Researcher: Özbakır, Yaprak
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Özbakır, Yaprak
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Publication Metadata only Versatile liquid-core optofluidic waveguides fabricated in hydrophobic silica aerogels by femtosecond-laser ablation(Elsevier, 2015) Yalizay, Berna; Morova, Yagiz; Dincer, Koray; Jonas, Alexandr; Akturk, Selcuk; N/A; Department of Chemical and Biological Engineering; Department of Physics; Özbakır, Yaprak; Erkey, Can; Kiraz, Alper; PhD Student; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Physics; Graduate School of Sciences and Engineering; College of Engineering; College of Science; N/A; 29633; 22542We report on the fabrication and characterization of versatile light waveguides exploiting filaments of a polar liquid confined within hydrophobic silica aerogels. Aerogels are highly porous materials with extremely low refractive index which makes them suitable as rigid cladding of liquid-core optofluidic waveguides based on total internal reflection of light. In this article, we introduce a new microfabrication technique that allows direct and precise processing of monolithic silica aerogels by ablation with femtosecond laser pulses. Using fast scanning of the focused laser ablation beam synchronized with the motion of the processed aerogel sample, we created high-quality straight microchannels of similar to 5 mm length with controlled cross-sections inside monolithic aerogels. After the ablation, we filled the channels with high-refractive index ethylene glycol, forming multimode liquid core - solid cladding optofluidic waveguides. Subsequently, we carried out light-guiding experiments to measure overall optical attenuation of these waveguides. The characterization of waveguide transmission yielded values of propagation losses lower than 10 dB cm(-1), demonstrating that the liquid-core waveguides with laser-ablated aerogel cladding represent an attractive alternative in optofluidic applications targeting controlled routing of light along arbitrary three-dimensional paths.Publication Metadata only Total internal reflection-based optofluidic waveguides fabricated in aerogels(Springer, 2017) Jona, Alexandr; Department of Physics; Department of Chemical and Biological Engineering; N/A; Kiraz, Alper; Erkey, Can; Özbakır, Yaprak; Faculty Member; Faculty Member; PhD Student; Department of Physics; Department of Chemical and Biological Engineering; College of Sciences; College of Engineering; Graduate School of Sciences and Engineering; 22542; 29633; N/ALiquid-core optofluidic waveguides based on total internal reflection of light were built in water-filled cylindrical microchannels fabricated in hydrophobic silica aerogels. Silica aerogels with densities ranging from 0.15 to 0.39 g/cm(3) were produced by aging of alcogels in tetraethylorthosilicate solution for various time periods, followed by supercritical extraction of the solvent from the alcogel network. Subsequently, the resulting hydrophilic aerogel samples were made hydrophobic by hexamethyldisilazane vapor treatment. The synthesized samples retained their low refractive index (below similar to 1.09) and, hence, they could serve as suitable optical cladding materials for aqueous waveguide cores (refractive index n(core) = 1.33). Hydrophobic silica aerogel samples produced by the above technique also had low absorption coefficients in the visible part of the spectrum. Fabrication of microchannels in aerogel blocks by manual drilling preserving nanoporous and monolithic structure of aerogels was demonstrated for the first time. Long channels (up to similar to 7.5 cm) with varying geometries such as straight and inclined L-shaped channels could be fabricated. Multimode optofluidic waveguides prepared by filling the channels in the drilled aerogel monoliths with water yielded high numerical aperture values (similar to 0.8). Efficient guiding of light by total internal reflection in the water-filled channels in aerogels was visually revealed and characterized by monitoring the channel output. The presented technique is expected to open up further possibilities for creating three-dimensional networks of liquid channels in aerogels for optofluidic applications.Publication Metadata only Aerogels for optofluidic waveguides(MDPI, 2017) Jonas, Alexandr; N/A; Department of Physics; Department of Chemical and Biological Engineering; Özbakır, Yaprak; Erkey, Can; Kiraz, Alper; PhD Student; Faculty Member; Faculty Member; Department of Physics; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; N/A; 29633; 22542Aerogels-solid materials keeping their internal structure of interconnected submicron-sized pores intact upon exchanging the pore liquid with a gas-were first synthesized in 1932 by Samuel Kistler. Overall, an aerogel is a special form of a highly porous material with a very low solid density and it is composed of individual nano-sized particles or fibers that are connected to form a three-dimensional network. The unique properties of these materials, such as open pores and high surface areas, are attributed to their high porosity and irregular solid structure, which can be tuned through proper selection of the preparation conditions. Moreover, their low refractive index makes them a remarkable solid-cladding material for developing liquid-core optofluidic waveguides based on total internal reflection of light. This paper is a comprehensive review of the literature on the use of aerogels for optofluidic waveguide applications. First, an overview of different types of aerogels and their physicochemical properties is presented. Subsequently, possible techniques to fabricate channels in aerogel monoliths are discussed and methods to make the channel surfaces hydrophobic are described in detail. Studies in the literature on the characterization of light propagation in liquid-filled channels within aerogel monoliths as well as their light-guiding characteristics are discussed. Finally, possible applications of aerogel-based optofluidic waveguides are described.Publication Metadata only Monolithic composites of silica aerogel with poly(methyl vinyl ether) and the effect of polymer on supercritical drying(Elsevier, 2015) N/A; N/A; N/A; Department of Chemical and Biological Engineering; Department of Electrical and Electronics Engineering; Özbakır, Yaprak; Ülker, Zeynep; Erkey, Can; PhD Student; PhD Student; Faculty Member; Department of Chemical and Biological Engineering; Department of Electrical and Electronics Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 262388; 29633Novel monolithic and crack-free poly(methyl vinyl ether) (PMVE)-silica aerogel composites were synthesized using a modified sol gel procedure. The presence of PMVE in solid network of the composite samples was confirmed by a variety of techniques including IR spectroscopy, TGA and BET. Extraction of ethanol from alcogel composite rods using scCO(2) was investigated both by experiments and simulations. Partial differential equations representing mass transfer within the composite alcogel phase and in the flowing scCO(2) phase were developed and solved using finite difference method. Mass of ethanol removed from the samples as a function of time were found to be in good agreement with model results using mass transfer coefficients regressed from the experimental data. Effect of the polymer content on the drying could be estimated by taking into account of porosity change with polymer incorporation. Drying time of the alcogel increased with increasing weight fraction of polymer in the silica network and gel thickness.Publication Metadata only An aerogel-based photocatalytic microreactor driven by light guiding for degradation of toxic pollutants(Elsevier Science Sa, 2021) Jonas, Alexandr; N/A; Department of Chemical and Biological Engineering; Department of Physics; Özbakır, Yaprak; Erkey, Can; Kiraz, Alper; PhD Student; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Physics; Graduate School of Sciences and Engineering; College of Engineering; College of Sciences; N/A; 29633; 22542Efficient utilization of light in photocatalytic chemical processes requires careful optimization of the photocatalytic reactor layout to maximize the interaction between the incident light, photocatalyst and reactant molecules. Herein, we report a new type of photocatalytic flow microreactor with an integrated light guide, formed by a channel fabricated inside a hydrophobic composite aerogel monolith made of silica and titania (TiO2). The liquid-filled channel simultaneously acts as a reaction vessel and as a liquid-core optofluidic waveguide, distributing the incident light over the whole reaction volume. Anatase TiO2 nanoparticles embedded in the channel walls then serve as a photocatalyst that can efficiently interact with both the guided light and the reactant solution along the channel length. Composite aerogels were synthesized with TiO2 content between 1 and 50 wt %, retaining their interconnected mesoporous network, low refractive index, and waveguide propagation losses below -3.9 dB/cm over this range of compositions. Using photocatalytic degradation of phenol - an organic compound with harmful environmental effects - as a model chemical reaction, the performance of the microreactor was systematically investigated. Reactant conversion was observed to increase with increasing incident light power, decreasing reactant flow rate and increasing mass fraction of TiO2 in the composite. An analytical model of the reactor/light guide system was developed that predicted successfully the scaling of the reactant conversion with the incident light power and reactant flow rate. The presented concept of aerogel-based optofluidic photocatalytic microreactors is readily scalable and possesses great potential for carrying out other photocatalytic reactions in both polar and non-polar solvents.Publication Metadata only Experimental and theoretical investigation of supercritical drying of silica alcogels(Elsevier Science Bv, 2015) N/A; N/A; Department of Chemical and Biological Engineering; Özbakır, Yaprak; Erkey, Can; PhD Student; Faculty Member; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/AExtraction of ethanol from pores of cylindrical silica alcogel samples using supercritical CO2 (scCO(2)) in a tubular extraction vessel was investigated both by experiments and simulations. Partial differential equations representing mass transfer within the cylindrical silica alcogel phase and in the external scCO(2) phase were developed and solved using finite difference method. the percent removal data as a function of time were found to be in good agreement with model results using mass transfer coefficients regressed from the experimental data. the mass transfer coefficients were found to be higher by about a factor of three from two of the correlations tested which were developed for supercritical extraction. Effect of scCO(2) flow rate on the drying was investigated through experiments and simulations. increasing flow rate led to a decrease in effluent concentration at a specific time but did not significantly affect rate of extraction of ethanol. Effects of the magnitude of the effective diffusion coefficient and alcogel thickness on drying were investigated through simulations. Drying time of the alcogel decreased with decreasing gel thickness and increasing effective diffusion coefficient.Publication Open Access Kinetics of supercritical drying of gels(Multidisciplinary Digital Publishing Institute (MDPI), 2018) Ülker, Zeynep; Department of Chemical and Biological Engineering; Şahin, İbrahim; Özbakır, Yaprak; İnönü, Zeynep; Erkey, Can; PhD Student; Faculty Member; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; N/A; N/A; N/A; 29633Supercritical drying of gels is considered as the most important step of aerogel production since it enables preservation of the three-dimensional pore structure which lead to unique material properties such as high porosity, low density, and large surface area. An understanding of the kinetics of supercritical drying is necessary to provide insight into material development, scale-up, and optimization of the aerogel manufacturing process. Thus, investigation of supercritical drying is gaining increased attention in recent years. This review paper covers the experimental considerations and techniques to study the kinetics of supercritical drying, fundamental mass transfer mechanisms during the drying process and modeling efforts to predict the drying kinetics for varying operating conditions and gel properties. Transport phenomena involving diffusion, convection, spillage by volume expansion, and axial dispersion are discussed by providing the fundamental equations and empirical correlations to predict transfer coefficients. A detailed review of literature covering experimental and theoretical studies on kinetics of supercritical drying is presented.Publication Open Access Optofluidic waveguides written in hydrophobic silica aerogels with a femtosecond laser(Society of Photo-optical Instrumentation Engineers (SPIE), 2015) Yalızay, B.; Morova, Y.; Jonas, A.; Aktürk, S.; Department of Physics; Department of Chemical and Biological Engineering; Kiraz, Alper; Erkey, Can; Özbakır, Yaprak; Faculty Member; Faculty Member; Department of Physics; Department of Chemical and Biological Engineering; College of Sciences; 22542; 29633; N/AWe present a new method to form liquid-core optofluidic waveguides inside hydrophobic silica aerogels. Due to their unique material properties, aerogels are very attractive for a wide variety of applications; however, it is very challenging to process them with traditional methods such as milling, drilling, or cutting because of their fragile structure. Therefore, there is a need to develop alternative processes for formation of complex structures within the aerogels without damaging the material. In our study, we used focused femtosecond laser pulses for high-precision ablation of hydrophobic silica aerogels. During the ablation, we directed the laser beam with a galvo-mirror system and, subsequently, focused the beam through a scanning lens on the surface of bulk aerogel which was placed on a three-axis translation stage. We succeeded in obtaining high-quality linear microchannels inside aerogel monoliths by synchronizing the motion of the galvo-mirror scanner and the translation stage. Upon ablation, we created multimode liquid-core optical waveguides by filling the empty channels inside low-refractive index aerogel blocks with high-refractive index ethylene glycol. In order to demonstrate light guiding and measure optical attenuation of these waveguides, we coupled light into the waveguides with an optical fiber and measured the intensity of transmitted light as a function of the propagation distance inside the channel. The measured propagation losses of 9.9 dB/cm demonstrate the potential of aerogel-based waveguides for efficient routing of light in optofluidic lightwave circuits.Publication Open Access A new type of microphotoreactor with integrated optofluidic waveguide based on solid-air nanoporous aerogels(Royal Society of Chemistry (RSC), 2018) Jonas, Alexandr; Department of Chemistry; Department of Electrical and Electronics Engineering; Department of Physics; Özbakır, Yaprak; Erkey, Can; Kiraz, Alper; PhD Student; Faculty Member; Faculty Member; Department of Chemistry; Department of Electrical and Electronics Engineering; Department of Physics; College of Engineering; College of Sciences; N/A; 29633; 22542In this study, we developed a new type of microphotoreactor based on an optofluidic waveguide with aqueous liquid core fabricated inside a nanoporous aerogel. To this end, we synthesized a hydrophobic silica aerogel monolith with a density of 0.22 g cm(-3) and a low refractive index of 1.06 that-from the optical point of view-effectively behaves like solid air. Subsequently, we drilled an L-shaped channel within the monolith that confined both the aqueous core liquid and the guided light, the latter property arising due to total internal reflection of light from the liquid-aerogel interface. We characterized the efficiency of light guiding in liquid-filled channel and-using the light delivered by waveguiding-we carried out photochemical reactions in the channel filled with aqueous solutions of methylene blue dye. We demonstrated that methylene blue could be efficiently degraded in the optofluidic photoreactor, with conversion increasing with increasing power of the incident light. The presented optofluidic microphotoreactor represents a versatile platform employing light guiding concept of conventional optical fibres for performing photochemical reactions.Publication Open Access Photocatalytic transformation in aerogel-based optofluidic microreactors(Society of Photo-optical Instrumentation Engineers (SPIE), 2018) Department of Chemical and Biological Engineering; Department of Electrical and Electronics Engineering; Department of Physics; Özbakır, Yaprak; Jonas, Alexandr; Kiraz, Alper; Erkey, Can; PhD Student; Other; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Electrical and Electronics Engineering; Department of Physics; College of Sciences; College of Engineering; N/A; N/A; 22542; 29633Here, we demonstrate a new type of microphotoreactor formed by a liquid-core optofluidic waveguide fabricated inside aerogel monoliths. It consists of microchannels in a monolithic aerogel block with embedded anatase titania photocatalysts. In this reactor system, aerogel confines core liquid within internal channels and, simultaneously, behave as waveguide cladding due to its extremely low refractive index of similar to 1. Light is confined in the channels and is guided by total internal reflection (TIR) from the channel walls. We first fabricated L-shaped channels within silica aerogel monoliths (rho= 0.22 g/cm(3), n=1.06) without photocatalyst for photolysis reactions. Using the light delivered by waveguiding, photolysis reactions of methylene blue (MB) were carried out in these channels. We demonstrated that MB can be efficiently degraded in our optofluidic photoreactor, with the rate of dye photoconversion increasing linearly with increasing power of incident light. For photocatalytic transformation in this reactor system, titania particles were successfully embedded into the mesoporous network of silica aerogels with varying amount of the titania in the structure from 1.7 wt % to 50 % wt. The presence of titania and its desired crystalline structure in aerogel matrix was confirmed by XRF, XRD patterns and SEM images. Band gap of silica-titania composites was estimated from Tauc plot calculated by Kubelka-Munk function from diffuse reflectance spectra of samples as near expected value of approximate to 3.2 eV. Photocatalytic activity and kinetic properties for photocatalytic degradation of phenol in the channels were investigated by a constant flow rate, and longer-term stability of titania was evaluated.