Self-ordering Dynamics in Controlled Encapsulation of Single and Multiple Cells

Abstract

Droplet-based microfluidics involves the manipulation of small (often picoliter) volumes of fluid that are suspended within an immiscible carrier fluid. Encapsulating single biological cells within aqueous microdroplets reveals heterogeneities in cell populations by restricting secretions or lysates from diffusing among the bulk population. Furthermore, isolating cell pairs within droplets also provides a unique window into cell-cell interactions at a fundamental level. However, Poisson statistics places harsh limits on theoretical encapsulation efficiencies. Fortunately, as the particle Reynolds number approaches O(1), microspheres and biological cells self-order into equally spaced trains that can be controllably encapsulated using microdroplet generators. After expanding the application space for ordered encapsulation, this dissertation seeks to better understand the fluid mechanics that yield ordering across the boundary between inertial and viscous microfluidics.

In the first experimental section, we extended ordered particle encapsulation from single to multi-particle controlled encapsulation with a nearly threefold improvement over random encapsulation. In a second experimental study, we joined two separate ordering channels upstream of a droplet generation nozzle to load cells and particles of different types into one droplet. The result was a nearly fivefold improvement over random co-encapsulation. To show the device’s utility, we co-encapsulated plus and minus mating types of single-celled algae gametes. After cell mating and over two weeks of survival within droplets, the resulting offspring remained viable.

Then, to better understand the dynamics of inertial focusing and cell ordering, we designed a custom high-speed microscope stage to track particles and cells in high-speed micro-flows with a lagrangian reference frame. As a result, trajectory and interaction observation times were up to 50 times longer than those in stationary reference frames. To gather additional quantitative results, we integrated experimental results into a transient computational fluid dynamics model.

The combination of experiments and simulations provided unique insights on the effects of key design parameters on force scaling, interparticle spacing, ordering dynamics, particle rotations, and cell stresses. In total, this dissertation encapsulates a range of accomplishments in applied bioMEMS devices, additional understanding of two-phase particulate flows, and tools to guide studies in applied and fundamental microfluidics.