I modeled encapsulation of cells in a flow-focusing microchannel to investigate the effects of various fluid and flow properties on droplet size, frequency of droplet generation, and cell population per droplet. Four different modes were identified based on cell population per droplet: 1) mostly empty droplets with no cells; 2) at most one cell per droplet (single cell encapsulation); 3) at least one cell per droplet; and 4) no break-up to form cell-encapsulated droplets. Phase diagrams of successful/unsuccessful encapsulation for different flow conditions showed that physical conditions affects these encapsulation modes, with the single cell encapsulation interestingly occurring when velocities of dispersed and continuous fluids were in the same order of magnitude. Different patterns of droplet formation were also found for a combined non-dimensional parameter with specific trends in droplet volume and frequency of droplet generation.

Moreover, my parametric study on the influence of flow-focusing geometry on cell encapsulation process showed that, an orifice with a radius equal to its length, but one-fourth that of the outer tube radius and at least three radii away from the inner tube exit produces the best result of encapsulation: at most one cell per droplet at a high probability \cite{nooranidoost2019geometry}. Increasing orifice radius and decreasing its length enlarges droplets and increases numbers of cells per droplet. Also, a short nozzle-to-orifice distance results in a controlled droplet production, while a long distance negatively affected predictability of encapsulation process and made the droplet generation unpredictable and non-uniform.

As the physical fluid and flow properties, as well as geometry of flow-focusing microchannels found to change encapsulation mode, this work provided beneficial information for improving microencapsulation of cells. This work can guide researchers in medicine to precisely encapsulate cells without the need to fabricate various devices and using different fluids.