Droplet emitters

The topic of handling liquids of different viscosities for dispersion processes in microfluidic networks with two microreactors ( droplet emitters ) is addressed by Unilever R D (Vlaardingen, The Netherlands) in a patent [14,15]. The patent deals with the flow distribution in a microfluidic network on the background of dispersion processes with two supply sources for the two phases, which supply via upstream channel systems two microfluidic reactors called drop emitters (Figure 22.6). The dispersion leaves the microreactors via downstream channels. The major point of the patent is that the flow distribution is controlled by a design which creates a high pressure drop in the upstream channels. The pressure drop in the downstream... 

There are some variants that have emerged in the wake of DESI. By replacing the electrospray emitter by a metal needle and allowing solvent vapor into the coaxial gas flow desorption APCI (DAPCI) can be performed [106], Other versions are atmospheric-pressure solids analysis probe (ASAP) where a heated gas jet desorbs the analyte, which is subsequently ionized by a corona discharge [107], and electrospray-assisted laser desorption/ionization (ELDI) where a laser ablates the analyte and charged droplets from an electrospray postionizes the desorbed neutrals [108],... 

In general, color emitters used as components of pyrolants are metallic compounds rather than metal particles. Metal particles agglomerate to form liquid metal droplets and liberation of metal atoms in flames occurs only at the surface of the droplets. On the other hand, metallic compounds decompose at relahvely low temperatures compared with metal particles and liberate dispersed metal atoms. Table 12.5 shows typical salts used to obtain emissions of the requisite colors. 

A practical application coming out of field ion emission is the liquid metal ion source. Ion sources of a wide variety of chemical elements, most of them low melting point metals, can be produced by using either liquid metals131,132 or liquid alloys.133 The idea of extracting charged droplets out of liquid by application of an electrostatic field is perhaps older than field ion microscopy. But the development of liquid metal ion sources from liquid capillaries, from slit shaped emitter modules and from wetted field emission tips, etc., as well as the understanding of the mechanisms of ion formation in terms of field evaporation and field ionization theories,... 

Electrospray ionization involves the introduction of a liquid solution directly into the atmospheric pressure source through an emitter. The liquid forms a droplet at the end of the emitter, where it is exposed to a high electrical field (Fig. 1). This results in a buildup of multiple charges on the surface of the droplet. The coulombic forces from these charges ultimately result in the droplet s expulsion from the surface. The ions produced in the ion source are then extracted into the mass analyzer. ESI is now widely used for identifying small molecules, proteins, studying large non-covalent complexes, structural analysis, and as a detector for separation methods such as HPLC and capillary... 

Figure 26-3 A cloud chamber. The emitter is glued onto a pin stuck into a stopper that is mounted on the chamber wall. The chamber has some volatile liquid in the bottom and rests on diy ice. The cool air near the bottom becomes supersaturated with vapor. When an emission speeds through this vapor, ions are produced. These ions serve as seeds about which the vapor condenses, forming tiny droplets, or fog.

Fig. 32.6 Critical droplet size in the dripping mode. Electric field applied to a capillary containing distilled water. Experiment conditions Voltage applied at capillary tip was varied from 0 to 10,000 V flow rate set at 0.07 mL min , ground electrode is set at 10 mm below the emitter tip...

One of the reasons the electrospray is such a useful tool is that it produces relatively small and evenly distributed droplets. Using a Phase Doppler Anemometer (PDA) system, Keqi Tang has been able to characterize the droplets of an electrospray produced from Heptane and Methanol [11]. The results of these experiments show that the spray droplets are highly mono-dispersed as they travel from the emitter tip to the ground electrode as shown in Fig. 32.11. 

In Fig. 32.11 the y axis shows the number densities and the x axis is the distance radially from the center of the emitter orifice. The various curves of different z values show the results of various analyses performed at distances z from the emitter orifice along the spray axis. It is interesting to note the double hump that forms very close to the emitter orifice. This can be related to the formation of satellite and offspring droplets which are smaller in size than the parent droplets and hence are pushed away from the center due to mutual Coulombic repulsion. The spreading out of the droplets over a large radial distance from the spray axis can be attributed to the strongly divergent electric field near the capillary tip. 

Fig. 32.16 Graph showing the average diameter (d) of a heptane droplet as it travels away ftom the emitter orifice (Reprinted with permission [11])...

Sheath gas flow around the emitter orifice can be utilized when a stable electrospray in the cone-jet mode is difficult to achieve. One reason why a stable cone-jet mode electrospray can be difficult to obtain in a mass spectrometer application is an increased flow rate. An increase in flow rate causes the electrospray to operate outside the island of stability mentioned previously (Fig. 32.12). The effect that the sheath gas has on the electrospray is increased breakup of the spray droplets through the transfer of kinetic energy. 

Interfaces Between Microfluidics and Mass Spectrometry, Fig. 4 Digital microfluidics - nanoelectrospray interface, (a) Image showing the dried blood spot (DBS) and droplets sitting on actuation electrodes. The device couples directly with the nanospray (capillary) emitter, (b) Side-view schematic of the DMF device. The application... 

Berggren, M., et al. 1997. Solid-state droplet laser made from an organic blend with a conjugated polymer emitter. Adv Mater 9 968. 

FIGURE 15.2 Graphical interpretation of the structural model (a) electrical and mechanical analog of the microcollective or cluster (b) equivalent circuit for the emitter-coupled oscillator (c) the macrocollective a schematic cross-section of the droplet and its characteristics Vj, structural volumes or clusters 14, excluded surface volumes or interspaces 14, excluded bulk volumes or interspaces Si, internal separation external separation R, rigidity droplet boundary E, elasticity droplet boundary). 

The equivalent electrical circuit, rearranged under the influence of an apphed physical field, is considered as a parallel resonant circuit coupled to another circuit such as an antenna output circuit Thus, in Figure 15.4c, Wj, Cd, La, and Ra correspond to the circuit elements each Wd represents active emitter-coupled oscillator and Cd, Ld, and Rd, represent passive capacitive, inductive, and resistive elements respectively. The subscript d is related to the particular droplet diameter, that is, the droplet under consideration. Now, again the initial electromagnetic oscillation is represented by... 

FIGURE 15.4 Definition sketch for understanding the theory of electroviscoelasticity (a) rigid droplet (b) incident physical field, for example, electromagnetic (c) equivalent electrical circuit-antenna output circuit. Wd represents the emitter-coupled oscillator and Cd, and i d are capacitive, inductive, and resistive elements of the equivalent electrical circuit, respectively. Subscript d is related to the particular diameter of the droplet under consideration. (Courtesy of Marcel Dekker, Inc.) Spasic, A.M. Ref. 3., p. 854. 

An example of this process is shown diagrammatically in Figure 1. The product that emerges can be varied the control of several ctors llte first of these are the magnitude of the electric potential and the distance between the emitter and the collector. The surface tension at droplet surface is also important, and is determined by the viscosity of the polymer solution, in turn controlled by the solvent used, the molecular weight of the polymer, its concentration in solution, and the ambient temperature. Thus, there is considerable scope for the formation of subtly different materids. 

In nanospray, the initial droplets formed have a much smaller size than those formed in electrospray and result in increased total available surface area. Also, the diffusion time for solvated species to migrate to the droplet surface decreases in smaller droplets. In addition, the smaller sizes of the initial droplets in nanospray result in a reduced number of Columbic explosions required to form sufficiently small droplets suitable for ionization. All of these reduce the extent of preferential enrichment of any chemical species. Schmidt et al. (2003) first observed this equimolar phenomenon in nanospray at low flow rates using a nanospray emitter with a capillary tip with an outside diameter of less than 1 fim. Using a mixture of turanose and ra-oetyl-glueopyranoside at a 10 1 molar ratio, they observed the response ratio ehanged from approximately 2 1 at flow rates above 50 nL/min to approximately equimolar response (10 1) at flow rates of a few nanoliters/minute (Fig. 17.1). Turanose is very hydrophilic while -octyl-glucopyranoside is hydrophobic and has high surface... 

Recently, an even simpler version of PI-ESI [114] was proposed, in which the use of a capillary as sample emitter was circumvented. A sample droplet (4-10 pi) containing analytes was placed in front of the MS inlet connected to the source of electric potential ( 3 kV). The ions corresponding to the analyte molecules present in this droplet were instantly recorded by a mass spectrometer. Importantly, the sample droplet was deposited on a dielectric substrate. Polarization of electric charges on the surface of the dielectric and the sample contributes to detachment of smaller droplets which are directed toward the MS orifice (Figure 2.16). This step is followed by desolvation which may occur in a similar way to that in ESI. Because of the simplicity of the setup, Pl-ESI is suitable for use in TRMS studies (see Chapter 11). 

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