons of results with experimental data show a marked improvement over the previous rigid-support models for smaller L/b values. For the practical ranges of parameters considered the model indicates that, at smaller L/b values, support compliance may reduce Q by up to ~14% and fres and mass sensitivity (Sm) by up to ~21%. Conversely, for L/b\u3e15 the support compliance effects are no more than 2% on Q and 4% on fres and Sm

An Analytical Model of a Thermally Excited Microcantilever Vibrating Laterally in a Viscous Fluid

To achieve higher quality factors (Q) for microcantilevers used in liquid-phase sensing applications, recent studies have explored the use of the lateral (in-plane) flexural mode. In particular, we have recently shown that this mode may be excited electrothermally using integrated heating resistors near the micro cantilever support, and that the resulting increase in Q helps to make low-ppb limits of detection a possibility in liquids. However, because the use of electrothermally excited, liquid-phase, microcantilever-based sensors in lateral flexure is relatively new, theoretical models are lacking. Therefore, we present here a new analytical model for predicting the vibratory response of these devices. The model is also used to successfully confirm the validity of our previously derived Q formula, which was based on a single-degree-of-freedom (SDOF) model and a harmonic tip force. Comparisons with experimental data show that the present model and, thus, the analytical formula provide excellent Q estimates for sufficiently thin beams vibrating laterally in water and reasonable upper-bound estimates for thicker beams

Hydrogenotrophic Denitrification of Groundwater Using a Simplified Reactor for Drinking Water: A Case Study in the Kathmandu Valley, Nepal

High nitrate-nitrogen (NO3−–N) content is a typical feature of groundwater, which is the primary water source in the Kathmandu Valley, Nepal. Considering the Kathmandu Valley’s current problem of water scarcity, a user-friendly system for removing NO3−–N from groundwater is promptly desired. In this study, a simplified hydrogenotrophic denitrification (HD) reactor was developed for the Kathmandu Valley, and its effectiveness was evaluated by its ability to treat raw groundwater. The reactor operated for 157 days and showed stability and robustness. It had an average nitrogen removal efficiency of 80.9 ± 16.1%, and its nitrogen loading rate and nitrogen removal rate varied from 23.8 to 92.3 g–N/(m3∙d) and from 18.3 to 73.7 g–N/(m3∙d), respectively. Compared to previous HD reactors, this simplified HD reactor is a more user-friendly option for the Kathmandu Valley, as most of the materials used for the reactor were locally available and require less maintenance. The reactor is recommended for groundwater treatment at the household level. It has a current treatment capacity of 40 L/d, which can fulfill the daily requirements for drinking and cooking water in a household with 4–5 people