This thesis focuses on two topics involving electric field-fluid interactions. The first is the experimental realization of a coherent, macroscopic state of aligned dipolar filaments of water molecules under an applied electric field. The second is the conception and design of an electroosmotic micropump with gate voltage-induced giant Onsager coefficient. The first topic is relevant to a new kind of electrorheological (ER) fluids, purely composed of water molecules dispersed in silicone oil. This is inspired by the microscopic mechanism of giant ER (GER) effect as well as water’s evident polar characteristic when confined in nanoscale structures. We fabricated a setup including an active region with 60% porosity to permit only vaporized water molecules passing through. The vaporized wa...[ Read more ]

This thesis focuses on two topics involving electric field-fluid interactions. The first is the experimental realization of a coherent, macroscopic state of aligned dipolar filaments of water molecules under an applied electric field. The second is the conception and design of an electroosmotic micropump with gate voltage-induced giant Onsager coefficient. The first topic is relevant to a new kind of electrorheological (ER) fluids, purely composed of water molecules dispersed in silicone oil. This is inspired by the microscopic mechanism of giant ER (GER) effect as well as water’s evident polar characteristic when confined in nanoscale structures. We fabricated a setup including an active region with 60% porosity to permit only vaporized water molecules passing through. The vaporized water molecules from a thermostatic reservoir are shown to form filamentary or columnar structures, penetrating through the porous silicone oil, as a state with minimum energy in response to an external electric field. The formation of such structures is visualized directly by utilizing water-soluble fluorescent probe and captured by inverted confocal microscopy through a transparent electrode. The attendant GER effect, represented by a well-defined yield stress, is measured as a manifestation of such a coherent new molecular state. A phenomenological theory is presented to support the experimental data, with excellent quantitative agreement.

The second topic of this thesis is the conception and design of a novel micropump actuated by the electroosmotic (EO) effect. The efficiency for the traditional EO micropumps is limited by the non-slip boundary condition at the liquid/solid interface, typically not surpassing 1%. This ceiling can be broken by our new design of gate voltage-induced EO micropump coupled with slipping effect from superhydrophobic surfaces. The basic idea is first separating the bulk ions by using a pulsed gate voltage, then driving the separated bulk ions with tangential electric field from a subsequent time-coordinated bias voltage, that would propel the water to slip over nano-structured superhydrophobic surface. A theoretical model is shown to predict the surface potential, slip length, Onsager coefficients as well as the peak efficiency. Numerical simulations on this new EO micropump are performed by COMSOL Multiphysics in the time domain. The peak efficiency based on the simulated results are calculated for different channel heights, slip lengths, gate, and bias voltages to determine the optimal geometry and controlling parameters of the prototype. Both theoretical and numerical results have demonstrated the feasibility of this new design, with orders of magnitude larger Onsager coefficient and peak efficiency. We present the process flow to fabricate such a micropump based on semiconductor processing in the nanosystem fabrication facility (NFF) of HKUST. Preliminarily experimental results, including the methods of fabrication as well as characterization of patterns, are illustrated to support the validity of the design.