Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session CS: Drops III: Evaporation |
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Chair: John Saylor, Clemson University Room: Long Beach Convention Center Grand Ballroom A |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CS.00001: Hydrodynamics and evaporation of a sessile drop of capillary size Lev Barash, Terry Bigioni, Valerii Vinokur, Lev Shchur Several dynamical stages of the Marangoni convection of an evaporating sessile drop are obtained. We jointly take into account the hydrodynamics of an evaporating sessile drop, effects of the thermal conduction in the drop and the diffusion of vapor in air. The stages are characterized by different number of vortices in the drop and the spatial location of vortices. During the early stage the array of vortices arises near a surface of the drop and induces a non-monotonic spatial distribution of the temperature over the drop surface. The number of near-surface vortices in the drop is controlled by the Marangoni cell size, which is calculated similar to that given by Pearson for flat fluid layers. The number of vortices quickly decreases with time, resulting in three bulk vortices in the intermediate stage. The vortex structure finally evolves into the single convection vortex in the drop, existing during about 1/2 of the evaporation time. Simulation results agree well with the data of evaporation rate measurements for the toluene drop. Computed dependence of contact angle of colloidal sessile droplet during evaporation coincide well with available experimental time dependence of angle of nanocrystal superlattice domains orientation. [Preview Abstract] |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CS.00002: Contact Angle Hysteresis of an Evaporating Droplet Jeongeun Ryu, Sanghyun Lee, Kwan Hyoung Kang Contact angle and contact line dynamics of an evaporating droplet exhibit interesting features. During the evaporation process, contact angle decreases with time and becomes smaller than the static receding contact angle; and contact line is deformed and locally pinned. In this work, we attempted to explain the phenomenon in terms of contact angle hysteresis. We conjecture that impurities inside a droplet promote the heterogeneity of surface and induce the rapid decrease of contact angle lower than the receding contact angle. Based on our conjecture, we investigate experimentally and theoretically the effect of impurities as a source of contact angle hysteresis and subsequent change of contact angle. [Preview Abstract] |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CS.00003: Drying drops of blood David Brutin, Benjamin Sobac, Boris Loquet, Jos\'e Sampol The drying of a drop of human blood is fascinating by the complexity of the physical mechanisms that occur as well as the beauty of the phenomenon which has never been previously evidenced in the literature. The final stage of full blood evaporation reveals for a healthy person the same regular pattern with a good reproducibility. Other tests on anemia and hyperlipidemic persons were performed and presented different patterns. By means of digital camera, the influence of the motion of red blood cells (RBCs) which represent about 50{\%} of the blood volume, is revealed as well as its consequences on the final stages of drying. The mechanisms which lead to the final pattern of dried blood drops are presented and explained on the basis of fluid and solid mechanics in conjunction with the principles of hematology. Our group is the first to evidence that the specific regular patterns characteristic of a healthy individual do not appear in a dried drop of blood from a person with blood disease. Blood is a complex colloidal suspension for which the flow motion is clearly non-Newtonian. When drops of blood evaporate, all the colloids are carried by the flow motion inside the drop and interact. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CS.00004: Rush hour for particles suspended in evaporating drops Hanneke Gelderblom, \'Alvaro G. M\'arin, Jacco H. Snoeijer, Detlef Lohse In the late nineties Deegan et al. explained the formation contact-line deposits in a drying sessile droplet suspension of particles (Nature 389 (1997), Physical Review E 60, (2000)). It was found that if there is evaporation from the drop edge while the contact line is pinned, liquid and particles are dragged towards the contact line creating the well known \emph{coffee-stain ring}. Here, we analyze this process in detail by measuring the velocity field inside an evaporating drop using $\mu$-PIV. It was found that most of the particle transport occurs in the last moments of the droplet life-time. This rush explains the different characteristic packing of the particles in the layers of the ring, which is much more ordered in the thin outer part than in the thick inner one, since almost all particles arrive at the end. The rush-hour behavior of particles in evaporating drops can be attributed to the vanishing of the contact angle and follows from mass conservation. [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CS.00005: Surface wettability and triple line behavior controlled by nano-coatings: effects on the sessile drop evaporation Benjamin Sobac, David Brutin, Jer\^ome Gavillet Sessile drop evaporation is a phenomenon commonly came across in nature or in industry with cooling, paintings or DNA mapping. However, the evaporation of a drop posed on a substrate is not completely understood due to the complexity of the problem. Here we investigate, with several nano-coating of the substrate (SiOx, SiOc and CF), the wettability and the triple line dynamic of a sessile drop under natural phase change. The experiment consists in analyzing simultaneously the kinetics of evaporation, internal thermal motion and heat and mass transfer. Measurements of temperature, heat-flux and visualizations with visible and infrared cameras are performed. The dynamic of the evaporative heat flux appears clearly different for a drop evaporating in pinned mode than in receding mode. Moreover, the kinetics of evaporation, the internal flow structure and the evaporative heat flux are drastically influenced by the wettability the substrate. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CS.00006: Diffusion-Controlled Evaporating Stationary Meniscus in a Channel Jean-Pierre Njante, Stephen Morris Isochemical liquid evaporates into a mixture of its own vapor and an inert component. On one wall, the contact line is pinned; the other wall is perfectly wetted. These walls are at uniform temperature $T_o$ equalling that of the distant gas. Liquid evaporates because the partial pressure $p_\infty^v$ of the distant vapor is less than the saturation pressure $\textsl {P}$ evaluated at $T_o$ and pressure $p_\infty^\ell$ of the distant liquid. Evaporation draws liquid into the contact region; near the wetted wall, the resulting pressure differences distorts the interface, creating an apparent contact angle. $\theta$ is a flow property and increases with the control parameter $\textsl{P}(p_\infty^\ell,T_o) - p_\infty^v$. As a preliminary to finding $\theta$, we prove the following; $(a)$ The system is effectively isothermal; though evaporation induces liquid temperature differences, they are kinetically negligible. $(b)$ Whenever the continuum approximation holds within the gas, diffusion is rate-limiting. As a result, liquid and vapor at the interface are in local thermodynamic equilibrium; the vapor partial pressure is related to liquid pressure by Kelvin's equation $p^v = \textsl {P}(p^\ell,T_o)$. Given $(a)$ and $(b)$, the film thickness $h (x)$, is determined by a system comprising of the steady state diffusion equation for $p^v(x,y)$, the lubrication equation for $p^\ell(x)$, and the augmented Young-Laplace equation for $h$. These equations are coupled by Kelvin's equation. We use our solution to address the corresponding problem for a droplet on a substrate. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CS.00007: An evaporation model of multicomponent solution drops Silvana Sartori, Amable Li\~n\'an, Juan C. Lasheras Solutions of polymers are widely used in the pharmaceutical industry as tablets coatings. These allow controlling the rate at which the drug is delivered, taste or appearance. The coating is performed by spraying and drying the tablets at moderate temperatures. The wetting of the coating solution on the pill's surface depends on the droplet Webber and Re numbers, angle of impact and on the rheological properties of the droplet. We present a model for the evaporation of multicomponent solutions droplets in a hot air environment with temperatures substantially lower than the boiling temperature of the solvent. As the liquid vaporizes from the surface the fluid in the drop increases in concentration, until reaching its saturation point. After saturation, precipitation occurs uniformly within the drop. As the surface regresses, a compacting front formed by the precipitate at its maximum packing density advances into the drop, while the solute continues precipitating uniformly. This porous shell grows fast due to the double effect of surface regression and precipitation. The evaporation rate is determined by the rates at which heat is transported to the droplet surface and at which liquid vapor diffuses away from it. When the drop is fully compacted, the evaporation is drastically reduced. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CS.00008: Deposition of bi-dispersed particles in inkjet-printed evaporating colloidal drops Ying Sun, Abhijit Joshi, Viral Chhasatia In this study, the deposition behaviors of inkjet-printed evaporating colloidal drops consisting of bi-dispersed micro and nano-sized particles are investigated by fluorescence microscopy and SEM. The results on hydrophilic glass substrates show that, evaporatively-driven outward flow drives the nanoparticles to deposit close to the pinned contact line while an inner ring deposition is formed by microparticles. This size-induced particle separation is consistent with the existence of a wedge-shaped drop edge near the contact line region of an evaporating drop on a hydrophilic substrate. The replenishing evaporatively-driven flow assembles nanoparticles closer to the pinned contact line forming an outer ring of nanoparticles and this particle jamming further enhances the contact line pinning. Microparticles are observed to form an inner ring inside the nano-sized deposits. This size-induced particle separation presents a new challenge to the uniformity of functional materials in bioprinting applications where nanoparticles and micro-sized cells are mixed together. On the other hand, particle self-assembly based on their sizes provides enables easy and well-controlled pattern formation. The effects of particle size contrast, particle volume fraction, substrate surface energy, and relative humidity of the printing environment on particle separation are examined in detail. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CS.00009: Evaporation of a sessile droplet: Inside the coffee stain Anna Hoang, Guillaume Berteloot, Adrian Daerr, Pirouz Kavehpour, Francois Lequeux, Laurent Limat The deposition of uniform layers of colloids on a solid surface is a major challenge for several industrial processes such as glass surface treatment and creating optical filters. A possible strategy involves the deposition of the colloids behind a contact line that recedes due to hydrodynamic reasons and evaporation (drying). We have investigated a drop of colloidal suspension evaporating on a flat surface where the contact line remains strongly pinned on the surface. We have observed that the deposit grows from the contact line following a $t^{\frac{2}{3}}$ law and then accelerates with surprising spatial and temporal modulations. The power law can be recovered by a ballistic model, in which the particles are driven to contact line by the evaporation field that diverges near the contact line. [Preview Abstract] |
Sunday, November 21, 2010 2:57PM - 3:10PM |
CS.00010: Deposits of drying drops of a nanotube suspension Minerva Vargas, Caleb Limon, Oscar Sarmiento, Guillermo Hernandez-Cruz, Eduardo Ramos, Marina Rincon We have made observations of the pattern formed by deposits of an evaporating sessile drop of a carbon nanotube suspension. The nanotubes are chains of carbon molecules, 2 nm diameter and approximately 15 micrometers long.The suspension concentration is 0.25 mg/ml and initially, the drops volume is 2 $\mu$l. Nanotubes are transported by the flow generated by evaporation at the surface of the drops and the resulting patterns are the result of the drag of the filaments by the fluid motion. The pattern observed is composed of a circular band with several (order ten) spots with higher concentration of nanotubes. Also, the inner rim of the band displays a higher concentration of nanotubes. In contrast to similar observations where the suspensions are prepared with microspheres, no ring formation at the outer edge of the initial footprint of the drop (coffee effect) is clearly identified. Our observations are interpreted in terms of existing theories. [Preview Abstract] |
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