Radiometric force

Radiometric forces Weak forces that cause the motion of particulate matter, including diffusiophoresis, thermophoresis, and photophoresis. 

Numerous forces can be exerted on a microparticle, and direct measurement of the force-to-weight ratio follows that of the drag force. The examples of phoretic and radiometric forces should suffice to make the point. 

A force that is as large as the gravitational force can be used to suspend a particle against gravity, provided that it can be controlled and directed upward to balance gravity. One such force is the radiation pressure force or radiometric force. Ashkin and Dziedzic (1977), whose work is discussed in the next section, were the first to use the radiation pressure to levitate a microsphere stably. It was demonstrated by Allen et ai (1991) that the radiometric force can be measured with the electrodynamic balance, and they used the technique to determine the absolute intensity of the laser beam illuminating a suspended particle. This was accomplished in the apparatus displayed in Fig. 13. The laser illuminated the microparticle from below, and... 

Fig. 14. The ratio of the radiometric force to the gravitational force as a function of laser power for a levitated microdroplet of dibutyl phthalate. from Buehler (1991).

Forces acting on a particle as a result of a temperature gradient in a gas (thermophoresis) or nonuniform radiation (photophoresis) are known as radiometric forces. For a particle diameter much larger than the mean free path of a gas, the force due to the temperature gradient is given by [Hettner, 1926]... 

At room conditions and in air, the radiometric forces are important only for submicrometer particles. However, under conditions of high temperatures and/or large temperature... 

Fj Radiometric force vector Q bp Averaged charge transfer per impact Surface charge of a particle carried... 

Similar to the immunoassay techniques, radioimmunoassay, enzyme immunoassay, and chemiluminescence immunoassay are also applied and have proved useful. All techniques used for immunoassay represent coupling between the specificity of the antibody-antigen reaction and the sensitivity of the radiometric, electrometric, and chemiluminescence techniques. The limit of detection for immunoassay depends on the antibody affinity.128 130 Materials such as silica,131132 latex,133134 and alkylamine films135 are used for antibody absorption. SEM,136 scanning tunneling microscopy,137 and scanning force microscopy138 have been employed as tools for visualization of the immobilized antibodies. 

The acoustic pressure can be measured through the effects produced in the propagation liquid, namely (1) heating, measured calorimetrically and (2) the action of radiation pressure, determined by measuring the force exerted on a surface (the radiometric method). While the former allows the measurement of the mechanical power of the vibrating source, the latter is associated with the acoustic power transmitted by the acoustic radiation. The acoustic pressure can also be determined using calibrated hydrophones (the acoustic method). 

Particles activated by depositing a radioactive isotope on the particle surface from a solution may only be used for measuring adhesive forces in air, since, in a liquid the radioactive isotopes pass into solution, and this makes it impossible to determine the number of particles radiometrically. 

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