Droplet on Demand

Performing thousands of different reactions on a single chip will certainly require computer controlled execution of the pre-encoded protocols. Hiis execution necessitates the development of computer controlled modules for sorting (guiding) droplets and - above all - for introducing droplets of predesigned volumes at scheduled times of emission - a technique often called droplet on demand . In the last part of this lecture we will review also this subject. 

We first introduce the basic concepts in physics of flow of simple and multiphase fluid in networks of microchannels. We then go on to demonstrate the phenomenology of the flow of droplets through the simplest network - a single loop of channels - and then provide examples of experiments on more complicated systems. The third part of the lecture introduces the subject of modeling of the dynamics of flow of units of resistance through networks of conductors, and show the results of these efforts and their correspondence to microfluidic flows. Finally, we provide an introduction to the subject of automation of flows of droplets in microchannels and demonstrate an example of the droplet-on-demand system constmcted in our laboratory. 

The development of truly versatile chips for analytical, synthetic and biological chemistry will require not only the understanding of the flow in networks, but also the development of modules for active control over the flow, merging, splitting and above all, formation of droplets. Below we review the field of active control over formation of droplets and provide an example of a microfluidic droplet-on-demand system. 

Techniques for formation of droplets on demand must provide - in contrast to the traditional techniques of formation of droplets at constant rate of flow or constant pressure - for the ability to issue the droplet at arbitrary times and with arbitrarily prescribed volume. This goal involves incorporation of a valve. It is necessary to control the flow of the fluid-to-be-dispersed it needs to be stopped over arbitrarily long intervals and then opened with an external signal. In analogy to the classic division of hydraulic valves, there are two ways to constmct a droplet on demand chip. First is to actively force the flow of fluid with external stimuli at predefined instants (that is, the fluid is normally stopped ). The second way is to have the fluid... 

K. Churski, J. Michalski, and P. Garstecki, Droplet on demand system utilizing a computer controlled microvalve integrated into a stiff polymeric microfluidic device.. Lab on a Chip, (in press), (2009). 

Keywords Droplet-on-demand (DOD) droplet generators Electrohydrodynamics (EHD) droplet generators Microfluidic droplet generators Piezoelectric droplet generators Pneumatic droplet generators Thermal or bubble jet droplet generators... 

Tubular piezoelectric ceramics have been also used to generate droplets on demand [37]. Figure 25.6 shows a (glass) capillary in the core coupled with a piezoelectric ceramic. Almost the entire inner and outer surface of the piezoceramic tube has an electrode. A voltage between the inner and outer electrode causes the tube to contract in a longitudinal direction, thereby reducing its diameter. The piezoelectric tube is driven by a pulse generator. 

Loudspeakers may be employed as a source of disturbance generation in droplet on demand or continuous droplet stream generators [38]. An acoustic droplet generator is shown in Fig. 25.7 [39]. Typically, these droplet generators consist of a... 

Acoustic actuation Acoustic bubbles Droplet dispenser Droplet on demand Oscillating bubbles... 

Micrqet system for forming droplets on demand. The droplets are used to manufecture microlenses. (Source MacFarlane, D.L. et al. 1994. IEEE Photonics Technology Letters, 6(9), 1112-1114. With permission.)... 

The analysis of droplet streams is important in spray freezing which is often used in pharmaceutical atomization processes [63]. A major feature for the quality of the spray is the droplet size variation. Due to droplet-on-demand techniques the variation in size of droplets leaving the nozzle is very small [63]. However, the influence of atmospheric friction introduces a variation of the droplets in a jet [64]. This is shown in the image series in Fig. 8.19, which is comprised of several images captured at different vertical distances to the nozzle with a high-speed camera [64]. It shows that droplet collisions occur and lead to a merger of two subsequent droplets. This increases the size of a droplet and results in a loss of quality. 

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