Ph.D Thesis | |

Ph.D Student | Boyko Evgeniy |
---|---|

Subject | Non-Uniform Electro-Osmotic Flow: From Microscale Flow Patterning to Fluid-Structure Interaction and Instability |

Department | Department of Mechanical Engineering |

Supervisors | ASSOCIATE PROF. Moran Bercovici |

ASSOCIATE PROF. Amir Gat | |

Full Thesis text |

Electro-osmotic flow (EOF) is the fluid motion that arises over electrically charged surfaces due to the interaction of an externally applied electric field with the net charge in the electric double layer on a surface. Since its discovery by Reuss in 1809, EOF has become a common method to manipulate fluids at the microscale. When the surface charge on the walls is inhomogeneous, non-uniform EOF arises, resulting in internal pressure distribution, dictated by mass conservation.

This dissertation is concerned with the investigation of the use of non-uniform electro-osmotic flow as a driving mechanism to achieve desired flow fields and arbitrary deformations of microfluidic structures, with the aim of realizing dynamically configurable structures.

In the first part of the dissertation, we investigate the use of non-uniform EOF as a mechanism to create complex flow patterns in a Hele-Shaw configuration. We derive a pair of Poisson equations for the pressure and stream function where the source terms depend on the gradients of zeta potential. We show that the case of a disk-shaped region with uniform zeta potential results in a uniform flow within the disk and dipole flow around it. Investigating the inverse problem where the desired flow field is known and determining the associated zeta potential, we demonstrate a novel method to create complex microscale flows, without physical walls.

In the second part, we analyze the flow of non-Newtonian fluids in a
Hele-Shaw cell driven by non-uniform EOF. Motivated by their potential use for
increasing the characteristic pressure fields, we consider power-law fluids
with wall depletion properties and derive a *p*-Poisson equation governing
the pressure field, as well as its asymptotic approximation for weakly
non-Newtonian behavior. We show that the asymptotic approximation is in good
agreement with exact solutions even for fluids with significant non-Newtonian
behavior, allowing its use in the analysis and design of microfluidic systems
involving electro-kinetic transport of such fluids.

In addition to controlling flow fields, non-uniform EOF can also be used to generate desired pressure distributions at the microscale. In the third part, we study the use of non-uniform EOF as an actuation mechanism to create desired dynamic deformations in a lubricated elastic sheet bounding a fluid-filled chamber by inducing internal pressure in the fluid. We develop a theoretical model describing the fluid-structure interaction and predicting the deformation field and the typical time scales and magnitudes of the deformations observed experimentally.

In the last part of the dissertation, we study the nonlinear effects arising from the interaction of non-uniform EOF with an elastic substrate. We show that above a certain electric field threshold, such soft EOF-actuated systems may exhibit viscous-elastic interfacial instability. We develop a theoretical model describing the spatiotemporal evolution of the interfacial instability and show several distinct modes of instability depending on the electro-osmotic pattern, controlled by a non-dimensional parameter representing the ratio of electro-osmotic to elastic forces.