Particles droplet mode

As discussed earlier, the particle/droplet dynamics can be significantly modified by timing the fuel injection to be in- or out-of-phase with the large-scale vortex structures. To explore if timed fuel injection could alter the stability characteristics, the flow was forced at the quarter-wave mode of the inlet and droplet injection was timed to be in- or out-of-phase with the forcing. Results from these simulations show that the pressure fluctuations at the quarter-wave mode of the inlet can indeed be amplified or attenuated depending on the phasing of the droplet injection. 

Hering and Friedlander (1982) made similar observations for particle sulfate and attributed the smallest mode (referred to as the condensation mode) to formation from gas-phase S02 oxidation and the larger modes (the droplet mode) to oxidation in the condensed phase. Meng and Seinfeld (1994) have shown that the droplet mode particles cannot arise from growth of the smaller, condensation mode particles and propose that the condensation mode particles are activated to form fog or cloud droplets, followed by chemical reactions and subsequent evaporation to form the droplet particles. 

At low alkali addition levels, the bitumen was attached to air bubbles as discrete particles. This mode of bitumen attachment is also seen at reduced temperatures and is akin to the type of mechanism encountered in mineral flotation. At more reasonable alkali addition levels however, the bitumen will encapsulate the air bubble. The size of the droplets increases with increasing dosage of alkali. Bitumen droplet size was found to be invariant with depth in the separation vessel, and this invariance implies that bitumen droplets do not coalesce as they rise through the middlings phase. 

Processing of accumulation and coarse mode aerosols by clouds (Chapter 17) can also modify the concentration and composition of these modes. Aqueous-phase chemical reactions take place in cloud and fog droplets, and in aerosol particles at relative humidities approaching 100%. These reactions can lead to production of sulfate (Chapter 7) and after evaporation of water, a larger aerosol particle is left in the atmosphere. This transformation can lead to the formation of the condensation mode and the droplet mode (Hering and Friedlander 1982 John et al. 1990 Meng and Seinfeld 1994). 

Measurements of the urban aerosol mass distribution have shown that two distinct modes often exist in the 0.1 to 1.0 pm diameter range (Hering and Friedlander 1982 McMurry and Wilson 1983 Wall et al. 1988 John et al. 1990). These are referred to as the condensation mode (approximate aerodynamic diameter 0.2 pm) and the droplet mode (aerodynamic diameter around 0.7 pm). These two submicrometer mass distribution modes have also been observed in nonurban continental locations (McMurry and Wilson 1983 Hobbs et al. 1985 Radke et al. 1989). Hering and Friedlander (1982) and John et al. (1990) proposed that the larger mode could be the result of aqueous-phase chemical reactions. Meng and Seinfeld (1994) showed that growth of condensation mode particles by accretion of water vapor or by gas-phase or aerosol-phase sulfate production cannot explain existence of the droplet mode. Activation of condensation mode particles, formation of cloud/fog drops, followed by aqueous-phase chemistry, and droplet evaporation were shown to be a plausible mechanism for formation of the aerosol droplet mode. 

Ice Nuclei Ice particles can be formed through a variety of mechanisms. All of these require the presence of a particle, which is called an ice nucleus (IN). These mechanisms are (1) water vapor adsorption onto the IN surface and transformation to ice (deposition mode), (2) transformation of a supercooled droplet to an ice particle (freezing mode), and (3) collision of a supercooled droplet with an IN and initiation of ice formation (contact mode). 

Under conditions of high humidity, such as in a cloud or fog, the accumulation mode may itself have two submodes a condensation mode with MMAD of 0.2— 0.3 pm and a droplet mode with MMAD of 0.5-0.8 pm. The droplets are formed by the growth of hygroscopic condensation-mode particles. This process may be facilitated by chemical reactions in the droplet. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

Particulates are also classified as to their mode of formation. Some, designated sls primary particulates, are released directly to the atmosphere in the form of tiny particles or droplets. Droplets of salt and water blown off the surface of the ocean are examples of primary particulates. Other particulates are formed in the atmosphere as a result of chemical and/or physical reactions. For example, sulfur... 

Gillani, Leaitch, and co-workers (1995) carried out a detailed study of the fraction of accumulation mode particles (diameters from 0.17 to 2.07 /Am) that led to cloud droplet formation in continental stratiform clouds near Syracuse, New York. When the air mass was relatively clean, essentially all of the particles were activated to form cloud droplets in the cloud interior and the number concentration of cloud droplets increased linearly with the particle concentration. However, when the air mass was more polluted, the fraction of particles that were activated in the cloud interior was significantly smaller than one. This is illustrated by Fig. 14.40, which shows the variation of this fraction (F) as a function of the total particle concentration, Nun. In the most polluted air masses (as measured by large values of Nun), the fraction of particles activated was 0.28 + 0.08, whereas in the least polluted, it was as high as 0.96 + 0.05. The reason for this is likely that in the more polluted air masses, the higher number of particles provided a larger sink for water vapor, decreasing the extent of supersaturation. 

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