Droplet formation mechanisms

A number of investigations have been made to explain the droplet formation mechanisms associated with electrostatic atomization. 119][1201 It has been hypothesized that the dispersion of a liquid by electrostatic atomization occurs via the detachment of a single droplet from the capillary tip of the liquid. However, this mechanism has not been proven experimentally. Due to the complex physics involved, a generic theoretical model has not yet been established. 

Recently, the size and shape of a liquid droplet at the molten tip of an arc electrode have been studied,12151 and an iterative method for the shape of static drops has been proposed. 216 Shapes, stabilities and oscillations of pendant droplets in an electric field have also been addressed in some investigations. 217 218 The pendant drop process has found applications in determining surface tensions of molten substances. 152 However, the liquid dripping process is not an effective means for those practical applications that necessitate high liquid flow rates and fine droplets (typically 1-300 pm). For such fine droplets, gravitational forces become negligible in the droplet formation mechanism. 

Both direct and premix emulsification can be obtained with a continuous phase flowing along the membrane surface (i.e., crossflow, stirring) (Figure 21.2(b)). However, it is important to distinguish between the droplet-formation mechanism and the macroscopic operation procedure. In other terms, often, in the literature, the... 

The balances approaches have to necessarily incorporate approximations and fundamental hypotheses, which reduce the prediction capability of the latter. Every hypothesis comes from a postulated droplet-formation mechanism. The formation mechanism, however, depends significantly on the mentioned operating, membrane and phase parameters, thus, it is very difficult to find one mechanism valid for all possible parameters values. Consequently, more accurate computation procedures, such as the microscopic modeling or methods using the minimization of the droplet surface, are necessary for the detailed description of droplet formation and accurate predictions. 

Starting from this consideration, Rayner et al. [52] analyzed the spontaneous-transformation-based (STB) droplet-formation mechanism from the point of view of the surface Gibbs free energy with the help of the Surface Evolver code. Rayner et al. estimated the difference of surface free energy of the droplet before ( i) and after (E2)... 

Figure 15.15. Droplet sizes obtained with an SPG membrane, as measured by Yasuno et al. (2002). One would expect that the droplet size would depend on the cross-flow velocity of the continuous phase, but it does not. Apparently, another droplet formation mechanism is active. Two flow rates through of the dispersed phases (O ) are indicated.

Main Droplet Formation Mechanisms in Microstructured Systems... 

Various direct microfluidic emulsification geometries are discussed in literature, e.g., (straight-through) microcharuiels and T- and Y-junctions (see Eig. 1). Two droplet formation mechanisms can be distinguished one uses Laplace pressure differences for spontaneous droplet generation and the other uses shear to form droplets. 

Spontaneous Droplet Formation in Microchannels This phenomenon was originally demonstrated by Kawakatsu and coworkers [7], and the droplet formation mechanism was first proposed by... 

The objectives of this chapter are to review the studies on droplet formation mechanisms under the action of electrostatic forces and to determine key parameters critical for production of very small polymer microbeads (i.e., less than 100 p,m in diameter). Specifically, attention is given to the effects of applied potential, needle size, polymer concentration, and electrode spacing, and geometry. An overview of theoretical models and experimental correlations for predictions of droplet diameter is presented. 

Several parameters affecting the droplet-formation mechanism and the microbead diameter were investigated, including the needle size, electrode spacing, sodium alginate concentration, and applied potential [18-20]. 

When the alginate concentration was decreased from 1.5% to 0.8%, a difference was observed in droplet formation mechanism. For the low-viscosity alginate, elongation of the filament finking the new droplet and the meniscus at the tip of the needle was not as pronounced, resulting in more uniform bead sizes (Figure 31.7a-c, [18,26]). 

A comparative analysis of the two charge setups (Figure 2a and b) at the same applied potential and needle size, was carried out in order to get an insight into the droplet-formation mechanism [17]. 

FIGURE 31.8 Schematic presentation of the droplet formation mechanism. 

FIGURE 31.10 Comparative analysis of droplet formation mechanism (6kV applied potential, 2.5 cm electrode distance) (a) parallel plate setup and (b) positively charged needle setup. Note the formation of the jet spray in (h). (From Poncelet, D., Bugarski, B., Amsden, B., Zhu, J., Neufeld, R., and Goosen, M.F.A., Appl. Microbiol. BiotechnoL, 42 (2-3), 251-255, 1994. With permission.)... 

By microchanneling (Figure 20.8, left), monodisperse emulsions may be produced in the absence of shear forces [63]. A strongly non-cylindrical geometry at the microchannel exit followed by a terrace is responsible for this effect [64, 65]. The droplet formation mechanism is limited to a critical velocity of the dispersed phase, above which the droplet sizes are widely distributed [51]. 

Figure 9 Smooth disk atomizer droplet formation mechanisms (A) direct droplet formation, (B) ligament formation, and (C) sheet formation. (From Ref. 12.)...

Figure 13.19 Schematic droplet formation mechanism in microchannel emulsification [18].

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