Rupture Of Liquid Film Research Articles

Infrared laser induced rupture of thin liquid films on stationary substrates

We studied the deformation and destabilization of thin liquid films on stationary substrates via infrared illumination. The film thickness evolution was measured using interference microscopy. We developed numerical models for the temperature evolution and the liquid redistribution. The substrate wettability is explicitly accounted for via a phenomenological expression for the disjoining pressure. We systematically measured the film thinning- and rupture dynamics as a function of laser power, which are accurately reproduced by the simulations. While smaller laser spots generally lead to shorter rupture times, the latter can become independent of the spotsize for very narrow beams due to capillary suppression.

Thinning and Rupture of Liquid Films by Moving Slot Jets

We present systematic experiments of the rupture and dewetting of thin films of a nonvolatile polar liquid on partially wetting substrates due to a moving slot jet, which impinges at normal incidence. The relative motion was provided by a custom-built spin coater with a bidirectionally accessible axis of rotation that enabled us to measure film thickness profiles in situ as a function of substrate velocity using dual-wavelength interference microscopy. On partially wetting polymeric substrates, dry spots form in liquid films with a residual thickness well below 1 μm. We measured the density of dry spots as well as the density and size distribution of the residual droplets as a function of film thickness. In a certain parameter range, the droplet distributions exhibit pronounced anisotropy due to the effect of long-range shear stresses on the dewetting rim instability. We find robust power-law scaling relations over a large range of film thicknesses and a striking similarity to literature data obtained with ultrathin polymer melt layers on silicon substrates.

Crystal Patterns Created by Rupture of a Thin Film

We describe a method of generating remarkable crystal patterns on smooth surfaces such as mica, glass, or calcite. Very dilute droplets of aqueous salt solution (0.1 wt % NaCl) are deposited on the chosen substrate, and if the perimeter of these droplets becomes pinned by crystal deposition, they thin and concentrate by evaporation rather than shrink laterally. When thin enough, the liquid film ruptures by one of two mechanisms. In the first, crystals nucleate on the substrate and puncture the film, and as the holes grow radially from the rupturing crystal, the receding liquid rim deposits two-dimensional crystal dendrites. A second mechanism of rupture can occur with more dilute solutions (0.025 wt % NaCl), where attractive forces across the liquid film lead to rupture before crystals nucleate. This results in a pattern of bare patches surrounded by crystals, because supersaturation is only reached when the neighboring holes in the film are close to coalescence. The method is applicable to any solute in a volatile solvent on any smooth substrate, and the interplay of dewetting, crystal deposition, and symmetry of crystal and substrate leads to complex, system-specific patterns. The technique may be developed into methods of surface patterning with technologically important crystals.

A homotopy continuation approach for analysing finite-time singularities in thin liquid films

We consider self-similar solutions related to rupture of thin liquid films on a solid substrate that evolve solely under the stabilizing influence of surface tension and the destabilizing influence of effective van der Waals interactions between the free surface of the film and the substrate. Such solutions have been previously analysed in the literature and various numerical approaches to obtain such solutions have been proposed. Such approaches are based either on shooting or finite-difference schemes and require well-chosen initial guesses for solutions. We propose an alternative numerical method, which is based on a homotopy approach and continuation techniques and allows one to reach self-similar solutions from analytically known small-amplitude steady solutions of the related thin-film equation. We argue that this method is more robust than previously proposed methods and does not require initial guesses to obtain solutions. Although the present study focuses on the particular case of self-similar solutions related to planar rupture that have square-root far-field behaviour, our approach can also be used to obtain planar solutions having a different far-field behaviour and radially symmetric self-similar solutions for the considered thin-film equation. We expect the approach to be also valid for other equations of similar type that show a subcritical primary bifurcation and finite-time singularities.

Molecular-Level Modeling of Interfacial Phenomena in Boiling Processes

The thermophysics of liquid–vapor interfaces has long been recognized as a critical element in the physical mechanisms of boiling processes. This article will describe results of recent molecular dynamics simulation studies that explore the structure and stability of liquid–vapor interfacial regions using a hybrid analysis scheme that combines new formulations of capillarity theory with molecular dynamics simulations that use similar interaction potentials. Two forms of this type of hybrid scheme have been developed: one for non-polar fluids based on a Lennard–Jones interaction potential and a second specifically for water using a modified treatment of the extended simple point charge interaction potential that accounts for water dipole interactions. The hybrid model has the advantage that the capillarity theory provides theoretical relationships among parameters that govern interfacial region structure and thermophysical behavior, while the companion molecular dynamics simulations allow more detailed molecular level exploration of the interfacial region thermophysics. Predictions of interfacial region structure indicated by this kind of hybrid modeling will be described for pure non-polar and water liquid–vapor interfaces and for water with dissolved ionic solutes (i.e., salts). Extension of the methodology to thin liquid films will also be described. Rupture of a free liquid film dictates merging of adjacent bubbles, which is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and the onset of rupture at the molecular level. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for free liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (e.g., in nanostructured boiling surfaces), lack of film core stability is more likely to be the mechanism. Predictions of the hybrid models will be compared to results of experimental studies of the effects of ionic solutes on interfacial tension and bubble merging. The implications of the molecular dynamics model predictions for boiling processes in micro-channels and boiling in nanostructured surfaces are also discussed.

Rupture of thin liquid films sprayed on solid surfaces

An experimental study was done to observe the formation of thin films by spraying liquid onto a solid surface and to determine the conditions under which the films would rupture or remain stable. Water, or water mixed with 20–70 % by weight of glycerin, was sprayed for varying lengths of time onto a circular, 165-mm-diameter plate made of either Plexiglas, steel, or Parafilm-M and the motion of the liquid recorded using a high-speed camera. Water films ruptured immediately after the impact near the center of the surface. Then, if the film thickness was greater than a critical value, the water flooded back and the hole closed; otherwise, the hole remained in the water layer. The critical film thickness increases linearly with advancing liquid–solid contact angle. Increasing liquid viscosity by adding glycerin had little effect on critical film thickness, but inhibited spreading of the liquid and suppressed initial rupture of the liquid layer. A surface energy model was used to predict the variation of critical film thickness with surface wettability.

Investigation of Colloidal Stripe Formation Mechanism by In-situ Analysis of Meniscus Shape in Convective Self-assembly Process

Convective self-assembly technique is of growing importance as a robust technique to arrange various colloidal particles without any templates. Here we focus on stripe pattern formation by the convective self-assembly technique. To unveil the mechanism of the stripe formation process, we employed an in-situ observation of the variation of meniscus shape during the particle assembly process. The observation revealed a following new stripe formation mechanism, which is different from the "depinning" model. A meniscus is elongated downward to be concave toward a substrate as the solvent evaporation progresses. The convective flow toward the meniscus tip accelerates the deformation of the meniscus, resulting in the rupture of liquid film at the thinnest point of the meniscus.

INSTABILITY AND RUPTURE OF THIN LIQUID FILMS ON SOLID SUBSTRATES

Studies of rupture of thin liquid films on solid surfaces are important for modeling of multiphase flows in microfluidic devices, heat exchange systems, mining industry, and for biomedical applications such as dynamics of the tear film in the eye. In the present study we review theoretical work on film rupture and discuss its comparison with some recent experiments. Conditions for the break-up of thin liquid films by London-van der Waals dispersion forces, electrostatic effects, and thermocapillary instability are discussed. Mathematical models of instability of a liquid film between a deformable fluid interface and a solid wall are important for a number of applications of multiphase flow. The fluid interface in the present context is a boundary between liquid and vapor or gas, although some of the results discussed here are applicable to liquid-liquid interfaces as well. When the interface approaches the wall, the film can rupture, resulting in the formation of a three-phase contact line. The overall dynamics of many types of multiphase flows encountered in applications depends on these local phenomena. In microfluidic devices, drops and bubbles transported through a channel can slow down as a result of rupture of the film separating the fluid interface and the wall since the presence of such film is essential for maintaining the microfluidic transport efficiency, as discussed, e.g., in Ajaev and Homsy (2006). In cooling systems based on thin-film flows driven by either gravity or shear stresses at the liquid-gas interface, formation of dry spots results in significant reduction of heat flux from the heated wall, as shown experimentally by Kabov (2000). Liquid films flowing under gravity over a localized heater on vertical or inclined flat plates rupture for sufficiently high values of the heat flux generated by the heater. These experimental observations are important for developing guidelines to avoid rupture in cooling systems for industrial applications. Stability of thin films separating solid and gas phases is important for studies of interaction between solid particles and gas bubbles. Of particular importance for these studies are the conditions when particles become attached to air bubbles as a result of rupture of the liquid film separating them. The main application of this work is froth flotation (Nguyen and Schulze, 2004; Rao, 2004), an important step in mineral processing during which particles of valuable minerals in a liquid tank become attached to rising air bubbles and thus separated from the gangue, i.e., commercially worthless part of the raw ore deposit that remains near the bottom of the tank. Typical minerals in froth flotation are metal sulfides, which then undergo further processing until pure metals such as copper, lead, and zinc are produced. Similar techniques are successfully applied in the coal industry and, more recently, for the de-inking process in paper recycling (Drelich and Miller, 2001). Several recent studies of liquid film rupture have been motivated by dynamics of the tear film in the eye and are discussed in detail in Braun (2012). The tear film forms after each blink and is essential for providing a high-quality optical surface at the front of the eye and also for protection against dust and bacteria. A medical condition called the dry-eye syndrome (DES) occurs when the tear film ruptures too quickly after the blink, usually within a few seconds. Understanding the causes of this rupture is essential for developing treatment strategies for DES.

Filling a nanoporous substrate by dewetting of thin films

Following a simple thermodynamic model, which predicts that an array of non-wettable pores can be filled by dewetting of sufficiently thin films, we use molecular dynamics to simulate the rupture of nanometre-thick liquid Au films on nanoporous substrates. Our simulations clearly exhibit spinodal dewetting and hole nucleation, and some of the metal is indeed absorbed by non-wettable pores solely as a virtue of the Laplace pressure acting on dewetted droplets and rivulet-like structures. Finally, we show that the fraction of absorbed Au can be increased through patterning of the initial film.

Continuous Microwire Patterns Dominated by Controllable Rupture of Liquid Films

Controllable microwire patterns are prepared by dominating the rupture of liquid films. Regular rhombic-shaped micropillar arrays serve as wetting defects to pin or depin liquids, yielding continuous, herringbone, bead-shaped polystyrene microwire patterns or bead arrays. The results provide a deeper understanding of the controllable rupture of liquid films and offer a general strategy for the organization of polymers into structures needed for wiring, interconnects, and functional devices for future microfabrication.

Rupture of Thin Liquid Films Induced by Impinging Air-Jets

Thin liquid films on partially wetting substrates are subjected to laminar axisymmetric air-jets impinging at normal incidence. We measured the time at which film rupture occurs and dewetting commences as a function of diameter and Reynolds number of the air-jet. We developed numerical models for the air flow as well as the height evolution of the thin liquid film. The experimental results were compared with numerical simulations based on the lubrication approximation and a phenomenological expression for the disjoining pressure. We achieved quantitative agreement for the rupture times. We found that the film thickness profiles were highly sensitive to the presence of minute quantities of surface-active contaminants.

Rupture of thin liquid films on structured surfaces

We investigate stability and breakup of a thin liquid film on a solid surface under the action of disjoining pressure. The solid surface is structured by parallel grooves. Air is trapped in the grooves under the liquid film. Our mathematical model takes into account the effect of slip due to the presence of menisci separating the liquid film from the air inside the grooves, the deformation of these menisci due to local variations of pressure in the liquid film, and nonuniformities of the Hamaker constant which measures the strength of disjoining pressure. Both linear stability and strongly nonlinear evolution of the film are analyzed. Surface structuring results in decrease of the fastest growing instability wavelength and the rupture time. It is shown that a simplified description of film dynamics based on the standard formula for effective slip leads to significant deviations from the behavior seen in our simulations. Self-similar decay over several orders of magnitude of the film thickness near the rupture point is observed. We also show that the presence of the grooves can lead to instability in otherwise stable films if the relative groove width is above a critical value, found as a function of disjoining pressure parameters.

Thermophysics of the Interfacial Region and Its Impact on Instability and Rupture of Thin Free Liquid Films

Merging of adjacent bubbles is a particularly important process in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. The merging process is ultimately associated with the rupture of the liquid film separating two bubbles, and the stability of the liquid film dictates whether coalescence is likely to occur. This paper examines two methods of exploring how the molecular-level features of the thin film and its interfaces affect the stability of the film and its subsequent impact on bubble merging. One method is an extended molecular capillarity model that has been developed for a free liquid film bounded by two liquid–vapor interfacial regions. This model predicts that as film thickness diminishes, a point is reached at which the mean centerline density in the film is between the spinodal limits for the fluid, implying that the film core lacks intrinsic stability. This suggests that loss of intrinsic stability may be the mechanism for film rupture in some cases. The second method examined here is use of molecular dynamics simulations to examine film structure and stability. Simulations using a Lennard–Jones interaction potential for non-polar fluids and the SPC/E potential for water and salt water are examined. Results of these explorations indicate that for water–NaCl solution films, surface tension increases with increasing NaCl concentration. These predictions are shown to be consistent with measured surface-tension data for such solutions. The results also indicate that for pure water and NaCl solutions, below a threshold thickness, the surface tension decreases with film thickness. Wave interface stability theory implies that this tends to make the film less stable. The implications of the results of these investigations for bubble merging in two-phase flow and boiling processes are also examined.

Features of the rupture of free hanging liquid film under the action of a thermal load

We consider a deformation and a rupture of a thin liquid film which is hanging between two solid flat walls under the action of concentrated thermal load action. A two-dimensional model is applied to describe the motion of thin layers of viscous non-isothermal liquid under micro-gravity conditions. For flow simulation, two-dimensional Navier–Stokes equations are used. A computational analysis of the influence of thermal loads on the deformation and the rupture behavior of the thin freely hanging film is carried out. It is shown that the rupture of the thin film with generation of a droplet can occur under the thermal beam of specific width acting on the free surface of the film. The results of the model problem solutions are presented.

Occurrence of hydrodynamic cavitation

In this paper, the conditions under which cavitation (or liquid film rupture) can or cannot occur in thin layers of moving liquid are derived for three typical cases. At the same time, expressions depending on geometrical and movement parameters, where cavitation might start, are given. The results are obtained using simple engineering terms, which can be used in cases whether it is necessary to avoid cavitation or to induce it.

Numerical assessment of plume heat and mechanical loads and contamination on multi-layer insulation in hard vacuum

The paper presents a numerical analysis of plume exhausted from the 10N bipropellant thruster. The computations have been performed for steady state and pulse mode firing of the thruster. The plume impinges a multi-layer insulation (MLI) that covers a satellite surface, where plume effects have been computed for ideal and bulged shapes of the MLI. The ideal shape is a circular cylinder surface, where the bulge is a deflection of the MLI due to satellite spin. Plume effects, pressure and heat flux, are almost doubled for the bulged MLI. An engineering model based on experimental data has been applied. It was shown that it is conservative against the numerical modeling in the plume backflow. An application of the pulse mode leads to a formation of droplets. One of the sources of the droplets is a rupture of liquid film at the nozzle lip. A parametric study with different droplet sizes and initial velocities has been performed for thrusters firing along and against the satellite spin. The thruster that fires against the satellite spin contaminates the MLI more than the other thruster. Droplets with an initial velocity smaller than 6m/s cannot contaminate the MLI.

Rupture of thin liquid films: Generalization of weakly nonlinear theory

In this paper, we investigate the rupture dynamics of thin liquid films driven by intermolecular forces via weakly nonlinear bifurcation analysis. The dynamic equations governing slow dynamics of the perturbation amplitude of the near-critical mode corresponding to several models describing the evolution of thin liquid films in different physical situations appear to have the same structure. When antagonistic (attractive and repulsive) molecular forces are considered, nonlinear saturation of the instability becomes possible, while the boundary of this supercritical bifurcation is determined solely by the form of the intermolecular potential. The rupture time estimate obtained in closed form shows an excellent agreement with the results of the previously reported numerical simulations of the strongly nonlinear coupled evolution equations upon fitting the amplitude of the small initial perturbation. We further extend the weakly nonlinear analysis of the film dynamics and apply the Galerkin approximation to derive the amplitude equation(s) governing the dynamics of the fastest growing linear mode far from the instability threshold. The comparison of the rupture time derived from this theory with the results of numerical simulations of the original nonlinear evolution equations shows a very good agreement without any adjustable parameters.

Dynamics of dewetting

The dynamics of dewetting is re-examined from a theoretical perspective and in the light of experiment. Two cases are considered: (a) spontaneous dewetting following rupture of a thin liquid film previously formed on a partially wetted solid surface; (b) steady dewetting when a partially wetted solid surface is withdrawn vertically from a pool of liquid. In the first, a rim of liquid forms at a circular dewetting front that expands at constant speed, while the remainder of the film remains quiescent. The second is characterised by the formation of a serrated contact line, in which each straight-line section also recedes at a constant normal velocity, but with no significant rim. These two cases are analysed in terms of the relevant dissipation processes. Hydrodynamic dissipation is estimated using a simple model proposed by de Gennes, while the molecular-kinetic theory accounts for contact-line friction. The new analysis suggests that for a given system, the dewetting velocities and dynamic contact angles seen in the two cases are likely to be different. In (a), they will be the outcome of both dissipation channels operating in parallel, whereas in (b), both parameters will be determined by a dynamic balance between the two sources of dissipation that avoids an inflection in the liquid–vapour interface near the receding contact line. These predictions are consistent with current observations and could be tested by well-designed experiments.

A novel time strip flow visualisation technique for investigation of intermittent dewetting and dryout in elongated bubble flow in a microchannel evaporator

A flow visualisation study of flow boiling of R245fa in silicon multi-microchannels at low mass flux and moderate heat flux has been carried out with a high speed digital camera. The micro-evaporator had 67 channels of length 20 mm, width 223 μm, and height 680 μm while the fin width between adjacent channels was 80 μm. The base heat flux ranged from 2 to 26 W cm −2 for a mass velocity of 100 kg s −1 m −2, resulting in exit vapour qualities ranging from 10% to 70%. In particular, a novel time strip technique was developed to analyse the recorded image sequences and significantly highlight the various phenomena occurring along given channels. Notably, this technique was able to reveal profound details regarding the intermittent dryout mechanism of liquid films trapped between the elongated bubbles and the heated channel walls. The results show that the intermittent dryout of the evaporating liquid film is comprised of four stages with distinct time scales and dynamics: (i) the growth of liquid film thinning perturbations to a critical amplitude causing the rupture of the metastable liquid film, (ii) a dewetting stage involving expanding dry spots leading to a rivulet flow regime, (iii) evaporation of the rivulets leading to full dryout, and (iv) a rewetting stage. This intermittent dryout mechanism appears to explain the many seemingly contradictory heat transfer coefficient trends observed with changes in vapour quality in microchannels, thus resolving an important heat transfer dilemma. Furthermore, since dryout is an undesirable event during the practical application of a microchannel evaporator, it is important to delay or even suppress the initial rupture of the liquid film that leads to dryout. This can be achieved by manufacturing or treating the channel surfaces to be highly wettable with the chosen refrigerant.

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

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.