Spreading Of Liquid Film Research Articles

A Study on Perforation of a Liquid Sheet Caused by Thermocapillary Effect

The perforation of a thin liquid sheet by thermocapillary effect has been investigated. In a radial liquid sheet, which is produced by release of a liquid film spreading on a disk into the air, a laminar-turbulent transition occurs just outside the disk edge. The coherent turbulent structure after the transition forms the extremely thin liquid sheet locally and perforates it. In addition, the perforation is promoted by the impingement of a hot air flow on the thin liquid sheet. This perforation promotion requires specific condition of liquid flow velocity and airflow temperature. This study proposed the theoretical model of thermocapillary induced perforation of the thin liquid sheet and predicted the specific condition for the thermal perforation promotion in the radial liquid sheet. This model representing time diminishing sheet thickness is derived from a momentum balance between inertial force, Marangoni force and Laplace pressure. A simplified model gives the simple algebraic equation expressing the time required to the perforation. The validity of the theory is verified by comparing the theoretical perforation time with that calculated from a numerical simulation based on CIP method. The specific condition for the perforation promotion, which is estimated from the theoretical perforation time, agrees well with experimentally observed one. This agreement demonstrated that the perforation promotion is caused by thermocapillary effect.

Selective Spreading and Jetting of Electrically Driven Dielectric Films

We study the electrically driven spreading of dielectric liquid films in wedge-shaped gaps across which a potential difference is applied. Our experiments are in a little-studied regime where, throughout the dynamics, the electrical relaxation time is long compared to the time for charge to be convected by the fluid motion. We observe that at a critical normal electric field the hump-shaped leading edge undergoes an instability in the form of a single Taylor cone and periodic jetting ensues, after which traveling waves occur along the trailing thin film. We propose a convection-dominated mechanism for charge transport to describe the observed dynamics and rationalize the viscosity dependence of the self-excited dynamics.

Inertia dominated flow and heat transfer in liquid drop spreading on a hot substrate

The present work deals with computational modeling of the fluid flow and heat transfer taking place in the process of impact of a cold liquid drop ( T d = 20–25 °C) onto a dry heated substrate characterized by different thermophysical properties. The computational model, based on the volume-of-fluid method for the free-surface capturing, is validated by simulating the configurations accounting for the conjugate heat transfer. The simulations were performed in a range of impact Reynolds numbers (Re = 2000–4500), Weber numbers (We = 27–110) and substrate temperatures ( T s = 100–120 °C). The considered temperature range of the drop-surface, i.e. liquid–solid system does not account for the phase change, that is boiling and evaporation. The model performances are assessed by contrasting the results to the reference database originating from the experimental and complementary numerical investigations by Pasandideh-Fard et al. [Pasandideh-Fard, M., Aziz, S., Chandra, S., Mostaghimi, J., 2001. Cooling effectiveness of a water drop impinging on a hot surface. International Journal of Heat and Fluid Flow, 22, 201–210] and Healy et al. [Healy, W., Hartley, J., Abdel-Khalik, S., 2001. On the validity of the adiabatic spreading assumption in droplet impact cooling. International Journal of Heat and Mass Transfer, 44, 3869–3881]. In addition, the thermal field obtained is analyzed along with the corresponding asymptotic analytical solution proposed by Roisman [Roisman, I.V., 2010. Fast forced liquid film spreading on a substrate: flow, heat transfer and phase transition. Journal of Fluid Mechanics, 656, 189–204]. Contrary to some previous numerical studies, the present computational model accounts for the air flow surrounding the liquid drop. This model feature enables a small air bubble to be resolved in the region of the impact point. The reported results agree reasonably well with experimental and theoretical findings with respect to the drop spreading pattern and associated heat flux and temperature distribution.

Stability of a volatile liquid film spreading along a heterogeneously-heated substrate

The dynamics and stability of a thin, viscous film of volatile liquid flowing under the influence of gravity over a non-uniformly heated substrate are investigated using lubrication theory. Attention is focused on the regime in which evaporation balances the flow due to gravity. The film terminates above the heater at an apparent contact line, with a microscopically thin precursor film adsorbed due to the disjoining pressure. The film develops a weak thermocapillary ridge due to the Marangoni stress at the upstream edge of the heated region. As for spreading films, a more significant ridge is formed near the apparent contact line. For weak Marangoni effects, the film evolves to a steady profile. For stronger Marangoni effects, the film evolves to a time-periodic state. Results of a linear stability analysis reveal that the steady film is unstable to transverse perturbations above a critical value of the Marangoni parameter, leading to finger formation at the contact line. The streamwise extent of the fingers is limited by evaporation. The time-periodic profiles are always unstable, leading to the formation of periodically-oscillating fingers. For rectangular heaters, the film profiles after instability onset are consistent with images from published experimental studies.

Fast forced liquid film spreading on a substrate: flow, heat transfer and phase transition

This theoretical study is devoted to description of fluid flow and heat transfer in a spreading viscous drop with phase transition. A similarity solution for the combined full Navier–Stokes equations and energy equation for the expanding lamella generated by drop impact is obtained for a general case of oblique drop impact with high Weber and Reynolds numbers. The theory is applicable to the analysis of the phenomena of drop solidification, target melting and film boiling. The theoretical predictions for the contact temperature at the substrate surface agree well with the existing experimental data.

Spreading of Liquid AuSi on Vapor−Liquid−Solid-Grown Si Nanowires

Vapor-liquid-solid growth of high-quality Si nanowires relies on the stability of the liquid metal seed. In situ transmission electron microscopy shows that liquid AuSi seed spreads along the sidewalls of Si nanowires for some growth conditions. This liquid thin film phase separates to form solid Au clusters as the nanowire is quenched below the solidus temperature. The length that the liquid film spreads from the seed and its thickness can be explained by considering the spreading thermodynamics of droplets on cylinders.

A Thin Liquid Film and Its Effects in an Atomic Force Microscopy Measurement

Recently, it has been observed that a liquid film spreading on a sample surface will significantly distort atomic force microscopy (AFM) measurements. In order to elaborate on the effect, we establish an equation governing the deformation of liquid film under its interaction with the AFM tip and substrate. A key issue is the critical liquid bump height y0c, at which the liquid film jumps to contact the AFM tip. It is found that there are three distinct regimes in the variation of y0c with film thickness H, depending on Hamaker constants of tip, sample and liquid. Noticeably, there is a characteristic thickness H* physically defining what a thin film is; namely, once the film thickness H is the same order as H*, the effect of film thickness should be taken into account. The value of H* is dependent on Hamaker constants and liquid surface tension as well as tip radius.

Development of a PLIC-VOF method for the dynamic simulation of entry region flow in a laminar falling film

The present work describes a numerical procedure to simulate the development of hydrodynamic entry region in a gravity-driven laminar liquid film flow over an inclined plane. It provides a better insight into the physics of developing film in entry region. A novel numerical approach is proposed which has the potential to provide solutions for the complex physics of liquid film spreading on solid walls. The method employs an incompressible flow algorithm to solve the governing equations, a PLIC-VOF method to capture the free surface evolution and a continuum surface force (CSF) model to include the effect of surface tension. To account for the moving contact line on the solid substrate, a precursor film model based wall treatment is implemented. Liquid film flow has been simulated for the Reynolds number range of 5 ≤ Re ≤ 37.5, and the predicted results are found to agree well with the available analytical and experimental data.

Spreading of a viscous liquid film over the corrugated surface and the heat and mass transfer calculations

There are a lot of industrial applications of structured packing. Distillation columns are one of the examples where the liquid flows over the corrugated surface as a thin film to provide a good mass-transfer surface between the liquid and vapor phase. The purpose of the present paper is to study the hydrodynamics and the heat-mass transfer of the liquid film spreading down the corrugated surfaces when the corrugation amplitude is comparable with Nusselt’s film thickness (the amplitude corresponds to a small texture of the structured packing). As a result, a nonlinear type diffusion equation is obtained to describe the evolution of the film thickness profile. The nonlinear diffusion coefficient is obtained for three cases: a smooth inclined plate, a corrugated plate with large ribs, and an inclined corrugated plate with small ribs. The equations are solved numerically. As a result, it has been obtained that the small texture significantly increases the rate of the film thickness evolution in comparison with a smooth plate. To obtain the nonlinear diffusion coefficient in the case of a small texture, the hydrodynamics of the film flow over an inclined corrugated surface are studied. The viscosity, inertia, and surface tension forces are taken into account. The calculations were carried out on the basis of the Navier-Stokes equations. The influence of the microcorrugations on both the heat transfer from the wall and the mass transfer through the free surface was investigated.

Transient Interfacial Patterns and Instabilities Associated with Liquid Film Adhesion and Spreading

A surface force apparatus was used to study surface shape changes during the adhesion and spreading of a polymer melt on a bare mica surface. Transient fingers were observed during the initial, rapid spreading process, pointing radially out from the initial adhesive contact point. The fingers had microscopic widths and lengths but submicroscopic thicknesses. They eventually disappeared, leaving a more slowly growing circular neck with a smooth, featureless polymer-air surface. The mean radius of the spreading meniscus (neck) was found to follow a scaling relationship with time of the form (ri + ro)/2 proportional, variant tn, with n = 0.128, while the ends of the fingers grew according to ro proportional, variant tn, with n = 0.10. These rates agree with the values of n = 0.100-0.125 predicted by classical wetting theories for circular macroscopic droplets (i.e., radially symmetric, without fingers) spreading on a solid surface. The lifetime of the transient fingering patterns increases with the polymer viscosity as tau proportional, variant etan, with n = 2.1 +/- 0.2. A circular trough or depression in the film was observed just beyond where the fingers ended, which appears to be a source of the material for the advancing fingers. In addition, beyond the trough, circular ripples/waves were observed on the polymer melt film surface. Such patterns may arise quite generally whenever a perturbation occurs that changes the local forces, thereby inducing a bulge or depression in a liquid film or surface. Thus, we observe similar fingers and ripples/waves during the spreading of liquid polybutadiene on (the immiscible and more viscous) liquid poly(dimethylsiloxane), suggesting that the phenomenon may exist in various liquid adhesion and spreading situations. For low viscosity liquids such as water and low molecular weight oils, our scaling relations suggest that the transient patterns will exist for only a few microseconds; this is likely the reason for why they have not yet been observed.

Slip effect for thin liquid film spreading characteristics

The spreading behaviour of thin liquid polymer perfluoropolyethers on hard disc surface was investigated theoretically. The impact of slip at the boundary was taken into consideration. The diffusion coefficient Ds(h) was modified by incorporating the effective Knudsen number and molecular weight. It is found that slip has to be negative to ensure a maximum value of Ds(h) at lubricant thickness of 1 nm. Replenishment behaviour was also investigated and it was shown that a positive slip would quicken the lubricant movement, whereas a negative slip would slow down the replenishment. Finally, the spreading profiles were examined to investigate lubricant mobility.

Axisymmetric spreading of a thin drop under gravity and time-dependent non-uniform surface tension

A second-order non-linear partial differential equation modelling the gravity driven spreading of a thin viscous liquid film with time-dependent non-uniform surface tension Σ ( t , r ) is considered. The problem is specified in cylindrical polar coordinates where we assume the flow is axisymmetric. Similarity solutions describing the spreading of a thin drop and the flattening of a thin bubble are investigated.

Capillarity driven spreading of power-law fluids

We investigate the spreading of thin liquid films of power-law rheology. We construct an explicit travelling wave solution and source-type similarity solutions. We show that when the nonlinearity exponent λ for the rheology is larger than one, the governing dimensionless equation h t + ( h λ+2 | h xxx | λ−1 h xxx ) x =0 admits solutions with compact support and moving fronts. We also show that the solutions have bounded energy dissipation rate.

Spreading Of A Viscous Liquid Film Over TheCorrugated Surface

Spreading Of A Viscous Liquid Film Over TheCorrugated Surface

Optical manipulation of microscale fluid flow.

A novel optical method is used both to probe and to control dynamics in experiments on the spreading of microscale liquid films over solid substrates. The flow is manipulated by thermally induced surface-tension gradients that are regulated by controlling the absorption of light in the substrate. This approach permits, for the first time, the measurement of the dispersion relation for the well-known contact line instability; the measurements are compared with theoretical predictions from the slip model for spreading films. The experiments also demonstrate the use of feedback control to suppress instability. These results show that optical control can provide dynamically reconfigurable manipulations of fluid flow, thereby suggesting a general approach for constructing reprogrammable microfluidic devices.

Experimental study of the replenishment of ultrathin liquid perfluoropolyether films on carbon surfaces

An experimental study is performed on the spreading and replenishment of ultrathin liquid films of perfluoropolyether Zdol on the same carbon coated disk. Carbon coated disks are partially dip coated, producing a spreading edge of the lubricant film, using Zdol with molecular weights of 2000 and 4000 g/mol. Away from the spreading edge of the film on the same carbon coated disk, the film is scratched by a semispherical slider. An ellipsometer is then used to measure the spreading and replenishment profiles consecutively with time. Diffusion coefficients are calculated from the spreading profiles and then used to simulate the replenishment profiles from the scratched profiles by solving the diffusion equation. The results are compared with the experimentally measured replenishment profiles with time from the same initially scratched profiles. The scratch depths are 1–3 nm for the sample with an initial film thickness of 5–7 nm. The comparison shows that the replenishment of ultrathin liquid films on solid surfaces can be calculated using the diffusion coefficients extracted from their spreading profiles at least for the scratch depth of more than 0.5 nm.

Polymer solution film formation process in spin coating

The results of numerical modeling of polymeric films formation under mass forces action are presented. The rheology laws of neutral and charged polymer solutions are discussed The instability of non-Newtonian liquid front edge at the initial stage of flow over disk is studied. Quasistationary shapes of the liquid film spreading over surface with some rheology laws in front edge vicinity are determined. For modeling of the second stage of coating flow a special attention was paid to the effects connected with two-dimensional character of the flow. Using complex codes that calculate two-dimensional non-stationary flow of non-Newtonian liquid the impact of rheological properties of a liquid to free surface shape was studied. The factors that determine final shape of polymeric coating free surface are discussed.

Dynamic processes occurring during the spreading of thin liquid films produced by drop impact on hot walls

Abstract When liquid drops impinge on a wall heated up above the boiling temperature of the liquid, bubbling and convection are observed in the emerging film which spreads along the wall. Experimental investigations of these processes and of the connected evaporation of the liquid are aimed at an improved understanding of the underlying mechanisms important for many technical applications. Typical Weber numbers of the impinging drops were varied between 50 and 700, the temperature of the polished aluminium wall between 70°C and 400°C, i.e., above the boiling temperature of ethanol used in the experiments. The growth rate of bubbles, their maximum diameter and their collapse as well as the formation and destruction of convection cells in the liquid film were recorded and analysed.

Dissipation Processes at the Mesoscopic and Molecular Scale. The Case of Polymer Films

The spontaneous spreading of liquid films results from the balance between an energetic driving term and dissipative processes. For films of mesoscopic thickness, the dissipative term is proportional to the bulk viscosity η of the liquid. For thinner films, the observed dynamics depends also on the friction coefficient ζ 1 of the first molecular layer of liquid on the solid. For high friction, the overall film growth should be mainly controlled by the value of η. For low friction, the friction coefficient ζ 1 should become the leading parameter. This behavior is investigated in the framework of the model of a stratified droplet recently proposed by de Gennes. Complementary information is provided by numerical simulations when solid-liquid interactions (which control the value of ζ 1 ) are modified keeping the liquid-liquid ones the same (i.e., the bulk viscosity η). Moreover, the numerical simulations provide information on molecules displacements inside the droplet. Experiments were performed with short polymer chains below the three-dimensional disentanglement threshold, where the polymer behaves as a simple, nonvolatile liquid. High and low friction correspond to different thickness profiles. The dynamics of the first layer in both cases agrees with the theoretical expectations.

Liquid Atomization of a Radial Liquid Sheet Due to Transition to Turbulence.

In this paper, the mechanisms of breakup and frop formation of a radially thinning liquid sheet, which are attributed directly to the transition from laminar to turbulent flow, are described. A radial liquid film flow is generated by water discharge from one end of a cylindrical nozzle through a small gap present between the end of the nozzle and the flat surface of a disk. The liquid film spreads radially outward on the disk as a sheet, slowing from the edge of the disk into the air. Sudden laminar-turbulent transition occurs in the liquid sheet when the Reynolds number exceeds a critical value, resulting in both perforation and disintegration of the sheet under an extremely high Reynolds number. The breakup process through perforation, ligament formation and drop formation is observed both by zoom-in photography using a single pulse strobe and a steel camera and high-speed photography using a multipulse laser and a drum streak camera. These photographs clearly show that droplets a few tens of micrometers in size are produced from the liquid sheet. Flow conditions for the occurrence of atomization induced by the transition are also presented.