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Bouncing or sticky droplets: impalement transitions on superhydrophobic micropatterned surfaces  [PDF]
Denis Bartolo,Farid Bouamrirene,Emilie Verneuil,Axel Buguin,Pascal Silberzan,Sebastien Moulinet
Physics , 2005, DOI: 10.1209/epl/i2005-10522-3
Abstract: When a liquid drops impinges a hydrophobic rough surface it can either bounce off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel droplets). The analysis of drop impact and quasi static ''loading'' experiments on model microfabricated surfaces allows to clearly identify the forces hindering the impalement transitions. A simple semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir non-wetting states. Motivated by potential applications in microfluidics and in the fabrication of self cleaning surfaces, we finally propose some guidelines to design robust superhydrophobic surfaces.
Dynamics of Ferrofluidic Drops Impacting Superhydrophobic Surfaces  [PDF]
D. A. Bolleddula,H. E. Dillon,A. Alliseda,P. Bhosale,J. C. Berg
Physics , 2010,
Abstract: This is a fluid dynamics video illustrating the impact of ferrofluidic droplets on surfaces of variable wettability. Surfaces studied include mica, teflon, and superhydrophobic. A magnet is placed beneath each surface, which modifies the behavior of the ferrofluid by applying additional downward force apart from gravity resulting in reduced droplet size and increased droplet velocity. For the superhydrophobic droplet a jetting phenomena is shown which only occurs in a limited range of impact speeds, higher than observed before, followed by amplified oscillation due to magnetic field as the drop stabilizes on the surface.
How water droplets evaporate on a superhydrophobic substrate  [PDF]
Hanneke Gelderblom,Alvaro G. Marin,Hrudya Nair,Arie van Houselt,Leon Lefferts,Jacco H. Snoeijer,Detlef Lohse
Physics , 2010, DOI: 10.1103/PhysRevE.83.026306
Abstract: Evaporation of water droplets on a superhydrophobic substrate, on which the contact line is pinned, is investigated. While previous studies mainly focused on droplets with contact angles smaller than 90^\circ, here we analyze almost the full range of possible contact angles (10^\circ -150^\circ). The greater contact angles and pinned contact lines can be achieved by the use of superhydrophobic Carbon Nanofiber substrates. The time-evolutions of the contact angle and the droplet mass are examined. The experimental data is in good quantitative agreement with the model presented by Popov (Physical Review E 71, 2005), demonstrating that the evaporation process is quasi-static, diffusion-driven, and that thermal effects play no role. Furthermore, we show that the experimental data for the evolution of both the contact angle and the droplet mass can be collapsed onto one respective universal curve for all droplet sizes and initial contact angles.
Modelling droplets on superhydrophobic surfaces: equilibrium states and transitions  [PDF]
A. Dupuis,J. M. Yeomans
Physics , 2005, DOI: 10.1021/la047348i
Abstract: We present a lattice Boltzmann solution of the equations of motion describing the spreading of droplets on topologically patterned substrates. We apply it to model superhydrophobic behaviour on surfaces covered by an array of micron-scale posts. We find that the patterning results in a substantial increase in contact angle, from $110^o$ to $156^o$. The dynamics of the transition from drops suspended on top of the posts to drops collapsed in the grooves is described.
Adhesion Mechanism of Water Droplets on Hierarchically Rough Superhydrophobic Rose Petal Surface
Hannu Teisala,Mikko Tuominen,Jurkka Kuusipalo
Journal of Nanomaterials , 2011, DOI: 10.1155/2011/818707
Abstract: Extremely hydrophobic surfaces, on which water droplets sit in a spherical shape leaving air entrapped into the roughness of the solid, are often called superhydrophobic. Hierarchically rough superhydrophobic surfaces that possess submicron scale fine structures combined with micron scale structures are generally more hydrophobic, and water droplet adhesion to those surfaces is lower in comparison with surfaces possessing purely micrometric structures. In other words, usually a fine structure on a superhydrophobic surface reduces liquid-solid contact area and water droplet adhesion. Here we show that this does not apply to a high-adhesive superhydrophobic rose petal surface. Contrary to the present knowledge, the function of the fine structure on the petal surface is to build up the high adhesion to water droplets. Understanding of the specific adhesion mechanism on the rose petal gives insight into an interesting natural phenomenon of simultaneous superhydrophobicity and high water droplet adhesion, but, in addition, it contributes to more precise comprehension of wetting and adhesion mechanisms of superhydrophobic surfaces overall.
A study on the dynamic behaviors of water droplets impacting nanostructured surfaces
Geunjae Kwak,Dong Woog Lee,In Seok Kang,Kijung Yong
AIP Advances , 2011, DOI: 10.1063/1.3662046
Abstract: We have investigated the influence of impact velocity and intrinsic surface wettability of nanostructures on the impact dynamic behaviors of water droplets on nanostructure surfaces. Nanowires array surfaces with tunable wettabilities ranging from superhydrophilic to superhydrophobic were fabricated by the deposition of surface modifiers differing in alkyl chain length. The transition criteria of rebound/wetting state and rebound/splashing state based on the relationship between the Webber (We) number and the surface free energy were determined. We have confirmed that the critical We number that determines the transition of the rebound/wetting increased as surface energy decreased. Additionally, the We number at which fragmentation occurred on our superhydrophobic surface was relatively low compared to previously reported values.
Extreme resistance of super-hydrophobic surfaces to impalement: reversible electrowetting related to the impacting/bouncing drop test  [PDF]
P. Brunet,F. Lapierre,V. Thomy,Y. Coffinier,R. Boukherroub
Physics , 2008, DOI: 10.1021/la801268v
Abstract: The paper reports on the comparison of the wetting properties of super-hydrophobic silicon nanowires (NWs), using drop impact impalement and electrowetting (EW) experiments. A correlation between the resistance to impalement on both EW and drop impact is shown. From the results, it is evident that when increasing the length and density of NWs: (i) the thresholds for drop impact and EW irreversibility increase (ii) the contact-angle hysteresis after impalement decreases. This suggests that the structure of the NWs network could allow for partial impalement, hence preserving the reversibility, and that EW acts the same way as an external pressure. The most robust of our surfaces show a threshold to impalement higher than 35 kPa, while most of the super-hydrophobic surfaces tested so far have impalement threshold smaller than 10 kPa.
Microstructured surfaces engineered using biological templates: a facile approach for the fabrication of superhydrophobic surfaces
Journal of the Serbian Chemical Society , 2008,
Abstract: The fabrication of microstructured surfaces using biological templates was investigated with the aim of exploring of a facile and low cost approach for the fabrication of structured surfaces with superhydrophobic properties. Two soft lithographic techniques, i.e., replica moulding and nano-imprinting, were used to replicate the surfaces of a biological substrate. Leaves of the Agave plant (Agave attenuate), a cost-free biological template, were used as a model of a biosurface with superhydrophobic properties. The replication process was performed using two polymers: an elastomeric polymer, poly(dimethylsiloxane) (PDMS), and a polyurethane (PU) based, UV-curable polymer (NOA 60). In the first replication step, negative polymer replicas of the surface of leaves were fabricated, which were used as masters to fabricate positive polymer replicas by moulding and soft imprinting. The pattern with micro and nanostructures of the surface of the leaf possesses superhydrophobic properties, which was successfully replicated into both polymers. Finally, the positive replicas were coated with a thin gold film and modified with self-assembled monolayers (SAMs) to verify the importance of the surface chemistry on the hydrophobic properties of the fabricated structures. Wetting (contact angle) and structural (light microscopy and scanning electron microscopy) characterisation was performed to confirm the hydrophobic properties of the fabricated surfaces (> 150°), as well as the precision and reproducibility of the replication process.
Droplet bouncing and skipping on microstructured hydrophobic surfaces  [PDF]
Rajat Saksena,Andrew Cannon,Julio Manuel Barros Jr.,William Paul King,Kenneth T. Christensen
Physics , 2009,
Abstract: The impact of a jet of droplets upon surfaces of varying hydrophobicity is studied via high-speed imaging. Microstructures on silicone surfaces consisting of cylindrical pillars of varying sizes and spacings are utilized to enhance hydrophobicity. Comparison of droplet motion after impact with these microstructured surfaces is contrasted with that noted for plain glass (hydrophilic) and flat silicone surfaces. Fluid dynamics videos are captured at 6000 fps and played back at 30 fps over a field of view of 1.35 cm (height) X 2.7 cm (width) that is back-illuminated with an LED array for 800-micron diameter droplets impinging the surfaces at 2.5 m/s with an angle of incidence of 38 degrees (relative to the surface). Bouncing of droplets after impact is not apparent for the glass and unstructured silicone cases, though many droplets were observed to roll along the surface in the latter case which is consistent with its slightly hydrophobic nature. In contrast, droplets were found to both skip and bounce upon impacting the microstructured surfaces which indicates a significant enhancement in hydrophobicity due to these surface features.
Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces  [PDF]
Hrudya Nair,Hendrik J. J. Staat,Tuan Tran,Arie van Houselt,Andrea Prosperetti,Detlef Lohse,Chao Sun
Physics , 2013,
Abstract: Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the temperature of the surface and kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher temperature compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher temperatures than on smooth surfaces. We also show that, as long as the impact is in the film boiling regime, the spreading factor of impacting droplets follows the same $\We^{3/10}$ scaling (with $\We$ the Weber number) found for smooth surfaces, which is caused by the vapor flow underneath the droplet.
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