Foam mechanics: spontaneous rupture of thinning liquid films with Plateau borders

2010 ◽  
Vol 658 ◽  
pp. 63-88 ◽  
Author(s):  
ANTHONY M. ANDERSON ◽  
LUCIEN N. BRUSH ◽  
STEPHEN H. DAVIS
Keyword(s):  
Thin Liquid Film ◽  
Liquid Films ◽  
Two Dimensions ◽  
Metallic Foams ◽  
Film Rupture ◽  
Drainage Flow ◽  
Curved Boundaries ◽  
Capillary Suction ◽  
Aqueous Foams ◽  
Foam Mechanics

Spontaneous film rupture from van der Waals instability is investigated in two dimensions. The focus is on pure liquids with clean interfaces. This case is applicable to metallic foams for which surfactants are not available. There are important implications in aqueous foams as well, but the main differences are noted. A thin liquid film between adjacent bubbles in a foam has finite length, curved boundaries (Plateau borders) and a drainage flow from capillary suction that causes it to thin. A full linear stability analysis of this thinning film shows that rupture occurs once the film has thinned to ‘tens’ of nanometres, whereas for a quiescent film with a constant and uniform thickness, rupture occurs when the thickness is ‘hundreds’ of nanometres. Plateau borders and flow are both found to contribute to the stabilization. The drainage flow leads to several distinct qualitative features as well. In particular, unstable disturbances are advected by the flow to the edges of the thin film. As a result, the edges of the film close to the Plateau borders appear more susceptible to rupture than the centre of the film.

Heat Transfer and Evaporation of Falling Liquid Films on Surfaces With Advanced Three-Dimensional Periodic Structures: Experiments and Numerical Simulations

2010 ◽  
Author(s):  
Karsten Lo¨ffler ◽  
Hongyi Yu ◽  
Steffen Hardt ◽  
Tatiana Gambaryan-Roisman ◽  
Peter Stephan
Keyword(s):  
Heat Transfer ◽  
Numerical Simulations ◽  
Wall Temperature ◽  
Three Dimensional ◽  
Liquid Films ◽  
Film Rupture ◽  
Reduced Pressure ◽  
Capillary Suction ◽  
Structured Surfaces ◽  
Falling Liquid Films

Thin liquid films are widely used in many technological applications. Heat and mass transfer in falling liquid films can be controlled and enhanced by using walls with advanced three-dimensional topographies that influence the film hydrodynamics, stability and wavy pattern or promote evaporation in a very thin film region. Furthermore, capillary suction on structured surfaces leads to a significant increase of the critical heat flux. In this work, heat transfer in laminar falling water films on heated plates with herringbone structure and with meandering grooves has been studied experimentally for different heat fluxes (up to 24 kW/m2), inclination angles and flow rates under reduced pressure, so that evaporation has a significant impact on heat transfer. The flow patterns and temperature gradients on the liquid-gas interface are visualized by high-speed infrared thermography. The wall temperature distribution is measured with thermocouples. The experimental data are compared with the results of numerical simulations. The predicted effect of micro region evaporation on heat transfer has been confirmed experimentally for the first time for partially wetting films on a plate with meandering grooves. This effect manifests itself in a significant decrease of the local wall temperature after the film rupture and consequent transition from a continuous film flow to rivulet flow regime.

scholarly journals Visualization of Surface Acoustic Waves in Thin Liquid Films

2016 ◽  
Vol 6 (1) ◽  
Author(s):  
R. W. Rambach ◽  
J. Taiber ◽  
C. M. L. Scheck ◽  
C. Meyer ◽  
J. Reboud ◽  
...  
Keyword(s):  
Liquid Film ◽  
Acoustic Waves ◽  
Low Cost ◽  
Thin Liquid Film ◽  
Surface Acoustic Waves ◽  
Liquid Films ◽  
Theoretical Description ◽  
Propagation Path ◽  
Microfluidic Channels ◽  
Potential Applications

Abstract We demonstrate that the propagation path of a surface acoustic wave (SAW), excited with an interdigitated transducer (IDT), can be visualized using a thin liquid film dispensed onto a lithium niobate (LiNbO3) substrate. The practical advantages of this visualization method are its rapid and simple implementation, with many potential applications including in characterising acoustic pumping within microfluidic channels. It also enables low-cost characterisation of IDT designs thereby allowing the determination of anisotropy and orientation of the piezoelectric substrate without the requirement for sophisticated and expensive equipment. Here, we show that the optical visibility of the sound path critically depends on the physical properties of the liquid film and identify heptane and methanol as most contrast rich solvents for visualization of SAW. We also provide a detailed theoretical description of this effect.

Viscosity Effect on Thermocapillary Rupture of Falling Liquid Films

2010 ◽  
Author(s):  
Dmitry Zaitsev ◽  
Andrey Semenov ◽  
Oleg Kabov
Keyword(s):  
Heat Flux ◽  
Film Thickness ◽  
Inclination Angle ◽  
Theoretical Models ◽  
Liquid Films ◽  
Liquid Viscosity ◽  
Inclined Plate ◽  
Film Rupture ◽  
Generalize Data ◽  
Wide Range

Rupture of a subcooled liquid film flowing over an inclined plate with a 150×150 mm heater is studied for a wide range of liquid viscosity (dynamic viscosity μ = (0.91–17.2)x10−3 Pa·s) and plate inclination angle with respect to the horizon (Θ = 3–90 deg). The main governing parameters of the experiment and their respective values are: Reynolds number Re = 0.15–54, heat flux q = 0–224 W/cm2. The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them. As the heat flux grows the film thickness between rivulets gradually decreases, and, upon reaching a certain threshold heat flux, qidp, the film ruptures in the area between the rivulets. The threshold heat flux increases with the flow rate of liquid and with the liquid viscosity, while the plate inclination angle has little effect on qidp. Criterion Kp, which is traditionally used in the literature to predict thermocapillary film rupture, was found to poorly generalize data for high viscous liquids (ethylene glycol, and aqueous solutions of glycerol) and also data for Θ≤45 deg. The criterion Kp was modified by taking into account characteristic critical film thickness for film rupture under isothermal conditions (no heating), deduced from existing theoretical models. The modified criterion has allowed to successfully generalize data for whole ranges of μ, Re, Θ and q, studied.

Three-dimensional surfactant-covered flows of thin liquid films on rotating cylinders

2018 ◽  
Vol 844 ◽  
pp. 61-91 ◽  
Author(s):  
Weihua Li ◽  
Satish Kumar
Keyword(s):  
Free Surface ◽  
Flow Visualization ◽  
Evolution Equations ◽  
Thin Liquid Film ◽  
Three Dimensional ◽  
Axial Direction ◽  
Liquid Films ◽  
Practical Applications ◽  
Rotation Rates ◽  
Visualization Experiments

The coating of discrete objects is an important but poorly understood step in the manufacturing of a broad variety of products. An important model problem is the flow of a thin liquid film on a rotating cylinder, where instabilities can arise and compromise coating uniformity. In this work, we use lubrication theory and flow visualization experiments to study the influence of surfactant on these flows. Two coupled evolution equations describing the variation of film thickness and concentration of insoluble surfactant as a function of time, the angular coordinate and the axial coordinate are solved numerically. The results show that surface-tension forces arising from both axial and angular variations in the angular curvature drive flows in the axial direction that tend to smooth out free-surface perturbations and lead to a stable speed window in which axial perturbations do not grow. The presence of surfactant leads to Marangoni stresses that can cause the stable speed window to disappear by driving flow that opposes the stabilizing flow. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and reduce the growth rate of rings that form at high rotation rates. Flow visualization experiments yield observations that are qualitatively consistent with predictions from linear stability analysis and the simulation results. The visualizations also indicate that surfactants tend to suppress dripping, slow the development of free-surface perturbations, and reduce the shifting and merging of rings and droplets, allowing more time for solidifying coatings in practical applications.

The mechanism for the long-wave instability in thin liquid films

1990 ◽  
Vol 217 ◽  
pp. 469-485 ◽  
Author(s):  
Marc K. Smith
Keyword(s):  
Physical Mechanism ◽  
Thin Liquid Film ◽  
Liquid Films ◽  
Thin Liquid Films ◽  
The Other ◽  
Wave Instability ◽  
Inclined Plane ◽  
Long Wave ◽  
The Many ◽  
Large Groups

A physical mechanism for the long-wave instability of thin liquid films is presented. We show that the many diverse systems that exhibit this instability can be classified into two large groups. Each group is studied using the model of a thin liquid film with a deformable top surface flowing down a rigid inclined plane. In the first group, the top surface has an imposed stress, while in the other, an imposed velocity. The proposed mechanism shows how the details of the energy transfer from the basic state to the disturbance are handled differently in each of these cases, and how a common growth mechanism produces the unstable motion of the disturbance.

Evaporation of Gravity- and Gas Flow-Driven Thin Liquid Films in Micro- and Minigrooves

2004 ◽  
Author(s):  
T. Gambaryan-Roisman ◽  
P. Stephan
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Gas Flow ◽  
Thin Liquid Film ◽  
Flow Region ◽  
Wave Theory ◽  
Liquid Films ◽  
Wall Material ◽  
Wall Structure ◽  
Ultra Thin Film

Using microstructured wall surfaces may improve the heat transfer performance of falling film or shear-driven film cooling devices enormously. The advantages of the structured surface include the prevention of the formation of dry patches on hot surfaces, the promotion of ultra-thin film evaporation, and a wavy motion of the film that enhances mixing of the liquid. We develop a model describing the hydrodynamics and heat transfer by evaporation of gravity- and gas flow-driven liquid films on grooved surfaces. For low Reynolds numbers or low liquid mass fluxes the heat transfer is governed by the evaporation of the ultra-thin film at a micro region, in the vicinity of the three-phase contact line. We investigate the hydrodynamic stability of the film flow using the long-wave theory. In addition to the films completely covering the wall structure, we study the stability characteristics of a thin liquid film partly covering the grooved wall, so that the flow region is bounded by contact lines. Two cases are analyzed: fully wetting liquids and liquids which form a small but finite contact angle with the wall material.

A phase-field-based lattice Boltzmann method for moving contact line problems on curved stationary boundaries in two dimensions

2019 ◽  
Vol 30 (06) ◽  
pp. 1950044
Author(s):  
Weifeng Zhao
Keyword(s):  
Boundary Conditions ◽  
Lattice Boltzmann Method ◽  
Contact Line ◽  
Phase Field ◽  
Lattice Boltzmann ◽  
Two Dimensions ◽  
Curved Boundaries ◽  
Moving Contact Line ◽  
Moving Contact ◽  
Boltzmann Method

In this work, we propose a phase-field-based lattice Boltzmann method to simulate moving contact line (MCL) problems on curved boundaries. The key point of this method is to implement the boundary conditions on curved solid boundaries. Specifically, we use our recently proposed single-node scheme for the no-slip boundary condition and a new scheme is constructed to deal with the wetting boundary conditions (WBCs). In particular, three kinds of WBCs are implemented: two wetting conditions derived from the wall free energy and a characteristic MCL model based on geometry considerations. The method is validated with several MCL problems and numerical results show that the proposed method has utility for all the three WBCs on both straight and curved boundaries.

Magnetic torque-induced suppression of van-der-Waals-driven thin liquid film rupture

2017 ◽  
Vol 813 ◽  
pp. 991-1006 ◽  
Author(s):  
E. Kirkinis
Keyword(s):  
Gas Phase ◽  
Thin Liquid Film ◽  
Van Der Waals ◽  
Van Der Waals Forces ◽  
Solid Substrate ◽  
General Effect ◽  
Magnetic Torque ◽  
Film Rupture ◽  
Carrier Liquid ◽  
Collective Rotation

An ultra-thin film of a carrier liquid containing nanosize ferromagnetic particles sitting on a solid substrate and surrounded by an ambient gas phase can be acted upon, apart from viscous and capillary forces, by attractive van der Waals forces which may promote instability leading to film rupture and substrate dewetting. In this article we show that the collective rotation of the particles on the liquid–gas interface, due to a magnetic torque, competes against the instability induced by the van der Waals forces that tend to deepen depressions of the liquid. The competition between the two effects (forcing and instability) leads to the generation of a permanent nonlinear interfacial viscous–capillary wave. Thus, film rupture and substrate dewetting are both averted. This is a general effect that may also be employed to suppress other types of instabilities such as the Rayleigh–Taylor and thermocapillary instabilities. This effect has yet to be observed in experiment.

Thermal Conductance of Biporous Evaporator Wicks in Thin Film Evaporation and Boiling Regimes

2010 ◽  
Author(s):  
Vinod Srinivasan ◽  
Ming-Chang Lu ◽  
Dusan Coso ◽  
Arun Majumdar
Keyword(s):  
Surface Area ◽  
Thin Liquid Film ◽  
Thermal Conductance ◽  
Volume Ratio ◽  
Heat Fluxes ◽  
Vertical Orientation ◽  
Capillary Suction ◽  
Thin Film Evaporation ◽  
Pin Fin ◽  
Flow Drag

We present detailed data on the performance of microstructured geometries for use in the evaporator section of a vapor chamber heat pipe. The central innovation of the geometries is their hierarchical structure, involving the use of large microchannels in order to reduce liquid flow drag while fabricating microscale pin fin arrays whose small pores increase capillary suction. The overall conductance in such a geometry is dependent on the extent of thin liquid film (thickness ∼few microns), which is manipulated by increasing the surface area-to-volume ratio through the use of microstructuring. Experiments were conducted for a heater area of 1cm2, with the wick in a vertical orientation. Results are presented for fixed microchannel widths of 30–60 microns, with pin fin diameters ranging from 4 to 32 microns, and pin fin array widths of 150 to 300 microns. The competing effects of increase in surface area due to microstructuring, and the suppression of evaporation due to reduction in pore scale are explored. In the evaporative regime, conductances of the order of 6 W/cm2-K are attained at heat fluxes of up to 140 W/cm2, until the capillary limit is reached and the wick dries out.

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