The Analysis of Asymmetric Micro Droplets

The primary object of the research activities in this semester is to investigate how to adapt the Popov’s evaporation model to droplets with asymmetric geometries. Besides, extensive work with Surface Evolver were performed to analyze the equilibrium geometries of droplets on different solid substrate. Progress has been significant in the development of skills in Surface Evolver, specifically in investigating droplets at the point of bursting. Progress has been less significant in the adaptation of Popov’s Model as there are many aspects of this that require in-depth analysis, but the progress made has been promising. Outcomes of this semester include a much more substantial understanding of Surface Evolver as well as Popov’s model. Significant progress were made towards making Popov’s Model being applicable to asymmetric droplets

Numerical and Experimental Study of the Wetting Characteristics of Water Droplets on Solid Substrates

In this study, the author first reviewed the background and the mechanism of a highly efficient cooling method—i.e., the thin film evaporative cooling, in which the heat removal performance is highly dependent on the wetting characteristics of the working fluid. Then, the author studied the wetting behavior of water on different solid substrate both numerically and experimentally. By minimizing the free energy, Surface Evolver was used to explore the profile of the static liquid meniscus and the corresponding contact angle of water droplets on plain solid substrate and pillar substrate with sharp edge. Besides, goniometer experiments were performed to study the contact angle of a sessile water droplet on silicon, copper and aluminum substrates with graphene oxide (GO) and reduced graphene oxide (RGO) nanocoatings of different thicknesses. In addition, the author prepared a detailed list of components to be ordered for performing Micro-PIV experiments. An extensive literature study have been done to support the feasibility of the author’s work

Optimizing Nanoscale Heat Transfer for Novel Applications

Nanoscale surface treatments and their effect on liquid film pinning and thin film evaporative heat transfer was studied through published literature and experimental simulation. ANSYS Fluent was utilized to study relevant geometries and to confirm experimental results found in published literature. Vapor chambers were studied to compare their current performance to that of a vapor chamber with a proposed graphene integrated CIO, copper inverse opal, wicking structure. The role of graphene in altering the surface energy and conductive characteristics of a given substrate as well as its performance as a protective coating was studied, yielding results that require further study. Further research will be required to confirm published results on the wettability of graphene as well as building a two-phase fluid flow simulation to study the performance of copper inverse opal wicking structures

Fundamentals of microfluidics fabrication process and the machining of hollow micropillar array

This report firstly introduces the fundamental knowledge of microfabrication which will be used in producing micropillar array, such as photolithography, physical vapor deposition, lift-off and reactive ion etching. Then by using different instruments to do microfabrication, which involves four main steps: lift-off, etching holes from backside of wafer, etching holes from frontside of wafer and etching pillars from frontside of wafer. The details of these four steps will be shown in this report. Finally, the outcome will be discussed by comparing the difference between designed dimensions and real micropillar dimensions. The main drawback of this experiment is also discussed and found out how to solve this problem

Surface characterization of self-assembled monolayers for applications of selective ionic transport through nanoporous membranes

Electrokinetics is the study of ionized particles or molecules and their interactions under an applied electric field. Transport processes include those of colloids and other charged particles in electrolytes, whose motion under applied external electric fields is described in part by fluid mechanics. In particular, there is growing interest for the application of electromigration through nanoporous membranes to water desalination. For thin double layers, the volumetric flow ratio (VFR) between electroosmosis and pressure driven flow is inversely proportional to 1⁄r^2 , indicating the advantages of utilizing electrokinetics for systems with small characteristics length scales. Bare solid surfaces of metal and metal oxides tend to adsorb organic contaminants in order to lower the free energy between the metal and the environment. The adsorbed matter can alter the interfacial properties of microfluidic and nanofluidic systems. The adsorbed matter does not have any specific functional properties; therefore, it is hard to reproduce any physical properties (e.g., thermoconductivity, electroconductivity, hydrophobicity). Self-assembled monolayers (SAMs) provide a unique way to control and functionalize the interfacial properties of metal, metal oxide, and semiconductors for nanoscale devices. The control in functionality can improve the performance of nanoscale devices by improving process precision. In this dissertation, a focus of interfacial transport phenomena is proposed in order to achieve improved-charge selective nanofluidic systems. There have been numerous studies on the quality of organic SAMs as a blocking mechanism for prevention of ion adsorption with applications ranging from biosensors to chemical sensing through nanoporous membranes. The applications of these devices are often limited by the quality of the SAM. For transport studies utilizing a SAM on a gold-coated nanoporous membrane, electrochemical impedance spectroscopy (EIS) can be used as a probe to characterize insulative surface properties. Insulation is especially crucial for short length scales found in microchannels and nanopores. A well-grown monolayer can help reduce adsorption of ions on walls of nanopores/nanochannels, which can lead to lower irreversibilities for charge selective systems. There have been extensive studies, which demonstrate that by making a polymer membrane conductive mostly through a process called electroless gold plating, that charge selectivity can be accomplished by controlling the surface charge of the conductive membrane by applying a range of cathodic and anodic potentials. The membranes are functionalized with SAMs in order to prevent adsorption of ions. Past work has shown that selectivity of ion adsorption can vary, depending on whether anodic or cathodic potentials are applied across the membrane. A second motivation of this work has been to study and characterize the quality of SAMs with the intent to minimize adsorption ions on a membrane surface in order to maximize charge selectivity. By minimizing adsorption, smaller over-potentials can be applied to achieve charge selectivity. Lastly, the fabrication of a membrane permeate flow cell is described which was then utilized to study the transport of organic analytes through a conductive nano-capillary array membrane (NCAM) by UV absorption spectroscopy. The goals of the transport studies are to demonstrate improved charge selectivity when well-grown SAMs are used over a wide range of potentials applied across a membrane. In addition, the studies are to further implement chemical separations by applying potentials within the millivolt range (≤±400 mV). This work improves on previous studies of applying potentials across conductive NCAMs by determining the critical voltage range at which potentials can be applied with minimum ion conduction through the SAM. Future work will also be addressed where it is suggested to explore the idea of competing effects between the contributions of the diffuse layer potential at the membrane surface and nanopore wall

Electrochemical Cell Design and Experimental Setup for Passive Two-Phase Cooling

The goal for the three credit-hour independent study in the Nanoscale Energy and Interfacial Transport laboratory was to familiarize myself with research methodology, energy storage on the micrometer scale, passive cooling for advanced electronics, and provide general aid in setting up the new laboratory space. Dr. Damena Agonafer, the principal investigator, assigned two projects to undergraduate researchers from the months of January 2017 to May 2017: novel methods for increased nanocapacitor performance, and passive two-phase cooling for microelectronics. Initial work consisted of background research into hybrid nanocapacitors, including materials, ordered quasi 2-D structures, and synthesis methods. This project was cancelled in progress in favor of the cooling project. After transitioning projects, work primarily consisted of designing and prototyping an electrolyte cell for synthesizing the required micro-structures, modeling experimental setups in 3-D CAD software, and continuing to construct the laboratory. The outcomes of this independent study include a deep understanding of electrochemical energy storage and transfer and research methodology, a functional design for an electrochemical cell for synthesis of a copper inverse-opal structure, and a nearly complete physical laboratory space

Interfacial Transport Modeling Through Use of MatLab and Surface Evolver

The object of this research semester was to become more familiar with the research area of the Nanoscale Energy and Interfacial Transport (NEIT) lab led by Dr. Damena Agonafer, establish the physical lab and its materials, verify Popov’s model using MatLab , and finding the lowest energy state of a steady state meniscus using Surface Evolver. Progress on each of these tasks has been significant, and in the process valuable skills have been developed such as MatLab modeling, coding using c and operating Surface Evolver, collaborating , and holistically analyzing a population of journal articles to understand the subject. Outcomes of this semester include a much deeper understanding on the topic, knowledge in using both MatLab and Surface Evolver, and a lab that is near complete in terms of being set up