Μm Depth Research Articles

Cellulose Surface Nanoengineering for Visualizing Food Safety.

Food safety is vital to human health, necessitating the development of nondestructive, convenient, and highly sensitive methods for detecting harmful substances. This study integrates cellulose dissolution, aligned regeneration, in situ nanoparticle synthesis, and structural reconstitution to create flexible, transparent, customizable, and nanowrinkled cellulose/Ag nanoparticle membranes (NWCM-Ag). These three-dimensional nanowrinkled structures considerably improve the spatial-electromagnetic-coupling effect of metal nanoparticles on the membrane surface, providing a 2.3 × 108 enhancement factor for the surface-enhanced Raman scattering (SERS) effect for trace detection of pesticides in foods. Notably, the distribution of pesticides in the apple peel and pulp layers is visualized through Raman imaging, confirming that the pesticides penetrate the peel layer into the pulp layer (∼30 μm depth). Thus, the risk of pesticide ingestion from fruits cannot be avoided by simple washing other than peeling. This study provides a new idea for designing nanowrinkled structures and broadening cellulose utilization in food safety.

Improved precision at high-spatial resolution of strontium isotope analysis in apatite and apatite inclusions by LA-MC-ICP-MS with 1013 Ω amplifiers

Recent advancements in in situ analytical techniques have generated new interest in measuring strontium (Sr) isotopes in rock-forming apatite and apatite inclusions in zircon, because they can bring new insights into the primary signatures of magmatic sources. This paper introduces a novel approach based on laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), which allows to achieve higher precision in situ analysis of Sr isotopes in small volumes of samples. The use of 1013 Ω current amplifiers for the acquisition of the signal on m/z 83, 83.5, 84, 85, 85.5, 86, 86.5, 87 and 88, along with a specific data reduction protocol, enhance both the internal precision and the external reproducibility of the 87Sr/86Sr ratio by a factor of four, presenting a significant improvement over the conventional use of 1011 Ω amplifiers. An internal precision better than 0.45‰ (i.e. 450 ppm; 2 s.e.) on the 87Sr/86Sr ratio can be achieved on the Durango apatite standard with a 13 μm square-shaped laser beam and a laser ablation pit of 60 μm depth. With a 5 μm beam, the precision is 2.2‰ on average with a 10 μm ablation pit depth for this standard. This analytical procedure was applied to investigate Sr isotopes in apatite and apatite inclusions within zircons from seven high BaSr magmatic rocks in the Northern Highland Terrane, Scotland. For these well-characterised, geochemically undisturbed samples, the data reveals similar Sr isotope compositions between apatite inclusions armoured in zircon, matrix apatites, and the bulk rock. This highlights the ability of apatite inclusions to faithfully record primary signatures of host magmas, elucidating their utility as reliable indicators in petrological studies of ancient samples.

Estimate of Water and Hydroxyl Abundance on Asteroid (16) Psyche from JWST Data

Our understanding of solar system evolution is closely tied to interpretations of asteroid composition, particularly the M-class asteroids. These asteroids were initially thought to be the exposed cores of differentiated planetesimals, a hypothesis based on their spectral similarity to iron meteorites. However, recent astronomical observations have revealed hydration on their surface through the detection of 3 μm absorption features associated with OH and potentially H2O. We present evidence of hydration due mainly to OH on asteroid (16) Psyche, the largest M-class asteroid, using data from the James Webb Space Telescope (JWST) spanning 1.1–6.63 μm. Our observations include two detections of the full 3 μm feature associated with OH and H2O resembling those found in CY-, CH-, and CB-type carbonaceous chondrites, and no 6 μm feature uniquely associated with H2O across two observations. We observe 3 μm depths of between 4.3% and 6% across two observations, values consistent with hydrogen abundance estimates on other airless bodies of 250–400 ppm. We place an upper limit of 39 ppm on the water abundance from the standard deviation around the 6 μm feature region. The presence of hydrated minerals suggests a complex history for Psyche. Exogenous sources of OH-bearing minerals could come from hydrated impactors. Endogenous OH-bearing minerals would indicate a composition more similar to E- or P-class asteroids. If the hydration is endogenous, it supports the theory that Psyche originated beyond the snow line and later migrated to the outer main belt.

Dynamic optical coherence tomography for imaging acute wound healing.

The aim of this study was to investigate acute wound healing with dynamic optical coherence tomography (D-OCT). From 22 patients with 23 split skin graft donor sites, vessels at four wound edges, the wound bed, and adjacent and unaffected skin of the contralateral leg were measured by D-OCT at six time points from surgery to 4 weeks of healing. Changes in vessel orientation, density, diameter, morphology and pattern in horizontal, vertical and 3D images were analysed for wound healing and re-epithelialization. At 300 μm depth, there were significant differences of blobs and serpiginous vessels between normal and wounded skin. The wound had significantly more vertically oriented vessels, a higher degree of branching, vessel density and diameter compared with healthy skin. 3D images showed increased angiogenesis from healthy skin towards the wound centre, significantly higher vessel density at the wound than at normal skin and the highest at the interface. During wound healing blobs, coils and serpiginous vessels occurred significantly more frequently in lesional than healthy skin. Vessel density was greatest at the beginning, decreased and then increased by 4 weeks post-surgery. D-OCT helps to evaluate acute wound healing by visualizing and quantifying blood vessel growth in addition to re-epithelialization.

Micron-scale topographies affect phagocytosis of bacterial cells on polydimethylsiloxane surfaces

Many medical devices implanted in patients to mitigate diseases and medical conditions have different types of topographic features. While appropriate textures can promote the integration of host cells and reduce scar tissue formation, some textured implants with inappropriate topographies have been associated with inflammation, bacterial colonization, or even malignant complications. To better understand how surface topography affects host immune response to colonizing bacteria, a protocol was developed to investigate phagocytosis of bacterial cells attached on polydimethylsiloxane (PDMS) surfaces with different square-shaped recessive patterns. The interaction between activated RAW 264.7 macrophages and Escherichia coli in recessive wells was visualized in 3D using multiple fluorescent markers. The results revealed that there is a threshold dimension of topography, below which phagocytosis of attached bacterial cells is significantly impeded. Specifically, under our experimental condition, up to 100-fold reduction in phagocytosis was observed in square-shaped patterns with 5 µm side length and 10 µm depth compared to the flat control and patterns with 10 µm or longer side length. The spacing between wells also showed significant effects; e.g., phagocytosis in the wells further decreased when spacing increased to 50 µm. These results are helpful for understanding how undesired topographies may contribute to bacterial colonization and thus infection and other associated complications. Statement of significanceSurface topography plays an important role in bacteria-material infections and thus the safety of implantable medical devices. Undesired topographic features can cause biofilm formation and related complications. However, how surface topography affects the capability of host immune cells to clear colonizing bacteria is not well understood. In this study, the interaction between macrophage RAW264.7 and colonizing E. coli cells on polydimethylsiloxane (PDMS) with recessive features is investigated. It was discovered that the size of recessive features and the spacing between these features have significant effects on phagocytosis of bacteria by macrophages. These new results are helpful for understanding the complex interaction among host cells, bacteria, and implanted biomaterials, which will help guide the rational design of safer medical devices.

High-efficiency thermo-optic control of a longitudinal radiation angle in a silicon-based optical phased array by leveraging the backside air trench structure.

We proposed a 2D 1 × 64 silicon optical phased array with a backside silicon-etched structure to achieve high tuning efficiency and a wide longitudinal steering range. At the radiator array, the n-i-n heater was implemented to steer the light in a longitudinal direction through the thermo-optic effect. The deep reactive ion etching process was utilized to generate the 600 µm depth air trench with a 1.8 cm2 area from the backside of the radiator array. We achieved almost 100% increment in terms of tuning efficiency, which is 1.56°/W for the proposed structure and 0.78°/W for the conventional structure.

Silk fibroin microgrooved zirconia surfaces improve connective tissue sealing through mediating glycolysis of fibroblasts

The use of zirconia has significantly enhanced the aesthetic outcomes of implant restorations. However, peri-implantitis remains a challenge to long-term functionality of implants. Unlike the perpendicularly arranged collagen fibers in periodontal tissue, those in peri-implant tissue lie parallel to the abutment surface and contain fewer fibroblasts, making them more prone to inflammation. Studies have shown that microgroove structures on implant abutments could improve surrounding soft tissue structure. However, creating precise microgrooves on zirconia without compromising its mechanical integrity is technically challenging. In this study, we applied inkjet printing, an additive manufacturing technique, to create stable silk fibroin microgroove (SFMG) coatings of various dimensions on zirconia substrates. SFMG significantly improved the hydrophilicity of zirconia and showed good physical and chemical stability. The SFMG with 90 μm interval and 10 μm depth was optimal in promoting the proliferation, alignment, and extracellular matrix production of human gingival fibroblasts (HGFs). Moreover, the in vitro results revealed that SFMG stimulated key glycolytic enzyme gene expression in HGFs via the PI3K-AKT-mTOR pathway. Additionally, the in vivo results of histological staining of peri-abutments soft tissue showed that SFMG promoted the vertical alignment of collagen fibers relative to the abutment surface, improving connective tissue sealing around the zirconia abutment. Our results indicated that SFMG on zirconia can enhance HGF proliferation, migration and collagen synthesis by regulating glycolysis though PI3K-AKT-mTor pathway, thereby improving connective tissue sealing.

Synergistic interplay between residual stress, hardness, and microstructural evolution in laser peened stainless steel 347

This study investigates the correlation between the properties of stainless steel 347 samples processed using Laser Shock Peening without Coating (LSPwC). While optimizing the process under different surface confinement conditions, the primary focus is on exploring the synergistic interplay between residual stress, hardness, and microstructural evolution for the selected condition. LSPwC with a water confinement layer at 6 GW cm−2 power density, showed significant Compressive Residual Stress (CRS) of −447.50 ± 48.71 MPa at 60 μm, reaching approximately 76 % of the yield stress and the dislocation density at 60 μm depth is 9.830 × 1014 m−2. Additionally, hardness increased to 256.50 ± 14.15 HV0.1 (∼+53 %) at 50 μm depth. Moreover, there was an increase in the grain boundary fraction to 0.3766 (∼+81 %), along with a rise in the twin boundary fraction to 0.1480. At 6 GW cm−2, correlation analysis revealed a strong relationship between compressive residual stress and hardness (correlation coefficient of 0.972), and between hardness and dislocation density (correlation coefficient of 0.968). Consequently, the 6 GW cm−2 condition emerges as the most practical and effective choice, offering substantial compressive stresses, improved hardness, and efficient grain refinement, and thus favours the 6 GW cm−2 power density condition as the optimal choice for LSPwC of stainless steel 347.

Local relaxation of residual stress in high-strength steel welded joints treated by HFMI

This research studies the influence of high-peak loads on local relaxation of residual stress and fatigue damage in high-strength steel welded joints treated by high-frequency mechanical impact (HFMI) treatment. The joint behavior is simulated with elastic–plastic finite element analyses that account for the combined effect of geometry, residual stress, and material properties. This simulation uses two treated geometry models: with or without surface roughness on HFMI groove, and two material properties: S690QL and AH36 structural steels. The results show that surface roughness and load history, including high-peak loads, significantly influence fatigue response. It is revealed that the model neglecting the surface roughness cannot represent the amount of residual stress change and fatigue damage at less than 100 µm depth from the surface. In addition, the local yield strength in the HFMI-treated zone affects the plasticity behavior near the surface imperfection under the high-peak loads, which provides comparatively different fatigue damage between S690QL and AH36 in some cases. As a result, this study provides the further understanding needed to develop a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

Rapid non-destructive inspection of sub-surface defects in 3D printed alumina through 30 layers with 7 μm depth resolution

The use of additive manufacturing (AM) processes has grown rapidly over the last ten years like fused deposition modelling and stereolithography techniques. 3D printing offers advantages in ceramic component production due to its flexibility. To enhance quality and reduce resource consumption in ceramics industry, fast, integrated, sub-surface and non-destructive inspection (NDI) with high resolution is needed. This study demonstrates sub-surface monitoring of 3D printed alumina parts to a depth of ∼0.7 mm in images of 400 × 2048 pixels with a lateral resolution of 30 μm and axial resolution of 7 μm, using mid-infrared optical coherence tomography (MIR OCT) based on a 4 μm center wavelength MIR supercontinuum laser. We detected individual printed ceramic layers and tracked predefined defects through all four processing steps and demonstrated how a defect in the green phase could affect the final product. This research sets the stage for NDI integration in AM.

Characteristics of pebble shape and the amount of pebble abrasion measured with a replica reproduced on a curling rink

The shape of pebbles on a curling rink was measured using a replica of the ice surface of the rink to understand the characteristics of pebbles after being in contact with stones. We focused on pebbles with flat tops for which the average shape was 3.81 mm in diameter at the lower base, 1.16 mm in diameter at the upper surface, 0.12 mm in maximum height, and 5.4° in contact angle. A scratch of about 1 µm in depth and 40 µm in width (traces of pebbles cut by a running band at the bottom of the stone) was observed on the upper surface. The pebbles were also found to have a moderate lower base diameter that preferentially contacted the nipper or stone due to its large maximum height value immediately after formation. Experiments to determine the amount of pebble abrasion associated with the passing of stones revealed that the average height of their upper surface decreased by 1 µm and the area of the upper surface increased by 0.21 mm2 for each stone passing as the stone cut the pebbles.

Suppression of Enhanced Magnesium Diffusion During High‐Pressure Annealing of Implanted GaN

Activation of ion‐implanted p‐type dopants in gallium nitride has demonstrated great progress utilizing high pressures to enable novel and traditional device architectures; however, such conditions consistently exhibit anomalously enhanced diffusion up to several microns in very short periods of time for device relevant concentrations. Here, this diffusion is shown to be modulated by unintentional hydrogen content within the anneal ambient and thus controllable by inclusion of a high‐temperature hydrogen getter. Furthermore, diffusion is also shown to be greatly suppressed using co‐implanted oxygen at low concentrations while simultaneously maintaining characteristics of p‐type material in photoluminescence. Subsequently, after annealing at 1300 °C for 30 min in 3.8 kbar of nitrogen pressure, the magnesium concentration in the diffusion tail is suppressed by 28% at 1–1.5 μm in depth using a hydrogen getter alone, which reduces hydrogen uptake by 45% and fully suppressed at >1 μm in depth using co‐implantation alone and further reduced with concurrent use of a hydrogen getter. Co‐implantation alone reduces the in‐diffused magnesium dose by 60% compared to reference samples.

A novel method for air assisted nanosecond pulsed fiber laser beam transmission micro-channelling on thick PMMA material

The demand for scaling up polymeric materials for mass production or scaling down for microscale applications is at its peak in the modern era. Laser beam technology enables the formation of micro-channels on polymers, specifically polymethyl methacrylate (PMMA). This study examines the viability and cost-effectiveness of employing the diode pump fiber laser transmission technique to create micro-channels on thick, transparent PMMA under air-assisted circumstances for biomedical engineering and micro-fluidics applications. The analysis has included process variables such as pulse frequency, power, and cutting speed, as well as machining parameters including depth of cut, kerf width, and heat-affected zone (HAZ) width. The results of multi-objective optimization indicate that the following parameters—a pulse frequency of 51.30 kHz, a working power of 13 % of 50 W, and a cutting speed of 0.60 mm/s—contribute to the desired microfluidics response values: 12.54 µm depth of cut, 24.05 µm kerf width, and 5.98 µm HAZ width.

Void swelling in pure iron after sequential self-ion-irradiation at different energies: Effect of injected interstitials and additional damage in regions containing pre-existing voids

The use of ion irradiation to add displacement dose onto previously neutron-irradiated material is becoming popular, especially since ion irradiation precedes on neutron-typical microstructures and microchemical distributions that would not be produced with ion irradiation alone. One issue to be addressed in these “neutron-preconditioned” specimens concerns the consequences of ion-injected interstitials on preexisting neutron-produced voids, especially since voids have been observed to disappear over a significant portion of the ion range in some recent preconditioning experiments. This situation can be explored using innovative ion simulation, thereby avoiding working with radioactive material.Two self-ion irradiation series of pure single-crystal iron were performed at 475 °C. The first series used 5 MeV Fe2+, reaching 50, 100 and 150 peak dpa, producing voids to ∼2 µm depth. The second series used 2.5 MeV Fe2+ to introduce additional 50 peak dpa onto each of the three first-step specimens with the ion range reaching ∼1.2 µm. Importantly, 1.2 × 10−3 peak dpa/s was used for both ions to minimize the influence of dpa rate. The major objective was to measure the influence of injected interstitials from the 2.5 MeV Fe2+ irradiation on previously existing voids. Another objective was to observe the behavior of voids exposed to additional dose and time, focusing not only on injected interstitials, but also on the effects of coalescence, ripening, and aging.There were significant changes in depth distribution of swelling following the second irradiation, especially void density. Whereas injected interstitials are well-known to suppress void nucleation at the end of ion range, it was shown in this study that injected interstitials are important in another manner, reducing growth of previously existing voids, producing a ‘notch” in both the swelling vs. depth profile and void size profile. At depths where the injected atoms for both ion energies are not significant, swelling proceeds at ∼0.2 %/dpa.

Understanding the wear behavior and mechanism of gradient nanostructured M50 bearing steel through nanoscratching tests

A gradient nanostructured M50 bearing steel with gradient structural size, carbides, and dislocation density was successfully fabricated by ultrasonic shot peening (USP) technology at room temperature. The friction behaviour and mechanism of gradient nanostructure M50 steel were investigated by using nanoscratch technology under various normal applied loads and depths. Under low normal applied load, gradient samples at ∼5 μm depth exhibited a 26.5 % maximum reduction in wear rate compared to the tempered sample; under high normal applied load, wear rates for samples at ∼100 μm depth exhibited a maximum reduction of 44.6 %. At low normal applied load, the wear mechanism involves plowing; while at high normal applied load, the wear mechanism shifts to cutting. Under low normal applied load, wear resistance correlates positively with hardness and negatively with structural size. Under high normal applied load, the increase in hardness of the martensite matrix and the partial decomposition of coarse spherical carbides enable the carbides on the surface of the USPed sample to withstand higher shear stress and stronger stress concentration, preventing cracking of the matrix and carbide edges, and spalling of carbides. High dislocation density (resulting in residual compressive stress) will slow down or inhibit the generation and expansion of cracks during the scratching process, potentially explaining why samples at ∼100 μm depths exhibit superior wear resistance.

A Novel 3D High Resolution Imaging Method Using Correlative X-Ray and Electron Microscopy to Study Neodymium-Doped Yttrium Aluminum Garnet Laser-Induced Defects in Intraocular Lenses.

Cataract extraction is the most frequently performed ophthalmological procedure worldwide. Posterior capsule opacification remains the most common consequence after cataract surgery and can lead to deterioration of the visual performance with cloudy, blurred vision and halo, glare effects. Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser capsulotomy is the gold standard treatment and a very effective, safe and fast procedure in removing the cloudy posterior capsule. Damaging the intraocular lens (IOL) during the treatment may occur due to wrong focus of the laser beam. These YAG-pits may lead to a permanent impairment of the visual quality. In an experimental study, we intentionally induced YAG pits in hydrophilic and hydrophobic acrylic IOLs using a photodisruption laser with 2.6 mJ. This experimental study established a novel 3D imaging method using correlative X-ray and scanning electron microscopy (SEM) to characterize these damages. By integrating the information obtained from both X-ray microscopy and SEM, a comprehensive picture of the materials structure and performance could be established. It could be revealed that although the exact same energies were used to all samples, the observed defects in the tested lenses showed severe differences in shape and depth. While YAG pits in hydrophilic samples range from 100 to 180 µm depth with a round shape tip, very sharp tipped defects up to 250 µm in depth were found in hydrophobic samples. In all samples, particles/fragments of the IOL material were found on the surface that were blasted out as a result of the laser shelling. Defects in hydrophilic and hydrophobic acrylic materials differ. Material particles can detach from the IOL and were found on the surface of the samples. The results of the laboratory study illustrate the importance of a precise and careful approach to Nd:YAG capsulotomy in order to avoid permanent damage to the IOL. The use of an appropriate contact glass and posterior offset setting to increase safety should be carried out routinely.

Corrosion of austenitic and ferritic steels, Kovar, and molybdenum in liquid Sb-Bi alloys with different compositions

Sb-Bi alloys may be applied as positive electrode in liquid metal batteries. In this work, the material compatibility of two steels (SS 316L and T91), Kovar 4J29, and molybdenum with SbBi9, Sb2Bi8, and Sb3Bi7 is investigated in static corrosion tests for 740 h at around 450 °C. Kovar shows severe dissolution attack (800 µm depth for SbBi9), which is mitigated at higher Sb contents due to the formation of Fe-Sb compounds. Both steels exhibit moderate corrosion depths (10–20 µm) for all Sb-Bi alloys thanks to the additional formation of stable Fe-Cr-Sb compounds. Good corrosion resistance is obtained for molybdenum.

Protoplanetary Disk Polarization at Multiple Wavelengths: Are Dust Populations Diverse?

Millimeter and submillimeter observations of continuum linear dust polarization provide insight into dust grain growth in protoplanetary disks, which are the progenitors of planetary systems. We present the results of the first survey of dust polarization in protoplanetary disks at 870 μm and 3 mm. We find that protoplanetary disks in the same molecular cloud at similar evolutionary stages can exhibit different correlations between observing wavelength and polarization morphology and fraction. We explore possible origins for these differences in polarization, including differences in dust populations and protostar properties. For RY Tau and MWC 480, which are consistent with scattering at both wavelengths, we present models of the scattering polarization from several dust grain size distributions. These models aim to reproduce two features of the observational results for these disks: (1) both disks have an observable degree of polarization at both wavelengths; and (2) the polarization fraction is higher at 3 mm than at 870 μm in the centers of the disks. For both disks, these features can be reproduced by a power-law distribution of spherical dust grains with a maximum radius of 200 μm and high optical depth. In MWC 480, we can also reproduce features (1) and (2) with a model containing large grains (a max = 490 μm) near the disk midplane and small grains (a max = 140 μm) above and below the midplane.

Residual stress and crystallite size analysis of SMAT on AA7075-T6

The study examines how SMAT influences the microstructure, dislocation density, and residual stress of AA7075-T6 alloy. SMAT involves the repeated impact of shots on a material surface within a vibration chamber, inducing severe plastic deformation and grain refinement. The study explores the interplay between vibration-induced shot effects and their impact on surface properties, emphasizing the importance of optimizing process parameters for improved material performance. Experimental analyses, including surface and subsurface microstructure examinations, X-ray diffraction (XRD) characterization, and residual stress evaluations, are conducted to elucidate the underlying mechanisms of SMAT. Results indicate that SMAT leads to significant plastic deformation, grain size reduction, increased dislocation density, and microstrain. The treatment produces a nanocrystallite layer, enhancing material properties. Residual stress measurements reveal a surface stress of −245 MPa, increasing to −360 MPa at 50 µm depth. The proposed multiple impact model accurately predicts deformation behavior, highlighting the deep pit regions formed during SMAT. This comprehensive investigation provides insights into the dynamic nature of SMAT and its potential for enhancing material properties. By integrating experimental findings with numerical modeling approaches, the study advances our understanding of SMAT-induced plastic deformation mechanisms and residual stress profiles.

Effect of an artificial cavity on the microlayer and contact line dynamics during bubble growth in nucleate boiling

We present an experimental study on the near-wall phenomena during the growth of a single bubble in saturated pool boiling of water at atmospheric pressure. Our focus is on the dynamics of triple contact line and liquid microlayer that can form between the heater and the liquid-vapor interface of the bubble. The microlayer thickness, the wall temperature distribution and the bubble shape are measured simultaneously and synchronously at 4000 fps by white light interferometry, infrared thermography and sidewise shadowgraphy, respectively. To study the effect of cavities (artificial nucleation sites) we compare two experiments using different heaters. In the first experiment, the bubble grows on a smooth surface of nanometric roughness whereas in the second, the bubble grows over a cylindrical cavity of 25 µm diameter and 50 µm depth. We found that the cavity reduces three times the required wall superheating to trigger the bubble growth. Moreover, the radii of the bubble, microlayer and dry spot are smaller by half and the macroscale bubble dynamics is also slower. The microlayer is thinner and is measurable in a larger portion of its extent. Based on the absence of interference fringes near the contact line (due to high interface slopes) and on recent numerical simulations, we conclude that the microlayer consists in two regions: a dewetting ridge near the contact line that grows over time and a flatter and wider region that thins over time. The microlayer can be seen as a film deposited by the receding meniscus and its profile is controlled by the viscous and surface tension effects; its thinning over time is due to local evaporation only. The ridge is a result of liquid accumulation due to contact line receding and strong viscous shear in the film.