Dynamics of electrowetting-induced droplet motion

Electrowetting (EW) has drawn significant interests due to the potential applications in electronic displays, lab-on-a-chip microfluidic devices and electro-optical switches, etc. However, current understanding of EW is hindered by the inadequacy of available numerical and theoretical methods in properly modeling the transient behaviors of EW-actuated droplets.  In this work, a combined numerical and experimental approach was employed to study the EW response of a droplet subject to both direct current (DC) and alternating current (AC) actuating signals.  Computational fluid dynamics models were developed by using the Volume of Fluid (VOF)-Continuous Surface Force (CSF) method.  A dynamic contact angle model based on the molecular kinetic theory was implemented as the boundary condition at the moving contact line, which considers the effects of the contact line friction and the pinning force.  The droplet shape evolution under DC condition and the interfacial resonance oscillation under AC condition were investigated.  It was found that the numerical models were able to accurately predict the key parameters of electrowetting-induced droplet dynamics.

Tunable adaptive nucleate boiling heat transfer surfaces with electrowetting

Thermal energy transport is a critical aspect of power generation and energy transmission, and significant advances will be made in increasing the efficiency of various energy systems if nucleate boiling can be better controlled and improved.  This research aims at exploring a new venue to enhance nucleate boiling heat transfer by creating tunable adaptive boiling surfaces with the aid of electrowetting (EW).  It capitalizes on the complimentary roles of hydrophobicity and hydrophilicity played in nucleate boiling, and takes advantage of the ability of electrowetting to alter the surface wettability reversibly and robustly.  With this approach, the engineered boiling surface remains hydrophobic at low-to-moderate heat fluxes without the electric field on, so that onset of nucleate boiling commences spontaneously and excellent boiling heat transfer can be obtained.  When the dryout-causing bubble growth and merger intensify at high fluxes, electrowetting will be activated to change the surface to hydrophilic, thereby keeping the surface rewetted and delaying the critical heat flux (CHF).  Moreover, alternating current-driven electrowetting (ACEW) can be used to stimulate time-harmonic bubble shape oscillations that generate strong streaming flows in the surrounding liquid.  The streaming convection will superimpose with natural convection and bubble ebullition-induced microconvection in further augmenting nucleate boiling heat transfer.  The overriding objectives of this research are to elucidate the effects of electrowetting-modulated reversible surface wetting on nucleate boiling as well as to demonstrate the efficient and low-cost manufacturing of  such tunable adaptive boiling heat transfer surfaces. 

Cavitating flow in high-pressure flow systems 

To validate the applicability of basic cavitating flow theory for high-pressure flow systems, this work provides a systematic study of high-pressure flow through a thick orifice plate over a wide range of operating conditions.  A combined theoretical, numerical and experimental approach was employed to explore the two-phase flow characteristics of both the non-choked and choked flows with a maximum upstream pressure of 5,000 psi and a maximum Reynolds number of 2 × 106.  For the flow configuration used in this work, a critical downstream-to-upstream pressure ratio of 0.45 was identified below which cavitation and flow choking will occur. Furthermore, it was found from the numerical models that the conventional one-dimensional analysis is inadequate in predicting the discharge coefficient and the condition for the onset of cavitation.  Subsequently, new theoretical corrections suitable for high-pressure conditions were proposed, based on the numerical simulation results, and validated by the experimental measurements.      


Dielectrophoresis-directed assembly of micro/nanostructures

Dielectrophoresis (DEP)-based fluidic self-assembly of nanoscale building blocks, such as nanoparticles and nanowires, is a promising alternative to the current micro/nanofabrication techniques to manufacture functional micro/nanodevices.  While individual particles can be manipulated with reasonable precision, it remains a grand challenge to scale up the assembly process to reproducibly assemble a large number of particles.  This is partially due to the lack of a quantitative understanding of the complex fluid-particle dynamics when numerous nanostructures are interacting both electrically and hydrodynamically.  In this work, both experiment and numerical study were conducted to explore the electrohydrodynamic effects during the assembly of multiple nanostructures driven by DEP.

Direct numerical simulations were conducted that combine the Maxwell Stress Tensor (MST) approach and the Distributed Lagrange Multiplier/Fictitious Domain (DLM/FD) method to solve the conjugate fluid-particle interaction problem.  The MST approach was used to compute the DEP forces and torques exerted on the particles, which yields rigorous solutions even for highly non-uniform electric field and for particles of irregular shapes.  The DLM/FD method was then employed to simulate the hydrodynamic equations of the particle-fluid system involving multiple particles.  The motion of the individual particles and the subsequent aggregation of adjacent particles under three major driving mechanisms for directed self-assembly, namely, DEP, traveling-wave DEP and electrorotation, were studied in details.  In addition, microfluidic DEP devices were fabricated and self-assembly experiments were carried out for polystyrene microparticles suspended in colloidal solutions.  The observed particle motion and the assembly patterns were compared to the numerical simulation results.  The good agreement suggests the comprehensive numerical framework developed in this work can be used as a powerful tool for the fundamental study of colloidal hydrodynamics with coupled electrokinetic effects.  

Dielectrophoresis-enabled micropumping

The integration of pumping components into microfluidic systems has emerged as an important challenge, due to performance, reliability, packaging and cost issues.  One alternative is to generate the required flow directly in the microfluidic devices by inducing strong electromechanical forces in the fluid using integrated microelectrodes.  This approach allows precise microfluidic actuation and control as well as further downsizing of the microfluidic system.  In this work, a microfluidic pumping approach using traveling-wave dielectrophoresis (twDEP) of microparticles is presented.  The dielectrophoretic motion of small particles arises when the suspended particles in a fluid medium are exposed to non-uniform electric fields causing interaction between the induced dipole on the particles and the fields.  As the particles move, the surrounding fluid is dragged in the same direction due to viscous effects, thus leading to an effective pumping action.  The fluidic driving mechanisms due to the particle-fluid and particle-particle hydrodynamic interactions under twDEP are analyzed, and quantitative information of the induced flow field is obtained from numerical simulations.  Experimental measurements of the flow velocity in a prototype DEP micropumping device using micro-particle image velocimetry show satisfactory agreement with the numerical predications.  Results from this work indicate that the DEP-induced micropumping scheme holds great promise for devising versatile, self-contained microscale fluid delivery systems.

Virtual scaffolds for spinal cord injury repairing using magnetically labeled neural stem cells

This research aims at developing a novel technique to fabricate injectable, alignable, and bioactive scaffold that uses neural stem cells (NSCs) as building blocks for spinal cord injury (SCI) repair.  This work capitalizes on the ability to manipulate superparamagnetic iron oxide nanoparticles (SPIONs) with magnetic field remotely and noninvasively.  In this approach, the NSCs are labeled with nanoengineered cationic magnetoliposomes (CMLs) which encapsulate numerous SPIONs, and can be injected into the injured spinal cord in colloidal suspensions.  Upon the application of a magnetic field, magnetically labeled NSCs will spontaneously self-assemble into chain/column lattices and align along a virtual axis defined by the field flux lines, thereby forming a scaffold to guide the directional regrowth of axons.  Neurotrophic factors stored in the bilayer of the CMLs can be released by radio frequency electromagnetic triggering to promote NSC survival and axonal growth.  The overriding objective of this research is to demonstrate in vivo magnetic directed self-assembly of NSCs into highly aligned chain/column lattices for nerve regeneration after SCI, which are capable of guiding the directional growth of axons and creating a regeneration-promoting environment. 

Thermal transport of nanofluids

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage, electronics cooling to thermal processing of materials.  However, the thermal transport mechanisms for nanofluids are far from being well understood.  In particular, a consensus is lacking on if and how the dispersed nanoparticles alter the single-phase and two-phase heat transfer mechanisms of nanofluids in forced convective flows, and the applicability of established fluid mechanics and heat transfer theories for predicting thermal transport of nanofluids has also been called into question. The present research aims at conducting a systematic study of single-phase convective heat transfer and two-phase flow boiling of nanofluids in a circular minichannel.  The goals are to experimentally characterize the effective thermophysical properties, pressure drop and heat transfer behaviors of nanofluids with respect to their constituent base fluids, and to explore the effects of the particle-fluid interactions on the convective transport physics in nanofluids. 

Two-phase transport in a microchannel

Microfluidic two-phase devices and systems have been found increasingly in various engineering applications, where the gas-liquid two-phase flow is confined in microchannels with hydraulic diameters of less than 500 μm and exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels.  One important issue is how to accurately determine the two-phase flow patterns in microchannels, which is crucial to selecting appropriate predictive tools for pressure drop, heat and mass transfer for microfluidic applications.

In this work, adiabatic air-water two-phase flow was studied experimentally in circular microchannels with inner diameters of 100, 180 and 324 μm.  High-speed photographic technique was employed to visualize the two-phase flow patterns over a wide range of liquid and gas superficial velocities.  Two-phase flow maps were constructed and the transition boundaries between different flow regimes were identified.  In addition, the two-phase pressure drop was measured and compared with the models available in the literature.  The effects of surfactant on the two-phase flow pattern and pressure drop in microchannels were also studied with the motivation that addition of surfactant will help to eliminate intermittent two-phase flow patterns.  Air-water mixtures with trace quantities of sodium dodecyl sulfate (SDS) were used in the experiments.  The results were compared to the data obtained from water-air flow without surfactants. It was found that addition of surfactants results in significant modification to the two-phase flow regimes and their transition characteristics in microchannels, in particular, the most intermittent slug flow is effectively suppressed, however, the two-phase pressure drop is increased.

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