Research Project: Süperhidrofobik Yüzey Üzerinde Duran Sıvı Mikrodamlaların İnceltilmis Optik Fiber ile Ultrayüksek Çözünürlükte Optik Spektroskopisi
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Contributors
Funders
ID
TB.00029
Authors
Kiraz, Alper
Faculty Member
Publications
Determination of microdroplet contact angles using electrically driven droplet oscillations
(American Institute of Physics (AIP) Publishing, 2011) Kiraz, Alper; Karadağ, Yasin; Jonáš, Alexandr; Tasaltın, Nevin; Department of Physics; Yes; College of Sciences
Oscillatory deformations of micrometer-sized NaCl-water droplets by an AC electric field are used for contact angle measurements on superhydrophobic surfaces. Contact angles are determined from the dependence of the lowest-order resonant frequency of the electrically driven droplet oscillations on the droplet size. The resonant frequency and size of a droplet are found using whispering gallery mode spectroscopy. Measurements are compared with those performed with direct mechanical driving of the droplets using a piezoelectric transducer, and a good agreement is found. The demonstrated contact angle measurement method can be readily integrated into the planar architecture of microfluidic chips.
Probing microscopic wetting properties of superhydrophobic surfaces by vibrated micrometer-sized droplets
(American Chemical Society (ACS), 2011) Jonas, Alexandr; Kiraz, Alper; Taşaltın, Nevin; Karadağ, Yasin; Kücükkara, İbrahim; Department of Physics; Yes; College of Sciences
We determine contact angles of micrometer-sized NaCl-water droplets on superhydrophobic surfaces by analyzing their lowest-order axisymmetric vibrational resonances driven by vertical oscillations of the surface. Fluorescence spectra of the dye-doped droplets excited by laser light feature whisperinggallery modes (WGMs) whose spectral widths depend on the droplet vibration amplitude, thus enabling precise measurements of the droplet mechanical resonant frequency. Following droplet size determination byWGM mode-matching, we calculate the contact angles from the dependence of the measured mechanical resonant frequency on the droplet size for two surfaces with different superhydrophobicity levels, and find a good correlation with the values measured by direct imaging of millimeter-sized droplets.
Preparation and characterization of superhydrophobic surfaces based on hexamethyldisilazane-modified nanoporous alumina
(SpringerOpen, 2011) Erkey, Can; Jonas, Alexandr; Kiraz, Alper; Taşaltın, Nevin; Şanlı, Deniz; Department of Physics; Yes; College of Sciences
Superhydrophobic nanoporous anodic aluminum oxide (alumina) surfaces were prepared using treatment with vapor-phase hexamethyldisilazane (HMDS). Nanoporous alumina substrates were first made using a two-step anodization process. Subsequently, a repeated modification procedure was employed for efficient incorporation of the terminal methyl groups of HMDS to the alumina surface. Morphology of the surfaces was characterized by scanning electron microscopy, showing hexagonally ordered circular nanopores with approximately 250 nm in diameter and 300 nm of interpore distances. Fourier transform infrared spectroscopy-attenuated total reflectance analysis showed the presence of chemically bound methyl groups on the HMDS-modified nanoporous alumina surfaces. Wetting properties of these surfaces were characterized by measurements of the water contact angle which was found to reach 153.2 ± 2°. The contact angle values on HMDS-modified nanoporous alumina surfaces were found to be significantly larger than the average water contact angle of 82.9 ± 3° on smooth thin film alumina surfaces that underwent the same HMDS modification steps. The difference between the two cases was explained by the Cassie-Baxter theory of rough surface wetting.
Probing of ultrahigh optical Q-factors of individual liquid microdroplets on superhydrophobic surfaces using tapered optical fiber waveguides
(Optica Publishing Group, 2012) Jonas, Alexandr; Karadağ, Yasin; Kiraz, Alper; Mestre, Michael; Department of Physics; Yes; College of Sciences
We report measurements of ultrahigh quality factors (Q-factors) of the optical whispering-gallery modes excited via a tapered optical-fiber waveguide in single glycerol-water microdroplets standing on a superhydrophobic surface in air. Owing to the high contact angle of the glycerol-water mixture on the superhydrophobic surface (≥155°), microdroplets with the geometry of a truncated sphere minimally distorted by gravity and contact line pinning effects could be generated. Q-factors up to 2.3 × 106 were observed for such droplets with radii of 100-200 μm exposed to the ambient atmosphere in a closed chamber with controlled relative humidity. Placement of microdroplets in a constant humidity environment permitted prolonged characterization of Q-factors for individual microdroplets. We found that the Q-factors in air were stable over more than 1 h and their measured values were limited mostly by the thermally induced droplet shape fluctuations.
