Particle-substrate Adhesion Research Articles

Detailed analysis of particle–substrate interaction based on a centrifugal method

• Experiments to remove particles adhering to inclined substrates in a centrifuge. • Analyses using point-mass and rigid-body models. • Elucidation of the particle removal mechanism based on rolling rather than friction. • Determination of particle–substrate adhesion force and effective contact radius. • Estimation of the removal fraction curves at different inclination angles. The interaction between particles and inclined substrates in a centrifuge was investigated theoretically and experimentally. First, the balance of the force acting on a particle adhering to the substrate, with an inclination angle from 0 to 90° to the horizontal, was formulated separately in the normal and tangential directions. The adhesion force was then derived based on the point-mass model as a function of the angular velocity. Next, the balance of the moments of the forces acting on a particle adhering to the substrate was formulated; theoretical equations for the adhesion force and the effective contact radius were then derived from the angular velocities, obtained at any two inclination angles, based on the rigid-body model. Finally, the removal fraction curves of spherical/nonspherical particles with median diameters of less than 10 µm were experimentally obtained by increasing the angular velocity at each inclination angle. The experimentally obtained angular velocities were substituted into the theoretical equations to compare the point-mass and rigid-body models. The effects of the particle shape on the adhesion force and effective contact radius and that of the inclination angle on the removal fraction curves based on the theoretical equation were also investigated.

Thermographic Analysis of Composite Metallization through Cold Spray

Cold Spray is an innovative technology to create coatings through the impact of metallic particles on substrates. Its application to composites’ surfaces is recently attracting the attention of the scientific community thanks to the possibility to functionalize and improve their thermal and wear properties. Within this context, the generation of the first metal-to-composite layer is fundamental. This work presented an experimental investigation of a composite panel, reinforced with glass fibers and coated with aluminum particles. The coating investigation was carried out through active pulsed thermography, analyzing the thermal response of single and double hatches. The thermal outputs were compared with a standard microscopic analysis, with a critical discussion supporting the identification of factors that influence the thermal response to the pulse: (1) layer’s thickness; (2) cold spray coverage; (3) layer compactness; (4) particle-substrate adhesion; (5) particle’s oxidation; and (6) surface roughness.

Comparison of surface energy and adhesion energy of surface-treated particles

The mineral industry uses tremendous amounts of water every year in the processing of ores. Sustainable practices associated with the processing of ores are, therefore, of critical importance. The project described herein is the first step toward producing a dry, particle-separation process based upon control and exploitation of adhesive forces. In this research, the goal is to determine the surface energy of particles, and further, whether the solid surface energy can be used to understand the adhesion between these particles and surface-modified substrates. Glass spheres were chosen to represent silicate minerals, the most abundant type of minerals found in mineral deposits. The solid surface energy was found by using contact angle measurements and by applying the van Oss-Good-Chaudhury (VOGC) method. The VOGC method utilizes three-liquid triads to determine the Lifshitz-van der Waals, Lewis acid and Lewis base surface energy components. Surface energies from plasma-cleaned glass were between 40.2 and 60.2 mJ/m2; for the same glass with a hydrophobic chemical surface treatment, trichloro(octadecyl)silane (TCOD), the surface energy was between 20.8 and 20.9 mJ/m2; and for the glass with a hydrophilic chemical surface treatment (n1-(3-trimethoxysilylpropyl) diethylenetriamine (TMPA)) the surface energy was between 46.3 and 61.6 mJ/m2. The particle-substrate adhesion was also measured using a mechanical impact tester. Glass disks and beads were used, cleaned and surface treated with TCOD and TMPA. A custom horizontal impact tester was designed and used to measure the adhesion force between the glass spheres and a glass disk substrate. Impact of the disk/particle puck causes particle removal as tensile forces act on the particles. The tensile detachment force and adhesive force are equal at a critical particle size. Johnson-Kendall-Roberts (JKR) theory was used to determine the interfacial energy between the particles and the surface. The average interfacial energy of plasma cleaned glass, glass treated with TCOD and with TMPA were 44.8 mJ/m2, 21.6 mJ/m2, and 40.1 mJ/m2, respectively. These values are in good agreement with the literature values and with the interfacial energy determined using the VOGC method described above, demonstrating that two approaches compare favorably, despite the dramatically different methods (molecular vs mechanical) utilized.

Site-specific study of jetting, bonding, and local deformation during high-velocity metallic microparticle impact

In the cold-spray process, jetting is often considered a precursor to particle-substrate adhesion, but the conditions under which these phenomena occur remain unclear. This paper presents a systematic site-specific study of single-particle impact across a wide range of impact velocities for Cu particles impacting a Cu substrate. A high degree of velocity control enables identification of new behaviors that emerge in distinct velocity ranges. At the lowest velocities, we observe a fully plastic impact regime with rebound of the incident particle. As velocity increases, substrate jetting emerges even while the particle still rebounds. At the critical adhesion velocity, vcr, jetting only occurs on the substrate, leading to poor metallurgical bonding. Bonding improves as velocity increases and the particles also begin to jet, but there is an apparent maximum in bonding extent at ~1.3 vcr. This is followed by a drop in metallurgical bonding due to the ‘backward’ jetting of the particle that is associated with peeling forces on the interface. At ~1.6 vcr and above, we observe hydrodynamic penetration of the particle and possible erosion, as indicated by petalling of the substrate and burial of the particle below the substrate impact plane. Taken together, this catalog of phenomena, correlated to specific velocities, points to an impact-structure-based approach to establishing processing windows for cold spray.

Seeded Growth of Large-Area Arrays of Substrate Supported Au Nanoparticles Using Citrate and Hydrogen Peroxide.

While seeded growth of quasi-spherical colloidal Au nanoparticles (NPs) has been extensively explored in the literature, the growth of surface supported arrays of such particles has received less attention. The latter scenario offers some significant challenges, including the attainment of sufficient particle-substrate adhesion, growth-selectivity, and uniform mass-transport. To this end, a reaction system consisting of HAuCl4, citrate, and H2O2 is here investigated for the growth of supported arrays of 10 nm Au seeds, derived via block copolymer (BCP) lithography. The effects of the reagent concentrations on the properties of the resultant NPs are evaluated. It is found that inclusion of citrate in the growth medium causes substantial particle desorption from Si surfaces. However, the presence of citrate also yields NPs with more uniformly circular top-view cross sections (“quasi-circular”), motivating the exploration of particle immobilization methods. We demonstrate that atomic layer deposition (ALD) of a single cycle of HfO2 (∼1 Å), after the seed particle formation, promotes adhesion sufficiently to enable the use of citrate without the added oxide noticeably affecting the shape of the resultant NPs. The presented ALD-based approach differs from the conventional sequence of depositing the adhesion layer prior to the seed particle formation and may have advantages in various processing schemes, such as when surface grafting of brush layers is required in the BCP lithography process. A proof-of-concept is provided for the growth of large-area arrays of supported “quasi-circular” Au NPs, in a rapid one-step process at room temperature.

Self-Cleaning and Controlled Adhesion of Gecko Feet and Their Bioinspired Micromanipulators

ABSTRACTBioinspired micromanipulators have been made based on gecko dynamic self-cleaning mechanism. Various particles such as spherical SiO2/polystyrene, and short fibrous glass can be captured, transmitted and dropped on glass substrate with precisely predesigned patterns, by using the micromanipulator with the help of atomic force microscope (AFM). It has been demonstrated that particle-pad interface and particle-substrate interface exhibit diverse adhesion behaviors under different z-piezo retracting speed. The particle-substrate adhesion increases faster than the particle-pad adhesion with increasing the detaching velocity, which makes it possible to manipulate the particles by adjusting the retreating speed only. Probability tests was performed to better choose suitable parameters for picking and dropping operations. This work provides a potential solution to manipulation of micro/nano particles for precise assembly.

Adhesive contact mechanics: From deterministic to stochastic description

The theoretical models in adhesive contact mechanics have been successfully applied in fields such as colloidal assembly, drug delivery as well as virus-cell interaction. This report briefly reviews the classical models in describing particle-substrate adhesion. Johnson, Kendall and Roberts (JKR) first obtained the critical tensile load leading to ballistic particle pull off from a substrate. Derjaguin, Muller and Toporov (DMT) proposed an alternate theory with a different prediction on the critical load. Maugis was able to show the JKR-DMT transition using a Dugdale cohesive description of surface interaction. More extensions of adhesive contact models have involved material anisotropy, elasticity gradation, viscoelasticity, and large deformation in nonlinear elastomers. All of these existing results are inherently deterministic, sharing the viewpoint that a critical level of pulling force, commonly referred to as the pull-off force, is needed for particles to detach from substrates. Below the critical pull-off force, particle detachment never occurs.

Adhesive strength distribution of charged particles on metal substrate in external electric field

An improved airflow method to measure the distribution of adhesive strength between charged particles and a metal substrate in an external electric field is presented. In this study, toner particles were negatively charged with a corona charger and deposited on the substrate. The substrate with the particles on the surface was mounted in a rectangular air channel with parallel electrodes. Air velocity was increased at a constant rate, and entrained particles were detected by a laser particle monitor. By studying the relationships between particle entrainment efficiency and air velocity, the particle–substrate adhesion was analyzed in detail. It was found that particle adhesion increased with the increase in the initial charge of particles. It was also found that the particle adhesion increased in a vertically downward electric field but decreased in the upward electric field. These experimental results cannot be explained by the Coulomb force in the electric field. Therefore, a theoretical model based on charge transfer in the external electric field was proposed. This model explains the variation of the particle–substrate adhesion by considering the image force arising due to the transferred charges.

Statistical Pull Off of Nanoparticles Adhering to Compliant Substrates

It is widely known in adhesive contact mechanics that a spherical particle will not detach from an elastic half-space unless a critical level of pulling force is reached, as already revealed by JKR- or DMT-type deterministic models. This article focuses on the scenario of particle-substrate adhesion where the size of particles is down to the nanometer scale. A consequence of particle size reduction to this range is that the energy scale confining the state of system equilibrium becomes comparable to the unit of thermal energy, leading to statistical particle detachment even below the critical pull-off force. We describe the process by Kramers' theory as a thermally activated escape from an energy well and develop a Smoluchowski partial differential equation that governs the spatial-temporal evolution of the adhesion state in probabilistic terms. These results show that the forced or spontaneous separation of nanometer-sized particles from compliant substrates occurs diffusively and statistically rather than ballistically and deterministically as assumed in existing models.

Adhesion properties of nanoparticle-coated emulsion aggregation toner

Toner is the key material in printing and copying processes. Fundamental understanding of toner detachment and adhesion during the printing process is critical to improve both the efficiency of toner usage and the quality of print. To control their adhesion property, toner particles can be surface-coated with nanoparticle additives to modify their surface roughness, and consequently, to tune their adhesion properties. In this study, a technique based on the rolling resistance moment of the particle–substrate adhesion bond is used to quantify the effect of nanoparticle surface area coverage (SAC) on the effective work of adhesion of individual toner particles. Nanoparticle-coated model emulsion aggregation (EA) toner microparticles with the specified SAC levels of 0%, 10%, 50% and 100% were studied and the corresponding particle–substrate work of adhesion values were determined and compared. It is quantitatively demonstrated that the work of adhesion between a surface-coated toner particle and a flat silicon substrate decreases significantly with increasing nanoparticle SAC, which provides an effective means to tailor the adhesion performance of the EA toner. Also, based on the experimental data, for a nanoparticle-coated microparticle on a flat substrate, two possible modes of contact formation were identified and discussed.

Effects of Nanoparticle Coating on the Adhesion of Emulsion Aggregation Toner Particles

Toner is a key material for the xerographic printing and copying industry. Compared to conventional pulverized toner particles, emulsion aggregation (EA) toner particles could adhere more strongly to substrates and their property distributions are narrower because of their near-perfect spherical shape and higher surface free energy. However, in general, uncoated EA toner is more difficult to transfer and clean during printing/copying processes for the same reasons. Nanoparticle additives could be used as control agents to reduce the toner particle–substrate adhesion. In order to optimize their adhesion and handling performances, an accurate adhesion characterization of the toner particles coated with different levels of additives is required. In current study, two non-contact adhesion characterization techniques based on acoustic air-coupled excitation and ultrasonic base excitation are presented. Their use for the adhesion characterization of individual bare toner particles with an average diameter of 6.0 μm on silicon substrates, and their silica nanoparticle coated variants with a surface area coverage (SAC) of 50% is demonstrated. The techniques are based on the detection of the resonance frequencies of rocking motion of micro-spheres. The out-of-plane transient displacement responses of the rocking particles are captured by a laser interferometer, and the natural frequencies of particle rocking motion obtained from transient displacements at the top of particles and the substrate are related to the strength of the adhesion bond. It is reported that the work of adhesion values of the 50% SAC-coated toner on silicon substrates are almost an order of magnitude lower than those for the bare ones.

Effects of Nanoparticle Coverage on the Adhesion Properties of Emulsion Aggregation Toner Particles

Toner is a critical material in printing and copying processes. Compared to the conventional pulverized toner, polymerized emulsion aggregation (EA) toner provides more precise control over the shape and surface roughness of particles. Due to its roundness and low surface roughness, the near-spherical EA toner appears to adhere more strongly to flat substrates, which is desirable for efficient toner transfer and development. However, in the cleaning and transport processes, due to enhanced adhesion, it is difficult to remove the residual EA toner from surfaces of the photoreceptor and transport belt. To reduce the adhesion of the EA toner in a controllable manner, the surfaces of the toner particles are coated with silica nanoparticles with a diameter range of 15–32 nm. In the current study, a technique based on the rolling resistance moment of the particle-substrate adhesion bond is utilized for quantifying the effect of nanoparticle surface coating on the adhesion of individual toner particles. With the aid of a custom-made nanomanipulation system, an increasing lateral pushing force was applied to an individual toner particle adhered to a silicon substrate while the pre-rolling and rolling motions of the particle in response to the lateral pushing force were recorded. The work of adhesion between the toner particle and silicon substrate was extracted from the corresponding lateral force-particle displacement curve. The technique was used to characterize the adhesion properties of both uncoated (bare) and nanoparticle-coated model toner particles with a specified surface area coverage of 50%. It is found that the work of adhesion values between the surface-coated experimental toner particles and the silicon substrate are almost an order of magnitude lower than those for the bare ones.

Influence of removal time and particle size on the particle substrate adhesion force

An investigation was conducted on influence of removal time on the particle substrate adhesive force. The centrifuge technique was used to determine the adhesion force at different compression and removal rates. A microcentrifuge with a maximum rotation of 14000 rpm was used to both compress upon particles and remove them from the surface of the substrate. An image analysis program (Image-Pro Plus 4.5) was employed to monitor the number of particles adhering to and removed from the surface of the substrate after each increase in angular speed. The influence of removal time on the adhesion force was investigated, using removal times of 1, 3 and 5 minutes, which indicated that removal time does not interfere with the adhesion force within the diameter range analyzed here.

Micro/Nano-particle Manipulation and Adhesion Studies

In this paper recent progress in micro/nano-particle manipulation and particle–substrate adhesion studies is reviewed. Various particle manipulation and particle–substrate adhesion measurement techniques are summarized. An atomic force microscope (AFM) is the most commonly used tool for particle manipulation, and there are also some custom-made devices that employ different interactions such as van der Waals, electrostatic and capillary forces to manipulate micro/nano-particles. In particle–substrate adhesion study, the contact-based measurement techniques such as pull-off test and lateral push test explore the adhesion between a single particle and a substrate, while non-contact adhesion measurement techniques such as electric field detachment and centrifugal detachment methods simultaneously explore the adhesion properties of a group of particles. Also reviewed are recent studies on factors that affect the particle–substrate adhesion, including surface roughness, electrostatic charge and relative humidity.

Molecular dynamics (MD) simulation on the collision of a nano-sized particle onto another nano-sized particle adhered on a flat substrate

Adhesion of a nano particle on a flat substrate both with and without deformation and also the behaviors of bullet and target particles after collision are simulated using the MD technique. The bullet particle, a low-temperature solid Argon, is modeled by a Lennard–Jones (LJ) potential, and the target particle is modeled by a strong LJ potential. Parameters varied are the size of bullet and contaminant particles, adhesion force between the target particle and the substrate, and the velocity and collision angle of the bullet particle. Removal characteristics are different between weakly adhered and strongly adhered particles. For soft target particles high velocity at small angle favors removal. For hard particles the particle–substrate adhesion is the determining factor, and the ineffective angle is observed.

Principles and mechanisms of sub-micrometer particle removal by CO2 cryogenic technique

The removal of sub-micrometer particles from surfaces is critical in different manufacturing processes requiring precision cleaning, such as in integrated devices, microsystems, read/write head, magnetic recording media, etc. Defects caused by small particles can adversely affect device performance and yields. The efficiency required in removing increasingly small particles without affecting underlying film integrity is a major challenge in standard wet cleaning. This paper describes the theory and mechanism of a dry, non-etching cleaning method in the form of CO2 cryogenic aerosol. A theoretical model of particle removal is presented along with discussions on the forces of particle-substrate adhesion in air. Results of cleaning experiments using standard polydisperse particles and slurries provide the validity of the cleaning model.

The influence of particle charge and roughness on particle–substrate adhesion

In the electrophotographic process, charged toner particles are transferred from the bulk to the photoreceptor drum and from there to the paper. In high performance printing systems, more than 1000 pages/min are printed, equivalent to the transfer of more than 10 10 particles/min. The key to understand and to improve the printing system is particle–particle and particle–substrate interactions, respectively. We study the adhesion forces by means of atomic force microscopy (AFM) measurements complemented by centrifugal detachment measurements in an ultracentrifuge. The interpretation of AFM measurements requires careful calibration of the used cantilevers together with a sound statistical analysis. Particle charges have been determined from force–displacement curves. Silicon wafers, polished and PVD-coated aluminum substrates with tailored roughness are chosen as model materials. Spherical, smooth polystyrene and specially prepared alumina particles as well as rugged toner particles of irregular shape are investigated. We proceed from well-defined model systems (flat smooth substrate interacting with spherical particles) to technical systems where rugged particles adhere to rough substrates. The adhesion forces consist of electrostatic and of non-electrostatic (e.g. van der Waals forces) components. We show how the influence of electrostatic forces relative to the van der Waals forces can be controlled.

A study of single drug particle adhesion interactions using atomic force microscopy

This paper aims to use Atomic Force Microscopy (AFM) to characterise the interaction forces between micronised salbutamol particles, an active ingredient frequently used in metered dose inhalers, and also to glass, lactose and a fluoropolymer. The methodology used involves challenging a salbutamol functionalised AFM tip to the surfaces of interest and measuring the force experienced by the cantilever as a function of tip–sample separation. Analysis of this force–distance data allows quantification of the particle–substrate adhesion. This study yields a ranking of adhesion as glass>lactose>salbutamol>polytetrafluoroethylene (PTFE). An increase in the interaction force between the salbutamol particle and PTFE on repeated contact due to tribocharging is also observed.

Electrolyte-Induced Aggregation of Gold Particles on Solid Surfaces

An electron microscope study has been made of the aggregation of small particles of gold (ca. 2-nm radius) supported on the surfaces of different solid materials (amorphous carbon, silicon monoxide, and layer-lattice silicates) when immersed in aqueous electrolyte solutions. Particle aggregation on relatively rough surfaces of carbon and silicon monoxide, where weak particle-substrate adhesion allows reasonably high particle migration rates, occurs by a primary minimum interaction. On the smooth surfaces of silicate minerals, particle aggregates formed at room temperature in solutions of sodium chloride having concentrations ⩾ ca. 5 × 10-2 mol dm-3 and pH values within the range 8-10 were found to exhibit a reasonably uniform particle separation distance (1.7 ± 0.7 nm), indicating a secondary minimum interaction. Similar dispersions immersed in aqueous sodium nitrate under comparable conditions remained colloidally stable, even at electrolyte concentrations in the region of 4 mol dm-3. Secondary minimum aggregation was, however, found to occur in these latter systems at elevated temperatures (≥40°C). In the case of silicate-supported dispersions, lowering the pH of the electrolyte solution within the range of pH 7-2 was found to be accompanied by an increasing tendency toward particle coalescence. This effect became dominant in the region of the pH (PZC) of the gold surface. Whilst the aggregation behavior of particles supported on the surfaces of carbon and silicon monoxide can be described reasonably well in terms of the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO), this theory is entirely inadequate to account for particle aggregation on the surfaces of the silicate minerals. In these latter cases, particle aggregation behavior can be modeled satisfactorily by including an interparticle repulsive force, attributed to the structuring of solvent molecules at the particle surfaces, which decays in a simple exponential manner with increasing distance of separation. The observed effects of surface electrical potential, pH, and temperature on the colloid stability of these systems support the view that counterion hydration plays a major role in the development of structural interactions between electrically charged particles dispersed in aqueous media.

Direct observations of deformations resulting from particle-substrate adhesion

Deformations of planar substrates and contacting particles due to surface forces, such as the van der Waals interactions, were postulated long ago. Direct observations of such deformations between submicrometer Kynar 301 F™ particles and substrates consisting of polyester-siloxane block copolymers or polished silicon, obtained using scanning electron microscopy (SEM), are presented. The deformations observed in the polyester-siloxane substrate are too large to be explained using Hertzian theory of elastic behavior but are consistent with the plastic deformation model of Krupp and the tensile model of Johnson et al. Potential experimental artifacts, such as space charge effects and the effect of using conducting coatings, are discussed.