Surfaces with Patterned Wettability: Design and Applications.

Surfaces with patterned wettability have well-defined domains containing both wettable and non-wettable regions. One of the key features of the surfaces with patterned wettability is their ability to localize wetting of liquids preferentially within the patterned wettable regions. This ability of the patterned surfaces has been widely explored as a simple route to pattern both liquids, as well as, solids for various applications such as microfluidics, electronic and optical devices, surfaces with enhanced heat transfer properties, etc. However, most of the patterned surfaces exhibit wettability contrast only with high surface tension liquids such as water, thereby limiting the applications of the patterned surfaces to only aqueous systems. Herein, we utilize the design principles of superomniphobicity (repellency towards all liquids) to develop the first-ever patterned superomniphobic-superomniphilic surfaces that exhibit extremely wettability contrast with both high and low surface tension liquids. Utilizing these patterned surfaces, we demonstrate site-selective self-assembly of various liquids including: oils, alcohols, polymer solutions and solid dispersions. We also demonstrate site-selective condensation and boiling with low surface tension liquids, which is crucial when designing surfaces with significantly enhanced, phase-change, heat-transfer properties. We have further utilized surfaces with patterned wettability as templates for fabricating monodisperse, multi-phasic micro- and nano-particles. The developed technique termed WETS (Wettability Engendered Templated Self-assembly) provides us with an unprecedented ability to manufacture multi-phasic particles, on a large-scale, with precise control over the size (down to 25 nm), shape, chemistry and surface charge of the particles. We further demonstrate the utility of the WETS technique in developing amphiphilic building blocks for self-assembly and multi-functional cargo carriers. Finally, we have also studied stimuli-responsive shape reconfigurations of the multi-phasic WETS particles. Overall, this dissertation puts forward design principles for developing surfaces with patterned wettability that are universal to almost all liquids, thus enabling novel applications for the patterned surfaces, such as the WETS technique reported here.PhDMacromolecular Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116726/1/saireddy_1.pd

An Efficient Photoelectrochemical Hydrogen Evolution System using Silicon Nanomaterials with Ultra‐High Aspect Ratios

We fabricated ultra‐high aspect ratio silicon nanomaterials, including a silicon nanomesh and silicon nanowire array, on a wafer scale for efficient photoelectrochemical hydrogen production. These silicon nanomaterials (feature size≈20 nm) possess a high aspect ratio to increase the optical absorptivity of the cells to approximately 95 % over a broad range of wavelengths. The silicon nanomesh and Si nanowire cells achieved high photocurrent values of 13 and 28 mA cm −2 , respectively, which are increased by 200 % and 570 % in comparison to their bulk counterparts. In addition, these scalable Si nanomaterials remained stable for up to 100 min of hydrogen evolution. Detailed studies on the doping and geometrical structures of the resulting hydrogen evolution cells suggest that both the n +  pp + doping and thickness of nanostructures are keys to the enhancement of the hydrogen evolution efficiency. The results obtained in this work show that these silicon nanomaterials can be used for high‐performance water‐splitting system applications. The straight doping: Wafer‐scale ultra‐high aspect ratio Si nanomesh/nanowires (feature size≈20 nm) were fabricated and utilized to produce an efficient photoelectrochemical hydrogen evolution system. The Si nanomesh cell yielded extreme optical absorptivity, high external quantum efficiency, and high photocurrent. Detailed studies suggest that both the n +  pp + doping and thickness of nanostructures are keys to the enhancement of the hydrogen evolution efficiency.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109595/1/ente_201402074_sm_miscellaneous_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109595/2/889_ftp.pd

An Overview of Fully Integrated Switching Power Converters Based on Switched-Capacitor versus Inductive Approach and Their Advanced Control Aspects

This paper reviews and discusses the state of the art of integrated switched-capacitor and integrated inductive power converters and provides a perspective on progress towards the realization of efficient and fully integrated DC–DC power conversion. A comparative assessment has been presented to review the salient features in the utilization of transistor technology between the switched-capacitor and switched inductor converter-based approaches. First, applications that drive the need for integrated switching power converters are introduced, and further implementation issues to be addressed also are discussed. Second, different control and modulation strategies applied to integrated switched-capacitor (voltage conversion ratio control, duty cycle control, switching frequency modulation, Ron modulation, and series low drop out) and inductive converters (pulse width modulation and pulse frequency modulation) are then discussed. Finally, a complete set of integrated power converters are related in terms of their conditions and operation metrics, thereby allowing a categorization to provide the suitability of converter technologies

Transparent, Flexible, Superomniphobic Surfaces with Ultra‐Low Contact Angle Hysteresis

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/101812/1/ange_201307222_sm_miscellaneous_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/101812/2/13245_ftp.pd

Paper‐Based Surfaces with Extreme Wettabilities for Novel, Open‐Channel Microfluidic Devices

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134243/1/adfm201601821.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134243/2/adfm201601821-sup-0001-S1.pd

Experimental Evaluation of Internal Model Control Scheme on a DC-DC Boost Converter Exhibiting Nonminimum Phase Behavior

In this work, an internal model controller (IMC) with two-degree-of-freedom has been implemented as a voltage mode controller for the output voltage regulation of a boost-type dc-dc converter that exhibits nonminimum phase behavior due to occurrence of a right-half plane (RHP) zero. The IMC structure provides an alternate parameterization of the conventional feedback controller and is comparatively simple to tune to achieve satisfactory servo and regulatory behavior that are close to the performance limits set by the RHP zero. An internal model controller was designed using a linear model developed in the neighborhood of a nominal operating point for the converter. To assess the efficacy of the IMC scheme, simulation studies and experimental evaluations were carried. In majority of the problems investigated, the IMC was found to perform significantly better than a PID designed using the conventional approach. The responses obtained using the experimental setup were found to match closely with the responses obtained using the nonlinear dynamic model based closed-loop simulations, which corroborated the conclusions reached through the simulations. Thus, the simulation as well as experimental studies indicated that the IMC scheme is ideally suited for controlling a boost-type dc-dc converter exhibiting the nonminimum phase behavior