Boundary Film Research Articles

Three-dimensional periodic thermoelastichydrodynamic modeling of hydrodynamic processes of a thrust bearing

The article presents the basic principles of three-dimensional mathematical modeling of the operation of a thrust plain bearing with fixed pads of the compressor. The model is based on the periodic thermoelastichydrodynamic (PTEHD) theory which allows calculating the temperature at the inlet to the pad and considering the complete thermal pattern. A description of the main provisions of the numerical implementation is given. In the stationary mode of the bearings operation, using the Sm2Px3Tx program, numerical experiments were carried out aimed at studying different boundary conditions to the Reynolds equation, the physics of the hydrodynamic process in the lubricating and boundary films of the bearing and the heat propagation in the body of the pad and thrust collar.

Transient Finite Element Simulation of a Lithium-Ion Battery Pack Thermal Management System Based on Latent Heat System Materials

Battery packs in high-intensity use run the risk of overheating, potentially inducing catastrophic thermal runaway. Thermal runaway occurs when an individual cell in a connected system fails and subsequently applies increased stress on the remaining cells, leading to cascading failures and destruction of the battery pack as a whole. One method for controlling battery temperature during usage is utilizing the battery pack casing as a latent heat system (LHS), specifically incorporating phase-change materials (PCMs). Using this method, the transient thermal response of a 84-cell, 270 V lithium-ion battery pack for an aircraft application was simulated using ANSYS FLUENT. Heat generation rates and specific heat capacity of a single cell were experimentally measured and used as input to the thermal and mechanical model. A heat generation load was applied to the entire battery pack, and natural convection film boundary conditions were applied to the exterior of the enclosure. The maximum temperature of the battery pack without any casing reached 69.8 ᵒC after 800 s of usage at a simulated peak power draw of 1013 W or 16 A. This exceeds the manufacturer’s maximum recommended operating temperature of 60 ᵒC. For other materials, we found the maximum temperature of the batteries reached 57.0 ᵒC with plastic casing and 45.0 ᵒC with aluminum casing. We present a design of the pack that incorporates a passive thermal management system using a composite material consisting of phase-changing material, Puretemp60®, mixed with expanded graphite (EG). Simulations of the battery pack showed a decrease in the maximum temperature with the Puretemp60® as the sole material reaching a maximum of 54.2 ᵒC and the Puretemp EG composite matrix having a maximum temperature of 51.1 ᵒC, both after 800 s at the same power draw.The optimal material combination was then determined through evaluating several PCM and EG composites in several ratios using ANSYS FLUENT, followed by experimental testing. The mixture was cost effective, easy to mix, and demonstrated good mechanical properties. Results illustrated that a mixture of 25% expanded graphite with 75% Puretemp60® (PCM) was capable of maintaining battery temperatures within the acceptable operating range under moderate load conditions and therefore preventing thermal runaway. A resulting weight reduction of about 23.5% was also achieved using the Puretemp EG composite matrix in comparison to aluminum casing, and about a 7.2% reduction compared to plastic casing.Differential scanning calorimetry (DSC) was utilized to determine the phase transformation of the composites. The addition of expanded graphite minimally affected the phase transformation temperature, while decreasing the latent heat of transformation by about 40%. Scanning electron microscope (SEM) pictures visualized the morphology of the graphite during each stage of the composite fabrication process. The Puretemp60® and expanded graphite were easily mixed and the backscattering electron images indicated the composition was homogenous.The heat generation rate of the battery was calculated using the q̇gen= mCp(dT/dt) + UA∆T. The change in temperature with respect to time was experimentally determined from a series of discharges. Heat generation in the batteries was modeled as uniformly distributed. The average of the experimentally measured heat generation rates for a 1P constant power discharge were divided by the volume of the battery to yield a curve for the average body heat flux load in W/m3 over the discharge of the battery. A polynomial curve fit to the average heat generation rates was applied as a heat load to each battery in the simulation. The heat generation due to discharge was tested with a setup where the battery was insulated in polyurethane foam in order to reduce convection heat transfer during the experiments. A polyurethane foam block was cut such that the battery could be placed in the center, along with three T-type thermocouples to measure the surface temperature and external air temperatures. Two metal cylinders, one consisting of copper and the other of 6061 aluminum, were machined to the same cylindrical dimensions as the battery. These cylinders were then used to determine the heat loss, or UA value, of the foam insulation block setup. A National instruments NI 9147, Chroma Programmable DC Power Supply, and LabView software were used for the experiments. Figure 1

The equivalence of thermodynamic potentials for ferroelectric thin films

A ferroelectric thin film epitaxially deposited on a substrate is usually subjected to the mixed mechanical boundary conditions that can neither be treated as purely fixed-strain boundaries nor purely fixed-stress ones, thus causing the inconsistency or inaccuracy for choice of the Helmholtz or Gibbs thermodynamic potential. It would be of interest to clarify this inconsistency and set up some rules for such a choice. In this work, we discuss various roadmaps to construct two types of modified thermodynamic potentials under the epitaxial thin film boundary conditions. The equivalence of these thermodynamic potentials is then established. Subsequently, a set of misfit strain–strain phase diagrams by choosing K0.5Na0.5NbO3 thin films as an example of realistic calculations are constructed to check this equivalence. Finally, some scenarios on choosing various thermodynamic potentials for ferroelectric thin films are discussed.

Development of methods for evaluation of lubrication properties of hydraulic aviation oils

A method for evaluation of the lubricating and rheological properties of hydraulic oils in tribological contacts has been developed, which consists in online studying samples of commercial batches of oils on a software and hardware complex with visual evaluation of the kinetics of changes in the main tribological indicators of friction contact. Using a roller analogy, the operation of gears in the conditions of rolling with 30% sliding is simulated. Samples of AMG-10 oil from two producers are analyzed. It is established that with increasing temperature of lubricant for Sample 2 (“Kvalitet-Avia” AMG-10), a long-term restoration of protective boundary films of oil is observed and the period of their formation increases by 2.5 times, causing the implementation of a semidry mode of lubrication at start-up. The total thickness of the lubricating layer is 1.27 times less as compared with Sample 1 ("Bora B" AMG-10 oil), regardless of the lubricant temperature. Also, the rheological properties of the oils have been determined. Sample 1 exhibits low shear stresses at the level of 9.4 MPa and high effective viscosity, 4249 and 5039 Pa·s, at a volumetric oil temperature of 20 and 100 ºС, respectively. For Sample 2, with increasing oil temperature to 100 ºC shear stress increases by 1.15 times and the effective viscosity in contact decreases by 1.53 times. Additives present in Sample 1 are characterized by more effective antiwear properties and thus increase the wear resistance of contact surfaces in the conditions of rolling with sliding thanks to strengthening of the surface metal layers during operation, while Sample 2 undergoes strengthening-softening processes which reduce the wear resistance of friction pairs

Micro- to Nano- and from Surface to Bulk: Influence of Halogen-Free Ionic Liquid Architecture and Dissociation on Green Oil Lubricity

Four nonhalogenated ionic liquids (ILs) based on the same phosphonium cation are investigated in terms of the anion suitability for enhancing the lubricity of a biodegradable oil. For all test conditions, typical for industrial machine components, the lubrication is shown to be governed by nonsacrificial films formed by the physisorption of ionic species on the tribo-surfaces. The anionic structure appears to have an important role in the formation of friction modifying films. The orthoborate ILs exhibit the formation of robust ionic boundary films, resulting in reduced friction and better wear protection. On the contrary, the surface adsorption of phosphinate and phosphate ILs appears to antagonistically disrupt the intrinsic lubrication properties of the biodegradable oil, resulting in high friction and wear. Through additional investigations, it is postulated that the higher dissociation of orthoborate ILs in the biodegradable oil allows the formation of hierarchical and electrostatically overscreened layer structures with long-range order, whereas the ILs with phosphate and phosphinate anions exhibit low dissociation in biodegradable oil, possibly due to the ion pairs being surrounded by a hydrocarbon halo, which presumably results in weak adsorption to form a mixed interfacial layer with no long-range order.

Sliding friction and wear of human teeth against biocompatible polyether ether ketone (PEEK) under various wear conditions

For the application in dentistry, the friction and wear behaviour of the thermoplastic polymer polyether ether ketone (PEEK) was analysed. Therefore, the friction coefficient and specific wear rate of PEEK against human enamel/dentine was investigated under various wet conditions. The tooth specimens consisting of enamel and dentine were prepared from nine mandibular molars. The sliding friction as well as the material loss of the counterparts was determined by using a block-on-ring rotational test configuration with a boundary film lubrication (deionised water). The material loss was measured by profilometry. The results were statistically analysed by one-way analysis of variance (ANOVA). As a result, the counter surfaces of the tooth samples showed a higher hardness and were stiffer than the polymeric specimens made of PEEK. The initial sliding friction coefficient varied in the range of μ = 0.23-0.44 for the three investigated rotational velocities from 1 m/s to 0.1 m/s and the stationary values were measured in the range of μ = 0.54-0.58 for the total sliding path of 1500 m. The resulting specific wear rate of PEEK was significantly higher ws = 80 ± 65.5 10-6 mm³/Nm than the coefficient of enamel/dentine ws = 0.9 ± 0.7 10-6 mm³/Nm (p < 0.01). In most cases, a significant increase of averaged surface roughness of the tooth samples was determined after tribological tests. In general, the investigations obtained a low removal of dental hard tissue (enamel, dentine) compared to the material loss of PEEK.

Oxidational wear in lubricated contacts – Or is it?

This study examines the influence of inert gas atmosphere on the wear behaviour of rubbing steel-on-steel contacts lubricated by two hydrocarbon base fluids, isooctane and hexadecane. It is found that for both fluids, wear and mean friction in nitrogen and argon atmospheres are considerably lower than in dry air. As the oxygen content in nitrogen is increased, mean friction and wear both increase, to level out above about 10% oxygen (an O2 partial pressure of 10 kPa). Raman analysis of rubbed surfaces shows the presence of a carbon film on surfaces rubbed in inert gas and at low O2 levels. This film is not observed at high O2 levels.These findings indicate that the prevailing model of oxidational wear in lubricated contacts, that states that wear is greater in air than in inert gas because of corrosion by oxygen, is largely incorrect. Instead, the deleterious effect of oxygen on lubricated wear is primarily due to it preventing the formation of a lubricious, carbon-based boundary film that is generated from hydrocarbon base fluids on rubbing steel surfaces in inert gas conditions.The ability of organic fuels and lubricants to form carbon-based films on rubbing steel surfaces in inert atmospheres may provide a mechanism for reducing friction and wear of fuel- and oil-lubricated machine components. The study also provides a platform from which to design lubricant formulations for use in inert atmospheres.

Microstructure and mechanical properties of difficult to weld Rene 77 superalloy produced by laser powder bed fusion

Fabrication of γ′ precipitation strengthened nickel-based superalloys via laser powder bed fusion still remains a challenge. In this study, Rene 77, a high γ′ containing superalloy that is considered as difficult to weld, was processed by laser powder bed fusion. Crack-free parts with high density were fabricated without any compositional modifications or preheating of the built plate. This defect-free structure was maintained upon solutionizing and aging heat treatment. The microstructure of the samples has been characterized in detail following the fabrication and after the heat treatment. Scanning electron microscopy analysis revealed that the as-built microstructure consists of columnar grains mainly aligned in the <100> direction along with extremely fine γ′ precipitates and spherical cell boundary carbides. The grain structure and texture were unaffected by the applied heat treatment due to the pinning effect exerted by the carbide particles. Development of a bimodal γ′ distribution including cuboidal primary and spherical secondary precipitates was observed in the heat-treated sample. Additional carbide formation as a discontinuous grain boundary film was also seen. Tensile deformation behavior for both conditions was also tested at room temperature and 810 °C. Measured strength values for all test conditions were higher compared to a wrought and heat-treated alloy tested at room temperature. The as-built sample showed hardening and loss of ductility during elevated temperature testing due to γ’ precipitation at the test temperature. The microstructure of the heat-treated sample was not altered during testing at 810 °C. However, improved elongation behavior and transition in fracture mode from cleavage to ductile fracture were observed due to microtwin formation at elevated temperatures.

The mechanism of formation of boundary lubricating films during friction in a medium of di(2-ethylhexyl) sebacate

In this work, the mechanism of formation of boundary lubricating films during friction in synthetic oil di(2-ethylhexyl) sebacate (DEHS) was studied. The study of the kinetics of the interaction of DEHS with the metal surface showed that the formation of the boundary film proceeds in the first 40 s of friction. The unoxidized iron detected by the XPS method on the friction track accelerates the chemical transformations of lubricant molecules. Removing the oxide film from the friction surface is a mechanically activated process. When DEHS interacts with the iron surface, the reactions of decarboxylation of molecules proceed, as a result free radicals are formed. The sequence of transformations of free radicals and compounds with CC bonds leads to the formation of surface films upon friction. The retention of the film on the surface occurs due to the forces of interaction of the metal with free radicals or unsaturated bonds of organic molecules.

Elasto-hydrodynamic friction changes on steel surfaces arising from the modified surface energy of the steel due to additive boundary films

The effect of three different film-forming additives mixed in PAO oil on the elasto-hydrodynamic (EHD) friction was investigated and compared to cases with the base oil only. When the boundary films were formed on the surfaces, the organic friction modifier (OFM) decreased the EHL friction by up to 8.7%, the ionic liquid (IL) decreased it by up to 6.4%, while the polymeric organic friction modifier (pOFM) increased it by up to 4.2%. In contrast, if the boundary films were not formed, the EHL friction remained the same as with the base oils only. The oils’ surface tension and viscosity were analysed at 25 °C and 100 °C as potential influencing effects, and the most important parameter for friction changes was found to be the surface energy. The mechanism behind this EHL friction reduction is, therefore, the effect of the boundary films on the poor oil-surface wetting and the consequent boundary slip. This study confirms the poor wetting arising from additive boundary-film formation as a relevant EHL friction-reduction mechanism. This observation is similar to that observed previously for surface coatings, thus suggesting that the boundary films of the surface-energy-reducing additives is a parameter with the potential to reduce the EHL friction.

Formation of boundary lubricating films during friction in synthetic oil modified with naphthalene derivatives

The influence of additives of naphthalene derivatives on the formation of boundary lubricating layers during friction in a dioctyl sebacate synthetic oil is considered. It is determined, that the activity of the additives is determined by the presence of acidic groups in their molecules and the ability to form complex compounds with metals. Keywords: boundary film, dioctyl sebacate, naphthalene derivatives, complex formation, lubricating properties alekc_sap@mail.ru

Restricted growth and grain boundary reinforcement of MAPbBr3 film by graphene quantum dots with enhanced luminescence and stability

Organic-inorganic hybrid perovskite (OIHP) attracted much attention in the optoelectronic applications owing to its high yield, adjustable color and good mono-chromaticity. However, the defective sites at MAPbBr3 film boundaries greatly jeopardize their luminescent property. In this study, graphene quantum dots (GQDs) are incorporated into the MAPbBr3 film and pushed to the grain boundaries during perovskite crystallization via a modified freeze casting approach. The rich carboxyl groups of GQDs interact with the dangling lead atoms at grain boundaries, thus reducing non-radiative recombination centers within the film. On the other hand, GQDs create additional nucleation sites, which helps to form a uniformly covered luminescent film with small grains. As a result, the incorporation of GQDs increases the photoluminescence (PL) intensity of the emission film by about six times and well preserves the crystal phase for 36-day storage in ambient air.

Fluctuations of the critical Casimir force.

The critical Casimir force (CCF) arises from confining fluctuations in a critical fluid and thus it is a fluctuating quantity itself. While the mean CCF is universal, its (static) variance has previously been found to depend on the microscopic details of the system which effectively set a large-momentum cutoff in the underlying field theory, rendering it potentially large. This raises the question how the properties of the force variance are reflected in experimentally observable quantities, such as the thickness of a wetting film or the position of a suspended colloidal particle. Here, based on a rigorous definition of the instantaneous force, we analyze static and dynamic correlations of the CCF for a conserved fluid in film geometry for various boundary conditions within the Gaussian approximation. We find that the dynamic correlation function of the CCF is independent of the momentum cutoff and decays algebraically in time. Within the Gaussian approximation, the associated exponent depends only on the dynamic universality class but not on the boundary conditions. We furthermore consider a fluid film, the thickness of which can fluctuate under the influence of the time-dependent CCF. The latter gives rise to an effective non-Markovian noise in the equation of motion of the film boundary and induces a distinct contribution to the position variance. Within the approximations used here, at short times, this contribution grows algebraically in time whereas, at long times, it saturates and contributes to the steady-state variance of the film thickness.

Effect of low grain boundary crystallization Cu2ZnSnSxSe4−x counter electrode on the performance of dye-sensitized solar cells

The high surface area of porous Cu2ZnSnSxSe4−x (CZTSSe) counter electrodes (CEs) was commonly used for dye-sensitized solar cells (DSCs), because it can retard electron transport processes and form high quality crystallized film. Herein, we introduced a facile method to prepare CZTSSe film with bulk large-grained crystal by vacuum fast processing (VFP), which beneficial to form oriented crystalline film and reduce grain the film boundaries. Thus, performances of the DSCs devices showed an outstanding PCE of 8.3% with Voc, Jsc and fill factor of 0.729 V, 16.9 mA·cm−2, and 67.4%, respectively. With conducting spectral characterization and electrochemical techniques, it was found that the accelerating charge transfer speed and superior electrocatalytic activities contribute the majority part to DSCs photovoltaic performances. Furthermore, under AM1.5 illumination at 45 °C, the DSCs devices showed good stability and retained over 94% of its initial performance for 1000 h light soaking, indicating of their general applicability in fabricating high-performance DSCs.

Effects of UHMWPE viscoelasticity on the squeeze‐film lubrication of hip replacements

AbstractMany studies have been performed to analyse the lubrication of artificial joints since the pioneer work by the late Professor Duncan Dowson. However, the viscoelastic deformation of one of the most widely used bearing materials, ultra‐high‐molecular‐weight polyethylene (UHMWPE), has only been considered recently. The described study attempted to investigate the effect of UHMWPE viscoelasticity on the elastohydrodynamic lubrication of such a soft artificial hip replacement under squeeze‐film motion. A transient viscoelastic squeeze‐film lubrication model of a typical hip implant was developed and solved to obtain the film thickness and pressure distributions. A boundary film thickness was adopted to consider the direct and indirect lubricant contact conditions. The results showed that the viscoelasticity had marked effects on the squeeze‐film lubrication performance of UHMWPE artificial hip joints. The minimum film thickness in the viscoelastic model was smaller than that of the elastic model, causing an earlier direct contact. However, the film thickness within the central contact region in the viscoelastic model was greater than that of the elastic model due to the restricted flow of the lubricant, therefore enhancing the lubricating effect and particularly with a short relaxation time and mechanical loss factor.

The Influence of Steel Composition on the Formation and Effectiveness of Anti-wear Films in Tribological Contacts

The effectiveness of antiwear additives in laboratory tests is commonly evaluated using specimens made of AISI 52100 through-hardened bearing steel. However, many lubricated machine components are made of steels with significantly different material compositions, which raises an important practical question of whether the performance of antiwear additives with these other steel types is different from that established with AISI 52100. To help answer this question, this paper investigates the influence of steel composition on the formation and effectiveness of antiwear films. Four steels that are commonly used in tribological applications, namely AISI 52100 through-hardened bearing steel, 16MnCr5 case-carburised gear steel, M2 high speed steel and 440C stainless steel are tested in rolling-sliding, ball-on-disc contacts lubricated with three custom-made oils, one containing ZDDP and two containing different types of ashless antiwear additives. The relative effectiveness of their boundary films was assessed by measuring their thickness and associated wear and friction over 12 h of rubbing at two specimen roughness levels. For ZDDP it was found that the formation of antiwear film was not significantly influenced by steel composition or specimen surface roughness. A similar tribofilm thickness, final tribofilm roughness and friction was observed with all four steels. No measurable wear was observed. By contrast, for the ashless antiwear additives the thickness and effectiveness of their tribofilms was strongly influenced by steel composition, particularly at higher roughness levels. The exact trends in film thickness vs steel relationship depended on the specific chemistry of the ashless additive (ester-based or acid-based) but in general, relative to AISI 52100 steel, M2 steel promoted ashless tribofilm formation whilst 440C retarded ashless tribofilm formation. This behaviour is attributed to the presence of different alloying elements and the ability of the additives to extract metal cations from the rubbing surfaces to support the growth of a tribofilm. In all cases ZDDP films were thicker and rougher, and produced higher friction than those formed by the ashless additives. However, unlike ZDDP, ashless blends generally produced significant wear, particularly with 16MnCr5 and M2 steels. The results indicate that to ensure reliable performance of a given machine component, the chemistry of an ashless antiwear additive should be matched with the types of steel present in the lubricated machine.

Computer simulation of substrate effect on 2D ferromagnetic film state

In the article computer simulation of ground state for two-dimensional film on the substrate is performed. The influence of the substrate is modeled using a periodic potential. The potential contains a parameter defining the width of the maxima and minima. The film has a simple square crystal lattice. The substrate potential structure models a square lattice. We consider the case of different film coating ratio ratios. The lattice and substrate periods are also different. If the coating coefficient is different from one, mechanical stresses are present in the film. The case with one fixed edge for the film and periodic boundary conditions was investigated. The film atoms do not form a square lattice in this case. The displacements of atoms are periodic. The period depends on the coating factor. A superlattice is present in such film.

Lubricating Properties of Boundary Films in Tribosystems under Critical Operation Conditions

Lubricating Properties of Boundary Films in Tribosystems under Critical Operation Conditions

Modelling of a Torrefaction Process Using Thermal Model Object

So far, the torrefaction process has been merely discussed based on the physical and chemical characteristics of the final product, but the mechanism has not been yet pondered and investigated. Thus far, microwave torrefaction has been predominately used for thermal pre-treatment; therefore, a paradigm shift in the methodology has been introduced by using a Joule heating system. The article mainly focuses on the thermal engineering aspect of the torrefaction process. The densified black pine underwent thermal pre-treatment at a temperature of 523 K. The furnace used for torrefaction was initially improvised to carry out thermal degradation at quasi-static/dynamic conditions. A 3D PDE thermal model was developed to determine the numerical solution and temperature distribution across a black pine pellet. To compare the effect of the linear ramping profile, time-dependent as well as fixed Dirichlet conditions were applied to the proposed model. The mass distribution, duration of the torrefaction process, the effect of Nusselt and Reynold’s number of inert gas, and thermal history are some of the factors whose influence on the numerical solution was investigated. The simulation of thermal pre-treatment and its effect on the heat transfer characteristic was examined with help of a PDE thermal model, whereas the numerical solution of diffusion of the product of reactions was determined by solving the partial differential equations with the help of the discretisation method (PDEPE). The densification of black pine was performed in a ring die, whereas initial milling of biomass was carried out using a 1.5 mm sieve size. The system was found to have a homogeneous distribution in energy and temperature with time, whereas the amplitude of heat flux along the radial direction was reduced by 15% if the same pellet underwent torrefaction for a duration of 5 min in dynamic mode. Similarly, a 64.46% drop in amplitude of heat flux along the azimuth plane could be seen while performing torrefaction in a time-dependent thermal history. However, the relative amplitude of the heat flux at the centre of the pellet was estimated to be lowered by 98.41% along the vertical axis for heating a pellet in a quasi-static condition. The net change in the mass fraction of carbon dioxide across the boundary film was seen to be 40% higher than that of carbon monoxide. The rate of change of mass fraction of carbon monoxide across the boundary film was increased by 7–11% with the increase in torrefaction time. A 6.8% rise in the evaporation of water was noticed during the first half interval of torrefaction (from 5 min to 10 min). In the second half, from 10 min to 15 min, it was merely increased by 5.8%. A relative drop of 17.24% in water evaporation was estimated in the dynamic state of the system.

Controlled Spreading Rates to Distribute Nanoparticles as Uniform Langmuir Films.

We report on using a controlled spreading rate to create Langmuir films of nanoparticles with more uniform, macroscale packing. A dispersion of hydrophobic quantum dots in n-hexane was deposited on subphase solutions containing various compositions of water and glycerol. Fluorescence images were captured as the film spread radially. An average spreading rate was defined using film radius and time at maximum expansion. On water with the highest spreading rate, films have an open region surrounded by a coffee ring. At a well-defined slower spreading rate, a distinct inner compact region appears between the open film and coffee ring, now called an outer compact region. As the spreading rate decreases further, the relative position for the open film boundary moves inward while the relative areas for the inner and outer compact regions increase. Films are the smallest in size at the slowest spreading rate on glycerol. The patterns are button-like with a central depleted region (open film), compact inner and outer regions, and a less-dense outer edge region. Normalized radial profiles were used to generate a partition map for the relative radial positions marking each film region at different spreading rates. Area number densities were calculated in the highest-packed regions. The values give no conclusive evidence that nanoparticles stack as multilayers, even the most compactly covered regions. Films spreading on glycerol form the most uniform, circular-shaped, densely packed arrangement of nanoparticles as their final pattern.