Researcher:
Rashid, Muhammed Zeeshan

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PhD Student

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Muhammed Zeeshan

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Rashid

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Rashid, Muhammed Zeeshan

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2015 - 20196

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Researcher Of Publications: search.filters.isAuthorOfPublication.68ce1ac6-8328-4bfc-97d3-382606e3e5a4

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Now showing 1 - 6 of 6
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    Reversible switching of wetting properties and erasable patterning of polymer surfaces using plasma oxidation and thermal treatment
    (Elsevier Science Bv, 2018) Soydan, Seren; Jonas, Alexander; N/A; Department of Chemistry; N/A; Department of Chemistry; Department of Physics; Department of Chemistry; Rashid, Muhammed Zeeshan; Atay, İpek; Yağcı, Mustafa Barış; Yılgör, Emel; Kiraz, Alper; Yılgör, İskender; PhD Student; Post Doctorate Student; Researcher; Researcher; Faculty Member; Faculty Member; Department of Physics; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; N/A; College of Sciences; College of Sciences; College of Sciences; College of Sciences; N/A; N/A; N/A; N/A; 40527; 22542; 24181
    Polymer surfaces reversibly switchable from superhydrophobic to superhydrophilic by exposure to oxygen plasma and subsequent thermal treatment are demonstrated. Two inherently different polymers, hydrophobic segmented polydimethylsiloxane-urea copolymer (TPSC) and hydrophilic poly(methyl methacrylate) (PMMA) are modified with fumed silica nanoparticles to prepare superhydrophobic surfaces with roughness on nanometer to micrometer scale. Smooth TPSC and PMMA surfaces are also used as control samples. Regardless of their chemical structure and surface topography, all surfaces display completely reversible wetting behavior changing from hydrophobic to hydrophilic and back for many cycles upon plasma oxidation followed by thermal annealing. Influence of plasma power, plasma exposure time, annealing temperature and annealing time on the wetting behavior of polymeric surfaces are investigated. Surface compositions, textures and topographies are characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and white light interferometry (WLI), before and after oxidation and thermal annealing. Wetting properties of the surfaces are determined by measuring their static, advancing and receding water contact angle. We conclude that the chemical structure and surface topography of the polymers play a relatively minor role in reversible wetting behavior, where the essential factors are surface oxidation and migration of polymer molecules to the surface upon thermal annealing. Reconfigurable water channels on polymer surfaces are produced by plasma treatment using a mask and thermal annealing cycles. Such patterned reconfigurable hydrophilic regions can find use in surface microfluidics and optofluidics applications. (C) 2018 Elsevier B.V. All rights reserved.
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    Passive sorting of emulsion droplets with different interfacial properties using laser-patterned surfaces
    (Springer Heidelberg, 2019) Erten, Ahmet; Jonas, Alexandr; N/A; Department of Physics; Department of Mechanical Engineering; Department of Physics; Rashid, Muhammed Zeeshan; Morova, Berna; Muradoğlu, Metin; Kiraz, Alper; PhD Student; Researcher; Faculty Member; Faculty Member; Department of Mechanical Engineering; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; College of Sciences; N/A; 152935; 22542
    We demonstrate passive sorting of emulsion microdroplets based on differences in their interfacial tension and contact angle. The sorted droplets are flowing inside a microfluidic channel featuring a shallow guiding track (depth similar to 0.6 mu m) defined by femtosecond laser micromachining in polydimethylsiloxane coating deposited on glass. Under these flow conditions, the droplets experience a confinement force that pulls them into the track; this force depends on the interfacial tension and the difference between the contact angles inside and outside the ablated track. The interplay between the confinement force, fluid drag, and wall friction then determines the trajectory of the droplet along the guiding track. We investigate experimentally the droplet trajectory as a function of droplet velocity and angle between the track and the channel axis and demonstrate precise control of droplet direction by adjusting the track angle. Moreover, we show that droplets of liquids with different interfacial tensions and contact angles travel different distances along the guiding track at a constant flow rate, which can be used for droplet sorting. We develop a theoretical model that incorporates the droplet position with respect to the ablated track, interfacial tension, and contact angles to predict the droplet trajectory under given experimental conditions. Thus, the dynamic behavior of the droplets leading to different guiding scenarios can be studied without the need of computationally expensive fluid dynamics simulations. The presented study paves the way for designing and optimizing new systems for advanced manipulation of droplets of different content using potentially reconfigurable guiding tracks.
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    Femtosecond laser ablated tracks on smart surfaces for droplet manipulation applications
    (Institute of Electrical and Electronics Engineers (IEEE), 2018) Coskun, Umut Can; Morova, Yaǧız; Bozkurt, Asuman Asikoglu; Erten, Ahmet Can; Jonas, Alexandr; Aktürk, Selcuk; N/A; Department of Physics; Department of Physics; Rashid, Muhammed Zeeshan; Morova, Berna; Kiraz, Alper; PhD Student; Researcher; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 22542
    In droplet based microfluidics, ability of controlling the motion of the droplets insidechips have great importance for the applications such guiding, sorting, mixing and dividing of liquid droplets. In this study, we fabricated hydrophilic microchannels inside the chips by femtosecond laser ablation technique in order to control droplet motion. The laser beam focused on PDMS converts the irradiated regions to hydrophilic by ablating the coating entirely from the surface. Droplets follow the tracks because of the difference in wettability of ablated region and PDMS surfaces together with the topographical profile of the ablated tracks. Within this study, we investigated the manipulation of three kind of liquid droplets (water/surfactant mixture, pure water and pure etylene glycol) along microchannels with depths of 1μm, 1.5μm and 2μm. We verified the experimental results with simulations and analyzed the trajectories of the moving droplets with the different wettability along the tracks of different depths. Our results showed that, while topographical effects play role in droplet guiding for 1.5μm and 2μm tracks, for the case of 1μm depth of channel, surface energy modifications cause the guiding rather than the topographical step on the surface. These results will pioneer for sorting applicaitons of different microdroplets mixed in the same microfluidic chip. / Damla temelli mikroakışkan sistemlerinde, damlaların yonga içerisindeki hareketlerinin kontrolü, yönlendirme, ayrıştırma, birleştirme ve bölme gibi uygulamalarda oldukça önemlidir. Bu çalışmada, damlaların hareketinin kontrolünü sağlamak için mikroakışkan yonga içerisinde hidrofilik mikrokanalları femtosaniye lazer ablasyon tekniği ile ürettik. Polydimethylsiloxane (PDMS) üzerine odakladığımız fs lazer hüzmesi uyarılan bölgeyi tamamen aşındırarak hidrofilik hale getirmektedir. Damlalar, aşındırılan hidrofilik bölge ile PDMS yüzeyi arasındaki ıslanabilirlik ve topografik farklılık sebebiyle bu kanallara eşlenerek ilerlemektedir. Bu çalışma kapsamında, üç farklı polar akışkandan (su/yüzey aktif madde karışımı, saf su ve saf etilen glikol) yağ içerisinde ürettiğimiz damlaların, 1μm, 1.5μm ve 2μm derinliklerine sahip işlenmiş mikrokanallar aracılığıyla yönlendirilmesini araştırdık. Deneysel olarak gerçekleştiridiğimiz çalışmaların sonuçlarını simülasyon değerleri ile karşılaştırdık ve farklı yapıdaki akışkan damlaların farklı derinliklerdeki kanallara sahip yongalardaki hareket yörüngelerini inceledik. Elde ettiğimiz sonuçlarla 1.5μm ve 2μm derinliğindeki kanallarda damlaların yönlendirilmesi hususunda kanal topografisinin etkili olduğunu, 1μm derinliğindeki kanallar için ise kanalın yüzeyde sahip olduğu derinlik yerine yüzey enerjisindeki değişimin etkili olduğunu gösterdik. Elde ettiğimiz bu sonuçlar, aynı yonga içerisinde bulunan farklı ıslanabilirlik özeliklerine sahip akışkanların ayrıştırılması gibi uygulamalara imkan sağlayacaktır.
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    Guiding of emulsion droplets in microfluidic chips along shallow tracks defined by laser ablation
    (Springer Heidelberg, 2017) Coskun, Umut Can; Morova, Yagiz; Bozkurt, Asuman Asikoglu; Jonas, Alexandr; Akturk, Selcuk; N/A; Department of Physics; Department of Physics; Rashid, Muhammed Zeeshan; Erten, Ahmet Can; Kiraz, Alper; PhD Student; Teaching Faculty; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; 233923; 22542
    We demonstrate controlled guiding of nanoliter emulsion droplets of polar liquids suspended in oil along shallow hydrophilic tracks fabricated at the base of microchannels located within microfluidic chips. The tracks for droplet guiding are generated by exposing the glass surface of polydimethylsiloxane (PDMS)-coated microscope slides via femtosecond laser ablation. The difference in wettability of glass and PDMS surfaces together with the shallow steplike transverse topographical profile of the ablated tracks allows polar droplets wetting preferentially the glass surface to follow the track. In this study, we investigate guiding of droplets of two different polar liquids (water/ethylene glycol) with and without surfactant suspended in an oil medium along surface tracks of different depths of 1, 1.5, and 2 mu m. The results of experiments are also verified with computational fluid dynamics simulations. Guiding of droplets along the tracks as a function of the droplet composition and size and the surface profile depth is evaluated by analyzing the trajectories of moving droplets with respect to the track central axis, and conditions for stable guiding are identified. The experiments and numerical simulations indicate that while the track topography plays a role in droplet guiding using 1.5-and 2-mu m deep tracks, for the case of the smallest track depth of 1 ae m, droplet guiding is mainly caused by surface energy modification along the track rather than the presence of a topographical step on the surface. Our results can be exploited to sort passively different microdroplets mixed in the same microfluidic chip, based on their inherent wetting properties, and they can also pave the way for guiding of droplets along reconfigurable tracks defined by surface energy modifications obtained using other external control mechanisms such as electric field or light.
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    Electromagnetic simulation of coupled silicon and diamond microdisks and slab waveguides in the mid-infrared
    (Electromagnetics Academy, 2015) Uysallı, Yiğit; Kurt, Adnan; N/A; N/A; N/A; Department of Physics; Chaudhry, Muhammad Rehan; Rashid, Muhammed Zeeshan; Gökay, Ulaş Sabahattin; Serpengüzel, Ali; PhD Student; PhD Student; PhD Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 27855
    Electromagnetic numerical studies of silicon and diamond microdisks coupled with silicon and diamond slab waveguides are performed in the CO2 laser emission region in the mid-infrared. Microdisk is the 2D analog of the microsphere and the slab waveguide is the 2D analog of the rectangular optical waveguide. The evanescent coupling between the waveguide and the microdisk results in efficient pumping of the whispering gallery modes of the microdisk. On-resonant and off-resonant studies are performed by tuning the laser wavelength to the microdisk whispering gallery modes.
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    Optofluidic dye lasers based on holey fibers: modeling and performance analysis
    (Institute of Electrical and Electronics Engineers (IEEE), 2018) Jonas, Alexandr; Buczynski, Ryszard; Department of Physics; N/A; Kiraz, Alper; Rashid, Muhammed Zeeshan; Faculty Member; PhD Student; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 22542; N/A
    Holey optical fibers with air-filled openings can be turned into unconventional optofluidic components by selectively introducing a suitable liquid into their internal holes. Enhanced interaction between the light guided by the fiber and molecular species suspended in the confined liquid then forms the basis for designing unique optofluidic devices-filters, sensors, or lasers-whose operating characteristics are extremely sensitive to minute changes in the liquid composition. We present a comprehensive mathematical analysis of dye lasers based on suspended-core and hollow-core fibers, with aqueous solution of Rhodamine B dye injected into the internal holes of the fibers acting as the gain medium. Our model consists of coupled first-order differential equations that characterize the population of lasing levels and the variation of pump and signal laser powers along the fiber length in the steady state. We obtain optimum operating conditions of the proposed holey fiber dye lasers, such as the optimum fiber length and dye concentration, and compare their performance to conventional dye jet lasers. We conclude that the fiber variant of dye laser using the hollow-core fiber is superior in terms of low lasing threshold and high slope efficiency. The presented theoretical framework can find applications in designing practical holey fiber dye lasers serving as fiber-based alternatives of conventional dye jet lasers as well as novel biological/chemical sensors and bio-lasers.