Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H22: Drops: Heat Transfer and Evaporation I |
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Chair: Thomas Seon, CNRS Room: 604 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H22.00001: On the effect of wicking on droplet cooling Manuel Auliano, Damiano Auliano, Maria Fernandino, Pietro Asinari, Carlos Dorao Enhancing the droplet cooling is important for an efficient and safe design of thermal management applications, such as electronics, nuclear and aeronautics industry. For enhancing the droplet cooling heat transfer, it is desirable that the droplet spreads as much and as fast as possible. In this regard, tailoring the surface with micro/nanostructures is a promising approach that controls the surface wettability and the heat transfer performance. This work focuses on the effect of the wicking of super-hydrophilic stochastic Si nanowires on droplet evaporation from low to high temperatures. The research method consisted in fabricating Si nanowires with different heights and characterizing the wickability of the surfaces prior to the heat transfer experiments. The evaporation performance of the processed samples has been discussed in terms of the droplet evaporation time. A significant reduction of the evaporation time and shift of the Leidenfrost point were observed and attributed to the strong wicking provided by the nanostructured surfaces. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H22.00002: A Switchable Wettability Surface for Condensation Heat Transfer Manipulation Jonathan Ludwicki, Paul Steen The development of variable heat transfer devices is critical to enabling human exploration missions beyond low-Earth orbit (2015 NASA Technology Roadmaps). Here, we show the ability to manipulate condensation heat transfer performance via a surface with temperature-dependent wettability. Specifically, a nanostructured poly(N-isopropylacrylamide) surface was developed to switch the surface wettability between hydrophobicity and hydrophilicity via temperature stimulus. The hydrophobic surface conformation produces flooded dropwise condensation, which shows a two-times larger heat transfer coefficient versus filmwise condensation in the hydrophilic state. Such surfaces hold promise as a means to control heat transfer during flow condensation in spacecraft two-phase flow thermal control systems. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H22.00003: Freezing a rivulet Antoine Monier, Axel Huerre, Christophe Josserand, Thomas Séon We study experimentally the solidification of a water rivulet flowing down an inclined plane cooled to subzero temperatures. The growth of the ice is described by two successive regimes with different dynamics. First, the ice grows as if the water was not flowing, well described by a classical 1D Stefan problem. Then, thermal convection from the constant water supply starts to play a role in the solidification process. Finally, the system reaches a stationary state where water flows on a peculiar ice structure. The surprising linear geometry of this structure, with distance from the water injection point, is explained trough scaling arguments. The physical explanation relies on the growth of the water boundary layer with respect to the finite size of the water rivulet. When the thickness of the thermal boundary layer reaches the free surface, the surface temperature of the water decreases with the plane position, allowing us to recover the linear behavior of the ice structure. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H22.00004: Ice patterns formation in freezing drop impacts Thomas Seon, Virgile Thievenaz, Christophe Josserand We investigate the different shapes taken by a water drop frozen during its impact on a cold surface. The capillary hydrodynamics of a water film dewetting on ice, coupled with its vertical solidification, is quantitatively characterized. This allows us to understand and predict the formation of the emerging patterns. Finally, we show that this experiment enables a measurement of the contact angle of water on ice, whose value is still debated. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H22.00005: Frost pattern on surfaces with millimetric serrated features Kyoo-Chul Park, Yuehan Yao, Tom Zhao, Emma Feldman, Christian Machado, Neelesh Patankar Numerous studies have focused on a low surface energy coating and a micro/nanoscale surface texture to design functional surfaces that delay frost formation and reduce ice adhesion. However, the scientific challenges for long-term icephobic surfaces have not been fully addressed because of degradation such as mechanical wearing. Inspired by the suppressed frost formation on concave regions of natural leaves, here we report findings on the frosting process on surfaces with various serrated structures. Dropwise condensation, the first stage of frosting, is enhanced on the peaks and suppressed in the valleys when the wavy surface is exposed to humid air, causing frosting to initiate from the peak. The condensed droplets in the valley are then evaporated, resulting in a non-frost band. The effects of surface topography on the frost pattern are systematically studied by varying the serrated geometry defined as the vertex angle, and numerically modeling the spatial distribution of diffusion flux of water vapor on the wavy surface. Under different ambient humidity levels, the magnitudes of diffusion flux at the non-frost boundaries of the surfaces are nearly identical, implying that the critical value of diffusion flux is the key to understanding the non-frost pattern. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H22.00006: Final fate of a Leidenfrost droplet: Explosion or takeoff Sijia Lyu, Varghese Mathai, Yujie Wang, Benjamin Sobac, Pierre Colinet, Detlef Lohse, Chao Sun When a liquid droplet is placed on a very hot solid, it levitates on its own vapor layer, a phenomenon called the Leidenfrost effect. Although the mechanisms governing the droplet's levitation have been explored, not much is known about the fate of the Leidenfrost droplet. Here we report on the final stages of evaporation of Leidenfrost droplets. While initially small droplets tend to take off, unexpectedly, the initially large ones explode with a crack sound. We interpret these in the context of unavoidable droplet contaminants, which accumulate at the droplet-air interface, resulting in reduced evaporation rate, and contact with the substrate. We validate this hypothesis by introducing controlled amounts of microparticles and reveal a universal 1/3-scaling law for the dimensionless explosion radius versus contaminant fraction. Our findings open up new opportunities for controlling the duration and rate of Leidenfrost heat transfer and propulsion by tuning the droplet's size and contamination. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H22.00007: Pattern formation from dried drops can be used to identify adulterated medicines Yojana Carreon, Ivan Cipriano-Urbano, Francisco Solorio-Ordaz, Jorge González-Gutiérrez, Roberto Zenit It is estimated that in Europe, from 2009 to 2012, 12.8 million of patients were exposed to counterfeit medicines. In Mexico, it is suspected that, between 2006 and 2013, many children suffering from cancer received distilled water instead of chemotherapeutic drugs. Fluid mechanics can be used to prevent such vile acts. Here we propose a simple method to detect the authenticity of medical substances, considering the patterns formed by dried drops. The method is based on the structural analysis of patterns generated after the solvent of a drop of liquid medicine has evaporated over a solid substrate. We exploit the capillary flux and the ionic interactions as a natural aggregation mechanism to form patterns, which reveals unique structures for each active ingredient or drug. We demonstrate that the first-order statistics (FOS) and gray level co-occurrence matrix (GLCM) are sufficient metrics to distinguish among deposits generated by pure and adulterated drugs with an accuracy greater than 90{\%}. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H22.00008: Crystal Critters: Growth and ejection of crystals from heated, superhydrophobic surfaces Samantha McBride, Henri Louis Girard, Kripa Varanasi Evaporating a drop of a volatile liquid containing a non-volatile solute leads to crystallization of said solute due to rising concentrations exceeding the solubility limit. When a drop of salty water is evaporated on a hydrophobic surface, ``salt globes'' that mirror the shape of the drop form due to nucleation at the air/water interface. Here, we present an unusual phenomenon in which salt globes grown from an evaporating drop on a heated superhydrophobic surface proceed to self-eject from that surface via growth of crystalline tubules. We show that this phenomenon is due to a specific superhydrophobic texture that combines minimal contact points and an assortment of channels that allow vapor to escape. A large temperature gradient across the height of the drop concentrates vaporization near the substrate, and escaping vapor at contact points between substrate and liquid leads to growth of crystalline tubules. These tubules grow into "legs," causing the entire salt globe -- and any remaining water -- to lift off from the surface. We call these structures composed of salt globes balanced on tubule legs ``crystal critters'' due to their resemblance to biological forms. Following complete evaporation, the crystal critters have minimal contact with the substrate and can be easily removed; and could find application for anti-fouling surfaces for spray heat exchangers. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H22.00009: Drop impact on a coated inclined surface Suhas Rao Tamvada, Varun Kulkarni, Nikhil Subhash Shirdade, Gaurav Kulkarni, Sushant Anand The impact of a drop onto a surface is useful in assessing the water repelling attributes of substrates with a given wettability. Most studies have focused on droplet impact on horizontal flat surfaces however oblique impact of a droplet onto a surface, representing more accurate scenarios such as impact onto turbine blades leading to ice accretion and pesticide spraying has not been investigated. To this end we study the gentle deposition of a droplet at different temperatures on paraffin wax coated on an inclined copper surface. We measure the distance and average velocity of droplet until it comes to a halt to evaluate the role of the impact conditions and surface inclination on the observed sliding behavior. As we increase droplet temperature enhanced melting of the wax layer promotes sliding of the droplet to larger distances eventually being arrested by the viscous drag due to the underlying melted wax layer. Of significant interest is the deviation from the typical stick-slip motion reported on inclined hydrophobic surfaces. Our studies evaluate the efficacy of wax coatings in promoting self-cleaning on inclined surfaces and their possible use in applications which demand expeditious shedding of droplets. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H22.00010: Stability of evaporating sessile drops comprising binary mixtures Adam Williams, George Karapetsas, Pedro Saenz, Omar Matar, Khellil Sefiane, Prashant Valluri Spreading and evaporation of a binary mixture sessile drop is a highly dynamic and complex process governed by the interplay between capillary stress, evaporation, hydrodynamic flow, mass diffusion and surface tension, with both thermal and solutal Marangoni stresses also present. We examine the behaviour and stability of volatile wetting ethanol-water drops deposited onto heated substrates using both experiments and modelling. We take a one-sided approach, utilising lubrication theory to obtain a base state before assessing stability by performing a linear stability analysis evoking the quasi-steady-state approximation. Evolution equations are derived for the interface height, temperature and concentration fields, assuming that the mixture comprises two ideally mixed volatile components with a surface tension linearly dependent on both temperature and concentration. Singularity at the contact line is avoided by releasing the drop over a precursor film. Results from both simulations and experiments indicate that concentration gradients in binary drops give rise to super-spreading, and, at high ethanol concentrations, contact line instability. Our initial stability analysis also confirms that the process is highly unstable. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H22.00011: Dynamics of evaporating and non-evaporating drops falling in air Jorge Chavarria, Saul Piedra, Juan Arcenegui-Troya, Lilia Margarita Hernandez, Guillermo Hernandez-Cruz, A.Alfonso Castrejon-Pita, Eduardo Ramos We report experimental observations of the dynamics of evaporating and non-evaporating drops falling in the air under the influence of gravity. We use water and acetone drops of approximately 1 mm initial diameter. The experimental rig allows us to observe and acquire the motion for more than 2 m which corresponds to almost one second of flight-time and 80\% of its terminal velocity. Under these conditions, evaporation reduces the initial volume by approximately 50\% when acetone is used. We find that the motion is rectilinear up to the point where the Reynolds number is 190 and 220 for evaporating and non-evaporating drops respectively; we suggest that under this regime the motion becomes helicoidal in both cases. It is observed that this Reynolds number coincides with the formation of hairpin vortices in the wake. The geometrical properties of the wake of the evaporating drops are visualized using a Schlieren technique. We developed a simple model to predict the position as a function time of evaporating and non-evaporating falling drops in rectilinear motion regime. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H22.00012: Drying water droplets: Suppression of the coffee-stain effect by letting them dry on a thin oil film Yaxing Li, Christian Diddens, Tim Segers, Herman Wijshoff, Michel Versluis, Detlef Lohse We systematically study the evaporation of a water microdroplet put on a thin oil film, both experimentally and theoretically. First, the absence of an intercalated film between droplets and substrates is demonstrated by interferometry. The interfacial energies between the droplet, the oil film and the solid surface are the key parameters to determine the wetting characteristics. During evaporation, we measure the flow field with $\mu$PIV, which shows that it is controlled by the contact line behavior and the wetting state of the film with the droplet. Once the microdroplet contains particles, they accumulate during the evaporation process. We experimentally find that the final deposit of the particles is determined by the flow and by the movement of the contact line. We derive an analytical expression for the radial velocity profile in the flow field near the substrate, which proves that the hindering of evaporation at the rim of the droplet by the non-volatile oil meniscus prevents the flow towards the edge, and therefore suppresses the ``coffee-stain” effect. We finally demonstrate that the final particle deposition can be manipulated by tuning the surface energy of the droplet by adding a specific amount of a surfactant. [Preview Abstract] |
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