Cloud chamber, diffusion

In cloud chambers (Wilson chambers) the tracks of ionizing particles are visible by condensation of droplets on the ions produced. The gas in the chamber is saturated with the vapour of water, alcohol or other volatile liquids. By sudden expansion supersaturation is obtained and condensation occurs along the ion tracks. Dust or other condensation centres must be eliminated, to avoid interferences. Cloud chambers can be operated in cycles by a piston or diaphragm (expansion chamber) or by diffusion of a saturated vapour into a colder region (diffusion cloud chamber). 

The diffusion cloud chamber has been widely used in the study of nucleation kinetics it is compact and produces a well-defined, steady supersaturation field. The chamber is cylindrical in shape, perhaps 30 cm in diameter and 4 cm high. A heated pool of liquid at the bottom of the chamber evaporates into a stationary carrier gas, usually hydrogen or helium. The vapor diffuses to the top of the chamber, where it cools, condenses, and drains back into the pool at the bottom. Because the vapor is denser than the carrier gas, the gas density is greatest at the bottom of the chamber, and the system is stable with respect to convection. Both diffusion and heat transfer are one-dimensional, with transport occurring from the bottom to the top of the chamber. At some position in the chamber, the temperature and vapor concentrations reach levels corre.sponding to supersaturation. The variation in the properties of the system are calculated by a computer solution of the onedimensional equations for heat conduction and mass diffusion (Fig. 10.2). The saturation ratio is calculated from the computed local partial pressure and vapor pressure. 

Figure 10.2 Variation with height of the properties of a mixture in the diffusion cloud chamber. Shown are the rnas.s densities of the carrier gas. ph, and the vapor,, the equilibrium vapor pressure, p, the partial pre.ssure of (he vapor, p, the temperature, 7/, and the saturation ratio, S. The highest temperature, vapor pressure, and gas density are at the chamber bottom, above the heated pool. The distributions with respect to chamber height are calculated by integrating expressions for the steady-state fluxes of heat and mass through the chamber...

FIGURE 11.5 Upward thermal diffusion cloud chamber. Typical cloud chamber profiles of total gas-phase density, temperature, partial pressure p (n-nonane in this example), equilibrium vapor pressure p saturation ratio 5, and nucleation rate J, as a function of dimensionless chamber height at T = 308.4 K, S = 6.3, and total p = 108.5 kPa. (Reprinted with permission from Katz, J. L., Fisk, J. A. and Chakarov, V. M. Condensation of a Supersaturated vapor IX. Nucleation of ions, J. Chem. Phys. 101. Copyright 1994 American Institute of Physics.)... 

The presence of ions has been shown experimentally to enhance the rate of nucleation of liquid drops in a supersaturated vapor. Katz et al. (1994) showed, for example, that the nucleation rate of n-nonane, measured in an upward thermal diffusion cloud chamber, at an ion density of 16 x 106 ions cm-3, increased by a factor of 2500 over that in the absence of ions. These investigators also confirmed experimentally that the nucleation rate is directly proportional to the ion density. The phenomenon of ion-induced nucleation plays an important role in atmospheric condensation, particularly in the ionosphere. While both positive and negative ions increase the nucleation rate, many substances exhibit a preference for ions of one sign over the other. 

Heist, R. H. (1986) Nucleation and growth in the diffusion cloud chamber, in Handbook of Heat and Mass Transfer, Gulf Publishing, Houston, TX, pp. 487-521. 

The contentious issue of quantum-particle trajectories is put into perspective by the Bohmian model. One interpretation is that the quantum electron has an unspecified diffuse structure, which contracts into a classical point-like object when confined under external influences. The observed trajectory, as in a cloud chamber, may be considered to follow the centre of gravity. 

The capture rate is the collision rate of a cluster of size i with a monomer times the probability that the monomer will stick. If the concentration of monomers does not change significantly during the experiment, Ci will be independent of time. A typical nucleation experiment in a cloud chamber or diffusion chamber is carried out in such a way that the pressure in the region observed remains essentially constant during the time of observation. Therefore we can assume that rii is nearly constant and hence that Ci is a constant. 

R. E. Heist and H. Reiss, Investigation of homogeneous nucleation of water vapor using a diffusion cloud chamber, J. Chem. Phys. 59,665-671 (1973). 

A simpler version of this apparatus is the diffusion cloud chamber, developed by Cowan, Needels, and Nielsen in 1950, in which super saturation is achieved by placing a row of felt strips soaked in a suitable alcohol at the top of the chamber. The lower part of the chamber is cooled by solid carbon dioxide. The vapour continuously diffuses downwards, and that in the centre (where it becomes supersaturated) is almost continuously sensitive to the presence of ions created by the radiation. 

Dropwise condensation of a vapor diluted in a carrier gas occurs whenever the vapor becomes supersaturated due to a change of state of the gas/vapor mixture. By supersaturation we define the actual vapor pressure over the equilibrium vapor pressure (over a flat surface) at the same temperature. The change of state may be an adiabatic expansion as in cloud chambers, supersonic nozzles and shock tubes or a diffusion process as in diffusion cloud chambers [1]. 

Figure 3. Experimental test of classical nucleation theory. Comparison between experimental (exp) and theoretical (th) nucleation rates for droplet condensation from n-nonane vapor. Subscript int denotes the integrated theoretical nucleation rate along the height of the thermal diffusion cloud chamber [37].

The above trends are illustrated in Figure 3. It shows thermal diffusion cloud chamber measurements of homogeneous nucleation rates for n-nonane. The fact that isotherms aie parallel to the 45" line along which experiments and classical theory agree demonstrates that the dependence on supersaturation is correctly captured by the theory. The fact that the isotherms don t collapse on this line demonstrates that the theory underpredicts nucleation rates at low temperatures, and overpredicts them at high temperatures. 

A method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and metal carbide particles (10-20 nm). The silica nanoparticles aggregate into a novel web-like microstructure. These aggregates are very porous and have a very large surface area (460 m /g). Bright blue photoluminescence from the nanoparticle silica has been observed upon irradiation with UV light. The photoluminescence is... 

The chapter consists of three major sections. The first is a brief review of the processes of nucleation and growth in supersaturated vapors for the formation of clusters and nanoparticles. The second section deals with the application of laser vaporization for the synthesis of nanoparticles in a diffusion cloud chamber. In the third section, we present some examples of nanoparticles synthesized using this approach and discuss some selected properties. 

In the experiments, a modified upward thermal diffusion cloud chamber is used for the synthesis of the nanoscale particles (10,31,32), A sketch of the chamber with the relevant components necessary for the synthesis of nanoparticles is shown in Figure 2. This chamber has been commonly used for the production of steady state supersaturated vapors for the measurements of homogeneous and photo-induced nucleation rates of a variety of substances (33). Detailed description of the chamber and its major components can be found in several references (33,34), Here we only offer a very brief description of the modifications relevant to the synthesis of the... 

Fig. 7-8. Influence on cloud nuclei formation of the mass fraction e (water-soluble material/particle dry mass). Left Critical supersaturation of aerosol particles as a function of particle dry radius. Right Cloud nuclei spectra calculated for e = 0.1 and 1 on the basis of two size distributions each for continental and maritime aerosols (solid and dashed curves, respectively). [Adapted from Junge and McLaren (1971).] The curves for the maritime cloud nuclei spectra are displaced downward from the original data to normalize the total number density to 300 cm-3 instead of 600 cm-3 used originally. The curves for e = 1 give qualitatively the cumulative aerosol size distributions starting from larger toward smaller particles (sk = 10 4 corresponds to r0 0.26 p.m, sk = 3 x 10 3 to rs 0.025 Atn). Similar results were subsequently obtained by Fitzgerald (1973, 1974). The hatched areas indicate the ranges of cloud nuclei concentrations observed in cloud diffusion chambers with material sampled mainly by aircraft [see the summary of data by Junge and McLaren (1971)] the bar represents the maximum number density of cloud nuclei observed by Twomey (1963) in Australia.

Thermal gradient static cloud diffusion chamber... 

---