Browsing by Subject "Droplet evaporation"

Now showing 1 - 4 of 4

Open Access
Capillary origami as a new method for printing nanoparticles on 3D surfaces
(Chemical and Biological Microsystems Society, 2016) Özkazanç, Gökçe; Erdem, Emine Yegan
We introduce capillary origami as a new method to print nanoparticles on curved, three dimensional polymer surfaces. When a liquid droplet containing nanoparticles is placed on a thin PDMS (polydimethylsiloxane) membrane, as the droplet evaporates, it folds the surface and leaves nanoparticles on it. We produced curved surfaces covered with a layer of magnetic iron-oxide (Fe3O4) nanoparticles with this method. These coated surfaces can be moved in a magnetic field.
Open Access
An iterative solution approach to coupled heat and mass transfer in a steadily fed evaporating water droplet
(American Society of Mechanical Engineers, 2019) Akkuş, Y.; Çetin, Barbaros; Dursunkaya, Z.
Inspired by the thermoregulation of mammals via perspiration, cooling strategies utilizing continuously fed evaporating droplets have long been investigated in the field, yet a comprehensive modeling capturing the detailed physics of the internal liquid flow is absent. In this study, an innovative computational model is reported, which solves the governing equations with temperature-dependent thermophysical properties in an iterative manner to handle mass and heat transfer coupling at the surface of a constant shape evaporating droplet. Using the model, evaporation from a spherical sessile droplet is simulated with and without thermocapillarity. An uncommon, nonmonotonic temperature variation on the droplet surface is captured in the absence of thermocapillarity. Although similar findings were reported in previous experiments, the temperature dip was attributed to a possible Marangoni flow. This study reveals that buoyancy-driven flow is solely responsible for the nonmonotonic temperature distribution at the surface of an evaporating steadily fed spherical water droplet.
Open Access
Organically modified silica nanostructures based functional coatings for practical applications
(2015) Tuvshindorj, Urandelger
In the past decades, the fabrication of superhydrophobic surfaces have received considerable attention due to the variety of potential applications ranging from biology to industry. Although significant progress has been made in their fabrication and design, there is still need to solve some problems in real-life use of these coatings, such as low stability against external pressure, lack of long term robustness, challenges in presice control over the degree of wettability and the need for facile fabrication methods. In this context, this thesis seeks simple solutions for mentioned problems based on organically modified silica superhyrophobic coatings. First, we investigate the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer micro-porous coating with a double-layer micro/nanoporous coating. The stability of the Cassie state in coatings were investigated with droplet compression and evaporation experiments, where external pressures as high as a few thousand Pa are generated at the interface. A droplet on a microporous coating gradually transformed to the Wenzel state with increasing pressure. The resistance of the micro/nano-porous surfaces against Wenzel transition during the experiments were higher than microporous single-layer coating and even higher than leaves of Lotus; prevalent natural superhydrophobic surface. Then, we reported a facile method for the preparation hydrophilic patterns on the superhydrophobic ormosil surfaces. On the defined areas of the superhydrophobic ormosil coatings, wetted micropatterns were produced using Ultraviolet/Ozone (UV/O) treatment which modifies the surface chemistry from hydrophobic to hydrophilic without changing the surface morphology. The degree of wettability of the patterns can be precisely controlled depending on the UV/O exposure duration and extremely wetted spots with water contact angle (WCA) of nearly 0º can be obtained. The ormosil coatings and modified surfaces preserve their wettability for months at room conditions. Furthermore, we demonstrated selective and controlled adsorption of protein and adhesion of bacteria on the superhydrophilic patterns which could be potentially used for biological applications.
Open Access
A theoretical framework for comprehensive modeling of steadily fed evaporating droplets and the validity of common assumptions
(Elsevier, 2020) Akkuş, Y.; Çetin, Barbaros; Dursunkaya, Z.
A theoretical framework is established to model the evaporation from continuously fed droplets, promising tools in the thermal management of high heat flux electronics. Using the framework, a comprehensive model is developed for a hemispherical water droplet resting on a heated flat substrate incorporating all of the relevant transport mechanisms: buoyant and thermocapillary convection inside the droplet and diffusive and convective transport of vapor in the gas domain. At the interface, mass, momentum, and thermal coupling of the phases are also made accounting for all pertinent physical aspects including several rarely considered interfacial phenomena such as Stefan flow of gas and the radiative heat transfer from interface to the surroundings. The model developed utilizes temperature dependent properties in both phases including the density and accounts for all relevant physics including Marangoni flow, which makes the model unprecedented. Moreover, utilizing this comprehensive model, a nonmonotonic interfacial temperature distribution with double temperature dips is discovered for a hemispherical droplet having internal convection due to buoyancy in the case of high substrate temperature. Proposed framework is also employed to construct several simplified models adopting common assumptions of droplet evaporation and the computational performance of these models, thereby the validity of commonly applied simplifying assumptions, are assessed. Benchmark simulations reveal that omission of gas flow, i.e. neglecting convective transport in gas phase, results in the underestimation of evaporation rates by 23–54%. When gas flow is considered but the effect of buoyancy is modeled using Boussinesq approximation instead of assigning temperature dependent density throughout the gas domain, evaporation rate can be underestimated by up to 16%. Deviation of simplified models tends to increase with increasing substrate temperature. Moreover, presence of Marangoni flow leads to larger errors in the evaporation rate prediction of simplified models.