161 research outputs found
Precision mass measurements in solution reveal properties of single cells and bioparticles
Precise characterization of biological materials ranging from single cells (~1-20 microns) to extracellular vesicles (20-200 nm) is of fundamental interest because of their biological and translational value. Here we discuss the value of precision mass measurements in solution for informing various physical and biological parameters, such as mass accumulation rate, longitudinal cell growth or cell density. We introduce how the limits of the single-particle mass measurements can be pushed down to nano-scale dimensions enabling the resolution of extracellular vesicles and viruses in solution. We believe with future advancements on the precision and throughput of this approach, the capability of analyzing biologically relevant particles in solution will have broad biological and translational impact
Radiation Impedance of Collapsed Capacitive Micromachined Ultrasonic Transducers
Cataloged from PDF version of article.The radiation impedance of a capacitive micromachined ultrasonic transducer (CMUT) array is a critical parameter to achieve high performance. In this paper, we present a calculation of the radiation impedance of collapsed, clamped, circular CMUTs both analytically and using finite element method (FEM) simulations. First, we model the radiation impedance of a single collapsed CMUT cell analytically by expressing its velocity profile as a linear combination of special functions for which the generated pressures are known. For an array of collapsed CMUT cells, the mutual impedance between the cells is also taken into account. The radiation impedances for arrays of 7, 19, 37, and 61 circular collapsed CMUT cells for different contact radii are calculated both analytically and by FEM simulations. The radiation resistance of an array reaches a plateau and maintains this level for a wide frequency range. The variation of radiation reactance with respect to frequency indicates an inductance-like behavior in the same frequency range. We find that the peak radiation resistance value is reached at higher kd values in the collapsed case as compared with the uncollapsed case, where k is the wavenumber and d is the center-to-center distance between two neighboring CMUT cells
Radiation Impedance of an Array of Circular Capacitive Micromachined Ultrasonic Transducers
Cataloged from PDF version of article.The radiation impedance of a capacitive micromachined ultrasonic transducer (cMUT) with a circular membrane is calculated analytically using its velocity profile for the frequencies up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. A higher radiation resistance improves the bandwidth as well as the efficiency of the cMUT. The radiation resistance is determined to be a strong function of the cell spacing. It is shown that a center-to-center cell spacing of 1.25 wavelengths maximizes the radiation resistance, if the membranes are not too thin. It is also found that excitation of nonsymmetric modes may reduce the radiation resistance in immersion applications
Interest Rate Smoothing and Macroeconomic Instability under Post-Capital Account Liberalization Turkey
Cataloged from PDF version of article.This paper considers the interest rate policy of the Central Bank of the Republic of Turkey (CBRT) in the post-financial liberalization and deregulation era. We find that (1) the Bank's interest rate smoothing tendency is the main determinant of its monetary policy in this period, (2) the CBRT does not seem to be responsive to the developments in real economy (output), and (3) although inflation targeting central banks are not supposed to pay attention to exchange rates, the CBRT appears to be slightly responsive to changes in real exchange rate. In answer to the question of whether there is a deeper underlying Structural constraint binding the CBRT's "independence," it seems clear that the global financial system is restricting the ability of the central banks to pursue "independent" policy objectives
High-speed multiple-mode mass-sensing resolves dynamic nanoscale mass distributions
Simultaneously measuring multiple eigenmode frequencies of nanomechanical resonators can determine the position and mass of surface-adsorbed proteins, and could ultimately reveal the mass tomography of nanoscale analytes. However, existing measurement techniques are slow (<1 Hz bandwidth), limiting throughput and preventing use with resonators generating fast transient signals. Here we develop a general platform for independently and simultaneously oscillating multiple modes of mechanical resonators, enabling frequency measurements that can precisely track fast transient signals within a user-defined bandwidth that exceeds 500 Hz. We use this enhanced bandwidth to resolve signals from multiple nanoparticles flowing simultaneously through a suspended nanochannel resonator and show that four resonant modes are sufficient for determining their individual position and mass with an accuracy near 150 nm and 40 attograms throughout their 150-ms transit. We envision that our method can be readily extended to other systems to increase bandwidth, number of modes, or number of resonators.United States. Army Research Office (Grant W911NF-09-0001)Center for Integration of Medicine and Innovative Technology (Contract 09-440)National Science Foundation (U.S.) (Grant 1129359
Improved performance of cMUT with nonuniform membranes
When capacitive micromachined ultrasonic transducers are immersed in water, the bandwidth of the device is limited by the membrane's second resonance frequency. At this frequency no mechanical power to immersion medium can be transferred. We present a membrane shape to shift the second resonance frequency to a higher value. The structure consists of a very thin membrane at the outer rim with a rigid mass at the center. The stiffness of the central region moves the second resonance to a higher frequency. This membrane configuration is shown to work better in terms of gain and bandwidth as compared to conventional uniform membranes in both transmission and reception. © 2005 IEEE
The measurement of Navier slip on individual nanoparticles in liquid
The Navier slip condition describes the motion of a liquid, relative to a
neighboring solid surface, with its characteristic Navier slip length being a
constitutive property of the solid-liquid interface. Measurement of this slip
length is complicated by its small magnitude, expected in the nanometer range
based on molecular simulations. Here, we report an experimental technique that
interrogates the Navier slip length on individual nanoparticles immersed in
liquid, with sub-nanometer precision. Proof-of-principle experiments on
individual, citrate-stabilized, gold nanoparticles in water give a constant
slip length of 2.70.6 nm (95% C.I.) - independent of particle size.
Achieving this feature of size independence is central to any measurement of
this constitutive property, which is facilitated through the use of individual
particles of varying radii. This demonstration motivates studies that can now
validate the wealth of existing molecular simulation data on slip.Comment: 12 pages, 4 figure
High Power CMUTs: Design and experimental verification
Cataloged from PDF version of article.Capacitive micromachined ultrasonic transducers
(CMUTs) have great potential to compete with piezoelectric
transducers in high-power applications. As the output pressures
increase, nonlinearity of CMUT must be reconsidered
and optimization is required to reduce harmonic distortions.
In this paper, we describe a design approach in which uncollapsed
CMUT array elements are sized so as to operate at the
maximum radiation impedance and have gap heights such that
the generated electrostatic force can sustain a plate displacement
with full swing at the given drive amplitude. The proposed
design enables high output pressures and low harmonic
distortions at the output. An equivalent circuit model of the
array is used that accurately simulates the uncollapsed mode
of operation. The model facilities the design of CMUT parameters
for high-pressure output, without the intensive need
for computationally involved FEM tools. The optimized design
requires a relatively thick plate compared with a conventional
CMUT plate. Thus, we used a silicon wafer as the CMUT
plate. The fabrication process involves an anodic bonding process
for bonding the silicon plate with the glass substrate. To
eliminate the bias voltage, which may cause charging problems,
the CMUT array is driven with large continuous wave
signals at half of the resonant frequency. The fabricated arrays
are tested in an oil tank by applying a 125-V peak 5-cycle
burst sinusoidal signal at 1.44 MHz. The applied voltage is increased
until the plate is about to touch the bottom electrode
to get the maximum peak displacement. The observed pressure
is about 1.8 MPa with −28 dBc second harmonic at the
surface of the array
Interaction between a cMUT cell and a liquid medium around the parallel resonance frequency
In this paper, we present how a capacitive micromachined ultrasonic transducer (cMUT) couples to the immersion medium, based on an accurate parametric model. We show that the velocity of cMUT membrane can be written as a sum of an average velocity term and a residual term. We demonstrate that this residual term carries non-zero energy at the parallel resonance frequency by investigating the interaction between the cMUT cell and a liquid medium. We develop a model that is also applicable around the parallel resonance frequency. © 2007 IEEE
Stagger tuned cMUT array for wideband airborne applications
In this study, we explore the limits of cMUTs in air-borne applications. First we investigate the ways of increasing the bandwidth of a single cMUT cell in air. The effect of array operation is also considered in order to increase the radiation resistance seen by the transducer. We calculate the bandwidth of a stagger tuned cMUT array. It is shown in this paper that more than 60% bandwidth can be obtained by three staggered frequencies. © 2006 IEEE
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