Rustom B. Bhiladvala

Bottom-up assembly of large-area nanowire resonator arrays

Directed-assembly of nanowire-based devices will enable the development of integrated circuits with new functions that extend well beyond mainstream digital logic. For example, nanoelectromechanical resonators are very attractive for chip-based sensor arrays because of their potential for ultrasensitive mass detection. In this letter, we introduce a new bottom-up assembly method to fabricate large-area nanoelectromechanical arrays each having over 2,000 single-nanowire resonators. The nanowires are synthesized and chemically functionalized before they are integrated onto a silicon chip at predetermined locations. Peptide nucleic acid probe molecules attached to the nanowires before assembly maintain their binding selectivity and recognize complementary oligonucleotide targets once the resonator array is assembled. The two types of cantilevered resonators we integrated here using silicon and rhodium nanowires had Q-factors of approximately 4,500 and approximately 1,150, respectively, in vacuum. Taken together, these results show that bottom-up nanowire assembly can offer a practical alternative to top-down fabrication for sensitive chip-based detection.

Operation of nanomechanical resonant structures in air

We report on the resonant operation of high-quality-factor silicon nanomechanical structures in air and at room temperature. We describe techniques used to actuate and detect nanomechanical structures in atmosphere, resulting in the enhancement of the effective quality factor to above 1000 and demonstrate the potential for successful sensor operation of resonant nanomechanical structures under ambient conditions.

Nanoresonator Chip-Based RNA Sensor Strategy for Detection of Circulating Tumor Cells: Response using PCA3 as a Prostate Cancer Marker

There is widespread interest in circulating tumor cells (CTCs) in blood. Direct detection of CTCs (often < 1/mL) is complicated by a number of factors, but the presence of ∼10(3) to 10(4) copies of target RNA per CTC, coupled with simple enrichments, can greatly increase detection capability. In this study we used resonance frequency shifts induced by mass-amplifying gold nanoparticles to detect a hybridization sandwich bound to functionalized nanowires. We selected PCA3 RNA as a marker for prostate cancer, optimized antisense binding sites, and defined conditions allowing single nucleotide mismatch discrimination, and used a hybrid resonator integration scheme, which combines elements of top-down fabrication with strengths of bottom-up fabrication, with a view to enable multiplexed sensing. Bound mass calculated from frequency shifts matched mass estimated by counting gold nanoparticles. This represents the first demonstration of use of such nanoresonators, which show promise of both excellent specificity and quantitative sensitivity.

Noncontinuum drag force on a nanowire vibrating normal to a wall: Simulations and theory

Nanoelectromechanical oscillators are very attractive as sensing devices because of their low power requirements and high resolution, especially at low pressures. While many experimental studies of such systems are available in the literature, a fundamental theoretical understanding over the entire range of operating conditions is lacking. In this article, we use our newly developed Bhatnagar–Gross–Krook based low Mach number direct simulation Monte Carlo method to study the noncontinuum drag force acting on a cylinder oscillating normal to a wall. We explore quasisteady flows in which ωτf⪡1 as well as unsteady flows for which ωτf=O(1). Here ω is the oscillation frequency and τf is the characteristic time for the development of the gas flow. The drag force per unit length acting on a long cylindrical wire is studied as a function of the Knudsen number, defined in terms of the mean free path λ and the radius of the cylinder R as Kn=λ/R. For quasisteady flows, we also present theoretical calculations for th...

Small Scale Sensing for Wind Turbine Active Control System

In large wind-turbine parks (farms), gusts, turbulence and interaction of wakes cause significant variation in the wind velocity over rotor blades, with a range of temporal and spatial scales. Arrays of sensors for pressure and wall shear stress at a few key locations, would directly provide data on local wind forces at these locations on the blade surface. Augmented by wind speed field data, wall shear data would be the most directly relevant input for “smart rotor” Active Flow Control (AFC), reducing power intermittency due to localized or large-area boundary-layer separation. Actuators run by a control system with such input could reduce extreme and fatigue loads, power fluctuations, and could help extend the wind speed envelope for power extraction. For measuring fluctuations of turbulent wall shear stress, flush-mounted single-hot-film sensors are non-intrusive, but suffer from unacceptably large errors caused by significant heat conduction to the substrate. To mitigate this problem, we introduce multi-element guard-heated sensors. Results from our analytical/numerical study show that it is possible to eliminate substrate conduction errors, with careful geometric design of the guard heater. A microfabricated prototype is also presented in this entry.

A new approach to highly resolved measurements of turbulent flow

In this paper we present the design and principle of a new anemometer, namely the 2d-Laser Cantilever Anemometer (2d-LCA), which has been developed for highly resolved flow speed measurements of two components (2d) under laboratory conditions. We will explain the working principle and demonstrate the sensor’s performance by means of comparison measurements of wake turbulence with a commercial X-wire. In the past we have shown that the 2d-LCA is capable of being applied in liquid and particle-laden domains, but we also believe that other challenging areas of operation such as near-wall flows can become accessible.

Can We Rescue Thermal Wall Shear Stress Sensing

The use of single flush-mounted thin-films for thermal sensing of wall shear stress fluctuations in turbulent flows has seen a decline, in spite of their non-intrusiveness, and the availability of microfabrication technologies to create very small films. The limitations of such single-element sensors are quite severe—their spatial resolution is not determined by their size alone, but modified by substrate heat conduction which creates variations in the effective sensor size (heat exchange area), dependent on strength and timescale of the fluctuations. Here a two-element design is investigated—with the hot-film sensor element surrounded by an electrically isolated guard heater film maintained at the same temperature as the sensor, but controlled by a separate anemometer circuit. Numerical studies are used to examine such guard heater designs over a range of shear stress values. The results show that if the sensor film center-location is biased towards the downstream end (75% and 65% of guard-heater length for water and air, respectively), with an appropriately-sized guard heater, 95% of the total heat generated in the sensing film can be transferred directly to the fluid, for strong turbulent fluctuations (Peclet number Pe > 8000) when the working fluid is water.© 2011 ASME

Fluid-Structure Coupling in Gas Damping Response of Nanowire Resonators

Nanowires vibrating at resonance in a gas can serve as sensitive detectors of mass (< 10−18 grams), of use to molecular diagnosis of disease. They can also serve as sensitive detectors of damping force in an ambient gas environment. The Q-factor of resonance spectra, quantifies the sharpness of the peak and is a measure of the ratio of inertial to dissipative (damping) forces. Q-factor data enable quantification of the gas damping force in different regimes of rarefied gas dynamics. Measurements were made with silicon and rhodium nanowires of comparable size, in pure dry nitrogen, with pressure increasing from high vacuum (10−10 atm) to one atmospheric pressure. The data show that, for the silicon nanowires, the Q-factor begins to decrease from its high-vacuum value at a lower pressure and reaches a lower minimum value at one atmosphere, compared to the rhodium nanowires. We show that nanowire structural properties, namely the elastic modulus and intrinsic damping, are responsible for these differing sensitivities to a similar gas damping force range. The results show an important coupling of fluid and structural interaction for rarefied gas dynamics at nanoscale. For practical sensing applications in an ambient gas, this coupling indicates that silicon nanowires are better suited for gas damping force sensing, while rhodium nanowires would fare better as mass sensors for molecular diagnosis.Copyright