Aerodynamic focusing pressure

The study of molecule-surface interactions is very old because of its importance in a myriad of applications (Somorjai 1981 Gasser 1985 Zangwill 1988). The first ideas can be found as far back as the turn of the century in relationship to heterogeneous catalysis later work was concerned with aerodynamics and, more recently, in this age of silicon, many investigations have been driven by the needs of the microelectronics industry. Nevertheless, it is only in the last two decades that detailed information has been learned about the microscopic details. This has occurred for a number of reasons. First, the early theoretical work mainly used thermodynamics and kinetics, which of course provide no information on the microscopic dynamics (except for fanciful interpretations). The few dynamical treatments focused on simplistic one-dimensional models and qualitative descriptions (Lennard-Jones 1932). Second, the early experiments utilized polycrystalline surfaces and relatively high pressures, both of which lead to confusion about the state of the surface, or the impinging molecule, or both. These observations are not intended to be critical comments about the early work. The fact is that the necessary theoretical and experimental tools were simply unavailable to these early researchers. 

As mentioned earlier the trajectory of the liquid jet before and after the CBL is of importance for design purposes. As we will see, it is also a critical piece of information needed by some empirical-numerical models to simulate the atomization process. A considerable number of research studies have been merely focused on measurements and predictions of the jet trajectory and its variation with change in different parameters such as the pressure and the temperature. To develop a simple model for predicting the jet trajectory, we can think of the jet as a stack of thin cylindrical elements piled on top of each other to form a jet. One such element with infinitesimal thickness h is shown in Fig. 29.1b. Then, one can treat the motion of the element like that of a projectile moving up with initial y-direction velocity j and zero x-direction velocity. In the simplest approximation, the only force acting on the element is the aerodynamic drag force... 

Under carefully engineered condition with low pressure, however, nanoaerosol particles can be separated from the gas phase because of the relatively small amount of molecules surrounding the nanoaerosol. The nanoaerosol can then be focused to a beam after expansion through an orifice, referred to as aerodynamic particle focusing. In this case, as depicted in Fig. 1, the trajectories of the aerosol particles depend on their corresponding Stokes number... 

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