1,721,003 research outputs found
Silk based nanocomposites for biophotonic and optical devices
Full text linkIn the last decade silk fibroin (SF), the protein extracted from the silk fibers, emerged as an attractive material for biophotonic applications due to its biocompatibility combined with unique mechanical and optical properties. A strategy to enhance the optical properties of silk, and to simultaneously introduce specific functionalities, is to combine silk with some specific inorganic, organic or biological compounds.
The work here presented reports the results achievable by combining silk with two different inorganic nanoparticles for the fabrication of optical nanostructured devices.
The first strategy investigated was to combine silk with titanate nanosheets (TNSs), a 2D precursor of TiO2, in order to significantly increase the refractive index of silk while preserving all its specific properties. The structural and functional characterizations of the SF-TNSs composites were performed to correlate the material structure with its properties. In particular, the ion exchange process was investigated as a strategy to easy functionalize post-process the material.
A second strategy that was investigated exploits the combination of gold nanoparticles within the silk matrix in order to introduce plasmonic functionalities in the material. Specifically, in this first phase, the inclusion of gold nanoparticles was exploited to induce localized heating thanks to the excitation of the nanoparticles plasmon resonance, an effect that can be potentially used in biomedical applications in the treatment of bacterial infections.
Finally, the fabrication of simple optical and photonic devices with the nanocomposites, such as multilayer Bragg reflectors and inverse opals, was demonstrated. In particular, a bioinspired multilayer optical structured was fabricated with the SF-TNSs material, showing a stimuli-responsive behavior, which results in a reversible change of structural coloration in response to humidity
VAPOR VELOCITY AND DROPLET DYNAMICS DURING DROPWISE CONDENSATION OF STEAM FLOWING OVER HYDROPHILIC SURFACES
No full textDropwise condensation (DWC) is a complex phase-change process that involves the nucleation, growth and removal of randomly distributed drops on the condensing surface. It is widely established that the promotion of dropwise condensation can significantly improve the heat transfer coefficient (HTC) as compared to filmwise condensation (FWC). The interaction between the condensing fluid and the surface (wettability) defines the condensation mode. Low wettability coatings with small contact angle hysteresis and low thermal resistance are a possible solution to obtain high heat transfer coefficients during DWC on metals. In energy applications, the condensing steam usually has a non-negligible velocity, but experimental data taken with flowing vapor are rare in the literature. Therefore, the investigation of DWC in presence of steam velocity would help to understand the physical mechanisms governing the phenomenon and to develop comprehensive dropwise condensation models.
In the present study, hydrophilic (advancing contact angle θa < 90°) sol-gel coated aluminum samples with reduced contact angle hysteresis (Δθ < 30°) were tested during DWC of pure steam. The engineered surfaces were characterized by dynamic contact angles and film thickness measurements. The experimental apparatus used for DWC investigation is a thermosyphon loop operating in steady-state conditions. The test rig is equipped with an optical system for the study of the droplet population and droplet dynamics. Heat transfer measurements and droplet population analyses were carried out at constant saturation temperature (~ 107.5 °C) while varying the heat flux in the range 290-1020 kW m-2 and increasing the inlet vapor velocity from 3 m s-1 to 13.5 m s-1. As a further step, the collected experimental data were compared against DWC models accounting for the effect of vapor velocity
Understanding the Role of Superhydrophobicity on Heat Transfer Enhancement During Dropwise Condensation in Humid Air Flow
No full textSuperhydrophobic surfaces have been extensively studied to enhance heat transfer during moisture condensation. However, the existing literature presents conflicting results, with some studies reporting enhanced performance while others observe a decline compared to hydrophilic surfaces. Furthermore, the effect of air velocity has been marginally addressed. In this work, superhydrophobic aluminum surfaces (advancing contact angle of 160°, contact angle hysteresis <1°) are fabricated by chemical etching followed by fluorosilane coating. Condensation tests are performed at constant air temperature (28 °C), while varying relative humidity (70%, 90%), dew-to-wall temperature difference (7–13 K) and air velocity (0.4–6 m s−1). It is found that, compared to the hydrophilic untreated surface, superhydrophobic surfaces do not offer any advantage at low air velocities (0.4 and 1 m s−1), while a condensation heat transfer coefficient increased by 30%–40% is achieved at high air velocities (4 and 6 m s−1). The performance is attributed to a more efficient droplet removal mechanism and enhanced vapor mass transfer through the non-condensable gas layer, which is also associated with droplet-induced vorticity, as confirmed by video analysis. The results clarify the operating conditions under which superhydrophobic surfaces are advantageous for applications of condensation from humid air, including dehumidification
Functionalization of Titanates–Silk Nanocomposites via Cation Exchange for Optical Applications
No full textA simple and efficient method is developed to introduce plasmonic and luminescence functionalities in titanate nanosheets (TNSs)–silk nanocomposites by direct cation-exchange process. First, the cation exchange properties such as exchange kinetic and capacity are studied to verify the behavior of the material and determine the best condition of exchange. In particular, the effect of the valence on the kinetic is investigated through elemental analysis, focusing on three target cations (Ag+, Cu2+, and Eu3+) in water. It is demonstrated that the cation exchange capability of the composite is strictly dependent of the amount of TNSs. By acting on the time of ion exchange or by changing the TNSs concentration in the silk fibroin matrix it is possible to tune the metal ions doping. The process is applicable both on flexible free-standing membranes and thin films deposited on an appropriate substrate. By exposing the material to UV radiation, it is possible to synthesize in situ metallic nanoparticles, exhibiting a characteristic plasmonic peak in the visible spectrum. Furthermore, it is verified that the europium ions preserve their photoluminescence properties when introduced in the nanocomposite, showing a characteristic red emission under UV light
Investigation of surface inclination effect during dropwise condensation of flowing saturated steam
Full text linkWhen a pure vapor condenses over a surface, it can form a continuous liquid film or a multitude of discrete droplets, thus realizing the so-called dropwise condensation (DWC). In the literature, most of the experimental data refer to DWC on vertical condensing surfaces with quiescent vapor. However, in many applications, the condensing vapor usually has a non-zero flow velocity with a consequent effect on the sliding motion of droplets. Moreover, the drag force due to vapor velocity may be the only mechanism for liquid removal on a horizontal surface or in space applications. A systematic investigation of the effects of vapor drag and surface inclination on the heat transfer and droplet population during DWC is needed and is addressed in the present paper.
Here, DWC of flowing steam is experimentally studied on sol-gel silica-based coated aluminium substrates at three different inclinations: vertical, inclined at 45°, and horizontal. Heat transfer coefficient (HTC) and droplet population measurements are performed in a wide range of heat flux (260–610 kW m−2) and average vapor velocity (3.3–13.8 m s−1). When decreasing the tilt angle, from vertical to horizontal, due to the lower contribution of the gravity force, the average droplet size increases, and a strong HTC reduction is observed above all at low vapor velocities. Because of the vapor drag force, the HTC increases with steam velocity and, at the highest mass velocity, the HTC is independent from the surface inclination. A model for the droplet departing radius in the presence of vapor velocity, initially proposed by the present authors for the sole case of vertical surfaces, is here modified to account also for the effect of surface inclination and then assessed against the present experimental data. Hence, we propose to predict the heat flux during DWC by coupling the new equation for the departing radius with the available models of heat transfer through a single droplet and drop-size distribution. The developed calculation method is found to provide satisfactory predictions of the HTC for the whole range of vapor velocity, heat flux and surface inclination
Electrodeposition of SnSe thin film using an organophosphorus [(Me2)3N3PSe] precursor for photovoltaic application
Full text linkSimultaneous measurement of heat flux and droplet population during dropwise condensation from humid air flowing on a vertical surface
No full textA new experimental apparatus for the investigation of dropwise condensation from humid air flowing over a vertical aluminum surface at controlled velocity is presented. Differently from other works on this subject, here we measure simultaneously, during condensation of flowing moist air, the total heat flux (by a heat flux sensor), the latent heat flux (by weighing the mass of condensate), the small and the large droplet population. Experimental tests were performed on two aluminum specimens that display different wettability: a mirror-polished surface (56° advancing contact angle, 46° contact angle hysteresis) considered as a baseline, and a coated surface functionalized by using a sol–gel coating (87° advancing contact angle, 15° contact angle hysteresis). The effects of the wall subcooling, moisture content, relative humidity and air velocity on the heat and mass transfer during dropwise condensation (DWC) are investigated. Enhanced condensation is observed on the coated surface but, with the increase of relative humidity and wall subcooling, the advantage of using the coated surface diminishes. Time-lapse videos of the condensation process, featuring droplets detection down to three microns, are used to investigate droplet population (both large and small droplets) and nucleation sites density. Usually, DWC models assume tentative values of the nucleation site density and tentative trends of the droplet population. In this paper we measure and discuss such parameters. The determination of the nucleation site density is crucial because it affects the droplet interactions and the drop size distribution, determining the overall heat transfer.
The measurement of the nucleation site density is currently rare in the literature, especially during condensation of flowing humid air. Here the nucleation site density is determined with a double approach and it is found to vary between 3.3 × 108 sites/m2 and 6.1 × 108 sites/m2 in the investigated range of experimental conditions. The experimental droplet population is also compared against the predicted one, finding some disagreement which should be properly addressed for the development of improved DWC models
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