Phoretic forces

The development of the electrodynamic balance and other particle traps has made it possible to perform precise measurements of the properties of small particles by focusing on the single particle. The variety of processes and phenomena that can be investigated with particle traps is quite extensive and includes gas/liquid and gas/solid chemical reactions, chemical spectroscopies, heat and mass transfer processes, interfacial phenomena, thermodynamic properties, phoretic forces, and other topics of interest to chemical engineers. 

The particle is first levitated stably in the absence of convective forces or phoretic forces by adjusting the ac and dc voltages and the ac frequency to keep it stationary at the midplane of the balance, as observed through a microscope or video imaging system. The ratio g/m is written in terms of the measured value of applying Eq. (15), and the result is used to eliminate qlm in Eq. (19). The result is... 

In addition to using electrowetting (see Figure 1.42), electrophoretic and dielectro-phoretic forces can be used for moving droplets [99], Electrophoretic droplet movement depends on the application of large DC fields, which may pose problems for fluid systems such as suspensions. For dielectrophoretic operation, however, AC fields are sufficient, as for electrowetting. 

Phoretic forces Forces on suspended particles resulting from differential molecular collisions on the par-... 

Similar expressions can be derived for other phoretic forces reflecting different effects of gas nonuniformities. 

The removal of aerosol particles under dry weather conditions is caused by turbulent diffusion and gravitational sedimentation, which transport particles to the Earth s surface, as well as by impaction on vegetation, buildings and other objects. Turbulent diffusion itself does not remove particles (Twomey, 1977). Soil and other surfaces are bordered by a thin laminar layer ( 1 mm thick) across which particles must be transported by other processes (e.g. phoretic forces, molecular diffusion, sedimentation). 

Aerosol particles may also be removed in the clouds by the different phoretic forces, e.g. by diffusiophoresis. This phenomenon involves the motion of particles due to concentration gradient of condensing or evaporating vapour (Goldsmith et al., 1963). In the case of condensation, particles displace towards the drop surface. According to Goldsmith et al. (1963), velocity caused by diffusiophoresis is... 

Washizu M, Jones TB (1994) Multipolar dielectro-phoretic force calculation. J Electrost 33 187-198... 

As Fig. Ic shows additionally, the warming between two electrodes also depends on their size. The two horizontal solid lines represent electrodes (U = IVnns) which extend infinitely normal to the plane of the paper. They have a width of 40 pm and a distance between them of 40 pm. The dashed lines indicate the ceiling and floor of a channel formed in 55 pm thick glass and filled with aqueous buffer (0.27 Sm ). In Fig. Id, this relationship is quantified numerically. Obviously, AT scales almost linearly with the electrode width. In contrast, the dielectro-phoretic force experienced by a particle in the vicinity of the two electrodes approaches... 

Wherever particles are involved, the problem of fouling, or deposition on surfaces arises. In the case of heat exchangers for combustion gases, thermal transfer efficiency may be drastically reduced by the deposition of the relatively highly insulating soot and flyash particles. In other contexts sulfuric acid and other corrosive vapor droplets diffuse or impact upon conduit surfaces thereby shortening their useful lifetime [1.22,25]. In all of these cases, numerous questions of kinetic theory arise including all the phoretic forces. In addition, particle interaction forces are ultimately responsible for delivery to the surface in question. 

Kang KH, Xuan X, Kang Y, li D (2006) Effects of DC-dielectro-phoretic force on particle trajectories in microchannels. J Appl Phys 99 064702-1-064702-8... 

To describe such trajectories, consider the net photo-phoretic force F ad on a particle of radius tp by a light beam (whose waist size wa is much larger, i.e. tp << wa) here the refractive index of the particle ( par) is larger than that of the surrounding medium (wmed) so that the particle acts like a spherical lens (Helmbrecht et al, 2007). The net force Frad resulting from the refraction of incident light (a laser beam) which transfers momentum to the particle has been described by Helmbrecht et al (2007) as... 

Thus, in the absence of phoretic and other external forces, the dc voltage must satisfy... 

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. 

Grover, S. N., H. R. Pruppacher, and A. E. Hamielec (1977). A numerical determination of the efficiency with which spherical aerosol particles collide with spherical water drops due to inertial impaction and phoretic and electrical forces. J. Atmos. Sci. 34, 1655-1663. 

The term Foo,er represents other forces, usually important for submicron particles and/or at specific conditions, for example, phoretic. Basset, Saffman, Magnus forces, etc. more details are given in [27] all these forces have been omitted in the present work for simplicity. 

---