Gordon Christopher

Microfluidic methods for generating continuous droplet streams

Microfluidic technologies have emerged recently as a promising new route for the fabrication of uniform emulsions. In this paper, we review microfluidic methods for synthesizing uniform streams of droplets and bubbles, focusing on those that utilize pressure-driven flows. Three categories of microfluidic geometries are discussed, including co-flowing streams, cross-flowing streams, and flow focusing devices. In each category we summarize observations that have been reported to date in experiments and numerical simulations. We describe these results in the context of physical mechanisms for droplet breakup, and simple theoretical models that have been proposed. Applications of droplets in microfluidic devices are briefly reviewed.

Coalescence and splitting of confined droplets at microfluidic junctions

The ability to merge two droplets is an important component of droplet-based lab-on-a-chip devices, yet flow-induced coalescence is difficult to achieve due to long film drainage times compared with relatively short residence times. We examine droplet collisions at a simple microfluidic T-junction and characterize the response for a wide range of droplet sizes and speeds. We find that three primary responses occur, where coalescence occurs easily at low collision speeds, smaller droplets traveling faster slip past one another without coalescing, and larger and faster droplets can break one another into multiple segments. The critical capillary number for coalescence agrees well with previously reported scaling for isolated droplet pairs when local curvature and speed are taken into account. The critical capillary number for splitting of droplets agrees well with a previously reported stability condition for individual droplets stretching in an extensional flow. Quantifying the necessary conditions for coalescence and non-coalescence behavior should enable the informed design of lab on chip devices based on discrete liquid segments.

Passive breakup of viscoelastic droplets and filament self-thinning at a microfluidic T-junction

Droplets containing a dilute polymer solution enter a T-shaped microfluidic junction and stretch as they pass through the stagnation point. Depending on the initial aspect ratio and speed, droplets may break into two segments. We characterize the breaking-nonbreaking behavior of these droplets and find that viscoelastic droplets are less stable than Newtonian droplets of comparable shear viscosity. When droplets break, we observe that viscoelastic droplet segments are connected by persistent filaments that first undergo stretching due to drag of the outer viscous liquid on the large connected segments. Later, filaments undergo iterated stretching and develop a series of beads along their length. Secondary filaments between beads no longer stretch with the outer flow, but rather they exhibit an exponentially decreasing diameter consistent with elastocapillary breakup. The relaxation time obtained from the filament diameter profiles is close to the estimated Zimm relaxation time for the polymer solution, but depends on the global flow parameters, including the initial size and speed of the droplet prior to entering the T-junction. Based on the microscale diameters of the filaments, we suggest that splitting of droplets at microfluidic T-junctions may be a useful way to characterize the extensional rheology of low viscosity elastic liquids.Droplets containing a dilute polymer solution enter a T-shaped microfluidic junction and stretch as they pass through the stagnation point. Depending on the initial aspect ratio and speed, droplets may break into two segments. We characterize the breaking-nonbreaking behavior of these droplets and find that viscoelastic droplets are less stable than Newtonian droplets of comparable shear viscosity. When droplets break, we observe that viscoelastic droplet segments are connected by persistent filaments that first undergo stretching due to drag of the outer viscous liquid on the large connected segments. Later, filaments undergo iterated stretching and develop a series of beads along their length. Secondary filaments between beads no longer stretch with the outer flow, but rather they exhibit an exponentially decreasing diameter consistent with elastocapillary breakup. The relaxation time obtained from the filament diameter profiles is close to the estimated Zimm relaxation time for the polymer solution, bu...

Modifying interfacial interparticle forces to alter microstructure and viscoelasticity of densely packed particle laden interfaces

HYPOTHESIS It is possible to control the absolute and relative magnitude of repulsive and attractive interactions and hence microstructure of interfacial particles at and air/water interface by adjusting subphase composition. It should be possible to modify interfacial viscoelasticity from elastic to viscous behavior through these changes to interfacial microstructure. EXPERIMENTS Particle laden interfaces are made from micron sized polystyrene at an air/water interface. The inter-particle interactions are controlled by the subphase salt concentration and addition of both non-ionic and ionic surfactants. These interfaces are then characterized using an interfacial rheometer with a custom visualization system. FINDINGS Three distinct microstructures are observed. Low repulsion and high attraction systems exhibit a soft glassy rheology with a disordered but dense microstructure. Creating high repulsion results in a dense hexagonal crystal. Finally, in systems with reduced repulsion and attraction, a hexatic phase can be observed. Each of these microstructures exhibit unique interfacial viscoelastic behavior. These results indicate that control over the properties of these interfaces, and hence Pickering emulsions, is possible through manipulation of interparticle forces.

Droplet Breakup in Shear and Elongation Dominated Flows in Microfluidic Devices

Microfluidic devices have recently been demonstrated as an effective platform for generating monodisperse drops and bubbles, which is important for applications from emulsification to drug delivery and lab on a chip. Here we compare drop formation mechanisms in microfluidic devices in which flows can be either predominantly shear flows, or predominantly elongational flows. In either case, drops of an aqueous liquid form due to viscous stresses imposed by a second oil phase. We show that the two flow types lead to dramatically different ability to control droplet sizes. We characterize the drop size over a large number of experiments by varying capillary number, volume fraction, and viscosity ratio. We observe distinct breakup modes depending on these three dimensionless parameters, and the flow type.Copyright