SUMMARY / RELATED TOPICS

Atomic force microscopy

Atomic force microscopy or scanning force microscopy is a very-high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. Atomic force microscopy is a type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit; the information is gathered by "feeling" or "touching" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on command enable precise scanning; the AFM has three major abilities: force measurement, topographic imaging, manipulation. In force measurement, AFMs can be used to measure the forces between the probe and the sample as a function of their mutual separation; this can be applied to perform force spectroscopy, to measure the mechanical properties of the sample, such as the sample's Young's modulus, a measure of stiffness.

For imaging, the reaction of the probe to the forces that the sample imposes on it can be used to form an image of the three-dimensional shape of a sample surface at a high resolution. This is achieved by raster scanning the position of the sample with respect to the tip and recording the height of the probe that corresponds to a constant probe-sample interaction; the surface topography is displayed as a pseudocolor plot. Although the initial publication about the atomic force microscopy by Binnig and Gerber in 1986 speculated about the possibility of achieving atomic resolution, profound experimental challenges needed to be overcome before atomic resolution of defects and step edges in ambient conditions was demonstrated in 1993 by Ohnesorge and Binnig. True atomic resolution of the silicon 7x7 surface - the atomic images of this surface obtained by STM had convinced the scientific community of the spectacular spatial resolution of scanning tunneling microscopy - had to wait a little longer before it was shown by Giessibl.

In manipulation, the forces between tip and sample can be used to change the properties of the sample in a controlled way. Examples of this include atomic manipulation, scanning probe lithography and local stimulation of cells. Simultaneous with the acquisition of topographical images, other properties of the sample can be measured locally and displayed as an image with high resolution. Examples of such properties are mechanical properties like stiffness or adhesion strength and electrical properties such as conductivity or surface potential. In fact, the majority of SPM techniques are extensions of AFM; the major difference between atomic force microscopy and competing technologies such as optical microscopy and electron microscopy is that AFM does not use lenses or beam irradiation. Therefore, it does not suffer from a limitation in spatial resolution due to diffraction and aberration, preparing a space for guiding the beam and staining the sample are not necessary. There are several types of scanning microscopy including scanning probe microscopy.

Although SNOM and STED use visible, infrared or terahertz light to illuminate the sample, their resolution is not constrained by the diffraction limit. Fig. 3 shows an AFM, which consists of the following features. Numbers in parentheses correspond to numbered features in Fig. 3. Coordinate directions are defined by the coordinate system; the small spring-like cantilever is carried by the support. Optionally, a piezoelectric element oscillates the cantilever; the sharp tip is fixed to the free end of the cantilever. The detector records the motion of the cantilever; the sample is mounted on the sample stage. An xyz drive permits to displace the sample and the sample stage in x, y, z directions with respect to the tip apex. Although Fig. 3 shows the drive attached to the sample, the drive can be attached to the tip, or independent drives can be attached to both, since it is the relative displacement of the sample and tip that needs to be controlled. Controllers and plotter are not shown in Fig. 3.

According to the configuration described above, the interaction between tip and sample, which can be an atomic scale phenomenon, is transduced into changes of the motion of cantilever, a macro scale phenomenon. Several different aspects of the cantilever motion can be used to quantify the interaction between the tip and sample, most the value of the deflection, the amplitude of an imposed oscillation of the cantilever, or the shift in resonance frequency of the cantilever; the detector of AFM measures the deflection of the cantilever and converts it into an electrical signal. The intensity of this signal will be proportional to the displacement of the cantilever. Various methods of detection can be used, e.g. interferometry, optical levers, the piezoresistive method, the piezoelectric method, STM-based detectors. Note: The following paragraphs assume that'contact mode' is used. For other imaging modes, the process is similar, except that'deflection' should be replaced by the appropriate feedback variable.

When using the AFM to image a sample, the tip is brought into c

Moriah Aviation Training Centre

Moriah Aviation Training Centre, is an aviation training school in Uganda, that provides training for prospective pilots, cabin crew staff, aviation customer managers and related courses in the aviation industry. The headquarters of the school are located along Bubuli Road, in Nkumba Parish, Katabi sub-county, Busiro County, Wakiso District 7 kilometres, by road, north-east of the central business district of Entebbe, the nearest large town; this is 29 kilometres, by road, south-west of downtown Kampala, the capital and largest city of Uganda. The geographical coordinates of the campus of Moriah Aviation Training Centre are: 00°06'07.0"N, 32°30'16.0"E. The training institution s owned; the school was established to address the shortage of flight professionals in the region. Graduates of MATC find employment with local and regional airlines; as of March 2018, the following courses are offered at MATC: Flight Operations Dispatch Course Aircraft Maintenance Engineers' Course Flight Instructor Techniques Course Technical Stores Management Course Basic Airport Fire Fighting Course.

Uganda Aviation School East African Civil Aviation Academy List of aviation schools in Uganda Website of Moriah Aviation Training Centre

Magnetohydrodynamic turbulence

Magnetohydrodynamic turbulence concerns the chaotic regimes of magnetofluid flow at high Reynolds number. Magnetohydrodynamics deals with what is a quasi-neutral fluid with high conductivity; the fluid approximation implies that the focus is on macro length-and-time scales which are much larger than the collision length and collision time respectively. The incompressible MHD equations are ∂ u ∂ t + u ⋅ ∇ u = − ∇ p + B ⋅ ∇ B + ν ∇ 2 u ∂ B ∂ t + u ⋅ ∇ B = B ⋅ ∇ u + η ∇ 2 B ∇ ⋅ u = 0 ∇ ⋅ B = 0. Where u, B, p represent the velocity and total pressure fields, ν and η represent kinematic viscosity and magnetic diffusivity; the third equation is the incompressibility condition. In the above equation, the magnetic field is in Alfvén units; the total magnetic field can be split into two parts: B = B 0 + b. The above equations in terms of Elsässer variables are ∂ z ± ∂ t ∓ z ± + z ± = − ∇ p + ν + ∇ 2 z ± + ν − ∇ 2 z ∓ where ν ± = 1 2. Nonlinear interactions occur between the Alfvénic fluctuations z ∓; the important nondimensional parameters for MHD are Reynolds number R e = U L / ν Magnetic Reynolds number R e M = U L / η Magnetic Prandtl number P M = ν / η.

The magnetic Prandtl number is an important property of the fluid. Liquid metals have small magnetic Prandtl numbers, for example, liquid sodium's P M is around 10 − 5, but plasmas have large P M. The Reynolds number is the ratio of the nonlinear term u ⋅ ∇ u of the Navier–Stokes equation to the viscous term. While the magnetic Reynolds number is the ratio of the nonlinear term and the diffusive term of the induction equation. In many practical situations, the Reynolds number R e of the flow is quite large. For such flows the velocity and the magnetic fields are random; such flows are called to exhibit MHD turbulence. Note that R e M need not be large for MHD turbulence. R e M plays an important role in dynamo problem; the mean magnetic field plays an important role in MHD turbulence, for example it can make the turbulence ani