摘要:

The design of a device for rheological orthogonal superposition measurements on fluids is discussed. Superposing a small amplitude oscillatory motion on a steady or transient shear flow provides a technique to probe flow-induced microstructural changesin situby means of mechanical spectroscopy. Oscillations perpendicular to the main shear flow possess intrinsic advantages over the parallel case. It is shown that the closed loop system of a force rebalanced transducer of an existing rheometer can be modified to drive an orthogonal oscillatory motion and to measure the material response. Nonhomogeneous flow, annular pumping or cavitation can occur and have to be avoided by means of a suitable flow cell. A double walled Couette cell, open at the bottom is suggested for that purpose. The device is evaluated with Newtonian fluids and with a viscoelastic polymer solution. The apparatus has two major advantages. First the forces associated with the main shear flow can be measured simultaneously with the orthogonal moduli. Second, the present design is implemented on an instrument already capable of performing parallel superposition measurements, hence the two superposition modes are available on a single instrument. The relative simplicity of the proposed modification could boost the use of orthogonal superposition measurements and facilitate the development of new applications, e.g., to probe the time evolution of the flow-induced anisotropy in complex systems. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148351

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Fluorescent dye dissolved in a liquid flow was used to outline liquid-gas free boundaries and, with digital imaging, to observe quantitatively surface wave propagation and pattern formation, as well as contact-line velocity and contact angle in thin film flows on horizontal and inclined substrates. Using the relatively inexpensive system described here, a fluid depth measurement with a precision of ±0.02 mm is obtained routinely in flows of several millimeters depth over an area of approximately one square meter, and essentially unlimited continuous time spans. Dynamic contact angles are measured, for the first time, on liquid fronts with significant three-dimensional curvature such as rivulets draining down an inclined plate at any speed or global location. Procedures to normalize results quantitatively for any nonuniformities of the incident illumination are given. Estimates of the contribution to the experimental error by other effects, such as variations in dye concentration and temperature, and image digital register capacity, are also discussed. Illustrative results for two fluids and several dyes are given. Refinements to decrease the local error further to ±0.005 mm or less are described. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148352

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This article reports the development of a two-dimensional analyzer, which enables us to observe the distribution of hydrogen on surfaces. A micro-focused electron beam with low primary electron energy (<1 keV) is scanned over a sample surface, in conjunction with a time-of-flight type electron-stimulated desorption spectroscope, to obtain clear H+ion images of a specimen surface. A line scan analysis of H+ions on an integrated circuit and a scanning electron-stimulated desorption image of H+ions on a Cu mesh are presented as demonstrations. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148353

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Fluorescence lifetime-resolved imaging (FLI) is a relatively new technique of fluorescence imaging whereby the spatial distribution of fluorescence decay times can be determined directly at every pixel of an image simultaneously. The fluorescence decay times of many chromophores can act as sensitive gauges of their molecular environments. By employing measurement techniques that are quantitatively related to the radiative dynamics of the dye molecules (in the nanosecond time range), additional physical parameters are available for discerning different fluorophores with disparate lifetimes, or for characterizing a single fluorophore in different surroundings. Many physical processes such as molecular aggregation, binding of dyes to macromolecular species, inclusion of chromophores in specific cellular organwelles, fluorescence resonance energy transfer, and dynamic quenching determine the excited-state lifetime of a fluorophore. The FLI technique provides a way to measure these processes directly at103–106pixels in an image. In addition, if image domains differ with respect to the mean fluorescence lifetime, FLI can be used to improve the contrast of a fluorescence image. By measuring the fluorescence lifetime one can determine whether fluorescence intensity differences from different locations in an image can be attributed to differences in dye concentration or whether physical spectroscopic effects such as local differences in the rate of dynamic quenching are responsible. All the above applications provide new possibilities for biology and medical diagnostics. However the speed of data acquisition and analysis in current FLI instrumentation is limited in general to several minutes; for real-time applications (in order to follow rapid changes of microscopic samples or makein vivoendoscopic medical diagnosis) the present instruments are too slow. We present here a FLI apparatus that is capable of acquiring, processing, and displaying fluorescence lifetime-resolved images in quasi-real time. We also present rapid algorithms for analyzing the data in real time. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148354

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We describe a simple, piezoelectric shear-force detection for controlling the tip–sample distance in near-field optical microscopes. The fiber is glued to a V-shaped piezoelectric cantilever and the assembly is fixed to a piezoelectric plate. The piezoelectric plate excites the fiber at resonance, while the piezoelectric cantilever of small mass detects the fiber motion with a high sensitivity. When the fiber approaches within, typically, 10–20 nm above the sample, shear forces cause the signal to reduce. The only signal processing before injecting it into the feedback loop is demodulation by a lock-in amplifier. The high signal-to-background allows the fiber resonance to be identified easily. We demonstrate the performance of our system with shear-force images of two test patterns. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148355

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In their present embodiment, sensors used in interfacial force microscopy do not have the necessary mechanical bandwidth to be employed as free-running tapping mode devices. We describe an extremely stable method of obtaining tapping mode images using feedback on the sensor. Our method is immune to small dc drifts in the force signal, and the prospect of diminishing the risk of damaging fragile samples is realized. The feasibility of the technique is demonstrated by our imaging work on a Kevlar fiber-epoxy composite. We also present a model which accounts for the frequency dependence of the sensor in air when operating under closed loop control. A simplified force modulation model is investigated to explore the effect of contact on the closed loop response of the sensor. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148356

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present the development of an atomic force microscope (AFM) with an externally controllable force using an electromagnet. The position of the AFM cantilever with magnetic material at the end of the backside is controlled directly by the external magnetic field of an electromagnet. It is possible to use an optical detection system because the electromagnet is located inside the piezo tube on which the sample is mounted. A magnetic force feedback system has been implemented in this AFM. The effective stiffness of the cantilever is increased by the open loop gain of the feedback. We are able to control the motion of a soft cantilever (0.16 N/m) with this feedback system in air. Force feedback using an electromagnet allows the elimination of “snap-in” contact which may physically damage the tip and mica sample. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148357

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The action of viscous forces on the motion of an atomic force microscope cantilever operating in resonant mode in air is modelized. We demonstrate that for most applications, the vibration of a V shaped cantilever in the air can be approximated to a simple damped oscillator. The damping factor is distance dependent and includes terms issued of the interaction of both cantilever and tip with the sample. Expressions for the various damping forces have been derived and related to the geometry of the tip-cantilever system. They lead to an expression which quantifies the variations in oscillation amplitude versus tip sample distance. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148358

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The use of commercial atomic force microscopes (AFM) operating in the noncontact mode for surface force measurements is critically reviewed. Approach curves (i.e., vibration amplitude versus tip–surface distance) using standard microfabricated tips are discussed with respect to the basic theory of an equivalent harmonic oscillator. Different artifacts are addressed. In particular, we show theoretically and experimentally that the force exerted by the layer of air confined between the cantilever and the surface is a major contribution to the force on the cantilever. However, by carefully choosing the parameters (essentially the vibration amplitude) for the measurement of the approach curve, and by taking into account the damping within the confined air layer, we succeeded in measuring reliable surface force profiles with commercial AFM in the air and in describing them quantitatively by dispersion force interactions. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148359

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This article describes a low temperature scanning tunneling microscope (STM) system which is designed to study film growth at very low substrate temperatures(4 K<TS<77 K).A simple tripod design with the addition of a sample manipulator, is implemented as the STM head. In this system, a metal film can be thermally deposited on a conducting or an insulating substrate held at cryogenic temperatures and be probedin situby STM.In situand room temperature images of a Pb film grown on a 4 K substrate are presented. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148360

出版商:AIP    

年代:1997

数据来源: AIP