Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R16: Flow Instability: Interfacial and Thin Films V |
Hide Abstracts |
Chair: Lou Kondic, New Jersey Institute of Technology Room: 204 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R16.00001: On the influence of initial geometry on the evolution of liquid filaments Lou Kondic, Kyle Mahady, Shahriar Afkhami Recent developments in nanofabrication allow for thin liquid films on substrates whose initial conditions can be specified with an unprecedented degree of precision. In recent work (Roberts etal., ACS Appl. Mater. Interfaces, vol. 5, 4450 (2014), it was demonstrated experimentally that the breakup of nanoscale strips with rectangular-wave perturbations can result in controlled arrays of nanodroplets, offering a means for bottom-up directed assembly of nanoparticle arrays. We present a computational study of the breakup of these rectangular-wave structures by means of direct simulation of the Navier-Stokes equations using the volume of fluid interface capturing method. By varying the parameters of the initial geometry, we find that the rectangular-wave structures can also undergo a range of different instabilities, leading to diverse structures consisting variously of droplet arrays, filaments, and combinations of the two. In particular, we show that structures can rupture into arrays of droplets even for wavelengths which are stable for sinusoidal perturbations. The short wavelengths for which this breakup occurs mean that his process can yield droplet arrays with center to center spacing smaller that what is expected based on Rayleigh-Plateau instability mechanism. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R16.00002: Features of the Interface Equation Coupling Thin and Thick Film Regimes in Conduction-Triggered Thermocapillary Flows Zachary Nicolaou, Sandra Troian An attractive feature of moving boundary problems involving the coupling of adjacent thin film regimes is the simplified form of the corresponding interface equation. For interfaces subject to conduction-triggered thermocapillary forces and damping by capillary forces, the evolution equation reduces to a 4$^{\mathrm{th}}$ order nonlinear PDE. The dispersion equation for linear instability of a uniform state then reduces to Type II, characterized by a vanishing growth rate at k$=$0, a positive k$^{2}$ contribution from the driving force and a negative k$^{4}$ from capillary damping. Here we generalize to a moving interface coupling thin and thick film regimes. The resulting 4th order, nonlinear integro-differential equation contains the usual form of the capillary term but a nonlocal thermocapillary term due to far field contributions from the lateral transport of conserved quantities. The dispersion equation in no longer of Type II since the destabilizing term is no longer quadratic. Despite these differences, the generalized form retains certain pleasing features which can be exploited for further analysis. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R16.00003: Self-similar rupture of thin free films of power law fluids Sumeet Thete, Christopher Anthony, Osman Basaran, Pankaj Doshi Rupture of a thin sheet (free film) of a power law fluid under the competing influences of destabilizing van der Waals pressure (vdWP) and stabilizing surface tension pressure (STP) is analyzed. In such a fluid, viscosity is not constant but decreases with the deformation rate raised to the $n-1$ power where $0 < n \le 1$ is the power law exponent ($n=1$ for a Newtonian fluid). It is shown that when $1 > n > 6/7$, film rupture occurs under a balance between vdWP, inertial stress (IS), and viscous stress (VS), and the film thickness decreases as $\tau ^{n/3}$ and the lateral length scale as $\tau ^{1-n/2}$ where $\tau $ is time remaining to rupture. When $n < 6/7$, the dominant balance changes so that VS becomes negligible and the film ruptures under the competition between vdWP, IS, and STP. In this new regime, film thickness and lateral length vary as $\tau^{2/7}$ and $\tau^{4/7}$. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R16.00004: Competing disturbance amplification mechanisms in two-fluid boundary layers Sandeep Saha, Jacob Page, Tamer Zaki The linear stability of boundary layers above a thin wall film of lower viscosity is analyzed. Appropriate choice of the film thickness and viscosity excludes the possibility of interfacial instabilities. Transient amplification of disturbances is therefore the relevant destabilizing influence, and can take place via three different mechanisms in the two-fluid configuration. Each is examined in detail by solving an initial value problem whose initial condition comprises a pair of appropriately chosen eigenmodes from the discrete, continuous and interface modes. Two regimes are driven by the lift-up mechanism: (i) The response to a streamwise vortex and (ii) the normal vorticity generated by a stable Tollmien-Schlichting wave. Both are damped due to the film. The third regime is associated with the wall-normal vorticity that is generated by the interface displacement. It can lead to appreciable streamwise velocity disturbances in the near-wall region at relatively low viscosity ratios. The results demonstrate that a wall film can stabilize the early linear stages of boundary-layer transition, and explain the observations from the recent nonlinear direct numerical simulations of this configuration by Jung {\&} Zaki (J. Fluid Mech., vol 772, 2015, 330-360). [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R16.00005: Instability of floating extensional flows Roiy Sayag, Grae Worster We study the propagation of a viscous fluid over a thin layer of a denser and inviscid fluid. The viscous fluid is released axisymmetrially at constant flux, and is driven by gravity. Near the origin, where the viscous layer is thick, the flow is dominated by vertical shear. In the outer region where the viscous layer is thinner, it floats over the inviscid layer and the dominant stress is extensional. The floating region of such flows remains axisymmetric when the viscous fluid is Newtonian. In contrast, when the viscous fluid is non Newtonian, the floating region can be distributed in an array of extensional tongues. We use experimental and theoretical analysis to study the symmetry breaking of the extensional region. Experiments using polymeric fluids show that the characteristic wavelength of the tongues increases with flux. Theoretically, we model the symmetry breaking as flow instability of a power-law fluid that becomes Newtonian at low strain rates. Our model predicts unstable modes at the strongly non-Newtonian limit, and stable, axisymmetric mode in the Newtonian limit. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R16.00006: Flow patterns in free liquid film caused by thermocapillary effect Ichiro Ueno, Linhao Fei, Yosuke Kowata, Toshihiro Kaneko, Donald Pettit The basic flow patterns realized in a thin free liquid film driven by the thermocapillary effect are focused. Spetial attention is paied to the effect of the volume ratio of the liquid film to the hole sustaining the film on the flow patterns. We prepare a thin liquid film of less than $0.5$ mm in thickness in order to stably realize the film under normal gravity. Liquid has in general negative temperature coefficient of it surface tension; that is, the fluid is driven to the colder to hotter regions by the non-uniform surface-tension distribution. In the case of thin free liquid film, however, it is found that a unique flow pattern is induced. One of the present authors, DRP, carried out a series of experiments under microgravity condition in the International Space Station (ISS) in $2003$. He prepared a ring made of metal, and formed a thin film of water inside the ring. Once he added a non-uniform temperature distribution to the film by placing a heated iron at one end of the ring, a net flow toward the heated iron was realized. In order to understand flow patterns, we focus on the flow structures of the thermocapillary convection in a cross section normal to the end walls as well as the surface temperature distributions. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R16.00007: A Combined Lagrangian-Thin Film Model for Investigating Film to Rivulet Transition on Surfaces with Various Wettabilities Moussa Tembely, Ian Dizon, Ali Dolatabadi Understanding rivulet formation which arises from cloud droplets impact is of interest for the aerospace industry, where in icing conditions rivulets and droplets run back along the aerodynamics surfaces. In the present work, a numerical model based on a coupling between a Lagrangian method for spray generation and thin film approach is used to investigate the rivulet formation on various surfaces. The thin-film approximation which results from the simplification of the Navier-Stokes equations accounts for the surface wettability through a contact line force model which enables to describe film-to-rivulet transition and film separation. After validating the thin film model with a Nusselt solution of a steady state laminar flow over a vertical plate, the transition from spray to rivulet is simulated on a cylinder with 3 wettabilities: hydrophilic, hydrophobic and superhydrophobic. The spray impingement on the cylinder is carried out in 2 configurations (i) vertical where a gravity-driven rivulet is formed on the cylinder side, and (ii) horizontal where the spray impacts on the cylinder under the effect of airflow. In addition to the simulations, these two configurations are investigated experimentally using a high speed camera and a small scale icing wind tunnel [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R16.00008: Breakup of partially wetting nanoscale nematic liquid films Michael Lam The breakup of nematic liquid crystals (NLCs) films with thicknesses less than a micrometer is studied. Particular attention is paid to the interplay between the bulk elasticity and the anchoring (boundary) conditions at the substrate and free surface. Within the framework of the long wave approximation, a fourth order nonlinear partial differential equation (PDE) is derived for the free surface height. Numerical simulations of a perturbed flat film show that, depending on the initial average thickness of the film, satellite droplets form and persist on time scales much longer than dewetting. Formulating the model in terms of an effective disjoining pressure (elastic response and van der Waals interaction), simulations further suggest that satellite droplets form when the initial average film thickness corresponds to a positive effective disjoining pressure. Our results may shed light on the so-called "forbidden film thicknesses" seen in experiments. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R16.00009: Surface tension gradient enhanced thin film flow for particle deposition James Gilchrist, Kedar Joshi, Tanyakorn Muangnapoh, Michael Stever We investigate the effect of varying concentration in binary mixtures of water and ethanol as the suspending medium for micron-scale silica particles on convective deposition. By pulling a suspension along a substrate, a thin film is created that results in enhanced evaporation of the solvent and capillary forces that order particles trapped in the thin film. In pure water or pure ethanol, assembly and deposition is easily understood by a simply flux balance first developed by Dimitrov and Nagayama in 1996. In solvent mixtures having only a few percent of ethanol, Marangoni stresses from the concentration gradient set by unbalanced solvent evaporation dominates the thin film flow. The thin film profile is similar to that found in ``tears of wine'' where the particles are deposited in the thin film between the tears and the reservoir. A simple model describes the 10x increase of deposition speed found in forming well-ordered monolayers of particles. At higher ethanol concentrations, lateral instabilities also generated by Marangoni stresses cause nonuniform deposition in the form of complex streaks that mirror sediment deposits in larger scale flows. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R16.00010: Two layer flow between corrugated electrodes Elizaveta Dubrovina, Richard V. Craster, Demetrios T. Papageorgiou In recent years there has been growing interest in the miniaturisation of electronic tools and much research has gone into finding appropriate techniques for patterning at small scales. One such technique exploits the electrohydrodynamic instabilities of a system to induce ordered structures. In this talk we present a model of the evolution of the interface between two perfect dielectric fluids flowing between two electrodes one of which is corrugated. With the help of a Floquet stability analysis and of full time dependant numerical simulations, we will show how the amplitude and the shape of the topography influence interfacial patterns. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R16.00011: Laminar flow over a three-dimensional thin film Thomas Ward Two-dimensional laminar flow over a three-dimensional thin liquid film will be investigated through mostly computational analysis. The boundary layer thin film evolution equations have been developed using the Blasius boundary layer solution for the external phase with lubrication analysis for the thin liquid film. The capillary length and the dynamic pressure are used as the length and pressure scale to non-dimensionalize the equations. The resulting dimensionless equations depend on the Reynolds, $Re$, and Weber, $We$, numbers and the two fluids viscosity ratio, $\lambda$. The dimensionless boundary layer thin film equation is then solved using a $4th$ order Runge-Kutta-Merson method. Sinusoidal perturbations of varying amplitude and wavelength in the initial thickness of the thin film are considered. Above certain values of the initial perturbation and parameters $Re$, $We$, and $\lambda$ the thin film's deformation is enhanced. [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