Novel Method for Analyzing Crack Growth in Polymeric Microtensile Specimens by In Situ Atomic Force Microscopy

In this paper a micro tensile test which allows the determination and observation of the crack growth behaviour in thin polymer layers is presented. The setup consists of micromanipulators and piezo actuators for straining the sample while an atomic force microscope (AFM) is used for scanning the crack tip area with high lateral resolution. The stress in the specimen is determined by an optical microscope for observation of the deflection of a force sensing beam. The material under investigation is an amorphous and strongly entangled thermoplastic polyimide which can be patterned photolithographically and is spin cast to form layers of 3μm thickness. The results show the potential of the setup to measure crack length, crack tip opening and nominal stress. The stress-crack length-diagram then allows to determine different stages during crack growt

High resolution measurement of FGM thin films using picosecond ultrasonics

AbstractFunctionally Graded Materials (FGMs) are materials with elastic properties λ, μ and density ρ depending on spatial coordinates. When between two homogeneous materials the transition layer thickness is of the same order of magnitude as the mechanical wave length, the wave scattering behavior at the interface becomes frequency dependent. The effect is of growing importance for micro- and nanostructures since the relative size of the interface layers is generally larger than in macroscopic structures. In this work a linear material grading is achieved with physical vapor deposition by magnetron sputtering. The used materials are aluminum (Al) and tantalum (Ta) due to their strong variation in acoustic impedance (bulk force reflection coefficient 0.6) and their good intermixing properties. In the sample investigated here the material transition layer thickness amounts to 60nm. With Rutherford Backscattering Spectroscopy (RBS) measurements and electron microscopy (SEM/TEM) the material properties of the sample are characterized. Mechanical waves in the specimen are excited and detected using a pump probe laser acoustic setup. The frequency dependent wave propagation in FGM is demonstrated by investigating the spectral response in theory and experiments. The entire experimental setup is modeled using a finite difference algorithm for better interpretation of the measurements. The frequency dependent wave propagation in FGM is analyzed to characterize the interface and finds applications in high frequency filters, semiconductor manufacturing or thermal barrier layers

Epitaxial Lead Chalcogenides on Si for Mid-IR Detectors and Emitters Including Cavities

Lead chalcogenide (IV-VI narrow-gap semiconductor) layers on Si or BaF2(111) substrates are employed to realize two mid-infrared optoelectronic devices for the first time. A tunable resonant cavity enhanced detector is realized by employing a movable mirror. Tuning is across the 4μm to 5.5μm wavelength range, and linewidth is <0.1μm. Due to the thin (0.3μm) PbTe photodiode inside the cavity, a higher sensitivity at higher operating temperatures was achieved as compared to conventional thick photodiodes. The second device is an optically pumped vertical external-cavity surface-emitting laser with PbTe-based gain layers. It emits at ∼5μm wavelength and with output power up to 50mW pulsed, or 3mW continuous wave at 100

Embedded Microbubbles for Acoustic Manipulation of Single Cells and Microfluidic Applications.

Acoustically excited microstructures have demonstrated significant potential for small-scale biomedical applications by overcoming major microfluidic limitations. Recently, the application of oscillating microbubbles has demonstrated their superiority over acoustically excited solid structures due to their enhanced acoustic streaming at low input power. However, their limited temporal stability hinders their direct applicability for industrial or clinical purposes. Here, we introduce the embedded microbubble, a novel acoustofluidic design based on the combination of solid structures (poly(dimethylsiloxane)) and microbubbles (air-filled cavity) to combine the benefits of both approaches while minimizing their drawbacks. We investigate the influence of various design parameters and geometrical features through numerical simulations and experimentally evaluate their manipulation capabilities. Finally, we demonstrate the capabilities of our design for microfluidic applications by investigating its mixing performance as well as through the controlled rotational manipulation of individual HeLa cells

Uplifting and Inflation with D3 Branes

Back-reaction effects can modify the dynamics of mobile D3 branes moving within type IIB vacua, in a way which has recently become calculable. We identify some of the ways these effects can alter inflationary scenarios, with the following three results: (1) By examining how the forces on the brane due to moduli-stabilizing interactions modify the angular motion of D3 branes moving in Klebanov-Strassler type throats, we show how previous slow-roll analyses can remain unchanged for some brane trajectories, while being modified for other trajectories. These forces cause the D3 brane to sink to the bottom of the throat except in a narrow region close to the D7 brane, and do not ameliorate the \eta-problem of slow roll inflation in these throats; (2) We argue that a recently-proposed back-reaction on the dilaton field can be used to provide an alternative way of uplifting these compactifications to Minkowski or De Sitter vacua, without the need for a supersymmetry-breaking anti-D3 brane; and (3) by including also the D-term forces which arise when supersymmetry-breaking fluxes are included on D7 branes we identify the 4D supergravity interactions which capture the dynamics of D3 motion in D3/D7 inflationary scenarios. The form of these potentials sheds some light on recent discussions of how symmetries constrain D term interactions in the low-energy theory.Comment: JHEP.cls, 35 pages, 3 .eps figure

de Sitter String Vacua from Supersymmetric D-terms

We propose a new mechanism for obtaining de Sitter vacua in type IIB string theory compactified on (orientifolded) Calabi-Yau manifolds similar to those recently studied by Kachru, Kallosh, Linde and Trivedi (KKLT). dS vacuum appears in KKLT model after uplifting an AdS vacuum by adding an anti-D3-brane, which explicitly breaks supersymmetry. We accomplish the same goal by adding fluxes of gauge fields within the D7-branes, which induce a D-term potential in the effective 4D action. In this way we obtain dS space as a spontaneously broken vacuum from a purely supersymmetric 4D action. We argue that our approach can be directly extended to heterotic string vacua, with the dilaton potential obtained from a combination of gaugino condensation and the D-terms generated by anomalous U(1) gauge groups.Comment: 17 pages, 1 figur