Nanoscale Liquid Film Research Articles

Mechanism of Friction Enhancement Induced by Nanoscale Liquid Film: A Brief Review

Mechanism of Friction Enhancement Induced by Nanoscale Liquid Film: A Brief Review

Fluctuation-driven instability of nanoscale liquid films on chemically heterogeneous substrates

The instability and dewetting of bounded liquid films at the nanoscale are shown to strongly depend on thermal fluctuations. In this work, this fluctuation-driven instability of films on a chemically heterogeneous substrate is investigated both by a stochastic lubrication equation (SLE), which models thermal fluctuations with white noise, and by molecular dynamics (MD). Linear instability analysis for the SLE, considering slip and intermolecular forces of the heterogeneous substrates, is employed to derive a spectrum of the thermal capillary waves and their corresponding interface roughness, which are then quantitatively confirmed by numerical solutions for the SLE and MD. The fluctuation-driven instability is found to be enhanced by the slip and intermolecular forces, with the latter becoming dominant toward the final rupture. Moreover, the results suggest that a heterogeneous substrate can be approximated as a homogeneous one with effective (averaged) slip lengths and Hamaker constants for the intermolecular forces. The gradients of the slip and intermolecular forces due to the heterogeneities are shown not to affect the instability significantly.

Molecular study on anisotropic thermal conductivity of nanoscale liquid argon films with different configurations

The thermodynamic and transport properties of thin films may be substantially divergent from their bulk materials. This is especially distinguished for nanoscale liquid films, as demonstrated by prior work about the specific heat capacity of nanoscale water films. Furthermore, the forms of thin liquid films, either freestanding, sessile or confined, also exhibit apparent impact on the properties. In this investigation, the influence of size and solid/liquid interface on the thermal conductivities of nanoscale liquid argon films with these different forms is studied by employing the molecular dynamics method. The results show that the anisotropy of the thermal conductivity is closely related to the configuration, but weakened while the film thickness increases. The relationships between the anisotropic thermal conductivities and the thin film size are revealed by considering the contribution of a solid/liquid interface to the thermal conductivity. The reason is analyzed in the aspect of size and interface by comparison of the densities, potential energy distributions, and vibrational densities of state of argon atoms, which show asymmetry in the directions parallel and perpendicular to the sessile film. Thus, insight is provided into the interface effect on the anisotropic thermal conductivity of nanoscale liquid argon films with different forms which can be found in many applications including interface separation, evaporation, and distillation.

Molecular dynamics simulations of R32/R1234yf nanoscale boiling on a smooth substrate

The boiling process of mixture fluids is of great significance in both natural research and industrial applications. Among the various mixtures, R32/R1234yf has a great application potential in power system such as refrigeration and Organic Rankine Cycle. Liquid film boiling is a key link to reveal the boiling heat transfer mechanism of mixtures. In this study, MD was used to study the boiling phenomenon of nanoscale liquid film with different concentrations of R32/R1234yf (xR32 = 0, 0.34, 0.58, 0.75, 1) on the heated smooth copper surface. The different stages of liquid film boiling were systematically described, and the differences in bubble inception time, concentration distribution near the wall, vertical density and temperature distribution of liquid film with different components were compared. The heat flux and surface wettability of different composition was regulated and the sensitivity of pure fluids boiling and mixtures boiling to the heat flux and wettability were compared. It was found that similar boiling processes occur on the heated surface of the thin liquid film with different compositions. With the increase of R32 concentration, the film boiling process becomes more intensely. The boiling process of the mixture liquid film is closer to that of pure R32 liquid film because R32 is more easily adsorbed by the solid surface than R1234yf. The vaporization rate obtained by simulation is significantly lower than the prediction value. The main reason is that the difference of the inertial forces between the two working fluids causes the saturation pressure at the bubble interface to be lower than that predicted by the nominal components. For mixture liquid film boiling, a more volatile component has a greater influence, it can be reasonably speculated that adding component of more solvophilic to the heat surface can effectively accelerate the boiling process.

Relaxation of Thermal Capillary Waves for Nanoscale Liquid Films on Anisotropic-Slip Substrates.

The relaxation dynamics of thermal capillary waves for nanoscale liquid films on anisotropic-slip substrates are investigated using both molecular dynamics (MD) simulations and a Langevin model. The anisotropy of slip on substrates is achieved using a specific lattice plane of a face-centered cubic lattice. This surface's anisotropy breaks the simple scalar proportionality between slip velocity and wall shear stress and requires the introduction of a slip-coefficient tensor. The Langevin equation can describe both the growth of capillary wave spectra and the relaxation of capillary wave correlations, with the former providing a time scale for the surface to reach thermal equilibrium. Temporal correlations of interfacial Fourier modes, measured at thermal equilibrium in MD, demonstrate that (i) larger slip lengths lead to a faster decay in wave correlations and (ii) unlike isotropic-slip substrates, the time correlations of waves on anisotropic-slip substrates are wave-direction-dependent. These findings emerge naturally from the proposed Langevin equation, which becomes wave-direction-dependent, agrees well with MD results, and allows us to produce experimentally verifiable predictions.

Atomistic Insight into the Effects of Depositional Nanoparticle on Nanoscale Liquid Film Evaporation.

Nanoscale liquid film evaporation plays an essential role in many engineering applications. This study carries out molecular dynamics simulations on the effects of the depositional nanoparticle's wettability and volume in base fluid on the evaporation process to understand how the depositional nanoparticle affects the evaporation heat transfer. Increasing the nanoparticle's wettability can enhance the evaporation heat transfer process, and the enhancement effect of the hydrophobic surface is more remarkable than that of the hydrophilic surface. This because the increasing wettability causes more significant solid-liquid interaction. However, the potential energy of argon atoms at the liquid-vapor interface is almost unaffected by wettability. Moreover, when the depositional nanoparticle locates below the free liquid film, increasing the nanoparticle volume has a better heat transfer performance. As the volume increases, the heat transfer through the nanoparticle becomes more obvious, which effectively enhances the heat transfer at the solid-liquid interface and the liquid-vapor interface. The latent heat of phase change at the liquid-vapor interface is almost unchanged so that the evaporation can be enhanced. This research provides an understanding of the effects of depositional nanoparticles on nanoscale evaporation, which can impact several engineering applications, including devices' cooling and fluid transport.

Thermal capillary wave growth and surface roughening of nanoscale liquid films

The well-known thermal capillary wave theory, which describes the capillary spectrum of the free surface of a liquid film, does not reveal the transient dynamics of surface waves, e.g. the process through which a smooth surface becomes rough. Here, a Langevin model is proposed that can capture this dynamics, goes beyond the long-wave paradigm which can be inaccurate at the nanoscale, and is validated using molecular dynamics simulations for nanoscale films on both planar and cylindrical substrates. We show that a scaling relation exists for surface roughening of a planar film and the scaling exponents belong to a specific universality class. The capillary spectra of planar films are found to advance towards a static spectrum, with the roughness of the surface $W$ increasing as a power law of time $W\sim t^{1/8}$ before saturation. However, the spectra of an annular film (with outer radius $h_0$ ) are unbounded for dimensionless wavenumber $qh_0<1$ due to the Rayleigh–Plateau instability.

Simulation of Wetting Phase Transitions in Thin Films

Based on the hydrodynamic model, the kinetics of wetting phase transitions in nanoscale liquid films on the substrate surface is analyzed. In the range of metastable states, the features of the formation of equilibrium clusters are studied and corresponding film thickness distributions are calculated. In the range of unstable states, the kinetics of the phase transition resulting in cluster formation is analyzed. In the early stage, regions shaped as holes with a film thickness close to the equilibrium one are formed. Hole coalescence leads to a film material redistribution followed by clustering. For this process, the kinetics of the average hole size and concentration is calculated. For formed clusters, the kinetics of the average radius, average height, concentration, and their radius and height distribution function are studied.

Dependence of the refractive and absorption indices on nanoscale liquid film thickness

Dependence of the refractive and absorption indices on nanoscale liquid film thickness

Strong competition between adsorption and aggregation of surfactant in nanoscale systems

HypothesisThe size-dependent behavior including surface tension, surface density (Γ), and critical micelle concentration (CMC) of a nanoscale liquid film containing surfactant has not been investigated until now. ExperimentsStrong competition between surface adsorption and bulk aggregation of surfactant in nanoscale systems was explored by Many-body Dissipative Particle Dynamics simulations. FindingsIn nanoscale systems, as the surfactant concentration increases, Γ continues rising even after CMC is exceeded. The saturation level of Γ is achieved only when the surfactant bulk concentration is over ten times of CMC. Moreover, both surface micelles formed by adsorbed surfactant and the sublayer below the adsorbed layer are clearly identified. The former can reduce the contacts of adsorbed surfactant with water, while the latter has the surfactant concentration significantly higher than that in bulk. The strong coupling between adsorption and micellization is attributed to large surface-to-volume ratio compared to macroscopic systems, and can be simply realized by the fact that the ratio of the numbers of surfactant distributed in bulk (nbulk) and at interface (nads) is always less than unity (nbulk/nads < 1) in nanoscale systems.

Molecular Dynamics Simulations of the Effects of Surface Sinusoidal Nanostructures on Nanoscale Liquid Film Phase-Change

Nanostructures on heat transfer surfaces can enhance micro/nanoscale phase-change heat transfer processes. To understand the enhancement mechanisms provided by nanostructures, evaporation processes of nanoscale liquid films on periodic sinusoidal-shaped surfaces were investigated via molecular dynamics simulations. By changing the amplitudes (A) and periods (T) of the curves describing the sinusoidal shapes, which possess shapes similar to j(x)=h0+Asin(2πx/T), sinusoidal surfaces of different sizes were constructed. An unevaporated region always existed on the copper plate for all surfaces during phase-change processes. Moreover, we calculated the interfacial thermal resistances and the mismatches between the vibrational density of states of the solid and liquid for different surfaces. The results show that the argon temperature changes and evaporation rates during the phase-change process on sinusoidal surfaces are higher than those on flat surfaces, and these results increase with the increase in amplitudes and the decrease in periods within certain limits mainly because of the thermal resistance decrease at the solid-liquid interface. Furthermore, the corresponding mismatches between the vibrational density of states of the solid and liquid also decrease, which indicates that the existence of sinusoidal nanostructures enhances the heat transfer of the phase-change process.

Molecular dynamics simulations of the effect of surface wettability on nanoscale liquid film phase-change

This article investigates the phase‐change of nanoscale liquid films on different wettability surfaces via molecular dynamics simulation, analyzes the thermal resistance between solid and liquid, investigates the thermal resistance at the solid‐liquid interface and studies the mismatch between the vibrational density of states of the solid and liquid for different wettability surfaces. The results show that a liquid region of 1.4 g/cm3 mass density always exists on the hydrophilic surface during phase‐change, whereas a liquid‐vapor‐liked region of 0.7–0.8 g/cm3 mass density exists on the hydrophobic surface. The thermal resistance at the solid‐liquid interface of the hydrophilic surface is less than that of the hydrophobic surface. The reason is that the mismatch between the vibrational density of states of the solid and liquid of the hydrophilic surface is less than that of the hydrophobic surface.

Automated formation of black lipid membranes within a microfluidic device via confocal fluorescence feedback-controlled hydrostatic pressure manipulations.

Black lipid membranes (BLMs) provide a biomimetic model system for studying cellular membrane processes, and are important tools in drug screening and biosensing applications. BLMs offer advantages over liposomes and solid-supported lipid bilayers in applications where access to both leaflets of the bilayer is critical. Reliable and repeatable formation of BLMs presents a major challenge, especially in systems that require interrogation of the membrane via optical microscopy. BLMs for optical interrogation are often formed by the manual painting method, which is tedious and has a high failure rate because it involves manual manipulation of nanoscale liquid films for membrane self-assembly. Here, we describe a fully automated technique for the formation of BLMs within the imaging plane of an inverted fluorescence microscope. The technique utilizes hydrostatic pressure manipulations within a simple microfluidic device, which are feedback controlled via confocal fluorescence monitoring of the BLM formation process. An algorithm for monitoring and precision control of BLM formation is devised and optimized to yield an 80% success rate for the formation of BLMs, with formation times on the order of 78min. Membranes formed via the automated procedure are confirmed to be fluid and biomimetic via spontaneous insertion of α-hemolysin pores with characteristic conductance of ca. 1nS. Graphical Abstract ᅟ.

Effects of wettability on explosive boiling of nanoscale liquid films: Whether the classical nucleation theory fails or not?

Abstract Two main reasons including practical and scientific significance continuously motivate the studies of explosive boiling of nanoscale liquid films. Whether the nanoscale explosive boiling agrees with classical nucleation theory is still an open issue. In this work, we study the effects of surface wettability on the explosive boiling of nanoscale liquid films using molecular dynamics simulations. A critical film thickness is proposed to address the debate of whether the classical nucleation theory fails in nanoscale boiling. The explosive boiling modes and dynamics are summarized with the phase diagram based on various simulation cases, considering the effects of surface wettability, film thickness, and surface superheat. When the films thickness exceeds the critical thickness, hydrophobic surfaces are more favorable for explosive boiling, which still agrees with the classical nucleation theory. However, a much higher superheat is required to trigger the explosive boiling of a thin liquid film when its thickness is less than the critical thickness on hydrophobic surfaces, which consists with previous molecular dynamics simulations. Furthermore, we also find that the explosive boiling is trigged faster for films on hydrophilic surfaces than those on hydrophobic surfaces with the same superheat owing to the lower Kapitza thermal resistance. With such heat transfer superiority, hydrophilic surfaces can heat liquid films faster to explosion. The deliveries of this work and the concept of the critical thickness might help to understand the still fledgling field of nanoscale boiling phase change and its relevant mechanisms.

Paradoxical Long-Timespan Opening of the Hole in Self-Supported Water Films of Nanometer Thickness.

The opening of holes in self-supported thin (nanoscaled) water films has been investigated in situ with the environmental scanning electron microscope. The opening of a hole occurs within a two-stage process. In the first stage, the rim surrounding a hole is formed, resembling the process that is observed under the puncturing of soap bubbles. In the second stage, the exponential growth of the hole is observed, with a characteristic time of a dozen seconds. We explain the exponential kinetics of hole growth by the balance between inertia (gravity) and viscous dissipation. The kinetics of opening a microscaled hole is governed by the processes taking place in the nanothick bulk of the self-supported liquid film. Nanoparticles provide markers for the visualization of the processes occurring in self-supported thin nanoscale liquid films.

Mechanics-Based Approach for Detection and Measurement of Particle Contamination in Proximity Nanofabrication Processes

In spite of the great progress made toward addressing the challenge of particle contamination in nanomanufacturing, its deleterious effect on yield is still not negligible. This is particularly true for nanofabrication processes that involve close proximity or contact between two or more surfaces. One such process is Jet-and-Flash Imprint Lithography (J-FIL™), which involves the formation of a nanoscale liquid film between a patterned template and a substrate. In this process, the presence of any frontside particle taller than the liquid film thickness, which is typically sub-25 nm, can not only disrupt the continuity of this liquid film but also damage the expensive template upon contact. The detection of these particles has typically relied on the use of subwavelength optical techniques such as scatterometry that can suffer from low throughput for nanoscale particles. In this paper, a novel mechanics-based method has been proposed as an alternative to these techniques. It can provide a nearly 1000 × amplification of the particle size, thereby allowing for optical microscopy based detection. This technique has been supported by an experimentally validated multiphysics model which also allows for estimation of the loss in yield and potential contact-related template damage because of the particle encounter. Also, finer inspection of template damage needs to be carried out over a much smaller area, thereby increasing throughput of the overall process. This technique also has the potential for inline integration, thereby circumventing the need for separate tooling for subwavelength optical inspection of substrates.

A nanosecond pulsed laser heating system for studying liquid and supercooled liquid films in ultrahigh vacuum.

A pulsed laser heating system has been developed that enables investigations of the dynamics and kinetics of nanoscale liquid films and liquid/solid interfaces on the nanosecond time scale in ultrahigh vacuum (UHV). Details of the design, implementation, and characterization of a nanosecond pulsed laser system for transiently heating nanoscale films are described. Nanosecond pulses from a Nd:YAG laser are used to rapidly heat thin films of adsorbed water or other volatile materials on a clean, well-characterized Pt(111) crystal in UHV. Heating rates of ∼10(10) K/s for temperature increases of ∼100-200 K are obtained. Subsequent rapid cooling (∼5 × 10(9) K/s) quenches the film, permitting in-situ, post-heating analysis using a variety of surface science techniques. Lateral variations in the laser pulse energy are ∼±2.7% leading to a temperature uncertainty of ∼±4.4 K for a temperature jump of 200 K. Initial experiments with the apparatus demonstrate that crystalline ice films initially held at 90 K can be rapidly transformed into liquid water films with T > 273 K. No discernable recrystallization occurs during the rapid cooling back to cryogenic temperatures. In contrast, amorphous solid water films heated below the melting point rapidly crystallize. The nanosecond pulsed laser heating system can prepare nanoscale liquid and supercooled liquid films that persist for nanoseconds per heat pulse in an UHV environment, enabling experimental studies of a wide range of phenomena in liquids and at liquid/solid interfaces.

Nano-scale liquid film sheared between strong wetting surfaces: effects of interface region on the flow

In this paper, we use molecular dynamics (MD) simulations to investigate changes in fluid flow at a solid/liquid interface. The flow is driven by shearing FCC structured solid molecular walls under isothermal conditions using previously developed interactive thermal wall models. For the nano-scale thin liquid film flows, a fluid molecular layer attached to the wall molecules behaves as an extended wall layer, which induces increased shearing in the middle of the fluid by reducing the width of the flow region. Small variations in molecular diameter length at the interface significantly affect flow characteristics. Shear locking on strong wetting surfaces caused by the dynamic structuring of fluid molecules (i.e., the fluid molecules layering on the solid surface due to the wall force field) increases the density and viscosity and decreases the shear rate and the heat dissipation ratio on the interface, which are important in nano-scale fluid flow analysis.

Thin liquid film lubrication under external electrical fields: Roles of liquid intermolecular interactions

One of the important features of the nanoscale liquid film lubrication is the formation of ordered layers at the solid/liquid interface. In this paper, the effect of the intermolecular interaction in liquid lubricant films on the formation of ordered layers after applying external electric fields (EEFs) has been investigated by measuring the central-film-thicknesses of liquids in concentrated point contacts and then inferring the thin film rheology. It has been found that the film formation properties of both pure liquid n-alkanes and liquid n-alcohols with relatively long chains have weak responses to EEFs, while those of their mixed solutions could be enhanced more notably by EEFs. In addition, the effect of the dispersive interactions between solvent molecules on the formation of ordered layers in thin lubrication films under EEFs was also discussed.

Nanoscale liquid-vapor phase-change physics in nonevaporating region at the three-phase contact line

Nanoscale liquid film evaporation is usually associated with super-high heat transport rates and can be found in natural processes and in many industrial and advanced technologies. In this paper, thin film evaporation is simulated in a nanochannel using molecular dynamics to study the effect of varying nanochannel height and film thickness. Three nanochannel heights (16.32, 25.5, and 35.7 nm; constant liquid film thickness=3 nm) and three liquid film thicknesses (2, 4, and 6 nm; constant nanochannel height=25.5 nm) are simulated to study six cases. A nonevaporating film is obtained for all six cases. Hamaker constant, vapor pressure, film thickness, and net evaporation and heat fluxes are evaluated. An additional simulation (case 7) is run with simultaneous evaporation-condensation; no nonevaporating film is obtained. Thus, the creation of a nonevaporating film, and its thickness (if the film forms), depends on the combination of three factors, namely, vapor pressure, substrate temperature, and solid-liquid molecular interaction strength.