Browsing by Subject "Relaxation"

Now showing 1 - 10 of 10

Open Access
Dissipation in a finite-size bath
(American Physical Society, 2011-07-18) Carcaterra, A.; Akay, A.
We investigate the interaction of a particle with a finite-size bath represented by a set of independent linear oscillators with frequencies that fall within a finite bandwidth. We discover that when the oscillators have particular frequency distributions, the finite-size bath behaves much as an infinite-size bath exhibiting dissipation properties and thus allowing irreversible energy absorption from a particle immersed in it. We also present a reinterpretation of the Langevin equation using a perturbation approach in which the small parameter represents the inverse of the number of oscillators in the bath, elucidating the relationship between finite-size and infinite-size bath responses.
Open Access
Effects of different lighting arrangements on space perception
(Taylor & Francis, 1999) Manav, B.; Yener, C.
The idea to improve the quality of lighting design in interior spaces has gained importance in the last years. This experimental study concentrateson the quality of lighting and the relation between lighting arrangements and perception. A room having four different lighting arrangements was prepared and the differences between the perception of the participants under each lighting arrangement was analysed. The results suggested that different lighting arrangements affect perception. Wall washing enhanced the impressions of clarity and order, cove lighting enhanced spaciousness and order and uplighting made the same space relaxing, private and pleasant.
Open Access
Effects of different lighting arrangements on space perception
(1997) Yücetaş, Banu
The importance given to the quality of lighting design has increased in recent years. This makes lighting design more an art than an engineering problem and it is closely related to the way a space is perceived. It is difficult to discuss perception as it is the subjective interpretation of a certain situation. This study concentrates on the quality of lighting and the relation between the lighting arrangements and perception. A room that has four different lighting arrangements was prepared and the differences between the perception of the participants under each lighting arrangement was analyzed in terms of the six impressions, clarity, spaciousness, relaxation, privacy, pleasantness and order. The variation in these terms due to the alterations in the lighting systems was studied. The age, gender and the eye deficiency of participants were collected as personal data and their relations to the process of perception were taken into account.
Open Access
Low drive field amplitude for improved image resolution in magnetic particle imaging
(Wiley-Blackwell Publishing, Inc., 2016) Croft, L. R.; Goodwill, P. W.; Konkle, J. J.; Arami, H.; Price, D. A.; Li, A. X.; Saritas, E. U.; Conolly, S. M.
Purpose: Magnetic particle imaging (MPI) is a new imaging technology that directly detects superparamagnetic iron oxide nanoparticles. The technique has potential medical applications in angiography, cell tracking, and cancer detection. In this paper, the authors explore how nanoparticle relaxation affects image resolution. Historically, researchers have analyzed nanoparticle behavior by studying the time constant of the nanoparticle physical rotation. In contrast, in this paper, the authors focus instead on how the time constant of nanoparticle rotation affects the final image resolution, and this reveals nonobvious conclusions for tailoring MPI imaging parameters for optimal spatial resolution. Methods: The authors first extend x-space systems theory to include nanoparticle relaxation. The authors then measure the spatial resolution and relative signal levels in an MPI relaxometer and a 3D MPI imager at multiple drive field amplitudes and frequencies. Finally, these image measurements are used to estimate relaxation times and nanoparticle phase lags. Results: The authors demonstrate that spatial resolution, as measured by full-width at half-maximum, improves at lower drive field amplitudes. The authors further determine that relaxation in MPI can be approximated as a frequency-independent phase lag. These results enable the authors to accurately predict MPI resolution and sensitivity across a wide range of drive field amplitudes and frequencies. Conclusions: To balance resolution, signal-to-noise ratio, specific absorption rate, and magnetostimulation requirements, the drive field can be a low amplitude and high frequency. Continued research into how the MPI drive field affects relaxation and its adverse effects will be crucial for developing new nanoparticles tailored to the unique physics of MPI. Moreover, this theory informs researchers how to design scanning sequences to minimize relaxation-induced blurring for better spatial resolution or to exploit relaxation-induced blurring for MPI with molecular contrast.
Open Access
A novel effect of Electron Spin Resonance on electrical resistivity
(Elsevier, 2018) Singh, N.; Rani, Luxmi
We extend the well known phenomenon of magnetoresistance (extra resistivity of materials in transverse magnetic field) to a regime where in addition to a transverse magnetic field, a transverse microwave field of resonant frequency is also applied. In a magnetic field, electron spin levels are Zeeman split. In a resonant microwave field, we uncover a new channel of momentum relaxation in which electrons in upper Zeeman level can deexcite to lower Zeeman level by generating spin fluctuation excitation in the lattice (similar to what happens in Electron Spin Resonance (ESR) spectroscopy). An additional resistivity due to this mechanism is predicted in which momentum randomization of Zeeman split electrons happen via bosonic excitations (spin fluctuations). An order of magnitude of this additional resistivity is calculated. The whole work is based upon an extension of Einstein’s derivation of equilibrium Planckian formula to near equilibrium systems.
Open Access
Probing viscosity via relaxation in magnetic particle imaging
(2017-01) Ütkür, Mustafa
Magnetic Particle Imaging (MPI) is a high-contrast imaging modality with applications such as angiography, stem cell tracking, and cancer imaging. In recent years, MPI was shown to be a potential functional imaging modality through \color MPI" techniques, where responses from different nanoparticles can be distinguished. These techniques can be extended to differentiate environmental conditions or states such as different viscosities. Increased viscosity in vivo was shown to be related with various diseases such as hypertension, atherosclerosis, and cancer. Through color MPI techniques, MPI shows a great promise for mapping viscosity and for helping in the diagnosis of these important diseases. This thesis demonstrates the capability of MPI to map viscosities through an estimation of relaxation time constant of nanoparticles. This capability is verified through an extensive experimental work with a magnetic particle spectrometer (MPS) setup that is custom designed. These experiments are conducted for the biologically important viscosity range between 0.89 mPa.s and 15.33 mPa.s, at four different frequencies (between 250 Hz and 10.8 kHz) and at three different field amplitudes (between 5 mT and 15 mT). The results demonstrate MPI's viscosity mapping capability in a biological range.
Open Access
Quantum efficiency enhancement in film by making nanoparticles of polyfluorene
(Optical Society of America, 2008) Huyal, I. O.; Ozel, T.; Tuncel, D.; Demir, Hilmi Volkan
We report on conjugated polymer nanoparticles of polyfluorene that were formed to exhibit higher fluorescence quantum efficiency in film (68%) and reduce undesired emission peak wavelength shifts in film (by 20 nm), compared to the solid-state polymer thin film made directly out of the same polymer solution without forming nanoparticles. Using the facile reprecipitation method, solutions of poly[9,9-dihexyl-9H-fluorene] in THF were added at different volume ratios to obtain different size distributions of nanoparticle dispersions in water. This allowed us to control the sizedependent optical emission of our polyfluorene nanoparticles. Such organic nanoparticles hold great promise for use as efficient emitters in optoelectronic device applications. (C) 2008 Optical Society of America.
Open Access
Relaxation and nonoccurrence of the Lavrentiev phenomenon for nonconvex problems
(Springer, 2013) Hüsseinov, F.
The paper studies a relaxation of the basic multidimensional variational problem, when the class of admissible functions is endowed with the Lipschitz convergence introduced by Morrey. It is shown that in this setup, the integral of a variational problem must satisfy a classical growth condition, unlike the case of uniform convergence. The relaxations constructed here imply the existence of a Lipschitz convergent minimizing sequence. Based on this observation, the paper also shows that the Lavrentiev phenomenon does not occur for a class of nonconvex problems. © 2013 Institute of Mathematics, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Chinese Mathematical Society and Springer-Verlag Berlin Heidelberg.
Open Access
Relaxation-based viscosity mapping for magnetic particle imaging
(Institute of Physics Publishing, 2017) Utkur, Mustafa; Muslu, Yavuz; Sarıtaş, Emine Ülkü
Magnetic particle imaging (MPI) has been shown to provide remarkable contrast for imaging applications such as angiography, stem cell tracking, and cancer imaging. Recently, there is growing interest in the functional imaging capabilities of MPI, where 'color MPI' techniques have explored separating different nanoparticles, which could potentially be used to distinguish nanoparticles in different states or environments. Viscosity mapping is a promising functional imaging application for MPI, as increased viscosity levels in vivo have been associated with numerous diseases such as hypertension, atherosclerosis, and cancer. In this work, we propose a viscosity mapping technique for MPI through the estimation of the relaxation time constant of the nanoparticles. Importantly, the proposed time constant estimation scheme does not require any prior information regarding the nanoparticles. We validate this method with extensive experiments in an in-house magnetic particle spectroscopy (MPS) setup at four different frequencies (between 250 Hz and 10.8 kHz) and at three different field strengths (between 5 mT and 15 mT) for viscosities ranging between 0.89 mPa • s-15.33 mPa • s. Our results demonstrate the viscosity mapping ability of MPI in the biologically relevant viscosity range.
Open Access
Simultaneous temperature and viscosity estimation capability via magnetic nanoparticle relaxation
(Wiley-Blackwell Publishing, Inc., 2022-04) Utkur, Mustafa; Sarıtaş, Emine Ülkü
Purpose: Magnetic particle imaging (MPI) is emerging as a highly promising imaging modality. Magnetic nanoparticles (MNPs) are used as imaging tracers in MPI, and their relaxation behavior provides the foundation for its functional imaging capability. Since MNPs are also utilized in magnetic fluid hyperthermia (MFH) and MPI enables localized MFH, temperature mapping arises as an important application area of MPI. To achieve accurate temperature estimations, however, one must also take into account the confounding effects of viscosity on the MPI signal. In this work, we analyze the effects of temperature and viscosity on MNP relaxation and determine temperature and viscosity sensitivities of relaxation time constant estimations via TAURUS (TAU estimation via Recovery of Underlying mirror Symmetry) at a wide range of operating points to empower simultaneous mapping of these two parameters. Methods: A total of 15 samples were prepared to reach four target viscosity levels (0.9–3.6 mPa (Formula presented.) s) at five different temperatures (25–45 (Formula presented.) C). Experiments were performed on a magnetic particle spectrometer (MPS) setup at 60 different operating points at drive field amplitudes ranging between 5 and 25 mT and frequencies ranging between 1 and 7 kHz. To enable these extensive experiments, an in-house arbitrary-waveform MPS setup with temperature-controlled heating capability was developed. The operating points were divided into four groups with comparable signal levels to maximize signal gain during rapid signal acquisition. The relaxation time constants were estimated via TAURUS, by restoring the underlying mirror symmetry property of the positive and negative half cycles of the time-domain MNP response. The relative time constants with respect to the drive field period, (Formula presented.), were computed to enable quantitative comparison across different operating points. At each operating point, a linear fit was performed to (Formula presented.) as a function of each functional parameter (i.e., temperature or viscosity). The slopes of these linear fits were utilized to compute the temperature and viscosity sensitivities of TAURUS. Results: Except for outlier behaviors at 1 kHz, the following global trends were observed: (Formula presented.) decreases with drive field amplitude, increases with drive field frequency, decreases with temperature, and increases with viscosity. The temperature sensitivity varies slowly across the operating points and reaches a maximum value of 1.18%/ (Formula presented.) C. In contrast, viscosity sensitivity is high at low frequencies around 1 kHz with a maximum value of 13.4%/(mPa (Formula presented.) s) but rapidly falls after 3 kHz. These results suggest that the simultaneous estimation of temperature and viscosity can be achieved by performing measurements at two different drive field settings that provide complementary temperature/viscosity sensitivities. Alternatively, temperature estimation alone can be achieved with a single measurement at drive field frequencies above 3 kHz, where viscosity sensitivity is minimized. Conclusions: This work demonstrates highly promising temperature and viscosity sensitivities for TAURUS, highlighting its potential for simultaneous estimation of these two environmental parameters via MNP relaxation. The findings of this work reveal the potential of a hybrid MPI–MFH system for real-time monitored and localized thermal ablation treatment of cancer.