Publications by authors named "Ward L Johnson"

5 Publications

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Sensing bacterial vibrations and early response to antibiotics with phase noise of a resonant crystal.

Sci Rep 2017 09 22;7(1):12138. Epub 2017 Sep 22.

Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, CO, 80305, USA.

The speed of conventional antimicrobial susceptibility testing (AST) is intrinsically limited by observation of cell colony growth, which can extend over days and allow bacterial infections to advance before effective antibiotics are identified. This report presents an approach for rapidly sensing mechanical fluctuations of bacteria and the effects of antibiotics on these fluctuations. Bacteria are adhered to a quartz crystal resonator in an electronic bridge that is driven by a high-stability frequency source. Mechanical fluctuations of cells introduce time-dependent perturbations to the crystal boundary conditions and associated resonant frequency, which translate into phase noise measured at the output of the bridge. In experiments on nonmotile E. coli exposed to polymyxin B, cell-generated frequency noise dropped close to zero with the first spectra acquired 7 minutes after introduction of the antibiotic. In experiments on the same bacterial strain exposed to ampicillin, frequency noise began decreasing within 15 minutes of antibiotic introduction and proceeded to drop more rapidly with the onset of antibiotic-induced lysis. In conjunction with cell imaging and post-experiment counting of colony-forming units, these results provide evidence that cell death can be sensed through measurements of cell-generated frequency noise, potentially providing a basis for rapid AST.
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http://dx.doi.org/10.1038/s41598-017-12063-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5610186PMC
September 2017

High-temperature electroacoustic characterization of Y-cut and singly-rotated Ca3TaGa3Si2O14 resonators.

IEEE Trans Ultrason Ferroelectr Freq Control 2014 Aug;61(8):1433-41

Synthetic piezoelectric crystals in the P321 crystal class have been a focus of substantial research that is largely driven by applications in high-temperature resonant BAW and SAW sensing. Fully ordered crystals in this class, such as Ca3TaGa3Si2O14 (CTGS), have been suggested as offering the potential of electroacoustic performance that is superior to more extensively studied langasite (LGS) and langatate (LGT), which are partially disordered. In this study, the resonant frequencies, acoustic damping, and electrical conductivity of CTGS bulk acoustic resonators with Y-cut and (YXl)-30° crystal orientations and fundamental frequencies near 5 MHz are investigated at temperatures between ambient and 1100°C. (YXl)-30° resonators are found to have turnover temperatures near 200°C for the third and fifth overtones, in contrast to a monotonic decrease in resonant frequencies of Y-cut crystals with increasing temperature. The maximum temperature derivative of fractional changes in fifth-overtone frequency of (YXl)-30° CTGS is 40 × 10-6K-1 (near 1100°C), and this value is not greatly different from the temperature derivative of Y-cut CTGS frequencies over a broader range of temperatures. At ambient temperatures, the acoustic loss Q-1 of CTGS with both crystal orientations is found to be greater than the lowest values previously reported for LGS and LGT. The electrical conductivity of the CTGS specimens between 500°C and 1100°C is substantially lower than that previously reported for LGS. Corresponding to this lower conductivity, the piezoelectric/conductive contribution to Q-1 at elevated temperatures is reduced. Additional anelastic relaxation peaks observed between 100°C and 700°C are similar to those previously reported for LGS and LGT.
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http://dx.doi.org/10.1109/tuffc.2014.3052DOI Listing
August 2014

Vibrational modes of nanolines.

Nanotechnology 2008 Apr 5;19(14):145707. Epub 2008 Mar 5.

Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80525, USA.

Brillouin-light-scattering spectra previously have been shown to provide information on acoustic modes of polymeric lines fabricated by nanoimprint lithography. Finite-element methods for modeling such modes are presented here. These methods provide a theoretical framework for determining elastic constants and dimensions of nanolines from measured spectra in the low gigahertz range. To make the calculations feasible for future incorporation in inversion algorithms, two approximations of the boundary conditions are employed in the calculations: the rigidity of the nanoline/substrate interface and sinusoidal variation of displacements along the nanoline length. The accuracy of these approximations is evaluated as a function of wavenumber and frequency. The great advantage of finite-element methods over other methods previously employed for nanolines is the ability to model any cross-sectional geometry. Dispersion curves and displacement patterns are calculated for modes of polymethyl methacrylate nanolines with cross-sectional dimensions of 65 nm × 140 nm and rectangular or semicircular tops. The vibrational displacements and dispersion curves are qualitatively similar for the two geometries and include a series of flexural, Rayleigh-like, and Sezawa-like modes.
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http://dx.doi.org/10.1088/0957-4484/19/14/145707DOI Listing
April 2008

Symmetrization of Ritz approximation functions for vibrational analysis of trigonal cylinders.

J Acoust Soc Am 2003 Apr;113(4 Pt 1):1826-32

Materials Reliability Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA.

In the Ritz method of calculating vibrational normal modes, a set of finite series approximation functions provides a matrix eigenvalue equation for the coefficients in the series and the resonant frequency. The matrix problem usually can be block-diagonalized by grouping the functions into subsets according to their properties under the symmetry operations that are common to the specimen geometry and crystal class. This task is addressed, in this study, for the case of cylindrical specimens of crystals belonging to one of the higher trigonal crystal classes. The existence of doubly degenerate resonant modes significantly complicates the analysis. Group-theoretical projection operators are employed to extract, from series approximation functions in cylindrical coordinates, the terms that transform according to each irreducible representation of the point group. This provides a complete symmetry-based block diagonalization and categorization of the modal symmetries. Off-diagonal projection operators are used to provide relations between the displacement patterns of degenerate modes. The method of analysis is presented in detail to assist in its application to other geometries, crystal structures, and/or forms of Ritz approximation functions.
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http://dx.doi.org/10.1121/1.1558372DOI Listing
April 2003

Traction-free vibrations of finite trigonal elastic cylinders.

J Acoust Soc Am 2003 Apr;113(4 Pt 1):1812-25

Department of Civil Engineering, Colorado State University, Fort Collins, Colorado 80523, USA.

The unrestrained, traction-free vibrations of finite elastic cylinders with trigonal material symmetry are studied using two approaches, based on the Ritz method, which formulate the weak form of the equations of motion in cylindrical and rectangular coordinates. Elements of group theory are used to divide approximation functions into orthogonal subsets, thus reducing the size of the computational problem and classifying the general symmetries of the vibrational modes. Results for the special case of an isotropic cylinder are presented and compared with values published by other researchers. For the isotropic case, the relative accuracy of the formulations in cylindrical and rectangular coordinates can be evaluated, because exact analytical solutions are known for the torsional modes. The calculation in cylindrical coordinates is found to be more accurate for a given number of terms in the series approximation functions. For a representative trigonal material, langatate, calculations of the resonant frequencies and the sensitivity of the frequencies on each of the elastic constants are presented. The dependence on geometry (ratio of length to diameter) is briefly explored. The special case of a transversely isotropic cylinder (with the elastic stiffness C14 equal to zero) is also considered.
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http://dx.doi.org/10.1121/1.1548159DOI Listing
April 2003