Radiation Force Measurements

Methods which have been developed for the monitoring of radiation forces can be very sensitive (up to 1 mW). They are better suited in free-field and noncavitating conditions, but involve errors due to acoustic streaming. The uncertainty for such methods ranges from 2.2% at 1 MHz to 12% at 30 MHz, and accuracy depends on the shape of the ultrasonic wave. Radiation force measurement provides a good method for the calibration of transducers in specifically devised chambers, but its use in chemical reactors of defined geometry could prove to be difficult. 

---

All of the methods which involve radiation force measurement have been extensively used to calibrate diagnostic ultrasound systems with acoustic intensities of only a few milliwatts per square centimeter for which radiation balances are readily applicable. An accuracy of 2% has been quoted and acoustic powers as low as 10 pw have been detected. Less commonly it has been used for therapeutic systems at much higher energy with outputs of the order of several watts, although in this case the accuracy is somewhat lower [17]. 

Ultrasound waves produce a variety of thermal and cavitational effects in biological tissues [1]. Given their widespread use in medical diagnostic and treatment since the sixties [2], stringent standards have been developed to keep the intensity of these waves within safe levels [3], Thus, calibration techniques have also been actively developed such as wideband hydrophones, calorimetric, optical and radiation-force measurement systems [3], When the intensity increases, as it is the case in HIFU (high intensity focused ultrasound) and lithotripters, calibration becomes more delicate [1],... 

Using this photon force measurement technique, radiation pressure induced by a focused laser beam and an evanescent field [12, 14, 19, 20] was investigated for polymer latexes and metallic particles. Electrostatic forces of charged particles in... 

In this section, we present a few examples of instruments available for visual observation and imaging of colloids and surfaces, for measurement of sizes and for surface force measurements. Such a presentation can hardly be comprehensive in fact, that is not our purpose here. Throughout the book, we discuss numerous other techniques such as osmotic pressure measurements, light and other radiation scattering techniques, surface tension measurements,... 

Class 3-Methods Based on Direct Mechanical Effects. These include the use of acoustical probes [57-71], acoustic impedance measurements [72—75], acoustic fluxmeter [76], the measurement of radiation forces [17,21,77—112], the distortion of liquid surface [ 113-115], surface cleaning, dispersive effects, emulsification [ 116-118], erosion [ 19,22,119-125], mass transfer measurements (electrochemical probe) [26,129], absorption methods [93,132], particle velocity measurements [132], and optical methods [133-141],... 

Methods based on nonlinear effects e.g. radiation forces. He also introduced other subdivisions depending on whether the method could be used for total or local power measurements, under free or restrained field conditions. 

There are two possible explanations which could account for this observation. The first is that although the liquid height was always adjusted to the same multiple of the wavelength, it was not sure that matching of the system was optimized. It is also a fact that the liquids do not all have identical absorption coefficient, viscosity, or thermal conductivity [23]. It is thus quite clear that in the search for more accurate measurements and absolute values of sound intensity, further precautions should be taken. This may involve calibration using another method (e.g. heating coil, radiation forces). The method then becomes more lengthy, but is still useful. 

Figure 20. Different configurations for the measurements of radiation force.

In some cases these lateral variations provide a desirable feature of the device operation, particularly in systems designed to trap particles, while in others, the variations are not desirable and may be detrimental to the device performance, such as flow-through concentrators, hi real devices lateral forces may well exist due to a combination of the above causes. The magnitude and distribution of the lateral forces within resonators have been measured and shown to compare well with calculations using Gorkov s formulation for radiation forces (described by Groschl in [1]). 

It is convenient to express this energy in eV-mL-i h-i according to Eq. 13, with V as the volume of the irradiated liquid. The sonochemical yield can be expressed in the G unit commonly used in radiolysis, i.e., representing the number of molecules or equivalents transformed per 100 eV of energy absorbed. Other force balances are described. A method of calibrating high-frequency transducers by measuring the radiation force on spherical objects set in an axisymmetric wave field is reported. ... 

Methods based on direct mechanical effects, including measurements using acoustic probes, optical methods and acoustic impedance, radiation forces, liquid surface distortion or particle velocity measurements. 

The assessment of RBCs deformability can also be performed by active methods, through the application of external force fields, such as acoustic, optical, electrical, and magnetic ones. Acoustic radiation force has been reported for fast and direct measurement of the mechanical properties of cells in a microfluidic chip. This method is based on the formation of an acoustic standing wave within a straight microchannel that moves the cells to the acoustic pressure nodes, with a movement speed dependent on the compressibility (Hartono et al., 2011). 

In this section we consider electromagnetic dispersion forces between macroscopic objects. There are two approaches to this problem in the first, microscopic model, one assumes pairwise additivity of the dispersion attraction between molecules from Eq. VI-15. This is best for surfaces that are near one another. The macroscopic approach considers the objects as continuous media having a dielectric response to electromagnetic radiation that can be measured through spectroscopic evaluation of the material. In this analysis, the retardation of the electromagnetic response from surfaces that are not in close proximity can be addressed. A more detailed derivation of these expressions is given in references such as the treatise by Russel et al. [3] here we limit ourselves to a brief physical description of the phenomenon. 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Liquid Level. The most widely used devices for measuring Hquid levels involve detecting the buoyant force on an object or the pressure differential created by the height of Hquid between two taps on the vessel. Consequently, care is required in locating the tap. Other less widely used techniques utilize concepts such as the attenuation of radiation changes in electrical properties, eg, capacitance and impedance and ultrasonic wave attenuation. 

Every method, with the exception of imaging technologies, provides the measurement of an equivalent spherical diameter in one form or another. The spherical diameter information can be deduced indirectiy from the behavior of the particles passing through restricted volumes or channels under the influence of gravity or centrifugal force fields, and from interaction with many forms of radiation. 

Dipol-kraft,/. dipole force, -messung,/. dipole measurement, -strahlung, /. dipole radiation. 

We can siunmarize all of the above research carried out over the last two centuries in that a photon is a qucuitum of radiation and a carrier of force between particles, whereas an electron is a quantum of matter. Now, let us examine the more mundane aspects of light measurement including color measurement. 

---