Acoustic manipulation and alignment of particles for applications in separation, micro-templating and device fabrication

This dissertation studies the manipulation of particles using acoustic stimulation for applications in microfluidics and templating of devices. The term particle is used here to denote any solid, liquid or gaseous material that has properties, which are distinct from the fluid in which it is suspended. Manipulation means to take over the movements of the particles and to position them in specified locations. ^ Using devices, microfabricated out of silicon, the behavior of particles under the acoustic stimulation was studied with the main purpose of aligning the particles at either low-pressure zones, known as the nodes or high-pressure zones, known as anti-nodes. By aligning particles at the nodes in a flow system, these particles can be focused at the center or walls of a microchannel in order to ultimately separate them. These separations are of high scientific importance, especially in the biomedical domain, since acoustopheresis provides a unique approach to separate based on density and compressibility, unparalleled by other techniques. The study of controlling and aligning the particles in various geometries and configurations was successfully achieved by controlling the acoustic waves. ^ Apart from their use in flow systems, a stationary suspended-particle device was developed to provide controllable light transmittance based on acoustic stimuli. Using a glass compartment and a carbon-particle suspension in an organic solvent, the device responded to acoustic stimulation by aligning the particles. The alignment of light-absorbing carbon particles afforded an increase in visible light transmittance as high as 84.5%, and it was controlled by adjusting the frequency and amplitude of the acoustic wave. The device also demonstrated alignment memory rendering it energy-efficient. A similar device for suspended-particles in a monomer enabled the development of electrically conductive films. These films were based on networks of conductive particles. Elastomers doped with conductive metal particles were rendered surface conductive at particle loadings as low as 1% by weight using acoustic focusing. The resulting films were flexible and had transparencies exceeding 80% in the visible spectrum (400-800 nm) These films had electrical bulk conductivities exceeding 50 S/cm. ^

Isolating the photovoltaic junction: atomic layer deposited TiO2-RuO2 alloy Schottky contacts for silicon photoanodes

We synthesized nanoscale TiO2-RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions >= 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13-46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2-RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.

Molecular dynamics study of a dual-cation ionomer electrolyte

The poly(N1222)xLi1-x[AMPS] ionomer system with dual cations has previously shown decoupled Li ion dynamics from polymer segmental motions, characterized by the glass transition temperature, which can result in a conductive electrolyte material whilst retaining an appropriate modulus (ie. stiffness) so that it can suppress dendrite formation, thereby improving safety when used in lithium metal batteries. To understand this ion dynamics behavior, molecular dynamics techniques have been used in this work to simulate structure and dynamics in these materials. These simulations confirm that the Li ion transport is decoupled from the polymer particularly at intermediate N1222+ concentrations. At 50 mol% N1222+ concentration the polymer backbone is more rigid than for higher N1222+ concentrations, but with increasing temperature Li ion transport is more significant than polymer or quaternary ammonium cation motions. Here we suggest an ion hopping mechanism for Li+, arising from structural rearrangement of ionic clusters that could explain its decoupled behavior. Higher temperatures favor an aggregated ionic structure as well as enhancing these hopping motions. The simulations discussed here provide an atomic-level understanding of ion dynamics that could contribute to designing an improved ionomer with fast ion transport and mechanical robustness.

Aligned nanofibers from polypyrrole/graphene as electrodes for regeneration of optic nerve via electrical stimulation

The damage of optic nerve will cause permanent visual field loss and irreversible ocular diseases, such as glaucoma. The damage of optic nerve is mainly derived from the atrophy, apoptosis or death of retinal ganglion cells (RGCs). Though some progress has been achieved on electronic retinal implants that can electrically stimulate undamaged parts of RGCs or retina to transfer signals, stimulated self-repair/regeneration of RGCs has not been realized yet. The key challenge for development of electrically stimulated regeneration of RGCs is the selection of stimulation electrodes with a sufficient safe charge injection limit (Q(inj), i.e., electrochemical capacitance). Most traditional electrodes tend to have low Q(inj) values. Herein, we synthesized polypyrrole functionalized graphene (PPy-G) via a facile but efficient polymerization-enhanced ball milling method for the first time. This technique could not only efficiently introduce electron-acceptor nitrogen to enhance capacitance, but also remain a conductive platform-the π-π conjugated carbon plane for charge transportation. PPy-G based aligned nanofibers were subsequently fabricated for guided growth and electrical stimulation (ES) of RGCs. Significantly enhanced viability, neurite outgrowth and antiaging ability of RGCs were observed after ES, suggesting possibilities for regeneration of optic nerve via ES on the suitable nanoelectrodes.

Probing synechocystis-arsenic interactions through extracellular nanowires.

Microbial nanowires (MNWs) can play an important role in the transformation and mobility of toxic metals/metalloids in environment. The potential role of MNWs in cell-arsenic (As) interactions has not been reported in microorganisms and thus we explored this interaction using Synechocystis PCC 6803 as a model system. The effect of half maximal inhibitory concentration (IC50) [~300 mM As (V) and ~4 mM As (III)] and non-inhibitory [4X lower than IC50, i.e., 75 mM As (V) and 1 mM As (III)] of As was studied on Synechocystis cells in relation to its effect on Chlorophyll (Chl) a, type IV pili (TFP)-As interaction and intracellular/extracellular presence of As. In silico analysis showed that subunit PilA1 of electrically conductive TFP, i.e., microbial nanowires of Synechocystis have putative binding sites for As. In agreement with in silico analysis, transmission electron microscopy analysis showed that As was deposited on Synechocystis nanowires at all tested concentrations. The potential of Synechocystis nanowires to immobilize As can be further enhanced and evaluated on a large scale and thus can be applied for bioremediation studies.

Procesos hopping a través del modelo difusional en materiales nanocristalinos usados para aplicaciones fotovoltaicas

Se presentan los modelos de hopping de rango variable (variable range hopping; VRH), vecinos cercanos (nearest neighbor hopping; NNH) y barreras de potencial presentes en las fronteras de grano; como mecanismos de transporte eléctrico predominantes en los materiales semiconductores para aplicaciones fotovoltaicas. Las medidas de conductividad a oscuras en función de temperatura fueron realizadas para región de bajas temperaturas entre 120 y 400 K con Si y compuestos Cu3BiS2 y Cu2ZnSnSe4. Siguiendo la teoría de percolación, se obtuvieron parámetros hopping y la densidad de estados cerca del nivel de Fermi, N(EF), para todas las muestras. A partir de los planteamientos dados por Mott para VRH, se presentó el modelo difusional, que permitió establecer la relación entre la conductividad y la densidad de estados de defecto o estados localizados en el gap del material. El análisis comparativo entre modelos, evidenció, que es posible obtener mejora hasta de un orden de magnitud en valores para cada uno de los parámetros hopping que caracterizan el material.

Inventário e análise sociológica das políticas públicas de turismo em Angola

As políticas públicas na área do turismo tornaram-se uma prioridade dos governos dos países desenvolvidos e menos desenvolvidos a partir da década de 1970, quando os organismos estatais ligados ao sector do turismo deram início à definição de amplas agendas nacionais para o desenvolvimento da indústria do turismo para períodos de longo prazo, com descrições claras sobre a posição que o turismo ocupa no âmbito do desenvolvimento da economia nacional; sobre as metas que se pretendem alcançar e a forma como elas serão atingidas. O enorme crescimento do turismo, o envolvimento dos governos e os impactos negativos do turismo que foram registados nos países em desenvolvimento ajudou a trazer ao debate académico a análise sobre as políticas públicas do turismo no final dos anos 80 e início de 90 do séc. passado. Neste estudo inventariou-se o quadro legislativo das políticas públicas do turismo em Angola e fez-se o enquadramento sociológico da Política Nacional e do Plano Diretor do turismo em Angola; identificaram-se os atores que intervêm na execução das políticas públicas do turismo em Angola e o seu sentido social, assim como os fatores centrais propiciadores do desenvolvimento social e económico em contexto local (Huíla). Isto permitiu determinar o perfil e tipologias resultantes da contribuição do quadro de Políticas Públicas de turismo para o nível de promoção do desenvolvimento social local em Angola, ajudando-nos a tentar perceber até que ponto as políticas públicas do turismo em Angola se constituem como fator de desenvolvimento social local. Partindo do perfil e tipologias resultantes da contribuição do quadro de Políticas Públicas de turismo e com base nos resultados obtidos através da análise de conteúdo das entrevistas, construiu-se uma proposta de modelo de políticas públicas de turismo propiciadoras do desenvolvimento local sustentável em Angola baseada numa lógica de ação coletiva, capaz de salvaguardar a sustentabilidade económica, política, social e territorial; ABSTRACT: Public policies in tourism became a priority of developed country governments and less developed countries from the 1970s, when the state bodies linked to the tourism sector began the definition of a broad national agenda for the development of the tourism industry for long-term periods, with clear descriptions of the position that tourism occupies in the national economic development; on the goals they want to achieve and how they will be achieved. The tremendous growth of tourism, the involvement of governments and the negative impacts of tourism were registered in developing countries and helped to bring the academic debate analysis on public tourism policies, in the late 80s and early 90s of the past century. In the legislative framework, we inventoried public tourism policies in Angola that later became the sociological framework of the National Policy and Plan for tourism in Angola; we identified the actors involved in the execution of public tourism policies in Angola and its social meaning, as well as the central factors conductive to social and economic development in the local context (Huila). This allowed to determine the profile and types resulting from the tourism Public Policy framework contribution to the level of promotion of local social development in Angola, helping us in this way to try to understand to what extent public tourism policies in Angola work as a local social development factor. From the profile and types resulting from the tourism Public Policy framework contribution and based on the results obtained from the interviews, we constructed a proposed model of public policies which encourage a more sustainable local development kind of tourism in Angola, based on a logic of collective action, able to safeguard the economic, political, social and territorial capitals.