Ultrathin Liquid Films Research Articles

Modeling reactive film flows down a heated fiber

We study the dynamics of an ultra-thin reactive liquid film flowing down a heated vertical fiber under the influence of gravity. A key focus is on the impact of the van der Waals attraction, which is proportional to h−3, where h denotes the film thickness. Linear stability analysis of the film flow reveals that the van der Waals attractions and the Marangoni effect enhance the instability. Furthermore, instability is reduced by exothermic chemical reactions, while it is strengthened in the case of endothermic chemical reactions. Moreover, the weakly nonlinear stability of the film flow is studied. The results indicate the possibility of both subcritical and supercritical stability in the system. Lastly, direct numerical simulations of the evolution equation are conducted for various flow parameters. These results enhance our understanding of the intricate interplay of chemical reactions, thermal effects, and intermolecular forces influencing the liquid film dynamics in complex systems.

Two-Dimensional Piezoelectric Nanofibrous Webs by Self-Polarized Assembly for High-Performance PM0.3 Filtration.

Particulate matter (PM) pollution has posed a serious threat to public health, especially the global spread of infectious diseases. Most existing air filtration materials are still subjected to a compromise between removal efficiency and air permeability on account of their stacking bulk structures. Here, we proposed a self-polarized assembly technique to create two-dimensional piezoelectric nanofibrous webs (PNWs) directly from polymer solutions. The strategy involves droplets deforming into ultrathin liquid films by inertial flow, liquid films evolving into web-like architectures by instantaneous phase inversion, and enhanced dipole alignment by cluster electrostatics. The assembled continuous webs exhibit integrated structural superiorities of nanoscale diameters (∼20 nm) of the internal fibers and through pores (∼100 nm). Combined with the wind-driven electrostatic property derived from the enhanced piezoelectricity, the PNW filter shows high efficiency (99.48%) and low air resistance (34 Pa) against PM0.3 as well as high transparency (84%), superlight weight (0.7 g m-2), and long-term stable service life. This creation of such versatile nanomaterials may offer insight into the design and upgrading of high-performance filters.

Efficient separation of CO2/CH4 by covering ultrathin ionic liquid film on COF membrane

Covalent organic framework (COF) membranes show great potential for gas separation, but their large pore size make it hard to effectively separate gases with similar kinetic diameters. A promising strategy is to cover the surface of COF membranes with an ultrathin film of ionic liquids (ILs) to improve the selectivity of gas permeation. Here, we investigated the CO2/CH4 separation performance of composite membranes composed of COF (CTF-1) and 20 kinds of ILs, respectively, using molecular simulations. The [C4mim][PF6]/COF membrane is found to exhibit the best separation performance, with CO2 permeability up to 7.93 × 104 GPU and CO2/CH4 selectivity of 12.33 when the IL thickness is 8 Å. By studying the microstructure of the composite membrane, it was found that there is a relatively dispersed distribution of cations and anions on the COF surface, which is beneficial to the diffusion of gas molecules. The gas permeation process results reveal that CO2 rapidly achieves equilibrium in both the adsorption and dissolution layers simultaneously, thereby enhancing the permeation rate of gas molecules. The analysis of interaction energy and PMF indicates that the improved selectivity is due to the stronger interaction between IL and CO2 than that for CH4.

Effect of Cross Nanowall Surface on the Onset Time of Explosive Boiling: A Molecular Dynamics Study

Explosive boiling is a fast-phase transition from an ultra-thin liquid film to vapor under an extremely high heat flux, which typically has been studied using the molecular dynamics simulation (MDS) method. The present MDS study investigated the explosive boiling of a liquid argon nanofilm over different solid copper surfaces with different nanowall patterns, including parallel and cross nanowalls. For each surface, atomic motion trajectories, the number of liquid and vapor argon atoms, heat flux, and, mainly, the onset time of explosive boiling were investigated. The simulation results indicated that explosive boiling occurs earlier on parallel and cross nanowall surfaces than on an ideally smooth surface, regardless of the topology and configuration of the nanowalls. Moreover, the results revealed that by using the cross nanowall surfaces, the onset time of explosive boiling decreased by 0.7–4% compared to the parallel nanowall surfaces. In addition, it was found that the onset time of explosive boiling strongly depends on the potential energy barrier and the movement space between nanowalls for both parallel and cross nanowall surfaces. Furthermore, the simulation findings showed that even though increasing the height of cross nanowalls increases the heat flux and temperature of the fluid argon domain, it does not necessarily result in a shorter onset time for explosive boiling. These findings demonstrate the capability of cross nanowall surfaces for explosive boiling, thereby being utilized in future surface design for thermal management applications.

Non-axisymmetric modes in ultrathin annular liquid films coating a cylindrical fiber

This paper presents a detailed analysis of the three-dimensional stability properties of an annular liquid film coating a cylindrical fiber in the presence of van der Waals (vdW) interactions, whose influence depends on the wettability of the solid by the liquid. Under wetting conditions, vdW interactions can stabilize a uniform annular film when its thickness is smaller than a critical value that depends only on the fiber radius, the Hamaker constant, and the surface tension coefficient [Quéré et al., Science 249(4974), 1256–1260 (1990)]. In contrast, under non-wetting conditions, both surface tension and vdW forces contribute to destabilize the interface, and non-axisymmetric modes may become dominant depending on the thickness of the film and the relative strength of the surface tension and vdW forces. We perform temporal stability analyses of both the Stokes and lubrication equations of motion, allowing us to reveal the dominant azimuthal mode, as well as the optimal axial wavenumber and the corresponding temporal growth rate, as a function of the relevant governing parameters.

The influence of an outer bath on the dewetting of an ultrathin liquid film

We report a theoretical and numerical investigation of the linear and nonlinear dynamics of a thin liquid film of viscosity μ sandwiched between a solid substrate and an unbounded liquid bath of viscosity λμ. In the limit of negligible inertia, the flow depends on two non-dimensional parameters, namely, λ and a dimensionless measure of the relative strengths of the stabilizing surface tension force and the destabilizing van der Waals force between the substrate and the film. We first analyze the linear stability of the film, providing an analytical dispersion relation. When the viscosity of the outer bath is much larger than that of the film, λ≫1, the most amplified wavenumber decreases as km∼λ−1/3, indicating that very slender dewetting structures are expected when λ becomes large. We then perform fully nonlinear simulations of the complete Stokes equations to investigate the spatial structure of the flow close to rupture revealing that the flow becomes self-similar with the minimum film thickness scaling as hmin=K(λ)τ1/3 when τ→0, where τ is the time remaining before the singularity. It is demonstrated that the presence of an outer liquid bath affects the self-similar structure obtained by Moreno-Boza et al. [“Stokes theory of thin-film rupture,” Phys. Rev. Fluids 5, 014002 (2020)] through the prefactor of the film thinning law, K(λ), and the opening angle of the self-similar film shape, which is shown to decrease with λ.

Effect of substrate roughness on dynamics of wetting and decay of an ultrathin liquid film

In this study, we analyze the dynamics of wetting and decay of an ultrathin liquid film on a rough substrate. The dependence of disjoining pressure on liquid film thickness is derived by the direct numerical calculation for the Lennard-Jones pair interaction potential and 3D rough substrate with a random relief. It is shown that disjoining pressure for a rough substrate can be approximated by a similar function for a smooth one with highly fluctuating parameters over the substrate surface. The equation describing wetting and decay of 3D liquid film is modified, accounting for the calculated coordinate dependence of disjoining pressure the general form of capillary pressure depending on substrate relief. The approach is used for the simulation of drop formation from a nanosized liquid film for substrates with various roughness parameters. It is demonstrated that substrate roughness is responsible for accelerated film rupture and formation of “detectable” drops, increase in their number density, and decrease in average size. At the later stage, liquid film transformation is described by the power-law dependences of number density, average height and size with the exponents depending on substrate roughness. The blocking point of the size distribution function shifts to a larger value for a substrate with a more fine-grained structure. These changes in the growth rate and size distribution function of drops are associated with the change in conditions of liquid flow, which becomes inhomogeneous due to substrate roughness.

Tailoring the Selectivity of 1,3-Butadiene versus 1-Butene Adsorption on Pt(111) by Ultrathin Ionic Liquid Films

Tailoring the Selectivity of 1,3-Butadiene versus 1-Butene Adsorption on Pt(111) by Ultrathin Ionic Liquid Films

Ultrathin liquid film nucleate boiling on grooved surfaces with variational aspect ratio

Ultrathin liquid film nucleate boiling on grooved surfaces with variational aspect ratio

Ultrathin liquid film phase change heat transfer on fractal wettability surfaces

Surface wettability is significant for the liquid–vapor phase change heat transfer. In this paper, patterned wettability surfaces are constructed from the perspective of fractal theory to comprehensively investigate the microscopic mechanism of surface wettability patterns affecting evaporation and bubble nucleation. The evaporation heat transfer rates of the hydrophilic fractal wettability surfaces are close to that of the pure hydrophilic surface. The solid–liquid interfacial heat transfer dominates the evaporation heat transfer. In contrast, the liquid–vapor interfacial heat transfer’s effect is insignificant. The hydrophobic points of the hydrophilic fractal wettability surfaces provide stable and abundant nucleation sites for the bubble nucleation process. The nucleation times of the hydrophilic fractal wettability surfaces are earlier than that of the pure hydrophilic surface. Hydrophobic fractal wettability surfaces can improve bubble nucleation. With increased hydrophilic points, nucleation sites are mainly concentrated in hydrophobic regions near the hydrophilic/hydrophobic contact boundary. The expanding low potential energy region near the contact boundary is the main reason for improving bubble nucleation. Critical heat flux (CHF) increases as the area proportion of the hydrophilic part increases, and surface superheats decrease when CHF occurs. The fractal wettability surfaces improve CHF, and the CHF of the hydrophilic fractal wettability surface is close to that of the pure hydrophilic surface.

Rapid boiling of ultra-thin liquid argon film on patterned wettability surface with nanostructure: A molecular dynamics investigation

Surface wettability and structure have been proved as two important influential factors to the thermal transport at the solid-liquid interface at nano scale, however, the combined enhancement mechanism has not been clearly understood till now. In this study, the rapid boiling behaviors of nano thin liquid argon film on the heterogeneous wetting surfaces were examined with the non-equilibrium molecular dynamics (MD) method. Meanwhile, the ring-patterned and stripe-patterned surfaces were designed and analyzed, respectively. By analyzing the trajectory of argon atoms, the bubble nucleation behavior, heat flux and interfacial thermal resistance, it is found that the lower hydrophobic area fraction is favorable for the bubble formation and the ring-patterned surface shows an advantage in the nucleate boiling compared with the stripe-patterned one. Meanwhile, the nanostructure has a great influence on the boiling phenomena, which accelerates the development of bubble nuclei and improves the maximum heat flux compared with the planar one. In present simulations, the ring-patterned surface with nanostructure of 40% hydrophobic area fraction is the optimal design for the efficiency enhancement of explosive boiling process. The findings in this work contribute to the design of the coating nanostructured surface to enhance the boiling heat transfer performance under the high heat fluxes.

Bubble wall confinement-driven molecular assembly toward sub-12 nm and beyond precision patterning.

Patterning is attractive for nanofabrication, electron devices, and bioengineering. However, achieving the molecular-scale patterns to meet the demands of these fields is challenging. Here, we propose a bubble-template molecular printing concept by introducing the ultrathin liquid film of bubble walls to confine the self-assembly of molecules and achieve ultrahigh-precision assembly up to 12nanometers corresponding to the critical point toward the Newton black film limit. The disjoining pressure describing the intermolecular interaction could predict the highest precision effectively. The symmetric molecules exhibit better reconfiguration capacity and smaller preaggregates than the asymmetric ones, which are helpful in stabilizing the drainage of foam films and construct high-precision patterns. Our results confirm the robustness of the bubble template to prepare molecular-scale patterns, verify the criticality of molecular symmetry to obtain the ultimate precision, and predict the application potential of high-precision organic patterns in hierarchical self-assembly and high-sensitivity sensors.

Pore-scale study of effects of relative humidity on reactive transport processes in catalyst layers in PEMFC

High surface area carbon (HSC) particles can be adopted to increase the specific surface area of catalyst layer (CL) in proton exchange membrane fuel cells. Relative humidity (RH) has a significant effect on the Pt activity inside HSC particles, and the underlying mechanisms need to be further investigated. In this study, a pore-scale model considering the effects of RH on the reactive transport processes inside the CLs is developed. Two kinds of liquid water distributions affected by the RH, including capillary condensation in pores and ultra-thin liquid film on Pt surface, are considered. The liquid water distribution, Pt activity and local oxygen transport resistance (Rlocal) under different RH are studied in detail. It is found that as RH increases from 0.3 to 1.0, the condensed water in micropores of HSC particles increases, resulting in an increase in reactive surface area by about 43 %. Combined effect of the RH, Pt loading, I/C ratio and different kinds of carbon particles is investigated. It is found that due to the lack of sufficient reaction sites, compared with that under a high Pt loading, Rlocal under a low Pt loading is more sensitive to the RH. Besides, since the Pt activity inside HSC particles depends on the condensed water, the Rlocal of HSC particles is more sensitive to the RH than solid carbon particles. Finally, the Rlocal at low ionomer content is more sensitive to RH due to low ionomer coverage on Pt particles. The present study provides a pore-scale model for investigating the coupled effects of RH and CL porous structures on local transport processes, and can facilitate the optimization of CL nanoscale structures for better cell performance.

Two-dimensional ionic liquids with an anomalous stepwise melting process and ultrahigh CO2 adsorption capacity

Ultrathin ionic liquid (IL) films have shown wide applications in the chemistry and materials fields. However, the structure feature and quantitative controlling mechanism of thin IL films have rarely been reported to date. Here, computational simulations combined with scanning tunnel microscope experiments are used to quantify the structure and function of the thinnest possible IL films, two-dimensional (2D) ILs, which consist of 2D ordered mono-ionic IL structures. Interestingly, the 2D ILs exhibit anomalous stepwise melting processes, involving localized rotated, out-of-plane flipped, and fully disordered states, which are different from 3D ILs. Meanwhile, a theoretical model and temperature-surface phase diagram are constructed to evaluate the critical stepwise melting behaviors. Furthermore, the 2D ILs also possess ultrahigh CO 2 adsorption capabilities and structural robustness during the CO 2 adsorption-desorption process. These findings are promising for the rational design of an IL-solid interface for CO 2 -capture-fixation chemistry and related applications. • 2D ionic liquid with ordered mono-ionic structure • The anomalous stepwise melting processes of 2D ionic liquids is revealed • A theoretical model is developed to evaluate the critical melting points • The 2D ionic liquids possess extremely high CO 2 adsorption capabilities Combining STM experiments and molecular dynamics simulations, Wang et al. investigate 2D ionic liquids that exhibit anomalous stepwise melting processes and superhigh CO 2 adsorption capacity. The 2D modification technique may provide a strategy for the precise control and function design of liquids.

Mesoscale Simulations on the Ultrahigh Heat Flux Evaporation of R143a within Ultrathin Nanoporous Membrane Using a Modified Dimensionless Lattice Boltzmann Method

A modified dimensionless pseudo-potential lattice Boltzmann method capable of restoring all related thermophysical properties of any working fluid and spatial/temporal scales of the computational domain is developed in this work. Direct numerical investigations of ultrathin liquid film evaporation of R143a within nanoporous membranes to its ambient pure vapor are carried out. The characteristics of evaporation-driven replenishing flow and the self-regulation of liquid-vapor interface profile under different heat fluxes are captured. In the case of intensive ultrathin liquid film evaporation within nanopore, the apparent contact angle could not reach as small as Young's contact angle θ0 for isothermal conditions due to the self-regulation of liquid-vapor interface. As a result, the maximum capability of nanoporous membranes to replenish the liquid is limited. Under the ambient temperature T0 = 318.19 K, before the three-phase contact line (TCL) recedes into the nanoporous membrane, a heat flux as high as qin,max = 1.52 kW/cm2 can be sustained with a superheat about Ts = 7.79 K for a membrane with pore radius rp = 38.71 nm, pore depth Lp = 294.96 nm, Young's contact angle θ0 = 23° and porosity η = 0.66. The maximum heat flux qin,max increases with the increase of rp and the decrease of Lp and θ0. The present work presents a promising numerical method to optimize thermal management devices using complex micro-nano composite structures.

Ultra-Thin Liquid Film with Shear and the Influences on Thermal Energy Transfer at Solid–Liquid Interfaces of Simple Liquid Methane in Contact With (110) Surface Structure of Face-Centred Cubic Lattice (FCC)

Thermal energy transfer (TET) is the main performance of contact interfaces which has been studied at a molecular level. Several investigations on TET were accomplished, however, the influences of liquid film thickness on TET have not been sufficiently examined. Thus, this paper analyses the influences of liquid film thickness on TET across solid–liquid (S-L) interfaces. Two liquid film thicknesses (Lz) of 30 Å and 60 Å have been evaluated, and two shear directions (x- and y-directions) have been tested in the simulation system. It has been found that there is no significant difference in the density distribution of liquid regardless of the shear directions for the same Lz. However, there are differences in the density distribution of liquid between Lz of 30 Å and 60 Å. Based on the results its suggests that, the cut-off of the temperature and velocity at the contact interfaces of solid and liquid is substantially influences by the liquid thickness of the simulation system. It is found that, there are a significant different in the thermal boundary resistance (TBR) for Lz of 30 Å and 60 Å for cases liquid sheared in the x-direction. Whereas TBR for Lz of 30 Å and 60 Å sheared in the y-direction have no significant difference. In conclusion, the TET is affected by the velocity cut-off at the contact interfaces of solid and liquid where larger velocity discontinuity exhibits higher TBR.

Surface thermal fluctuation spectroscopy study of ultra-thin ionic liquid films on quartz

We apply a surface thermal fluctuation spectroscopy technique to investigate the liquid nature of ionic liquid (IL) thin films and micro-droplets. While the spectrum for bulk IL was well explained by a simple Newtonian fluid model, a significant contribution of viscoelasticity was suggested for the vacuum-deposited thin films and micro-droplets. For a sample of IL micro-droplets with statistical distributions of their size and thickness, the scanning surface fluctuation spectroscopy was demonstrated to spatially map out the thickness and viscoelasticity values for effectively investigating the correlation between them, successfully reproducing a typical surface morphology of the sample as well.

Effects of Heat Source Temperature, Nanostructure, and Wettability on Explosive Boiling of Ultra-Thin Liquid Argon Film Over Graphene Substrate: A Molecular Dynamics Study

Background: The study on explosive boiling phenomenon has received increasing attention because it involves many industries, such as advanced micro-, nano-electromechanical and nano-electronic cooling systems, laser steam cleaning, and so on. Objective: In the present work, the explosive boiling of ultra-thin liquid film over two-dimensional nanomaterial surface in confined space with particular emphasis under the three different influencing factors: various heights of nanostructures, various wetting conditions of solid-liquid interface as well as various heat source temperatures. Methods: Molecular Dynamics simulations (MDs) in present work have been adopted to simulate the whole explosive boiling process. Results: For different heat source temperature case, the higher the heat temperature is, the less time the explosive boiling spends after relaxation. For nanostructure case, nanostructure surface significantly increases heat transfer rate and then leads to the increase of phase transition rate of explosive boiling. For different wetting property case, the increase of surface wettability results in an increase of phase transition to some degree. Conclusion: The addition of nanostructures, the higher heat source temperature and good wettability between thin liquid film and substrate surface dramatically improve thermal heat transfer from solid surface to liquid film, which will give rise to explosive boiling occur. In addition, the non-vaporized argon layer still exists in these three factors despite continuous thermal transmission from the substrate surface to liquid argon film adjacent to the solid surface even other vaporized argon atoms.

Nanostructure Determines the Wettability of Gold Surfaces by Ionic Liquid Ultrathin Films.

Ionic liquids are employed in energy storage/harvesting devices, in catalysis and biomedical technologies, due to their tunable bulk and interfacial properties. In particular, the wettability and the structuring of the ionic liquids at the interface are of paramount importance for all those applications exploiting ionic liquids tribological properties, their double layer organization at electrified interfaces, and interfacial chemical reactions. Here we report an experimental investigation of the wettability and organization at the interface of an imidazolium-based ionic liquid ([Bmim][NTf2]) and gold surfaces, that are widely used as electrodes in energy devices, electronics, fluidics. In particular, we investigated the role of the nanostructure on the resulting interfacial interactions between [Bmim][NTf2] and atom-assembled or cluster-assembled gold thin films. Our results highlight the presence of the solid-like structured ionic liquid domains extending several tens of nanometres far from the gold interfaces, and characterized by different lateral extension, according to the wettability of the gold nanostructures by the IL liquid-phase.

Electrocapillary effect in liquid films with an electrically charged interface

Abstract The electrocapillary effect is the change in interfacial tension between two immiscible liquids when subjected to an electric field across the interface. The change in interfacial tension occurs due to variations in the surface charge density of adsorbed ions or polar species at the interface. In the present work, we provide fluid mechanical insights into electrocapillarity-based dynamics of an interface between a passive air layer and a thin liquid film that has small, but finite electrical conductivity. We formulate and solve mathematical equations that model the situation in which a liquid film bounded by an air layer of controllable thickness is sandwiched between two electrode plates, and an electric field is applied across the layers. As the liquid and air have different conductivities, free charges would accumulate at the liquid–air interface to ensure that current is conserved across the layers. In our model, the interfacial tension is assumed to vary as per the classical Lippmann’s equation. The present mathematical model describes spatiotemporal variation of the film height and interfacial charge density as a function of the applied potential, air layer width, and the sensitivity of interfacial tension to the electric potential. Our results show that ultrathin liquid films (