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 GS: Drops VII |
Hide Abstracts |
Chair: James Feng, University of British Columbia Room: Long Beach Convention Center Grand Ballroom A |
Monday, November 22, 2010 8:00AM - 8:13AM |
GS.00001: Effect of neighboring particles on drop coalescence at an interface Ankur Bordoloi, Deepak Adhikari, Ellen Longmire The coalescence of a liquid drop in the presence of an adjacent solid particle or liquid drop is studied using high-speed visualization and Tomographic PIV. A drop of water/glycerin (W/G), surrounded by silicone oil of matched refractive index, is released onto an underlying W/G interface. A nylon sphere, neutrally buoyant with respect to the drop liquid, is placed adjacent to the drop. Three initial conditions are considered: the particle is wetted in W/G so that the interface maintains an angle of contact with the particle, the particle is wetted in oil so that it rests above the interface, and the particle is placed so that it maintains an angle of contact with the drop already resting above the interface. These cases are compared with that of two neighboring W/G drops. Off-axis rupture near the solid particle was found to be dominant in cases where the particle was wetted with W/G. However, when the particle was wetted with oil, the point of rupture occurred closer to the drop-axis. The film rupture in the drop is followed by retraction of the film and finally collapse of the drop. Both visualization and PIV results show that the trajectory of the collapsing drop depends on the initial contact condition as well as the rupture location. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GS.00002: Coalescent coalescence Nicolas Bremond, Hugo Dom\'ejean, J\'er\^ome Bibette Pulling apart two neighboring emulsion drops favors their coalescence. This counterintuitive phenomenon is then responsible for the propagation of coalescence among a concentrated emulsion. Indeed, the shape relaxation during the coalescence phase can induce a separation with the neighboring drops, a situation that is potentially destabilizing. We report an experimental investigation on such catastrophic phenomenon by using a microfluidic device where a calibrated two-dimensional emulsion is created and destabilized. The velocity of propagation as well as the probability of the coalescence are reported as a function of the size and the spatial distribution of the drops. We then propose a scenario for phase inversion of a concentrated emulsion based on this mechanism and discuss its efficiency by taking into account the existence of a drop size distribution. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GS.00003: Viscous to Inertial Crossover in Liquid Drop Coalescence Joseph Paulsen, Justin Burton, Sidney Nagel When two liquid drops coalesce, a dramatic topological transition occurs. We use an electrical method and high-speed imaging to probe the coalescence down to 10 ns after the drops touch. Immediately after contact, the resistance varies as $t^{-1}$ and later crosses over to $t^{-1/2}$. In the case of water drops [1], this behavior had been interpreted with a model in which coalescence occurs between slightly deformed interfaces. By varying the liquid viscosity over two decades, we conclude that at sufficiently low approach velocity where deformation is not present, the drops coalesce as spheres, but with an unexpectedly late crossover time between a regime dominated by viscous (i.e., $t^{-1}$) and one dominated by inertial (i.e., $t^{-1/2}$) effects. This interpretation is consistent with experiments in which we change the drop approach velocity and the surrounding gas pressure and molecular weight. We argue that the late crossover, not accounted for in the theory [2], is due to the flow field in the liquid and an additional length-scale present in the drop geometry. \\\ [1] S. C. Case, and S. R. Nagel, PRL ${\bf 100}$, 084503 (2008). \\\ [2] J. Eggers, J. Lister, and H. A. Stone, JFM ${\bf 401}$, 293 (1999). [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GS.00004: Mass-spring-damper dynamic system modeling for predicting drop-pair interaction outcomes Paul Van Noordt, Micah Bergman, Carlos Hidrovo In the present study, we investigate both theoretically and experimentally the process of two drops interacting through a head-on collision and the various outcomes that may result. The relationship between kinetic and surface energy of the colliding drop pair, as well as the viscosity of the intervening gaseous medium, are considered as factors that govern the outcome of the collision. A theoretical model is derived, which treats the collision process as a squeeze-film problem involving both planar and non-planar geometries. Based on the various mechanisms that influence the collision dynamics, an analogy is made between the fluidic system of liquid drops and a mechanical mass-spring-damper system. Examination of the analogous mechanical system yields an equivalent damping ratio, which is used to predict the outcome of the drop-pair collision. Our experimental setup allows drops of varying speed and size to interact with each other in a mid flight collision. The collision process is captured using high-speed photography, and the results obtained are used to validate our theoretical model and the effectiveness of our damping ratio in predicting the outcome of drop-pair collisions. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GS.00005: Modeling of droplet coalescence on non-uniform surfaces with the lattice Boltzmann method Krzysztof Kubiak, M.C.T. Wilson, J.L. Summers, N. Kapur, K. Hood Droplet coalescence is a key feature in a wide range of processes, e.g. spray coating, emulsion polymerization, inkjet printing, sintering etc. Significant progress has been achieved in experimental work on dynamic droplet coalescence, however proper and physically meaningful modeling is still a challenge. Most existing models are unable to capture the waves created on the free surfaces of droplets in the early stage of coalescence and/or dynamic oscillations of the neck between two droplets. This paper investigates droplet coalescence using a multiphase lattice Boltzmann method with a flexible wetting model. The well known Shan-Chen interparticle potential method, has been found to capture properly dynamic of sessile droplets coalescence. Oscillations of the neck present good agreement with experimental data. LBM simulations in 3D allow a comparison of the final footprint of droplet. Introduction of a non-uniform surface wettability into the model, in the form of hydrophilic patterns, helps to obtain final footprints of ellipsoidal form. Different size and distribution of patterns has been studied to analyze its influence on the coalescence process. The lattice Boltzmann method presents great potential for coalescence modelling especially on non-uniform surfaces. The dynamics of droplet coalescence can be properly simulated and the final footprint can be predicted. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GS.00006: Effects of small concentration surfactants on the coalescence of viscous drops Carolina Vannozzi Boundary integral simulations, employing Dai and Leal's code [Phys. Fluids \textbf{20, }040802 (2008)], are used to study the effects of small concentrations of insoluble surfactants C$_{s}$ on head-on collisions of two equal-sized viscous drops in a matrix of equal viscosity in a hyperbolic extensional flow, for low Reynolds numbers. The parameters were chosen to mimic the experiments of Yoon et al. [Phys. Fluids \textbf{19}, 023102 (2007)], which were performed with polymeric drops stabilized by block-copolymer insoluble surfactants in a polymer matrix, where both fluids acted as Newtonian viscous fluids. In these experiments a discontinuous transition in the coalescence process was found for low C$_{s}$ as the Capillary number Ca was increased. Thus, for Ca$>$Ca$_{c}$ a minimum surfactant concentration exists below which the system behaves like a clean interface system. Here, by varying C$_{s}$, i.e. the Marangoni number Ma, and the surface diffusivity, i.e. the interfacial Peclet number Pe$_{s}$, we explain the origin of the transition and its dependence on the parameters. The transition occurs if Pe$_{s}>$Pe$_{sc}$, Ca$>$Ca$_{c}$ and Ma$<$Ma$_{c}$. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GS.00007: Analysis of the formation of drops of a Bingham fluid Haijing Gao, Santosh Appathurai, Patrick Mcgough, Michael Harris, Osman Basaran Emulsions, dispersions, and foams are both of scientific interest and widely used in technological applications. A way to form such dispersed systems is to flow a liquid or a gas from a tube into a continuous phase of another fluid. In this talk, the dynamics of formation of drops of a Bingham fluid from a tube into a gas are studied computationally. The dynamics are governed by four dimensionless groups: Ohnseorge number, Oh (dimensionless drop viscosity), Weber number, We (square root of dimensionless flow rate), Bond number, G (ratio of gravitational to surface tension force), and yield stress parameter, Y (ratio of yield stress to capillary pressure). Tracking the evolution in time of yielded and unyielded regions in the drop is shown to be crucial for developing a good understanding of the fluid dynamics of the process. The effects of the governing dimensionless groups on the volumes of the primary drops that are formed and whether small satellites as well as large primary drops are produced are investigated. Similarities and differences between the dynamics of formation of drops of Bingham fluids and those of Newtonian fluids are also elucidated. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GS.00008: Analysis of the formation of drops of a Herschel-Bulkley fluid Patrick McGough, Santosh Appathurai, Haijing Gao, Osman Basaran Although viscoplastic liquids are widely used in technological applications, study of dynamics of drops of such liquids has received little attention to date. In this talk, the dynamics of formation of drops of a Herschel-Bulkley fluid from a tube into a gas are studied computationally and experimentally. The dynamics are governed by five dimensionless groups: Ohnesorge number, Oh (dimensionless drop viscosity), Weber number, We (square root of dimensionless flow rate), Bond number, G (ratio of gravitational to surface tension force), power-law exponent, n, and yield stress parameter, Y (ratio of yield stress to capillary pressure). Computational results are matched against experimental results. Tracking (computationally) the evolution in time of yielded and unyielded regions in the drop is shown to be crucial for developing a good understanding of the fluid dynamics of the process. The effects of the governing dimensionless groups on the volumes of the primary drops that are formed and whether small satellites as well as large primary drops are produced are investigated. Similarities and differences between the dynamics of formation of drops of Herschel-Bulkley fluids and those of Newtonian fluids are also elucidated. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GS.00009: Probing Interfacial Emulsion Stability Controls using Electrorheology Xiuyu Wang, Amy Brandvik, Vladimir Alvarado The stability of water-in-oil emulsions is controlled by interfacial mechanisms that include oil film rheology of approaching drops and the strength of drop interfaces. Film drainage is mainly a function of the continuous phase rheology. Temperature is used to regulate the viscosity of the continuous phase and hence determine its effect on emulsion stability through film drainage, in contrast with interfacial strength. In this study, one crude oil is used to formulate water-in-oil emulsions. Oil-water interfacial tension is measured to gauge other interfacial changes with temperature. The critical field value, used as proxy of emulsion stability, approaches a plateau value for each crude oil- aqueous solution pair, at sufficiently high temperature (50 $^{o}$C), which is interpreted to reflect the intrinsic drop-coating film resistance to coalescence. Interfacial tension does vary significantly with either aqueous phase composition or temperature. From comparison with previous results, we speculate that drop coating film is composed of a fraction of asphaltic compunds. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GS.00010: Stirring a Cahn-Hilliard fluid in moving microdroplets Saif A. Khan, S.H. Sophia Lee, Pengzhi Wang, Swee Kun Yap Biochemistry within living eukaryotic cells occurs in dynamic heterogeneous fluid environments containing macromolecules such as proteins, nucleic acids and sugars; most \textit{in-vitro} biochemical studies in dilute aqueous solutions do not capture this chemical and morphological complexity. Here, as an \textit{in-vitro} model for \textit{in-vivo} cellular environments, we investigate the dynamics of a phase separating aqueous polymer mixture within small moving droplets. We dispense aqueous mixtures of poly(ethylene glycol) (PEG) and dextran as droplets carried by an immiscible fluorinated oil at a microfluidic T-junction, and use high-speed optical microscopic imaging to observe dynamic phase behavior. In the static case, for off-critical compositions, this mixture separates via a spinodal mechanism into two phases- a PEG-rich phase and a dextran-rich phase. For moving drops, the polymer mixture exhibits a near continuum of flow and composition-dependent phase morphologies, from the `unmixed' static morphology to complex percolated morphologies resembling \textit{in vivo} cellular environments. We compare our measurements to previous theoretical and numerical studies of binary fluid mixing based on advective Cahn-Hilliard formulations. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700