Constantine M. Megaridis

University of Illinois at Chicago, Chicago, Illinois, United States

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Publications (116)296.67 Total impact

  • Sean Gart, Joseph E. Mates, Constantine M. Megaridis, Sunghwan Jung
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    ABSTRACT: Deciduous trees shed their leaves each autumn to survive cold weather. A first step in this process causes the leaves to lose their hydrophobic layer\char21{}a phenomenon that is hastened by pollution. Little is known about the physical details of raindrop-leaf interaction as hydrophobicity is changed, yet these details affect premature leaf loss and vulnerability to rainstorms. The authors model the dynamic response of a leaf to a raindrop as that of an elastic cantilever beam, and find that a hydrophobic surface can protect against raindrop impact better than a hydrophilic one. These results could also be used to optimize biomimetic piezoelectric devices for harvesting energy from falling rain.
    04/2015; 3(4). DOI:10.1103/PhysRevApplied.3.044019
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    ABSTRACT: An acrylic emulsion artists’ paint containing chlorinated copper phthalocyanine pigment was modified with variable-size multilayer graphene (exfoliated graphite) to induce low electrical resistance; composites were spray-cast on common printing paper, and subsequently polished under mild compression, to produce highly conductive paper. The mechanically robust conductive paint showed excellent adhesion to the underlying paper, as determined by Taber abrasion and tape peel tests, which displayed no adhesive failure under the test conditions studied. The conductivity of the paper substrates were tuned by changing the concentration and the size of the multilayer graphene particles. Detailed conductivity measurements showed stable Ohmic current-voltage behavior. The optimum graphene-in-paint formulations resulted in sheet resistances of the order of 10 Ω/sq. Standard electrostatic force microscopy measurements showed uniform surface electric field gradient distribution strongly correlating with the surface topography. Similarly, scanning Kelvin probe microscope measurements indicated stable work functions close to 5 eV, comparable to highly-ordered pyrolytic graphite. Furthermore, Kelvin probe measurements were more sensitive to surface charges related to copper phthalocyanine domains, which are known to have semiconducting properties. Finally, the conductive papers were also tested in the 0.50 to 0.75 terahertz frequency range for electromagnetic interference shielding (EMI) characterization and displayed quasi-metallic shielding performance.
    Carbon 02/2015; 87. DOI:10.1016/j.carbon.2015.01.056
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    ABSTRACT: A mild-temperature, nonchemical technique is used to produce a nanohybrid multifunctional (electro-conducting and magnetic) powder material by intercalating iron oxide nanoparticles in large aspect ratio, open-ended, hollow-core carbon nanofibers (CNFs). Single-crystal, superparamagnetic Fe3O4 nanoparticles (10 nm average diameter) filled the CNF internal cavity (diameter <100 nm) after successive steps starting with dispersion of CNFs and magnetite nanoparticles in aqueous or organic solvents, sequencing or combining sonication-assisted capillary imbibition and concentration-driven diffusion, and finally drying at mild temperatures. The influence of several process parameters-such as sonication type and duration, concentration of solids dispersed in solvent, CNF-to-nanoparticle mass ratio, and drying temperature-on intercalation efficiency (evaluated in terms of particle packing in the CNF cavity) was studied using electron microscopy. The magnetic CNF powder was used as a low-concentration filler in poly(methyl methacrylate) to demonstrate thin free-standing polymer films with simultaneous magnetic and electro-conducting properties. Such films could be implemented in sensors, optoelectromagnetic devices, or electromagnetic interference shields.
    Journal of Nanoparticle Research 01/2015; 17(1). DOI:10.1007/s11051-014-2856-6
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    ABSTRACT: Driven by its importance in nature and technology, droplet impact on solid surfaces has been studied for decades. To date, research on control of droplet impact outcome has focused on optimizing pre-impact parameters, e.g., droplet size and velocity. Here we follow a different, post-impact, surface engineering approach yielding controlled vectoring and morphing of droplets during and after impact. Surfaces with patterned domains of extreme wettability (high or low) are fabricated and implemented for controlling the impact process during and even after rebound -a previously neglected aspect of impact studies on non-wetting surfaces. For non-rebound cases, droplets can be morphed from spheres to complex shapes -without unwanted loss of liquid. The procedure relies on competition between surface tension and fluid inertial forces, and harnesses the naturally occurring contact-line pinning mechanisms at sharp wettability changes to create viable dry regions in the spread liquid volume. Utilizing the same forces central to morphing, we demonstrate the ability to rebound orthogonally-impacting droplets with an additional non-orthogonal velocity component. We theoretically analyze this capability and derive a We(-.25) dependence of the lateral restitution coefficient. This study offers wettability-engineered surfaces as a new approach to manipulate impacting droplet microvolumes, with ramifications for surface microfluidics and fluid-assisted templating applications.
    Scientific Reports 11/2014; 4:7029. DOI:10.1038/srep07029
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    ABSTRACT: Marine oil spills seriously endanger sea ecosystems, coastal environments, and result in a loss of energy resources. Environmental and economic demands emphasize the need for new methods of effectively separating oil-water mixtures, while collecting oil content at the same time. A new surface-tension-driven, gravity-assisted, one-step, oil-water separation method is presented for sustained filtration and simultaneous collection of oil from a floating spill. A bench-top prototype oil collection device uses selective wettability (superhydrophobic and superoleophilic) stainless steel mesh that attracts the floating oil, simultaneously separating it from water and collecting it in a container, requiring no pre-separation pumping or pouring. The collection efficiencies for oils with wide ranging kinematic viscosities (0.32 - 70.4 cSt at 40(o)C) are above 94%, including motor oil and heavy mineral oil. The prototype device also showed high stability and functionality over repeated use, and can be easily scaled for efficient clean-up of large oil spills on sea water. In addition, a brief consolidation of separation requirements for oil-water mixtures of various oil densities is presented to demonstrate the versatility of the material system developed herein.
    ACS Applied Materials & Interfaces 10/2014; DOI:10.1021/am505254j
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    ABSTRACT: Dropwise condensation (DWC) heat transfer depends strongly on the maximum diameter (Dmax) of condensate droplets departing from the condenser surface. This study presents a facile technique implemented to gain control of Dmax in DWC within vapor/air atmospheres. We demonstrate how this approach can enhance the corresponding heat transfer rate by harnessing the capillary forces in removal of the condensate from the surface. We examine various hydrophilic-superhydrophilic patterns, which, respectively sustain and combine DWC and filmwise condensation on the substrate. The material system uses laser-patterned masking and chemical etching to achieve the desired wettability contrast, and does not employ any hydrophobizing agent. By applying alternating straight parallel strips of hydrophilic (contact angle ~ 78(o)) mirror-finish aluminum and superhydrophilic regions (etched aluminum) on the condensing surface, we show that the average maximum droplet size on the less wettable domains is nearly 42% of the width of the corresponding strips. An overall improvement in condensate collection rate, up to 19%, (as compared to the control case of DWC on mirror-finish aluminum) was achieved by using an interdigitated superhydrophilic track pattern (on the mirror-finish hydrophilic surface) inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern design, particularly under higher humidity conditions in the presence of non-condensable gases (NCG), a condition that is more challenging for maintaining sustained DWC.
    Langmuir 10/2014; 30(43). DOI:10.1021/la5028866
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    ABSTRACT: Rough surfaces immersed under water remain practically dry if the liquid-solid contact is on roughness peaks, while the roughness valleys are filled with gas. Mechanisms that prevent water from invading the valleys are well studied. However, to remain practically dry under water, additional mechanisms need consideration. This is because trapped gas (e.g. air) in the roughness valleys can dissolve into the water pool, leading to invasion. Additionally, water vapor can also occupy the roughness valleys of immersed surfaces. If water vapor condenses, that too leads to invasion. These effects have not been investigated, and are critically important to maintain surfaces dry under water. In this work, we identify the critical roughness scale below which it is possible to sustain the vapor phase of water and/or trapped gases in roughness valleys - thus keeping the immersed surface dry. Theoretical predictions are consistent with molecular dynamics simulations and experiments.
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    ABSTRACT: Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiencies at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930's to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries.
    MRS Energy & Sustainability--A Review Journal 09/2014; 1. DOI:10.1557/mre.2014.9
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    ABSTRACT: Many important applications in fluid management could benefit from unidirectional transport through porous media via a simple, large-area, low-cost coating treatment; in essence, a fluid diode, demonstrated herein for water using common cellulosic paper substrates. In electronics, the diode is an electrical component with asymmetric current transfer characteristics. A light (< 2g/m2) superhydrophobic conformal coating applied onto one side of a porous substrate is shown to create liquid transport function analogous to the electronic diode, facilitating fluid movement in one direction under negligible penetration pressures, but opposing it in reverse up to greater pressures. The phenomenon is driven by capillary action, and can be observed using any similarly thin fluid barrier applied on only one side (i.e., wettability contrast) of an absorbent porous matrix. Diodic action and liquid transport rates are shown to be highly tunable, determined by fiber diameter and spacing, in combination with coating deposition amount. As an example, the device is used to separate an oil/water mixture relying upon the surface tension differences of the mixture constituents, and may be implemented in multicomponent fluid filtration/separation technologies.
    ACS Applied Materials & Interfaces 07/2014; 6(15). DOI:10.1021/am5028204
  • Lei Liu, Arindam Das, Constantine M. Megaridis
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    ABSTRACT: Electromagnetic interference (EMI) shielding reduces coupling between signals, crosstalk among electrical components, noise in cables and communication systems, etc. With the increasing speed of terahertz (THz) electronic circuits and systems, THz EMI shielding is becoming increasingly more important. We review recent and pioneering studies on shielding property–structure characterization and applications of carbon nanocomposite materials in the THz frequency domain. The theory of EMI shielding by nanocomposite materials is summarized first. A description of relevant fabrication methods follows, along with structural characterization details. THz probing and characterization of carbon nanofillers and their composites as EMI shielding and attenuation materials is presented next. Finally, the application of these materials in quasi-optical THz components, including polarizers and potentially mesh filters, as well as related manufacturing techniques are reviewed and discussed. Specific examples are presented in some detail to introduce the reader to this exciting and rapidly evolving technological area.
    Carbon 04/2014; 69:1–16. DOI:10.1016/j.carbon.2013.12.021
  • Aritra Ghosh, Ranjan Ganguly, Thomas M Schutzius, Constantine M Megaridis
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    ABSTRACT: Surface tension driven transport of liquids on open substrates offers an enabling tool for open micro total analysis systems that are becoming increasingly popular for low-cost biomedical diagnostic devices. The present study uses a facile wettability patterning method to produce open microfluidic tracks that - due to their shape, surface texture and chemistry - are capable of transporting a wide range of liquid volumes (~1-500 μL) on-chip, overcoming viscous and other opposing forces (e.g., gravity) at the pertinent length scales. Small volumes are handled as individual droplets, while larger volumes require repeated droplet transport. The concept is developed and demonstrated with coatings based on TiO2 filler particles, which, when present in adequate (~80 wt.%) quantities within a hydrophobic fluoroacrylic polymer matrix, form composites that are intrinsically superhydrophobic. Such composite coatings become superhydrophilic upon exposure to UV light (390 nm). A commercial laser printer-based photo-masking approach is used on the coating for spatially selective wettability conversion from superhydrophobic to superhydrophilic. Carefully designed wedge-patterned surface tension confined tracks on the open-air devices move liquid on them without power input, even when acting against gravity. Simple designs of wettability patterning are used on versatile substrates (e.g., metals, polymers, paper) to demonstrate complex droplet handling tasks, e.g., merging, splitting and metered dispensing, some of which occur in 3-D geometries. Fluid transport rates of up to 350 μL s(-1) are attained. Applicability of the design on metal substrates allows these devices to be used also for other microscale engineering applications, e.g., water management in fuel cells.
    Lab on a Chip 03/2014; 14(9). DOI:10.1039/c3lc51406d
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    ABSTRACT: We demonstrate the fabrication of free-standing polymeric nanocomposite films, which present magnetic and electrically conductive anisotropic properties. Magnetically functionalized carbon nanofibers are dispersed in a polymeric solution and, upon casting under a weak external magnetic field, are easily oriented and permanently assembled in a head-to-tail orientation in the polymer film during solvent evaporation. Magnetic and conductive property studies reveal that the resulting films have a high degree of anisotropy in both cases, thus allowing their use in functional complex devices. As a proof of concept, we demonstrate the potential application of these films as flexible THz polarizers. The detailed study shows that very high attenuation values per unit film thickness and fiber mass concentration are achieved, paving thus the way for cost-effective fabrication of substrate-free systems that have advantage over conventional devices realized so far.
    ACS Applied Materials & Interfaces 03/2014; 6(6). DOI:10.1021/am500335u
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    Mohamed Elsharkawy, Thomas M Schutzius, Constantine M Megaridis
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    ABSTRACT: We present a facile approach for the fabrication of low-cost surface biomicrofluidic devices on superhydrophobic paper created by drop-casting a fluoroacrylic copolymer onto microtextured paper. Wettability patterning is performed with a common household printer, which produces regions of varying wettability by simply controlling the intensity of ink deposited over prespecified domains. The procedure produces surfaces that are capable of selective droplet sliding and adhesion, when inclined. Using this methodology, we demonstrate the ability to tune the sliding angles of 10 μL water droplets in the range from 13° to 40° by printing lines of constant ink intensity and varied width from 0.1 mm to 2 mm. We also formulate a simple model to predict the onset of droplet sliding on printed lines of known width and wettability. Experiments demonstrate open-air surface microfluidic devices that are capable of pumpless transport, mixing and rapid droplet sampling (~0.6 μL at 50 Hz). Lastly, post treatment of printed areas with pH indicator solutions exemplifies the utility of these substrates in point-of-care diagnostics, which are needed at geographical locations where access to sophisticated testing equipment is limited or non-existent.
    Lab on a Chip 01/2014; 14(6). DOI:10.1039/c3lc51248g
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    ABSTRACT: The intense commercial demand for efficient fluid management (e.g., water barriers) has resulted in a recent proliferation of methods intended to impart liquid repellency to various substrates. Many such methods involve wet-based processing of hydrophobic polymers and thus rely heavily on organic solvents whose properties pose environmental challenges when scaled up from the laboratory bench. The current study presents a one-step, environmentally safe (>97 wt % water), room-temperature, low-cost, polymer-based technique that imparts superhydrophobicity to commercially relevant porous substrates. The method features aqueous dispersions of a commercially available fluoroacrylic copolymer and hydrophilic bentonite nanoclay and uses spray casting to apply coatings—which are subsequently dried in open air—to form thin conformal films. Wettability measurements demonstrate that the coating formulation imparts considerable resistance to water penetration in polymeric nonwoven and cellulosic substrates. In addition to the benign environmental impact of the aqueous formulation, all ingredients are commercially available, thus opening many technological opportunities in this area.
    Industrial & Engineering Chemistry Research 12/2013; 53(1):222–227. DOI:10.1021/ie402836x
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    ABSTRACT: Low-cost, large-area, superhydrophobic coating treatments are of high value to technological applications requiring efficient liquid repellency. While many applications are envisioned, only few are realizable in practice due to either the high cost or low durability of such treatments. Recently, spray deposition of polymer-particle dispersions has been demonstrated as an excellent means for producing low-cost, large-area, durable, superhydrophobic composite coatings/films; however, such dispersions generally contain harsh or volatile solvents, which are required for solution processing of polymers as well as for dispersing hydrophobic nanoparticles, thus inhibiting scalability due to the increased cost in chemical handling and environmental safety concerns. Moreover, such coatings usually contain fluoropolymers due to their inherent low surface energy -a requirement for superhydrophobicity- but concerns over their bio-persistence has provided an impetus for eliminating these chemicals. For spray coating, the former problem can be overcome by replacing organic solvents with water, but this situation seems paradoxical: Producing a highly water-repellent coating from an aqueous dispersion. We report a water-based, non-fluorinated dispersion for the formation of superhydrophobic composite coatings applied by spray on a variety of substrates. We stabilize hydrophobic components (i.e., polymer, nanoparticles) in water, by utilizing chemicals containing acid functional groups (i.e., acrylic acid, carboxylic acid) that can become ionized in aqueous environments under proper pH control (pH > 7). The polymer utilized in this study is a co-polymer of olefin and acrylic acid, while the particle filler is exfoliated graphite nanoplatelet (xGnP®), which contains acid functional groups at its periphery. Once spray deposited and dried, the components become insoluble in water, thus promoting liquid repellency. Such coatings can find a wide range of applications due to their benign processing nature as well as the variety of substrates on which they can be deposited.
    ACS Applied Materials & Interfaces 12/2013; 5(24). DOI:10.1021/am4043307
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    ABSTRACT: Carbon Nanotube (CNT) membranes hold the promise of extraordinary fast water transport for applications such as energy efficient filtration and molecular level drug delivery. However, experiments and computations have reported flow rate enhancements over continuum hydrodynamics, that contradict each other by orders of magnitude. We perform large scale Molecular Dynamics simulations emulating, for the first time, the micrometer thick CNTs membranes used in experiments. We find transport enhancement rates that are length dependent, due to entrance and exit losses, but asymptote to two orders of magnitude over the continuum predictions. These rates are far below those reported experimentally. The results suggest that the reported superfast water transport rates cannot be attributed to interactions of water with pristine CNTs alone.
    Nano Letters 03/2013; 13(5). DOI:10.1021/nl304000k
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    ABSTRACT: We report an approach for cost-effective manufacturing of THz quasi-optical polarizers by inkjet printing of polymer-carbon nanowhisker (CNW) dispersions. The electromagnetic interference properties of coatings with fixed CNW/polymer composition and varying thickness are quantified by a frequency domain THz spectroscopy system in the range 570–630 GHz. A shielding effectiveness of ∼40 dB is attained for 70 μm-thick coatings. A prototype THz polarizer printed on Mylar film displayed transmission and absorbance that varied with polarization orientation. The degree of polarization for film thickness of ∼1 μm was 0.35. This performance can be improved by refining grid dimensions, increasing coating thickness and adopting multi-layer polarizer structures.
    Applied Physics Letters 12/2012; 101(24). DOI:10.1063/1.4770368
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    ABSTRACT: Surface tension confined (STC) open tracks for pumpless transport of low-surface tension liquids (e.g., acetone, ethanol, hexadecane) on microfluidic chips are fabricated using a large-area, wet-processing technique. Wettable, paraffin-wax, submillimeter-wide tracks are applied by a fountain-pen procedure on superoleophobic, fluoroacrylic-carbon nanofiber (CNF) composite coatings. The fabricated anisotropic wetting patterns confine the low-surface-tension liquids onto the flow tracks, driving them with meniscus velocities up to 3.1 cm s(-1). Scaling arguments and Washburn's equation provide estimates of the liquid velocities measured in the STC tracks. These tracks are also shown to act as rails for directional sliding control of mm-sized water droplets. The present facile top-down patterned wettability approach can be extended to deposit micrometer-wide tracks, which bear promise for pumpless handling of low-surface tension liquids (e.g., aqueous solutions containing alcohols or surfactants) in lab-on-a-chip type applications or in low power, high-throughput bio-microfluidics for health care applications.
    Lab on a Chip 11/2012; 12(24). DOI:10.1039/c2lc40849j
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    ABSTRACT: Surfaces patterned with alternating (binary) superhydrophobic-superhydrophilic regions can be found naturally, offering a bio-inspired template for efficient fluid collection and management technologies. We describe a simple wet-processing, thermal treatment method to produce such patterns, starting with inherently superhydrophobic polysilsesquioxane-silica composite coatings prepared by spray casting nanoparticle dispersions. Such coatings become superhydrophilic after localized thermal treatment by means of laser irradiation or open-air flame exposure. When laser processed, the films are patternable down to ∼100 μm scales. The dispersions consist of hydrophobic fumed silica (HFS) and methylsilsesquioxane resin, which are dispersed in isopropanol and deposited onto various substrates (glass, quartz, aluminum, copper, and stainless steel). The coatings are characterized by advancing, receding, and sessile contact angle measurements before and after thermal treatment to delineate the effects of HFS filler concentration and thermal treatment on coating wettability. SEM, XPS and TGA measurements reveal the effects of thermal treatment on surface chemistry and texture. The thermally induced wettability shift from superhydrophobic to superhydrophilic is interpreted with the Cassie-Baxter wetting theory. Several micropatterned wettability surfaces demonstrate potential in pool boiling heat transfer enhancement, capillarity-driven liquid transport in open surface-tension-confined channels (e.g., lab-on-a-chip), and select surface coating applications relying on wettability gradients. Advantages of the present approach include the inherent stability and inertness of the organosilane-based coatings, which can be applied on many types of surfaces (glass, metals, etc.) with ease. The present method is also scalable to large areas, thus being attractive for industrial coating applications.
    Nanoscale 07/2012; 4(17):5378-85. DOI:10.1039/c2nr30979c
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    ABSTRACT: Electromagnetic waves in the frequency range of 0.1-10 THz have remained the least explored and developed in the entire spectrum, creating what is widely known as the " Terahertz Gap ". In recent years, THz waves have attracted much attention and continuous interests owning to their prospective applications in many important fields such as astronomy, chemical analysis, biological sensing, imaging and security screening. Among many in-development THz quasi-optical components, THz attenuators, polarizers and filters have been highly demanded in spectroscopy, polarization interferometry, polarimetric detection (e.g. polarization of Cosmic Microwave Background), and other systems. However, conventional methods for realizing the above components usually require complicated microfabrication processes such as photolithography or chemical etching. Therefore, new innovations in materials and processes for cost-effective, and potentially high performance THz quasi-optical components are of immense interest for a number of advanced technology applications. In this paper, we report a new approach for making cost-effective THz quasi-optical components based on inkjet printing of carbon nanocomposite coatings. The coating fabrication process is inherently low-cost, and all ingredients are commercially available. The chemical inertness of the coatings along with their water repellency and self-cleaning ability prevent contamination and corrosion when exposed to outdoor conditions. In addition, the patterning of the coatings can be achieved using inkjet-printing, bypassing the complicated photolithography, nanoimprinting, or chemical etching processes, thus holding the potential promise for cost-effective, flexible THz quasi-optical components such as polarizers and filters. In our previous work, we have demonstrated that large area carbon nanofiber (CNF)/PTFE polymer composite coatings as effective THz shielding and attenuation devices. The coating attenuation level can be modified by varying CNF loading content, and a THz shielding effectiveness (SE) of ~ 2 4 dB and ~32 dB were measured for the coating with the highest CNF content in the frequency range of 190-210 GHz and 570-630 GHz respectively. For a prototype demonstration, THz polarizers were first designed and inkjet-printed on Mylar thin films with nanocomposite coatings for operation near 600 GHz. The measured transmission, as well as calculated absorbance vary with polarization orientation as expected. An average degree of polarization of 0.35 has been demonstrated over the entire frequency range of 570-630 GHz, for a p olarizer with a coating thickness of only 10 μm. The polarizer performance, specifically the extinction ratio, can be potentially improved to be near 60 dB by increasing the coating thickness and adopting dual-layer polarizer structures. Prototype THz filters using single-and cross-slot structures will be soon fabricated for performance evaluation. The manufacture process and measurement results will be presented at the conference.
    23rd International Symposium on Space Terahertz Technology, Tokyo; 04/2012

Publication Stats

2k Citations
296.67 Total Impact Points

Institutions

  • 1993–2015
    • University of Illinois at Chicago
      • Department of Mechanical and Industrial Engineering
      Chicago, Illinois, United States
  • 2009
    • Technical University Darmstadt
      • Center of Smart Interfaces (CSI)
      Darmstadt, Hesse, Germany
  • 1989–1990
    • Brown University
      • School of Engineering
      Providence, RI, United States