Ph.D Research: Nanofluidics

I studied the dynamics of sub-micron and nano-scale bubbles growing in a super-saturated solution while confined between diverging plates and developed leading order models based on these observations. My analyses develops governing equations for the dynamics of the contact line, the region where the liquid, gas, and solid all meet. In this special regime, the model reveals that the dynamics of mass transfer, bubble geometry, and contact line movement are all highly coupled. The broken symmetry of the tapered conduit causes bubbles to migrate as they grow. Interestingly, the degree to which the footprint of the bubble appears non-circular is a function of the supersaturation and contact line dissipation: bubbles growing in small supersaturations or low contact line dissipation appear circular while those that grow faster develop a tear-drop shape. Rate-dependence was initially unexpected in this regime since both Capillary and Weber Numbers are vanishingly small; the finding suggests that nano-structuring of surfaces could be used to manipulate bubble transport in boilers and other processess where gas/vapor are produced at surfaces, such as electrolysis. arxiv.org/abs/1712.03427

The studies were made possible with electron microscopy expertise of Dr. Frances Ross (IBM T.J. Watson) using a novel liquid cell developed by a previous graduate student, Dr. Joseph Grogan. The bubbles are created by the very radiation that allows their visualization; the high energy electrons used to illuminated aqueous samples disassociate water into molecular hydrogen and oxygen (think of this as electrode-less electrolysis).

(Left) TEM image of a sub-micron bubble growing between two, diverging membranes (Middle) Top view of  Surface Evolver  prediction (Right) Intensity profile from the TEM image illustrating the anisotropic growth.

(Left) TEM image of a sub-micron bubble growing between two, diverging membranes (Middle) Top view of Surface Evolver prediction (Right) Intensity profile from the TEM image illustrating the anisotropic growth.

The results of a simulation describing the growth of  a bubble in a non-uniform Hele-Shaw device (a wedge). While the bubble begins as a sphere, as it grows the contact line evolves non-uniformly according to the  Blake-Haynes  mechanism. This leads to translation as well as the development of tear-drop-like geometries. The model is implemented using a Mathematica program I wrote to access  Surface Evolver . The essential physics can be captured in a simpler model by considering the limits of high confinement and small slope. 

The results of a simulation describing the growth of  a bubble in a non-uniform Hele-Shaw device (a wedge). While the bubble begins as a sphere, as it grows the contact line evolves non-uniformly according to the Blake-Haynes mechanism. This leads to translation as well as the development of tear-drop-like geometries. The model is implemented using a Mathematica program I wrote to access Surface Evolver. The essential physics can be captured in a simpler model by considering the limits of high confinement and small slope. 


Ph.D Research: Microfluidics

My microfluidics research has been concerned primarily with the creation of novel emulsions that feature solid cores. Understanding such emulsions is important for performing high through-put screenings of biological specimens and the creation of novel metal-materials. 

An ellipsoidal polystyrene particle (minor diameter is 100 microns) is encapsulated by water surrounded by a continuous phase of oil using a glass-capillary flow focusing device. Video was captured at ~10,000 frames a second.

An ellipsoidal polystyrene particle (minor diameter is 100 microns) is encapsulated by water surrounded by a continuous phase of oil using a glass-capillary flow focusing device. Video was captured at ~10,000 frames a second.

Encapsulation of a  C. elegans  nematode in water surrounded by a continuous phase of light mineral oil using a glass capillary flow-focusing device. The injection nozzles (left) is 80 microns is diameter.

Encapsulation of a C. elegans nematode in water surrounded by a continuous phase of light mineral oil using a glass capillary flow-focusing device. The injection nozzles (left) is 80 microns is diameter.

In "Ellipsoidal Particles Encapsulated in Droplets", I analyze the geometry of the interface between two immiscible phases when the disperse phase is pinned to an ellipsoidal particle. The ellipsoidal particle was intended to help us understand the encapsulation of slender bodies (such as C. elegans, above) using flow-focusing. The geometry of the particle coupled with its finiteness introduces the possibility of encapsulation hysteresis between geometries that are adhered to the particle (unduloidal geometry) and complete encapsulation (spherical geometry) as illustrated below. While the axisymmetric shapes are thoroughly understood, there are outstanding questions regarding the stability limit of solutions, especially when the axial symmetry constraint is relaxed. 

Free energy of all three states relative to the expelled state (ellipsoid completely encapsulated in the continuous phase). Insets indicate the morphology at the position of the contact line; the open circles present on the ellipsoids indicate the position of the contact line at point C (maximum volume that an ellipsoid can "hold").

Free energy of all three states relative to the expelled state (ellipsoid completely encapsulated in the continuous phase). Insets indicate the morphology at the position of the contact line; the open circles present on the ellipsoids indicate the position of the contact line at point C (maximum volume that an ellipsoid can "hold").