Adhesion Research Articles

Dynamic-Wetting Liquid Metal Thin Layer Induced via Surface Oxygen-Containing Functional Groups.

Enhancing the wettability of liquid metals (LMs) to address their high surface tensions is crucial for practical applications. However, controlling LMs wetting on various substrates and understanding the underlying mechanisms are challenging. Here, we present a facile dynamic-wetting strategy to modulate eutectic gallium-indium (EGaIn) wettability via chemical surface modification, spontaneously forming a stable and thin (∼18 μm) EGaIn layer. Polymer substrates exhibiting varying EGaIn wetting behaviors can be categorized by their sliding angles and adhesion force. X-ray photoelectron spectroscopy results demonstrate that the dynamic-wetting process occurs only on surfaces with sufficient oxygen-containing functional groups (content ≥18%) and confirm coordination interactions between the EGaIn oxide layer and surface functional groups. Furthermore, in EGaIn thermal management systems, the heat transfer rate in the wetting group is increased by up to 20% compared to that of the nonwetting group. This work will hasten the application of LMs in flexible circuits and thermal management.

Effect of Thermal Reflow on Microstructural Morphology and Contact Mechanics in the Photo-Lithographic Fabrication of Biomimetic Adhesive Materials.

Bionic adhesive materials with 3D complex micro/nanostructures have several advantages of low preload, strong adhesion, switchable adhesion, etc. As the primary high-precision fabrication method for such materials, lithography is inherently limited by its 2D processing capabilities. Achieving complex 3D morphologies typically requires auxiliary processes, such as dipping and double-sided separate UV exposures, which increase both the complexity and limitations of the fabrication process. In this work, an efficient dimensional regulation method-the photo-lithographic thermal reflow is proposed. The technique utilizes the intrinsic properties of photoresist materials, introducing thermal energy to transform microstructures from 2D to 3D. Mushroom-shaped morphology is taken as an example to fabricate bionic adhesive materials. The fabricated mushroom-shaped micropillar arrays exhibit different tendencies in adhesion force, friction, reversible adhesion, and repeatability, demonstrating the precise tunability of the micropillar geometry. The optimized mushroom-shaped adhesive material not only exhibits the adhesion force of up to 12.26 N on the silicon surface (superior to that of a single foot of gecko (10 N)) but also shows superior friction, easy peeling and high durability. The result demonstrates that this method enables rapid and efficient regulation of 3D morphology and provides a novel approach for the fabrication of complex micro/nanostructure.

Synergistic Adhesion and Shape Deformation in Nanowire-Structured Liquid Crystal Elastomers.

Nature provides many examples of the benefits of nanoscopic surface structures in areas of adhesion and antifouling. Herein, the design, fabrication, and characterization of liquid crystal elastomer (LCE) films are presented with nanowire surface structures that exhibit tunable stimuli-responsive deformations and enhanced adhesion properties. The LCE films are shown to curl toward the side with the nanowires when stimulated by heat or organic solvent vapors. In contrast, when a droplet of the same solvent is placed on the film, it curls away from the nanowire side due to nanowire-induced capillary forces that cause unequal swelling. This characteristic curling deformation is shown to be reversible and can be optimized to match curved substrates, maximizing adhesive shear forces. By using chemical modification, the LCE nanowire films can be given underwater superoleophobicity, enabling oil repellency under a range of harsh conditions. This is combined with the nanowire-induced frictional asymmetry and the reversible shape deformation to create an underwater droplet mixing robot, capable of performing chemical reactions in aqueous environments. These findings demonstrate the potential of nanowire-augmented LCE films for advanced applications in soft robotics, adaptive adhesion, and easy chemical modification, with implications for designing responsive materials that integrate mechanical flexibility with surface functionality.

Study of the modification mechanism and reinforcement performance of functionalized graphene-based structural adhesives

ABSTRACT This study investigates the effectiveness of modified graphene (MGE) for developing novel structural adhesives and their application in concrete beam reinforcement. The research aims to reveal the mechanism of MGE and evaluate the effect of different reinforcement methods on the flexural performance of concrete beams. Two adhesive formulations – original structural adhesive (OSA) and functionalized graphene-based structural adhesive (FGSA) – were examined alongside two reinforcement methods: the grouting reinforcement method and the adhesive fiber-reinforced polymer (FRP) method. The results show that the increased spacing of the functionalized MGE flakes improved their dispersion in epoxy resin, enhancing cohesive forces through bonding interactions and leading to the enhancement of the basic mechanical properties of FGSA by 6% to 12%. Compared to OSA, the application of FGSA improved the flexural properties of concrete beams, with the degree of improvement depending on the reinforcement method, while the failure mode of the beams remained unchanged. In addition, the FGSA/FRP combination showed the most significant improvement in the fracture parameters (220% to 480%) compared to the unreinforced control, highlighting its efficiency and cost-effectiveness as a reinforcement method. These findings offer valuable insights into reinforcement strategies for concrete beams, addressing the limitations of structural adhesive bond strength and optimizing reinforcement techniques.

Biofunctionalized patterned platform as microarray biochip to supervise delivery and expression of pDNA nanolipoplexes in stem cells via mechanotransduction

Biochips are widely applied to manipulate the geometrical morphology of stem cells in recent years. Patterned antenna-like pseudopodia are also probed to explore the influence of pseudopodia formation on gene delivery and expression on biochips. However, how the antenna-like pseudopodia affect gene transfection is unsettled and the underlying trafficking mechanism of exogenous genes in engineered single cells is not announced. Therefore, the engineered microarray biochips were conceptualized and prepared by the synthesized photointelligent biopolymer to precisely manage geometric topological structures (cell size and antenna-like protrusion) of stem cells on biochips. The cytoskeleton could be regulated in engineered cells and large cells with more antennas assembled well-organized actin filaments to affect cell tension distribution. The stiffness and adhesion force were measured by atomic force microscope to reveal cell nanomechanics on microarray biochips. Cytoskeleton-mediated nanomechanics could be adjusted by actin filaments. Gene transfection efficiency was enhanced with increasing cell nanomechanics, which was also confirmed by the evaluation of cell internalization capacity of nanoparticles and DNA synthesis ability. This work will provide a new strategy to study functional biomaterials, microarray chips and internal mechanism of gene transfection in patterned stem cells on biochips.Graphical abstract

Angle-Dependent Adhesive Mechanics in Hard-Soft Cylindrical Material Interfaces.

In this research, the adhesive contact between a hard steel and a soft elastomer cylinder was experimentally studied. In the experiment, the hard cylinder was indented into the soft one, after which the two cylinders were separated. The contact area between the cylinders was elliptical in shape, and the eccentricity of this increased as the angle between the axes of the contacting cylinders decreased. Additionally, the adhesive pull-off force and the contact area increased with a decrease in the angle between the cylinders. The use of a transparent elastomer allowed for observation of the shape of the contact in real time, which facilitated the creation of videos demonstrating the complete process of contact failure and the evolution of the ellipse shape, depending on the distance between the cylinders and normal force. These findings contribute to a better understanding of adhesive interactions in elliptical contacts between cylinders and can be applied to fields such as soft robotics, material design, and bioengineering, where precise control over adhesion and contact mechanics is crucial.

The influence of pectins and cellulose in the mechanical and adhesive properties of seed mucilage.

Several plant seeds release a mucilaginous envelope through hydration, rich in pectins and stabilized by cellulose fibers. This mucilage aids in seed protection, development, and adhesion for dispersal. This study aimed to separate the effects of pectins and cellulose fibers by using pectinase to remove mucilage pectins, leaving cellulose arrays, and performing wet and dry pull-off force measurements on seeds of three plant species: Salvia hispanica (Chia), Collomia grandiflora (Collomia) and Linum usitatissimum (Flax). We used light and scanning electron microscopy to confirm partial pectin removal and intact cellulose fibers. Pull-off force measurements revealed similar wet adhesive properties and E-moduli in S. hispanica and C. grandiflora seeds before and after pectin removal. L. usitatissimum seeds, lacking cellulose fibers, exhibited significantly lower wet and dry adhesion forces post-pectin removal. Desiccation dynamics showed shorter desiccation times after pectin removal in all three species. Results indicated that adhesion forces in seed mucilage with cellulose fibers did not change significantly after pectin removal, suggesting that cellulose fibers contribute to the adhesive properties of seed mucilage, while pectins might not play an exclusive role in adhering to surfaces.

Improving the Adhesion Forces of Mussel-Inspired Peptides through Inverse Design

Improving the Adhesion Forces of Mussel-Inspired Peptides through Inverse Design

Polydopamine-Mediated, Centrifugal Force-Driven Gold Nanoparticle-Deposited Microneedle SERS Sensors for Food Safety Monitoring Theoretical Study of the SERS Substrate Fabrication.

Microneedle (MN) sensors have great promise for food safety detection, but the rapid preparation of MNs for surface-enhanced Raman scattering (SERS) sensors with tunable and homogeneous nanoparticles remains a great challenge. To address this, a SERS sensor of gold nanoparticles@polydopamine@poly(methyl methacrylate) MN (AuNPs@PDA@PMMA-MN) was developed. The extended-Derjaguin-Landau-Verwey-Overbeek theory was applied to calculate the interaction energy between AuNPs and PDA. It was confirmed that an appropriate centrifugal force could be utilized to overcome the electrostatic repulsion between AuNPs and PDA. Together with the adhesion force of PDA, AuNPs can therefore be uniformly and densely deposited on the MN substrate. The AuNPs@PDA@PMMA-MN had an enhancement factor of up to 1.74 × 106 for R6G. Furthermore, a MN sensor for the selective detection of putrescine and cadaverine was successfully constructed by modifying 4-mercaptobenzaldehyde (4-MBA) on AuNPs@PDA@PMMA-MN substrates. This sensor could quantitatively detect putrescine and cadaverine in meat. It has been successfully applied to the in situ detection of putrescine and cadaverine in real meat samples. The AuNPs@PDA@PMMA-MN SERS sensor has the advantages of facile fabrication, high sensitivity, high specificity, and online, in situ detection capability. It is expected to have applications in food quality testing, environmental monitoring, and disease diagnosis.

Rose-Inspired Adhesive Surface Regulation for Enhanced Fog Water Collection Efficiency and Self-Cleaning.

Inspired by the adhesion differences on the surfaces of fresh and dried rose petals, a rose bionic self-cleaning fog collector (RBSC) was designed and prepared to realize a self-driven fog harvesting function. The droplet detachment iteration rate was revealed by the regulating mechanism of the surface adhesion force of the RBSC and the influence of bionic texture parameters, as demonstrated through the fog harvesting experiment and droplet detachment failure analysis. Through the surface adhesion force regulation, the probability of droplet dissipation with the airflow is reduced by increasing the falling droplets' mass, and the single surface fog capture efficiency is up to 740 mg cm-2 h-1. With the adhesion gradient regulation, the composite hexagonal patterned surface achieves a water collection efficiency of 960 mg cm-2 h-1, which is a 243% improvement compared with that of a slippery surface and is superior to the efficiencies of both circular and 90° square patterns. This is due to the fact that the composite hexagonal patterned surface ensures that the water droplets converge at the center while increasing the distance between the droplets, improving the stability of droplet detachment. Compared to the stainless steel surface, the RBSC exhibits excellent self-cleaning performance, with the impurity deposition rate reduced by 91.2%. The multi-inspired strategy optimizes the entire harvesting process, offering a promising solution to the water crisis in arid regions.

Influence of Lens Power on IOL/Posterior Lens Capsule Interactions and IOL's PCO Potential.

Severely myopic eyes have been associated with high posterior capsule opacification (PCO) incidence. Although it has been reported that myopic eyes have weaker or more delayed capsule adhesion than emmetropic eyes, it is unclear whether/how dioptric power and posterior curvature of IOLs affect IOLs' affinity for the posterior lens capsule (PLC) and their PCO potential. To investigate this, acrylic foldable IOLs with increasing dioptric power of 6.0D (for high myopia), 20.0D, and 30.0D (for low/non-myopia) were tested on their binding affinity toward PLC and their ability to inhibit the proliferation and infiltration of lens epithelial cells (LECs) using an in vitro simulated human PLC (sPLC) model. We found that IOL power and posterior radius of curvature (PRC) had significant impacts on IOL/sPLC adhesion forces, which are in the following order: 20.0D ≈ 30.0D > 6.0D. Optical coherence tomography (OCT) images showed that loose binding between 6.0D IOLs and sPLC contributed to larger interface spaces and significantly greater LEC infiltration, proliferation, metabolic activity, and transdifferentiation compared to 20.0D and 30.0D IOLs. Statistical analyses showed that IOLs' PRC may have a substantial influence on IOL/sPLC physical interactions, LEC responses, and PCO incidence. The overall results suggest that the high PRC of low-diopter (6.0D) IOLs reduces their binding affinity toward the PLC, facilitates LEC reactions, thus causes high PCO incidence in myopic eyes. These findings strongly support that a new design to increase IOL posterior surfaces' PLC affinity may reduce PCO potential and increase safety for myopic patients.

Effect of Electric Fields on the Mechanical Mechanism of Regorafenib-VEGFR2 Interaction to Enhance Inhibition of Hepatocellular Carcinoma.

The interaction between molecular targeted therapy drugs and target proteins is crucial with regard to the drugs' anti-tumor effects. Electric fields can change the structure of proteins, which determines the interaction between drugs and proteins. However, the regulation of the interaction between drugs and target proteins and the anti-tumor effects of electric fields have not been studied thoroughly. Here, we explored how electric fields enhance the inhibition of regorafenib with regard to the activity, invasion, and metastasis of hepatocellular carcinoma cells. We found that electric fields lead to an increase in the normal (adhesion) and transverse (friction) interaction forces between regorafenib and VEGFR2. In single molecule pair interactions, there are changes in specific and nonspecific forces. Hydrogen bonds, hydrophobic interactions, and van der Waals forces are the main influencing factors. Importantly, the increase in the adhesion force and friction force between regorafenib and VEGFR2 caused by electric fields is related to the activity and migration ability of hepatocellular carcinoma cells. The morphological changes in VEGFR2 prove that electric fields regulate protein conformation. Overall, our work proves the drug-protein mechanical mechanism by which electric fields enhance the anti-tumor effect of regorafenib and provides insights into the application of electric fields in clinical tumor treatment.

Binding mechanism of oligopeptides on solid surface: assessing the significance of single-molecule approach.

This paper addresses the complementarity and potential disparities between single-molecule and ensemble-average approaches to probe the binding mechanism of oligopeptides on inorganic solids. Specifically, we explore the peptide/gold interface owing to its significance in various topics and its suitability to perform experiments both in model and real conditions. Experimental results show that the studied peptide adopts a lying configuration upon adsorption on the gold surface and interacts through its peptidic links and deprotonated thiolate extremities, in agreement with theoretical predictions. Single-molecule force spectroscopy (SMFS) measurements revealed the existence of a wide panel of adhesion forces, resulting from the interaction between individual peptide moieties and the abundant surface sites. We therefore propose methodological developments for sorting the events of interest to understand the peptide adsorption mechanism. Thermodynamic and kinetic aspects of the peptide adsorption are probed using both static and dynamic force spectroscopy measurements. Specifically, we show the possibility of providing a reasonable estimate of the peptide free energy of adsorption ΔadsG° by exploring the fluctuations of the adhesion work, based on the Jarzynski equality, and by using a parametric Gamma estimator. The proposed approach offers a relevant method for studying the different factors influencing the peptide adsorption and evaluating their impact on ΔadsG° as an alternative to exploring adhesion forces that may lead to misinterpretations. This is illustrated by the comparison of the adsorption of two peptides with specific amino acids substitution. Our method provides insights into the overall mechanism by which peptides interact with the surface and allows an integration of the single-molecule versus ensemble-average points of view.

Buzz pollination: investigations of pollen expulsion using the discrete element method.

Buzz pollination involves the release of pollen from, primarily, poricidal anthers through vibrations generated by certain bee species. Despite previous experimental and numerical studies, the intricacies of pollen dynamics within vibrating anthers remain elusive due to the challenges in observing these small-scale, opaque systems. This research employs the discrete element method to simulate the pollen expulsion process in vibrating anthers. By exploring various frequencies and displacement amplitudes, a correlation between how aggressively the anther shakes and the initial rate of pollen expulsion is observed under translating oscillations. This study highlights that while increasing both the frequency and displacement of vibration enhances pollen release, the rate of release does not grow linearly with their increase. Our findings also reveal the significant role of pollen-pollen interactions, which account for upwards of one-third of the total collisions. Comparisons between two types of anther exits suggest that pore size and shape also influence expulsion rates. This research provides a foundation for more comprehensive models that can incorporate additional factors such as cohesion, adhesion and Coulomb forces, paving the way for deeper insights into the mechanics of buzz pollination and its variability across different anther types and vibration parameters.

Clotrimazole mucoadhesive films with extended-release properties for vaginal candidiasis-A hot-melt extrusion application.

Clotrimazole, an antifungal agent for treating vaginal candidiasis, faces challenges in localized delivery due to poor solubility, complexity of the vaginal environment, limited fluid for dissolution, and rapid self washout of the vagina. The study aimed to enhance clotrimazole solubility using hot-melt extrusion (HME) to develop vaginal films with adequate bioadhesion, mechanical strength, and extended-release properties. Different formulations were created by varying the ratios of polyethylene oxide (PEO) grades (N750 and N10) to adjust the films' properties. The films demonstrated extended-release profiles, prolonging clotrimazole release for up to eight hours, with a cumulative gradual and complete in- vitro release in 100 mL of simulated vaginal fluid with 0.5% sodium dodecyl sulfate. In contrast, the marketed vaginal ovules exhibited a rapid and complete release within 30 minutes of shell rupture. The release kinetics followed Krosmeyer-Peppas model, and zero-order release mechanism. Films containing 25% clotrimazole, 56.25% PEO N750, and 18.75% PEO N10 exhibited strength of 87.9 N, stiffness of 35 N/sec, and adhesive force of 3.85 N.mm. In conclusion, the novel clotrimazole-loaded vaginal films developed using HME technology enhanced the solubility and localized vaginal delivery of clotrimazole. The extended-release profile may reduce the dosing frequency, enhance patient adherence, and improve therapeutic outcomes.

Optimized reconstruction of adhesion force distribution from resuspension measurements using the Rock’n’Roll model

Optimized reconstruction of adhesion force distribution from resuspension measurements using the Rock’n’Roll model

Evolution mechanism of cold adhesion force between electrical contact determined by coating hardness

Evolution mechanism of cold adhesion force between electrical contact determined by coating hardness

Nanoparticle adhesion at liquid interfaces.

Nanoparticle adhesion at liquid interfaces plays an important role in drug delivery, dust removal, the adsorption of aerosols, and controlled self-assembly. However, quantitative measurements of capillary interactions at the nanoscale are challenging, with most existing results at the micrometre to millimetre scale. Here, we combine atomic force microscopy (AFM) and computational simulations to investigate the adhesion and removal of nanoparticles from liquid interfaces as a function of the particles' geometry and wettability. Experimentally, AFM tips with controlled conical geometries are used to mimic the nano-asperities on natural nanoparticles interacting with silicone oil, a model liquid for many engineering applications including liquid-infused surfaces. Computationally, continuum modelling with the Surface Evolver software allows us to visualise the interface configuration and predict the expected force profile from energy minimisation. Quantitative agreement between the experimental measurements and the computational simulations validates the use of continuum thermodynamics concepts down to the nanoscale. We demonstrate that the adhesion of the nanoparticles is primarily controlled by surface tension, with minimum line tension contribution. The particle geometry is the main factor affecting the length of the capillary bridge before rupture. Both the particle geometry and liquid contact angle determine the shape of the adhesion force profile upon removal of the particle from the interface. We further extend our simulations to explore more complex geometries, rationalising the results from experiments with imperfect AFM tips. Our results could help towards the design of smart interfaces, for example, able to attract or repel specific particles based on their shape and chemistry.

Studying a droplet impaction on a vibrating porous medium

This study investigates the influence of vibration on droplet dynamics when a droplet impacts a three-dimensional (3D) structured porous medium, focusing on intrinsic properties such as porosity and wettability, along with the effect of vibration phase angle. Using an Allen-Cahn equation-based lattice Boltzmann method (A-C LBM), the research analyzes multiphase flow dynamics. The results compare droplet spreading and penetration on vibrating versus non-vibrating porous media. Without vibration, droplets get trapped within the pores, reaching equilibrium due to adhesive, viscous, and capillary forces. However, sufficient vibrational forces can overcome surface adhesion, allowing droplets to pass through the porous medium. This phenomenon is influenced by the droplet's contact angle and initial impact inertia. The study finds that hydrophobic surfaces, characterised by higher contact angles, significantly reduce liquid infiltration into the medium under both vibrational and non-vibrational conditions. This reduction is due to dominant surface tension and repellent forces, which, in combination with vibrational forces, minimise droplet spreading and penetration, even in highly porous media. Conversely, on hydrophilic surfaces, adhesive and capillary forces enhance the droplet's inertia, causing sudden penetration into the porous medium. Further, the study reveals that the phase angle of vibration critically affects the droplet's behaviour. With sinusoidal vibration, lower phase angles cause the porous medium to move in the opposite direction to the droplet, enhancing the synergy between vibration and inertia forces, leading to rapid infiltration. Higher phase angles, aligning the medium's movement with the droplet's impact path, result in reduced infiltration. Overall, this research provides crucial insights into how porosity, wettability, and vibration phase angle collectively influence droplet dynamics on structured porous media, offering valuable implications for applications involving fluid transport and filtration in porous structures.

Force–Position Coordinated Compliance Control in the Adhesion/Detachment Process of Space Climbing Robot

Adhesion-based space climbing robots, with their flexibility and multi-functional capabilities, are seen as a promising candidate for in-orbit maintenance. However, challenges such as uncertain adhesion establishment, unexpected detachment, and body motion unsteadiness in microgravity environments persist. To address these issues, this paper proposes a coordinated force–position compliance control method that integrates novel adhesion establishment and rotational detachment strategies, integrated into the gait schedule for a space climbing robot. By monitoring the foot-end reaction forces in real time, the proposed method establishes adhesion without risking damaging the spacecraft exterior, and smooth detachment is achieved by rotating the foot joint instead of direct pulling. These strategies are dedicated to reducing unnecessary control actions and, accordingly, the required adhesion forces in all feet, reducing the possibility of unexpected detachment. Climbing experiments have been conducted in a suspension-based gravity compensation system to examine the merits of the proposed method. The experimental results demonstrate that the proposed rotational detaching method decreases the required pulling force by 65.5% compared to direct pulling, thus greatly reducing the disturbance introduced to the robot body and other supporting legs. When stepping on an obstacle, the compliant control method is shown to reduce unnecessarily aggressive control actions and result in a reduction in relevant normal and shear adhesion forces in the supporting legs by 44.8% and 35.1%, respectively, compared to a PID controller.