Aerodynamic focusing

FIGURE 3.2 Basic mechanism of aerodynamic focusing by air sheath flow. Undisturbed interaction of injected liquid flow with the channel walls and air sheath flow produces a stable two-phase flow configuration throughout the microchannel. Channel height is 100 pm [382]. Reprinted with permission from Springer Science and Business Media. 

A two-phase air-liquid flow was developed on a PDMS chip (see also Chapter 3, section 3.1.2). It was found that the focused two-phase flow was stable in hydrophobic channels (down to 6 mL/h) see Figure 8.38. Below this flow rate, the sample column no longer maintained its integrity and broke up. This method was used to provide aerodynamic focusing of myoblast cells (C2Ci2) for the flow cytometry study. The cells, which were labeled with Syto 9, was focused and counted in the chip at a rate of 100 cells/s [382]. 

FIGURE 53.3 A microchip-MS interface with a nebulizer that includes an aerodynamic focusing chamber for directing the ESI plume into the mass spectrometer sampling orifice. (Reprinted from Grym, J., et ah. Lab Chip, 6, 1306-1314, 2006. Copyright 2006. With permission from The Royal Society of Chemistry.)... 

Compared to size distribution instruments, much less is developed for online analysis of nanoaerosol chemical composition. By coupling aerodynamic focusing techniques (see above) with particle ionization and mass spectrometry. 

Isolated Droplet Breakup—in a Velocity Field Much effort has focused on defining the conditions under which an isolated drop will break in a velocity field. The criterion for the largest stable drop size is the ratio of aerodynamic forces to surface-tension forces defined by the Weber number, N (dimensionless). 

The cathode is continuously rotated by means of an external motor in order to allow constant ablation conditions for all pulses and a homogeneous consumption of the rod. Higher deposition rates can be obtained by substituting the simple cylindrical nozzle with a more complex one (called focuser) as described in Reference 28. Exploiting inertial aerodynamic effects [28,29], the focuser reduces the angular semiaperture of the beam from 12° to less than 1° concentrating the cluster on the center of the beam. 

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. 

Wang, X., McMurry, P.H., An experimental study of nanoparticle focusing with aerodynamic lenses. Int. J. Mass Spectrom. 2006, 258, 30. 

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... 

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