Abstract

Droplet Microfluidics can be utilized to control the production of single droplets and double emulsions in biomedical application. This dissertation analyzes the droplet formation in hydrophobic and hydrophilic microchannels, encapsulation of cells in flow-focusing channels, and deformation of cell-laden droplets in constricted microcapillaries. The formation of droplets is studied in hydrophobic and hydrophilic channels in order to investigate the role of contact angle on flow structures and droplet generation patterns. In the hydrophobic case, stable and well-structured droplet and slug generation is achieved. However, the flow structure in the hydrophilic case significantly depends on contact angles of the walls. A small discrepancy in contact angle can lead to formation of droplets from one subchannel, similar to what happens in a T-junction microchannel. Flow focusing microencapsulation is an important process to protect the cells in biomedical applications. Numerical simulations are performed to characterize different cell encapsulation modes and present the droplet volume distribution, frequency of encapsulation and cell population in terms of inner and outer fluid capillary ratios and viscosity of the shell fluid. The desired mode of at least one cell in a droplet is determined for different capillary number ranges and each viscosity ratio. The droplet volume and frequency of droplet generation are normalized for a combined nondimensional parameter to classify different patterns of compound droplet formation which helps improve single cell encapsulation process. The effects of flow-focusing geometry on the droplet size, frequency of droplet generation, and number of cells per droplet are also investigated. Orifice radius, orifice length, and nozzle-to-orifice distance are found to significantly influence the flow-field and manipulate droplet formation. As the orifice radius increases, the drop size and the number of cells in the droplet increase correspondingly. For a short orifice radius, increasing the orifice length results in the generation of smaller droplets at higher frequency and fewer cells per droplet. On the other hand, for a longer orifice, droplet production is invariant with respect to orifice length. A short distance between the nozzle and the orifice lead to a more controlled and uniform production of droplets. When the nozzle-to-orifice length is increased, the droplet formation becomes non-uniform and unpredictable. Migration of encapsulated cells through sudden contractions in a capillary tube are studied to investigate deformation and viability of the encapsulated leukemia cells (HL60, Neutrophil, Jurkat) during their passage through the microchannel. As the cell-laden droplet moves through the sudden contraction, shear stresses are experienced around the cell. These stresses along with the interfacial force and geometrical effects cause mechanical deformation which may result in cell death. It is found that different cell types deform at different rates, and increasing viscoelasticity of the shell fluid can help improve the cell viability. The deformation is enhanced for capillary tubes with narrow and long contraction. This study can be useful to characterize cell deformation in constricted microcapillaries and to improve cell viability in bio-microfluidics.

Notes

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Graduation Date

2020

Semester

Fall

Advisor

Kumar, Ranganathan

Degree

Doctor of Philosophy (Ph.D.)

College

College of Engineering and Computer Science

Department

Mechanical and Aerospace Engineering

Degree Program

Mechanical Engineering

Format

application/pdf

Identifier

CFE0008359; DP0023796

Language

English

Release Date

December 2023

Length of Campus-only Access

3 years

Access Status

Doctoral Dissertation (Campus-only Access)

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