Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session GG: Instability: Interfacial and Thin Films II |
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Chair: Lucien Brush, University of Washington Room: Salt Palace Convention Center 250 A |
Monday, November 19, 2007 10:30AM - 10:43AM |
GG.00001: Instability of a thin melt film Lucien Brush, Michael Beerman Lubrication theory is used to study the instability and nonlinear evolution of an ultra-thin, metallic melt film in contact with its non-premelting crystal and a gas phase. Competition between destabilizing van der Waals attractive forces and the stabilizing effect of an applied thermal gradient gives rise to finite wavelength oscillatory instability. The effect of the crystal-melt interfacial energy is shown to excite a slower-growing, longer-wavelength, stationary instability. Numerical results show the evolution of stationary and oscillatory instabilities, and the interactions between unstable modes. Linearly unstable stationary modes are shown to excite the growth of oscillatory modes that ultimately lead to rupture. [Preview Abstract] |
Monday, November 19, 2007 10:43AM - 10:56AM |
GG.00002: Laser-induced hydrodynamic instability and pattern formation in metallic nanofilms R. Sureshkumar, J. Trice, C. Favazza, R. Kalyanaraman Cost effective methodologies for the robust generation of nanoscale patterns in thin films and at interfaces are crucial in photonic, opto-electronic and solar energy harvesting applications. When ultrathin metal films are exposed to a series of short (ns) laser pulses, spontaneous pattern formation results with spatio-temporal scales that depend on the film height and thermo-physical properties of the film/substrate bilayer. Various self-organization mechanisms have been identified, including a dewetting instability due to a competition between surface tension and dispersion forces, and intrinsic and/or extrinsic thermocapillary effects. We will discuss these mechanisms as well as the evolution of surface perturbations which have been explored using experiments, linear stability analysis and nonlinear dynamical simulations (Trice et al. Phys. Rev. B, 75, 235439 (2007); Favazza et al. Appl. Phys. Lett., 91, 043105 (2007); 88, 153118 (2006)). [Preview Abstract] |
Monday, November 19, 2007 10:56AM - 11:09AM |
GG.00003: Static Stability of Helically Supported Fluid Interfaces at Zero Bond Number Jorge Bernate, David Thiessen When gravitational effects are negligible with respect to capillary effects, it is possible to stabilize an infinite channel of liquid with a helical wire. Capillary-driven flow in such minimal support structures may have applications for use in heat- or mass-transfer processes under microgravity conditions or at small scales in micro- and nano-fluidic applications. Stability issues limit the initial penetration of the meniscus into the structure as well as steady flow. The static stability of infinite-length helical interfaces is theoretically determined at zero Bond number as a function of the contact angle and two dimensionless geometric parameters. The theory predicts a minimum and maximum stable pressure and corresponding volumes at which respectively breakup and blowout of the interface occurs. An approximate theory for the equilibrium of finite-length, free-ended segments is also presented which predicts a critical value of the pitch beyond which no stable free-ended interfaces exist. Predictions of stability limits for infinite and free-ended equilibria are confirmed experimentally in a Plateau tank with satisfactory agreement. Observations of the bath fluid flow in the vicinity of the free interface suggest a screw-like flow in the creeping flow regime. High-speed imaging was used to capture the instability mechanisms. [Preview Abstract] |
Monday, November 19, 2007 11:09AM - 11:22AM |
GG.00004: The Stability of Two-Dimensional Droplets Dragged Across Surfaces Derek Bassett, Roger Bonnecaze The stability of a droplet pressed between a stationary and moving surface is studied theoretically using thin film equations including inertial, capillary and viscous forces. This so-called ``drag-a-drop'' method has been proposed as a means to suspend a droplet of high index of refraction fluid between a mask blank and lens in a laser mask writing system to greatly enhance the resolution in microelectronic lithographic mask writing. The droplet of fluid is held between the moving lens and the mask due to surface tension forces and must be to stable at high velocities and accelerations. Theoretical calculations show the limits of stability for a two-dimensional droplet. These limits are determined by several factors including the surface energies of the fluid and lens and mask surfaces, the fluid viscosity and density and the thickness of the gap between the cylindrical lens and stationary surface. The droplets are found to breakup by two different mechanisms. In the receding edge instability a thin film pulled behind the lens breaks up into a trail of smaller droplets. In an advancing edge instability, the front edge of the droplet initial shows signs of partial detachment from the lens followed by complete break-up of the attached droplet. A stability map is presented that correlates the onset of these two instabilities as a function of the dimensionless capillary and Weber numbers and compared to our previous experimental measurements. [Preview Abstract] |
Monday, November 19, 2007 11:22AM - 11:35AM |
GG.00005: Stability of finite and infinite fluid rivulets Javier Diez, Lou Kondic We discuss the stability and possible breakup of a finite fluid rivulet on a horizontal substrate under partial wetting conditions, recently considered experimentally (Euro. Phys. Lett. 77, 44001 (2007)). To better understand this configuration, we revisit the classical problem of an infinite rivulet and discuss the similarities and differences between these two problems, with particular emphasis on understanding finite size effects. The research is carried out by means of 3D simulations under lubrication approximation. Partial wetting conditions are modeled by using disjoining and conjoining pressure terms. The results show that the early stages of the instability are in agreement with the linear stability calculations, also performed in this work. The nonlinear evolution leads to the formation of isolated drops, similarly to what is observed in experiments. We compare and discuss the differences between the finite and infinite rivulets, in addition to discussing the differences between strip stability, and stability of (semi)infinite films considered recently (Phys. Fluids 19, 072107 (2007)). [Preview Abstract] |
Monday, November 19, 2007 11:35AM - 11:48AM |
GG.00006: Dynamics of water bells formed on the underside of a horizontal plate Eleanor Button, John Sader, Graeme Jameson We study the thin film flow generated when a vertical liquid jet impacts on the underside of a large horizontal plate, spreads radially to an abrupt point, and then falls of its own accord. The fluid falls in threads, which may coalesce to form a water bell. The radius of departure from the plate is seen to be strongly dependent on the flow rate of the impinging jet. The stability of the thin film flow along the plate is considered as a mechanism for the fluid's departure from the plate, and an analytical model for the departure radius is developed. When a water bell has been formed, and the flow rate is altered, many interesting shapes are produced, that are dependent on shapes at previous flow rates. We discuss the dependence of this hysteresis, and present a leading order theory for the bell shape under a regime of changing flow rate. All models are compared with experimental results spanning two orders of magnitude of viscosity. [Preview Abstract] |
Monday, November 19, 2007 11:48AM - 12:01PM |
GG.00007: Brittle-to-ductile transition for fracture in aqueous foam Shehla Arif, Jih-Chiang Tsai, Sascha Hilgenfeldt We use soap foam, a yield stress shear-thinning power-law material, as an experimental model system for failure (fracture) in crystalline condensed matter. A quasi-two-dimensional sample of foam is confined in a channel between parallel plates in a Hele-Shaw set-up, and is then stressed by injecting pressurized air. Depending on the magnitude as well as the rate of the applied air pressure, the foam yields by either brittle or ductile fracture. The former is characterized by breaking thin films and small strains, the latter involves defect formation and motion (T1 transitions) without film breakage. The millimetric bubble size and moderate crack speeds allow for a detailed dynamical study of these processes, down to the microstructure encoded in the foam geometry, rendering the interaction of material defects with the crack tip accessible. The observed rate dependence, as well as the existence of a velocity gap between the fracture modes, suggests that the model faithfully reproduces features seen in hard crystalline matter. In addition, however, foam allows us to observe the brittle-to-ductile transition dynamically within the same fracture event. Thus, the behavior on a microstructural level can be studied throughout the transition process, allowing quantification of the changing stress field around the crack tip in the material. [Preview Abstract] |
Monday, November 19, 2007 12:01PM - 12:14PM |
GG.00008: Secondary instability of a horizontally oscillating, viscous interface Shreyas Jalikop, Anne Juel When a closed vessel containing two stably stratified, immiscible liquids is oscillated in the horizontal direction, the flat interface separating the liquids can become unstable to two-dimensional 'frozen waves' driven by interfacial shear, similar to the Kelvin-Helmoltz instability. We find that the onset of the frozen waves in the experiment is accurately predicted by a two-dimensional viscous linear stability model based on Floquet theory. As the forcing acceleration is increased, the growth of the two-dimensional 'frozen waves' is followed by a subcritical bifurcation to three-dimensional oscillatory waves with a response frequency that is locked to the forcing frequency. Secondary transition to three-dimensional waves underpin the dynamics of a variety of fluid flows, e.g. the oscillatory intability of rolls in thermal convection and the formation of streamwise vortices in mixing layers. We characterize the secondary instability of our oscillating interface by comparison with these systems and discuss the physical mechanism that leads to the onset of three-dimensional waves. [Preview Abstract] |
Monday, November 19, 2007 12:14PM - 12:27PM |
GG.00009: Interfacial Instabilities in Evaporating Drops Ross Moffat, Khellil Sefiane, Omar Matar We study the effect of substrate thermal properties on the evaporation of sessile drops of various liquids. An infra-red imaging technique was used to record the interfacial temperature. This technique illustrates the non-uniformity in interfacial temperature distribution that characterises the evaporation process. Our results also demonstrate that the evaporation of methanol droplets is accompanied by the formation of wave-trains in the interfacial temperature field; similar patterns, however, were not observed in the case of water droplets. More complex patterns are observed for FC-72 refrigerant drops. The effect of substrate thermal conductivity on the structure of the complex pattern formation is also elucidated. [Preview Abstract] |
Monday, November 19, 2007 12:27PM - 12:40PM |
GG.00010: Helical instability of a rotating viscous liquid jet J.P. Kubitschek, P.D. Weidman Experimental results are presented for a rotating viscous liquid jet showing a clear preference for helical instabilities that evolve from initially planar disturbances at large rotation rates. In the ideal case of a uniformly rotating viscous liquid column with stress-free boundaries in the absence of gravity, the preferred modes of linear temporal instability are theoretically known over the entire physical domain. The relevant physical parameters are $L=\gamma $/\textit{$\rho $a}$^{3}\Omega ^{2}$ and \textit{Re} = $a^{2}\Omega $/\textit{$\nu $}, where $a$ is the column radius, $\Omega $ the uniform angular velocity and \textit{$\rho $}, \textit{$\nu $}, and $\gamma $ are fluid density, kinematic viscosity and surface tension, respectively. The theoretical results suggest that instability in different regions of $L$-\textit{Re} parameter space is dominated by three modes: the axisymmetric mode, $n\ge $ 2 planar modes, and the first $n$ = 1 spiral mode. For the rotating viscous liquid jet, experiments reveal that planar disturbances of the same mode numbers ($n\ge $ 2) spontaneously arise in the same regions of parameter space predicted by uniformly rotating viscous liquid column theory. However, these planar disturbances do not persist, but instead rapidly evolve into helical instabilities. Although fundamental differences exist between the rotating liquid jet and the uniformly rotating liquid column, some remarkable similarities associated with initial growth rates, disturbances frequencies, and mode transitions between the two systems are found. [Preview Abstract] |
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