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Optical Properties
Published in Ko Higashitani, Hisao Makino, Shuji Matsusaka, Powder Technology Handbook, 2019
The mechanism of photophoresis is similar to that of thermophoresis. When a very small particle is illuminated from one side, a temperature gradient is induced in the particle. The difference of strength of the gas molecule impact on the particle causes the particle to move in the direction opposite to the illuminated side. Because photophoresis is a result of an interaction between the particle and the surrounding gas molecules, it cannot occur in the cases of nonabsorbing particles or in a vacuum. The photophoresis force depends on the light intensity and wavelength, the size, shape, and material of the particle, and the gas pressure. Although the evaluation of the force is rather difficult, some analysis has been proposed,11 and measurement data have been obtained.12
Deposition of particle pollution in turbulent forced-air cooling
Published in Aerosol Science and Technology, 2021
Environmental pollutants, such as coarse and fine particle matter (PM10 and PM), adversely affect human activities in various ways. The most harmful particles are those with aerodynamic diameters below 2.5 (PM) and are projected to increase in concentration up to ten-fold by 2050 in a business-as-usual scenario (Pozzer et al. 2012). Of course, the most serious concern is the impact this has on health and mortality rates (Lelieveld et al. 2015), however, the presence of particle pollutants also adversely affects the reliability of technologies by accumulating on surfaces (Tencer 2008). The deposition of particle pollutants can lead to mechanical, chemical and electrical failures in electronic systems that are either located outdoor, or in an insufficiently controlled indoor environment. An example of the latter are free cooling or Direct Air Cooling Systems (DACS) which provide significant energy savings by utilizing external air, however, also introducing an added risk of exposing electronics to contaminants. Deposition mechanisms can include particle diffusion, gravitational settling, thermophoresis, electrophoresis and photophoresis (Tencer 2008). In forced cooling systems, however, advection is typically dominant over other transport mechanisms.