1,073 research outputs found
Subnanometer Translation of Microelectromechanical Systems Measured by Discrete Fourier Analysis of CCD Images
AbstractâIn-plane linear displacements of microelectromechanical systems are measured with subnanometer accuracy by observing the periodic micropatterns with a charge-coupled device camera attached to an optical microscope. The translation of the microstructure is retrieved from the video by phase-shift computation using discrete Fourier transform analysis. This approach is validated through measurements on silicon devices featuring steep-sided periodic microstructures. The results are consistent with the electrical readout of a bulk micromachined capacitive sensor, demonstrating the suitability of this technique for both calibration and sensing. Using a vibration isolation table, a standard deviation of Ï = 0.13 nm could be achieved, enabling a measurement resolution of 0.5 nm (4Ï) and a subpixel resolution better than 1/100 pixel. [2010-0170
Rate Dependence and Role of Disorder in Linearly Sheared Two-Dimensional Foams
The shear flow of two dimensional foams is probed as a function of shear rate
and disorder. Disordered foams exhibit strongly rate dependent velocity
profiles, whereas ordered foams show rate independence. Both behaviors are
captured quantitatively in a simple model based on the balance of the
time-averaged drag forces in the foam, which are found to exhibit power-law
scaling with the foam velocity and strain rate. Disorder modifies the scaling
of the averaged inter-bubble drag forces, which in turn causes the observed
rate dependence in disordered foams.Comment: 4 Figures, 4 page
Microfluidic applications of functionalized magnetic particles for environmental analysis: focus on waterborne pathogen detection
The continuous surveillance of drinking water is extremely important to provide early warning of contamination and to ensure continuous supplies of healthy drinking water. Isolation and detection of a particular type of pathogen present at low concentration in a large volume of water, concentrating the analyte in a small detection volume, and removing detection inhibiting factors from the concentrated sample, present the three most important challenges for water quality monitoring laboratories. Combining advanced biological detection methods (e.g., nucleic acid-based or immunology-based protocols) with microfluidics and immunomagnetic separation techniques that exploit functionalized magnetic particles has tremendous potential for realization of an integrated system for pathogen detection, in particular, of waterborne pathogens. Taking advantage of the unique properties of magnetic particles, faster, more sensitive, and more economical diagnostic assays can be developed that can assist in the battle against microbial pathogenesis. In this review, we highlight current technologies and methods used for realization of magnetic particle-based microfluidic integrated waterborne pathogen isolation and detection systems, which have the potential to comply in future with regulatory water quality monitoring requirement
Flow in linearly sheared two dimensional foams: from bubble to bulk scale
We probe the flow of two dimensional foams, consisting of a monolayer of
bubbles sandwiched between a liquid bath and glass plate, as a function of
driving rate, packing fraction and degree of disorder. First, we find that
bidisperse, disordered foams exhibit strongly rate dependent and inhomogeneous
(shear banded) velocity profiles, while monodisperse, ordered foams are also
shear banded, but essentially rate independent. Second, we introduce a simple
model based on balancing the averaged drag forces between the bubbles and the
top plate and the averaged bubble-bubble drag forces. This model captures the
observed rate dependent flows, and the rate independent flows. Third, we
perform independent rheological measurements, both for ordered and disordered
systems, and find these to be fully consistent with the scaling forms of the
drag forces assumed in the simple model, and we see that disorder modifies the
scaling. Fourth, we vary the packing fraction of the foam over a
substantial range, and find that the flow profiles become increasingly shear
banded when the foam is made wetter. Surprisingly, our model describes flow
profiles and rate dependence over the whole range of packing fractions with the
same power law exponents -- only a dimensionless number which measures the
ratio of the pre-factors of the viscous drag laws is seen to vary with packing
fraction. We find that , where , corresponding to the 2d jamming density, and suggest that this scaling
follows from the geometry of the deformed facets between bubbles in contact.
Overall, our work suggests a route to rationalize aspects of the ubiquitous
Herschel-Bulkley (power law) rheology observed in a wide range of disordered
materials.Comment: 16 pages, 14 figures, submitted to Phys. Rev. E. High quality version
available at: http://www.physics.leidenuniv.nl/sections/cm/gr
Chaotic mixing using source-sink microfluidic flows in a PDMS chip
We present an active fixed-volume mixer based on the creation of multiple source-sink microfluidic flows in a polydimethylsiloxane (PDMS) chip without the need of external or internal pumps. To do so, four different pressure-controlled actuation chambers are arranged on top of the 5ÎŒl volume of the mixing chamber. After the mixing volume is sealed/fixed by microfluidic valves made using âmicroplumbing technology', a virtual source-sink pair is created by pressurizing one of the membranes and, at the same time, releasing the pressure of a neighboring one. The pressurized air deforms the thin membrane between the mixing and control chambers and creates microfluidic flows from the squeezed region (source) to the released region (sink) where the PDMS membrane is turned into the initial state. Several schemes of operation of virtual source-sink pairs are studied. In the optimized protocol, mixing is realized in just a sub-second time interval, thanks to the implementation of chaotic advectio
Light focusing and additive manufacturing through highly scattering media using upconversion nanoparticles
Light-based additive manufacturing holds great potential in the field of
bioprinting due to its exceptional spatial resolution, enabling the
reconstruction of intricate tissue structures. However, printing through
biological tissues is severely limited due to the strong optical scattering
within the tissues. The propagation of light is scrambled to form random
speckle patterns, making it impossible to print features at the
diffraction-limited size with conventional printing approaches. The poor tissue
penetration depth of ultra-violet or blue light, which is commonly used to
trigger photopolymerization, further limits the fabrication of high
cell-density tissue constructs. Recently, several strategies based on wavefront
shaping have been developed to manipulate the light and refocus it inside
scattering media to a diffraction-limited spot. In this study, we present a
high-resolution additive manufacturing technique using upconversion
nanoparticles and a wavefront shaping method that does not require measurement
from an invasive detector, i.e., it is a non-invasive technique. Upconversion
nanoparticles convert near-infrared light to ultraviolet and visible light. The
ultraviolet light serves as a light source for photopolymerization and the
visible light as a guide star for digital light shaping. The incident light
pattern is manipulated using the feedback information of the guide star to
focus light through the tissue. In this way, we experimentally demonstrate that
near-infrared light can be non-invasively focused through a strongly scattering
medium. By exploiting the optical memory effect, we further demonstrate
micro-meter resolution additive manufacturing through highly scattering media
such as a 300-{\mu}m-thick chicken breast. This study provides a proof of
concept of high-resolution additive manufacturing through turbid media with
potential application in tissue engineering
Multi-photon polymerization using upconversion nanoparticles for tunable feature-size printing
The recent development of light-based 3D printing technologies has marked a
turning point in additive manufacturing. Through photopolymerization, liquid
resins can be solidified into complex objects. Usually, the polymerization is
triggered by exciting a photoinitiator with ultraviolet (UV) or blue light. In
two-photon printing (TPP), the excitation is done through the non-linear
absorption of two photons; it enables printing 100-nm voxels but requires
expensive femtosecond lasers which strongly limits their broad dissemination.
Upconversion nanoparticles (UCNPs) have recently been proposed as an
alternative to TPP for photopolymerization but using continuous-wave lasers.
UCNPs convert near-infrared (NIR) into visible/UV light to initiate the
polymerization locally as in TPP. Here we provide a study of this multi-photon
mechanism and demonstrate how the non-linearity impacts the printing process.
In particular, we report on the possibility of fine-tuning the size of the
printed voxel by adjusting the NIR excitation intensity. Using gelatin-based
hydrogel, we are able to vary the transverse voxel size from 1.3 to 2.8 {\mu}m
and the axial size from 7.7 to 59 {\mu}m by adjusting the NIR power without
changing the degree of polymerization. This work opens up new opportunities for
speeding up the fabrication while preserving the minimum feature size with
cheap light sources
Interplay between the potential waveform and diffusion layer dynamics determines the time-response of voltammetric detection in microchannels
The diffusion layer is a critical factor affecting the temporal response of electrochemical devices. In this article, we have investigated the effect of the relaxation of the diffusion layer during the potentiodynamic sensing of ferrocenemethanol (FcMeOH) in microchannels and compared these results to amperometry. First, the effect of the relaxation of the diffusion layer is described, both theoretically and experimentally. Then, chronoamperometric and voltammetric measurements were considered, and the rate of current increase as a plug of FcMeOH is injected into the device was studied for both cases. It was found that, for the oxidation of FcMeOH, the waveform maximising the duration of the anodic phase provided an improved response for potentiodynamic methods, even though amperometry was always found to show the best results. This was further established by extracting the impulse response and modulation transfer functions, which characterize the time and frequency responses, respectively, of the fluidic/electrochemical system. These findings can help designing potential waveforms improving the time response of the device, in systems where high temporal resolution is needed. This is particularly appropriate to bioelectrochemical analyses, where release and uptake phenomena can occur on the millisecond timescale. (C) 2015 Elsevier Ltd. All rights reserved
Delayed voltammetric with respect to amperometric electrochemical detection of concentration changes in microchannels
The time response of an electrode incorporated into a fluidic channel to variations in analyte concentration of the outer-sphere redox probe ferrocenemethanol was investigated both for amperometry (AMP) and cyclic voltammetry (CV). The experimental data show that the temporal resolution of CV is not as good as that of AMP, as CV cannot properly detect fast concentration transients. The delayed response of CV was previously reported, for neurotransmitters, and mostly attributed to the adsorption of the analyte on the electrode surface. By using an outer-sphere redox couple, we show that mass transport also significantly delays the response of CV. The experimental delay time in CV was understood from mass transfer limitations due to the relaxation of the diffusion layer during repeated potential scanning. Furthermore, a robust protocol for the analysis of fast concentration transients was established, using the impulse and modulation transfer functions of the system. This method was found to be more precise than the mere analysis of undifferentiated traces in the time domain. As a proof of concept, the effect of increased viscosity was investigated, showing that AMP was more sensitive than CV to these variations. Overall, this analysis underlines further the enhanced temporal sensitivity of AMP over CV, at the expense of decreased chemical resolution, potentially having implications for in situ electrochemical detection of biologically relevant molecules
Optical microscopy using a glass microsphere for metrology of sub-wavelength nanostructures
A technique that allows direct optical imaging of nanostructures and determines quantitatively geometric nanofeatures beyond the classical diffraction limit by using high-refractive index glass microspheres is introduced. The glass microsphere is put on a nanostructure that is immersed in oil and collects the sample's near-field evanescent wave and transforms it into a propagating one, thereby generating a magnified image in the far-field which is recorded by a conventional oil-immersion microscope objective. Experimental results on nanostructures demonstrates a resolution of similar to lambda/4-lambda/7, where lambda is the illumination wavelength, by using a 60 mu m glass microsphere and a normal wideband halogen lamp as illumination source. A two-dimensional numerical study of the light propagation through a glass microsphere using finite element method (FEM) is performed, providing key insight into the microsphere's superior imaging capability. (C) 2015 Elsevier B.V. All rights reserved
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