Engineering

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The Bilkent University Open Access Articles collection consists of scholarly articles written by Bilkent-affiliated authors that are made available through BUIR under the Bilkent Faculty Open Access Policy, or under related publisher agreements.

Version details are supplied for each paper in the collection:

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Post peer-reviewed scholarly articles are available through BUIR collections, such as those for departments, labs, and research centers.

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Now showing 1 - 20 of 6856

Open Access
On-chip flow rate sensing via membrane deformation and bistability probed by microwave resonators
(Springer Link, 8-04-2023) Seçme, Arda; Pisheh, Hadi Sedaghat; Tefek, Uzay; Uslu, H. Dilara; Küçükoğlu, Berk; Alataş, Ceren; Kelleci, Mehmet; Hanay, Mehmet Selim
Precise monitoring of fluid flow rates constitutes an integral problem in various lab-on-a-chip applications. While off-chip flow sensors are commonly used, new sensing mechanisms are being investigated to address the needs of increasingly complex lab-on-a-chip platforms which require local and non-intrusive flow rate sensing. In this regard, the deformability of microfluidic components has recently attracted attention as an on-chip sensing mechanism. To develop an on-chip flow rate sensor, here we utilized the mechanical deformations of a 220 nm thick Silicon Nitride membrane integrated with the microfluidic channel. Applied pressure and fluid flow induce different modes of deformations on the membrane, which are electronically probed by an integrated microwave resonator. The flow changes the capacitance, and in turn resonance frequency, of the microwave resonator. By tracking the resonance frequency, liquid flow was probed with the device. In addition to responding to applied pressure by deflection, the membrane also exhibits periodic pulsation motion under fluid flow at a constant rate. The two separate mechanisms, deflection and pulsation, constitute sensing mechanisms for pressure and flow rate. Using the same device architecture, we also detected pressure-induced deformations by a gas to draw further insight into the sensing mechanism of the membrane. Flow rate measurements based on the deformation and instability of thin membranes demonstrate the transduction potential of microwave resonators for fluid–structure interactions at micro- and nanoscales.
Open Access
Phase imaging in reflection with the acoustic microscope
(A I P Publishing, 1978-01) Atalar, Abdullah; Quate, C. F.; Wickramasinghe, H. K.
When a polished surface of a single crystal is examined with a converging acoustic beam the reflected signal has a characteristic response that is dependent upon the elastic properties of the reflecting surface. This property can be used in the acoustic microscope to monitor the thickness of layers deposited on these surfaces and the small‐scale variations of the elastic parameters in these materials.
Open Access
An angular‐spectrum approach to contrast in reflection acoustic microscopy
(A I P Publishing, 1978-11) Atalar, Abdullah
The scanning acoustic microscope in the reflection mode has proved to be a rather simple and direct means for monitoring the elastic properties of a solid surface. When smooth surfaces of crystalline material are examined in a liquid with a highly convergent sound beam they exhibit a distinct response. This characteristic response, which can be treated as a ’’signature’’, is obtained by recording the output of the microscope as the spacing between the acoustic lens and the object is varied. An angular‐spectrum approach is used to derive an expression for this output in terms of the reflectance function. This function has an angular dependence determined by the bulk constants of the material itself. The expression resulting from this treatment can be used to explain the source of contrast in acoustic images.
Open Access
Reflection coefficient for a lossy liquid–lossless isotropic solid interface
(A I P Publishing, 1979-05) Atalar, Abdullah
A theoretical treatment of the reflection problem at a lossy liquid–lossless isotropic solid is given. The calculation results indicate that the reflection coefficient has a peak greater than unity. A physical explanation is proposed which also leads to the prediction that at some critical liquid attenuation a Rayleigh wave propagating on the solid surface will not leak into the liquid.
Open Access
Modulation transfer function for the acoustic microscope
(The Institution of Engineering and Technology, 1979-05-24) Atalar, Abdullah
A simple method to measure the modulation transfer function of an acoustic microscope is described. Theoretical results are compared with experimental measurements. An explanation of `shadowing´ found in acoustic images is given.
Open Access
Acoustic microscopy: resolution of subcellular detail
(National Academy of Sciences, 1979-07) Johnston, R. N.; Atalar, Abdullah; Heiserman, J.; Jipson, V.; Quate, C. F.
Recent advances now permit the use of scanning acoustic microscopy for the analysis of subcellular components. By sequential viewing of identified fixed cells with acoustic, light, and electron microscopy, we have established that the acoustic microscope can readily detect such features as nuclei and nucleoli, mitochondria, and actin cables. Under optimal conditions, images can even be obtained of filopodia, slender projections of the cell surface that are approximately 0.1-0.2 micron in diameter. Small objects separated by as little as 0.5-0.7 micron can successfully be resolved. Three aspects of the acoustic micrographs prepared in this preliminary survey seem especially prominent. These are, first, the extraordinary level of acoustic contrast that can differentiate the various cytoplasmic organelles, even in regions of very thin cytoplasm; second, the reversals in acoustic contrast that occur when altering the plane of focus; and third, the sensitivity of the acoustic response to overall cytoplasmic thickness. The acoustic microscope uses a novel source of contrast that is based on local mechanical properties. In addition, it can provide a degree of resolution that is comparable to that of the light microscope.
Open Access
Acoustic microscopy with mechanical scanning—A review
(IEEE, 1979-08) Quate, C. F.; Atalar, Abdullah; Wickramasinghe, H. K.
Acoustic waves in liquids are known to have wavelengths comparable to that of visible light if the frequency is in the gigahertz range. The phenomena of Brillouin scattering in liquids is based on such waves. In helium near 2 K acoustic waves with a wavelength of 2000 Å were studied some ten years ago at UCLA. It follows from these observations that an imaging system based on acoustic radiation with a resolving power competitive with the optical microscope is within reach if an ideal lens free from aberrations could be found. Such a lens, which was so elusive at the beginning, is now a simple device and it is the basic component in the acoustic microscope that forms the basis for this review. In this article we will establish the characteristic properties of this new instrument. We will review some of the simple properties of acoustic waves and show how a single spherical surface formed at a solid liquid interface can serve as this ideal lens free from aberrations and capable of producing diffraction limited beams. When this is incorporated into a mechanical scanning system and excited with acoustic frequencies in the microwave range images can be recorded with acoustic wavelengths equal to the wavelength of visible light. We will present images that show the elastic properties of specimens selected from the fields of material science, integrated circuits, and cell biology. The information content in these images will often exceed that of the optical micrographs. In the reflection mode we illuminate the smooth surface of a crystalline material with a highly convergent acoustic beam. The reflected field is perturbed in a unique way that is determined by the elastic properties of the reflecting surface and it shows up in the phase of the reflected acoustic field. There is a distinct and characteristic response at the output when the spacing between the object and the lens is varied. This behavior in the acoustic ieflection microscope provides a rather simple and direct means for monitoring the elastic parameters of a solid surface. It is easy to distinguish between different materials, to determine the layer thickness, and to display variations in the elastic constants on a microscopic scale. These features lead us to believe there is a promising future for the field of acoustic microscopy.
Open Access
A physical model for acoustic signatures
(A I P Publishing, 1980-01) Atalar, Abdullah
A physical model is presented to explain the interference phenomenon that gives rise to the material‐dependent signature obtained from an acoustic reflection microscope. An approximate formula is derived for the peak separation of the characteristic response, and it agrees well with the experimental results.
Open Access
A backscattering formula for acoustic transducers
(A I P Publishing, 1980-06) Atalar, Abdullah
A backscattering formula for a single transducer geometry is derived. It expresses the output voltage of the transducer in terms of the angular spectra of scalar and vector acoustic potentials on a plane. The formulation is suitable for acoustic problems involving wide angular spectrum beams. The derived formula gives a simple expression for the problem of a circular transducer facing a plane reflector. The output voltage of a transducer receiving the backscattered waves from an arbitrary size spherical flaw is also presented.
Open Access
Photoacoustic effect as a liquid absorbance detector
(Optical Society of America, 1980-09) Atalar, Abdullah
A theoretical treatment of the photoacoustic effect for a liquid enclosed in a cylindrical cell is given. Simple analytical expressions for the photoacoustic signal are derived for cw and pulse modulation schemes. Modulation frequency or pulse width dependence of the photoacoustic signal is discussed. For the cw modulation scheme it is found that more photoacoustic power is generated in a resonant cell than a nonresonant cell at the expense of bandwidth reduction. Conditions leading to 1/f dependence of the signal on modulation frequency are given. Experimental results are presented that support the simplified treatments. Eventual sensitivities of various excitation schemes under optimum conditions are calculated.
Open Access
Reflection of ultrasonic waves at a liquid–cubic–solid interface
(A I P Publishing, 1983-02) Atalar, Abdullah
The results of numerical calculations are presented for the reflection coefficient of sound waves incident on a liquid–cubic–solid interface. The reflection coefficient is calculated numerically for the (001) face of various cubic crystals. It is found that for certain orientations there is a null in the reflection coefficient. At this orientation all the power is coupled into a quasishear wave inside the solid. An explanation based on impedance theory is presented. The explanation given predicts that there might be reflection nulls for other liquid–solid interfaces where the solid is any anisotropic solid not just cubic.
Open Access
Penetration depth of the scanning acoustic microscope
(IEEE, 1985-03) Atalar, Abdullah
A definition for the penetration depth of the scanning reflection acoustic microscope is given. With this definition it is possible to calculate the penetration depth of a given lens geometry for a given material. The penetration depth depends on the elastic parameters of the object, the signal-to-noise ratio, and the operation frequency of the acoustic microscope. Calculations show that for high-impedance materials, the penetration depthis limited by the wavelength of the surface waves and hence by frequency. For low-impedance materials the opening angle of the lens can be properly selected to make the longitudinal or shear wave penetration dominant, effectively increasing the penetration well above the wavelength limit of the surface wave.
Open Access
Focusing surface waves using an axicon
(A I P Publishing, 1985-12) Köymen, Hayrettin; Atalar, Abdullah
Axicons are generators of waves which focus on a line. They are used in various imaging and nondestructive testing applications as bulk wave focusing devices with a very long depth of focus. In this letter, a new type of conical axicon is introduced and it is shown that this axicon, immersed in a liquid, insonifying a plane solid surface can be used to excite surface waves on the solid surface provided that the cone angle of the axicon coincides with the Rayleigh critical angle of the liquid‐solid interface. The generated surface waves focus into a diffraction‐limited spot. This new surface wave focusing scheme is easy to use, has a conversion efficiency and sensitivity far better than other existing techniques.
Open Access
High‐performance acoustic microscope
(A I P Publishing, 1986-06) Atalar, Abdullah; Hoppe, Martin
A commercial scanning acoustic microscope operating in the frequency range of 50–2000 MHz is described. It has a stable scan mechanism suitable for the high‐frequency end to give high‐resolution images and modes to allow for material characterization at the low‐frequency end. The high‐frequency electronics permitting a wideband operation with a variety of acoustic lenses are described. Microprocessors control many parts of the the instrument to release the user from the routine adjustments, resulting in a user‐friendly interface. Images are presented to show the resolution and penetration abilities of the instrument at several frequencies, along with comparative optical images.
Open Access
A high performance acoustic microscope - technical aspects and selected applications
(Springer, Boston, MA, 1986-07) Atalar, Abdullah; Hoppe, M.
Technical aspects of a scanning acoustic microscope with broad frequency coverage (50…2000 MHz) are described. Images demonstrating the capabilities of the microscope are shown.
Open Access
A new surface acoustic wave imaging technique
(Springer, Boston, MA, 1986-07) Atalar, Abdullah; Köymen, Hayrettin
A new type of imaging technique is presented which incorporates focussed surface acoustic waves. Surface acoustic waves are generated on the surface to be imaged by use of conical wavefronts. The conical wavefronts are obtained through the reflection of planar wavefronts from a parabolic cylindrical mirror. An imaging system is built which uses focussed surface acoustic waves in a mechanical scanning arrangement controlled by a computer. The resulting images show subsurface features with diffraction limited resolution.
Open Access
V(Z) of the surface acoustic wave focusing system
(IEEE, 1986-11) Atalar, Abdullah; Köymen, Hayrettin
It is possible to define a V(Z) function for the surface acoustic wave (SAW) focusing system for a class of reflectors similar to that defined in acoustic microscopy. In this case V(Z) is a function which relates the transducer output voltage to the distance between the focal point and the reflection line. It is possible to express V(Z) in an analytical expression using angular spectrum techniques. V(Z) is measured for straight edge reflectors forming various angles with the surface. V(2) of straight step reflectors are investigated. These curves show a dependence on the type of reflector. The experiments are performed at 1.5 MHz in conjunction with a computerized data acquisition system. The phase information is also recorded in all these measurements in addition to the amplitude. It is possible to get an inversion of V(Z) to get the SAW reflection coefficient at a straight discontinuity.
Open Access
Imaging flaws close to surface using focused surface acoustic waves
(IEEE, 1986-11) Köymen, Hayrettin; Atalar, Abdullah; Çiloğlu, T.; Önder, Murat; Uzel, Ç.; Yavuz, H.
The resolving power and detection ability of the focused surface acoustic wave (SAW) imaging modality is investigated in this paper. In this mode of imaging, conical bulk acoustic waves are used to generate and focus leaky surface acoustic waves on smooth surfaces of materials. Imaging systems built using this technique has diffraction limited focusing property. An imaging system using this focusing principle has been built, operating at 1.5 and 20 MHz. A slow mechanical scanning system controlled by a personal computer scans the surface of the object, and the data is acquired by the computer to generate a color or a black and white image on its graphic screen. The results of the initial experiments show that the inaging system is very sensitive to the grain structure and possible residual stresses on the surface of the object. It can resolve subsurface gratings of spacing less than a SAW wavelength very close to surface. The imaging system is inherently zero background, providing a high sensitivity not found in similar systems.
Open Access
An Input/Output control system for the dynamic job shop
(Taylor & Francis, 1987) Onur, L.; Fabrycky, W. J.
A combined input/output control system is presented for periodically determining the set of jobs to be released (input variables) and the capacities of processing centers (output variables) in the dynamic job shop, so that a composite cost function is minimized. An interactive heuristic optimizing algorithm incorporating a 0-1 linear mixed integer program is formulated. The resulting control system is compared by simulation with an alternate system for which only input is subject to control. Significant improvements are obtained for most performance measures evaluated. © 1987 Taylor & Francis Group, LLC.
Open Access
Digital decoding of in-line holograms
(SPIE, 1987) Onural, L.; Scott, P. D.
Digitally sampled in-line holograms may be linearly filtered to reconstruct a representation of the original object distribution, thereby decoding the information contained in the hologram. The decoding process is performed by digital computation rather than optically. Substitution of digital for optical decoding has several advantages, including selective suppression of the twin-image artifact, elimination of the far-field requirement, and automation of the data reduction and analysis process. The proposed filter is a truncated series expansion of the inverse of that operator that maps object opacity function to hologram intensity. The first term of the expansion is shown to be equivalent to conventional (optical) reconstruction, with successive terms increasingly suppressing the twin image. The algorithm is computationally efficient, requiring only a single fast Fourier transform pair.