Droplet formation processes

In the microfluid dynamics approaches the continuity and Navier-Stokes equation coupled with methodologies for tracking the disperse/continuous interface are used to describe the droplet formation in quiescent and crossflow continuous conditions. Ohta et al. [54] used a computational fluid dynamics (CFD) approach to analyze the single-droplet-formation process at an orifice under pressure pulse conditions (pulsed sieve-plate column). Abrahamse et al. [55] simulated the process of the droplet break-up in crossflow membrane emulsification using an equal computational fluid dynamics procedure. They calculated the minimum distance between two membrane pores as a function of crossflow velocity and pore size. This minimum distance is important to optimize the space between two pores on the membrane... 

Droplet formation occurs primarily through the surface tension and viscosity dominated breakup of these liquid threads due to symmetric (or dilational) waves as described by Rayleigh (6) for inviscid liquids and by Weber (J) for viscous fluids. Figure 3 shows the double pulsed image of the droplet formation process for No. 2 and SRC-II fuel sprays under identical atomizer conditions. These two photographs illustrate typical differences seen between these two fuels. 

Double Pulsed Image of Droplet Formation Process Atomizer is Delavan 60A at 100 psig. 

Figure 11. Relaxation time vs. temperature for an epoxy PDLC shutter for droplets which are nearly spherical and for droplets elongated by shearing the shutter substrate during the droplet formation process. (Reproduced with permission from Ref. 18. Copyright 1989 International Society of Optical Engineering.)...

In this type of nozzle, spray liquid and air are mixed together as they pass through chambers in the nozzle body to form a spray that is delivered from a modified flood or reflex type nozzle. The flow of air contributes directly to the spray droplet formation process. By controlling air and liquid pressures, liquid flow rate and spray quality from the nozzle can be varied independently. Droplets generated from such nozzle designs can contain air inclusions if they are above... 

The state of the art in understanding and describing droplet formation processes is divided into two different perspectives flie cmitinuum mechanics or fluid dynamics approach and the particle-based approach. Whilst the cmitinuum mechanics approach is based usually on the Navier-Stokes equation, the continuum equation and the energy equation, the particle-based approach is based on the interconnecting forces between neighboring particles like molecules or pseudo particles. 

Sugiura and his team [8]. When taking a detailed look at spontaneous droplet formation in microchannels, see Fig. 2, it is clear that the droplet formation process consists of various phases. First the to-be-dispersed phase is pushed through the feed channel toward a wider shallow area called the terrace where it takes a disklike shape (Fig. 2a). This disk will keep growing through the supplied dispersed phase and eventually reach the end of the terrace, after which (part of) the disk can leap into the deeper channel to which the terrace is connected as shown in Fig. 2b. The droplet may still be connected to the feed, but after reaching a specific size, the neck keeping the droplet connected will break and a droplet will be released (Fig. 2c). This process was captured by Van Dijke and coworkers in CFD calculations, from which a relatively simple flux criterion was derived [5]. 

Fog Formation A vapor or vapor-gas mixture may be cooled below the dew point. If cooled sufficiently, droplets of condensate may form in the bulk vapor stream. The droplet formation process is known as homogeneous nucleation and significant nucleation will result in a noticeable fog in the vapor. Fog droplets... 

Basic configuration similar to hydrodynamic focusing can be used to form droplets of solvent and solute (see Figure 4.14(a)). The middle inlet is used to flow carrier fluid, which is immiscible to both solute and solvent. The solute and solvent enter from opposite sides. The droplet formation process depends on the capillary number Ca = Ufila and sample fraction (x) ... 

In order to discuss quantitatively the DoD droplet formation process, the positions of several representative points in the ejected liquid can be plotted as a function of time to produce the curves of the DoD droplet formation, as depicted in Figure 10. The axial distances of points 1-5 from the nozzle exit are denoted as Xi( ) to Xs( ), respectively, with measured from the first appearance of liquid from the nozzle. Initially, point 1 is the leading edge of the liquid ejected from the nozzle that later becomes the tip of the primary drop. Point 2 is the first pinch-off point of the liquid from the nozzle tip, and also the tail of the free liquid thread its first appearance corresponds to the initial breakup time bi. Points 3 and 4 are the lower and upper points produced by the second pinch-off, and the curves associated with these points initiated at the second breakup time b2) these curves form a closed loop if (in the case of a single satellite drop) the satellite recombines with the main drop (as is the case in Figure 10(a)) or may continue separately if the satellite survives as a discrete body of liquid. Later, point 3 becomes the tail of the primary drop, and point 4 becomes the head of the secondary free liquid thread or satellite. Between points 2 and 4, further pinch-off points may occur. Point 5 is the tip of liquid protruding from the nozzle orifice due to multiple reflections of the pressure wave inside the ink chamber. 

---