Generation of a droplet sequence

For generating a droplet sequence, the system was always completely flushed with HFE oil before starting a generation. All other liquids needed to be already connected as well. For the generation process, the valve V1 was open and V2 was closed. Thus, the entire flow went from the syringes to X1, into the coil C1, making a left turn at the T-junction T3, and leaving the network to the waste 1 . Normally, droplets were generated until the coil C1 was full and the first droplets were already flushed out to the waste. Then the flow of all the liq- uids was stopped and the valves V1 and V2 were switched. Finally, the droplets were pushed forward into the coil C2.

To have a long-time stable droplet sequence, the generation of it must be flawless. In a sequence of hundreds of droplets, one coalesced droplet might cause minor difference in its entire flow behaviour. This will have a small effect on the entire droplet sequence and it might be stabled for a few cycles but when it comes to many hundreds cycles it will escalate to bigger problems such as coalescence of neighbouring droplets or separation of the sequence.

A basic method for generating an emulsion is by using a T-junction to bring the continuous phase and the disperse phase together [88]. The continuous phase is running along the straight path and the disperse phase is coming from the

4.1 The fluidic network for droplet generation and shuttling

side. While working with two disperse phases, two T-junctions were used (Fig- ure 4.5 (a)) in a earlier version of the device. The two T-junctions were connected in a row. Later, they were replaced by a X-junction (Figure 4.5 (b)).

Figure 4.5: Showing the two junctions type and flow rate combination with water droplets (blue) and clear mineral oil droplets; (a) two T-junction, (b) X-junction, (c) 5/5/5 ml/h and (d) 1/5/5 ml/h.

The kind of the junction did not have any influence on the droplets volume dimensions. This can only be varied by changing the flow rate ration of the liquids. Since the cross section of the droplets was always the same and fixed by the tubing inner diameter, the droplet length has been the only parameter that was influencing the volume of the droplets. The droplet length can be changed by changing the flow rate ratio. By having a flow ratio of A/A/A (e.g. 5/5/5 mL/h) the flow rates of all liquids would be the same and so the length of each of the two droplet phases will be the same as well as the gap of HFE oil between them (Figure 4.5 (c)). By assuming a cylindrical shape, the volume of the water droplet equals to

V = π 4 · d

2_{· h. (5)}

where d is the diameter of the droplet which was always fixed and was consid- ered equal to the tubing inner diameter (500 µm). Therefore, a simplified equation can be used

V [nL] = 196, 3 · h[mm]. (6)

The height h is equal to the length of the droplet which was approximately 0,7 mm and the corresponding volume approximately 140 nL. With this flow ratio, it might happen that the droplets in the beginning and at the end will separated from the main droplet sequence. This can be avoided by adding a long

mineral oil drop in front and at the end [58]. The length of this droplet can have several lengths of the regular droplets. This method of using the same flow ration in combination with the larger mineral droplets at both ends was consuming a lot of time and resources. However, it was used for the first experiments with the Paramecium tetraurelia.

Generating droplets with the two T-junctions sometimes caused a synchroniz- ing problem. Normally, the water phase would enter the straight path while the continuous phase was present. This was possible because the HFE oil was the continuous phase supported by the surfactants. But it could also be that the wa- ter phase would enter while the mineral oil was present. Even under these adverse conditions, it would split up the mineral oil phase leading to a more unstable droplet sequence. For this reason, an X-Junction was chosen instead of two T- junctions (Figure 4.5 (b)). Here the two phases would alternately enter the HFE path resulting every time in a stable and flawless droplet sequence.

Furthermore, several experiments revealed a phase size ratio which was stable as well as the 1/1/1 but doesn’t need the bigger mineral oil droplets at the ends. A combination of the three flow rates of 1/5/5 would lead to bigger droplets with a small gap of HFE oil as unit cell (Figure 4.5 (d)). Here the droplet length was approximately 1 mm and its volume approximately 200 nL. In this way, only one or two droplets would separate during a long-term measurement and their loss was acceptable because of the great number of remaining droplets.

Additionally the optical coherence tomography (OCT) technique was used for a different kind of visualization (see Appendix F) and because of the different refractive index numbers of the phases an accurate measurement of the droplet was not possible.