Hardness of moist agglomerates in relation to interparticle friction, capillary and viscous forces

Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. Recent micro-scale studies have identified the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles and on the saturation of the agglomerate mass.On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not accounted for, the model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology.     

Relationship between particle scale capillary forces and bulk unconfined yield strength

One of the main problems in industrial applications is the handling of fine cohesive materials. This cohesion can cause hang-ups in process equipment and stop or slow down processing. Conversely, just a small quantity of cohesion in a bulk powder can prevent particle size segregation. It is obvious that understanding cohesion from a fundamental point of view is useful in describing process flow behavior. Capillary force between particles is one source of cohesion between particles. This paper presents a theory that can be used to model the cohesive behavior of bulk materials based on capillary forces between particles.     

Assessment of adhesion and autoadhesion forces between particles and surfaces

A centrifuge technique has been used to investigate the autoadhesion force between particles and a plane surface of the same material compacted into a disk, which is resistant to the centrifugal force. When measured by profilemetry, these compact surfaces are rougher than metal or plastic surfaces used in previous studies. This results in a change of the detachment force distribution from a log-normal to a right-shifted distribution. The relationship between press-on force and median autoadhesion force depends on particle size, shape and particle surface morphology of the powder particles autoadhered. The lower the autoadhesion force, the greater the possibility that the substance can be used in a single-component powder application. The bulk properties of the powders such as cohesiveness or free powder flow were found to be related to the autoadhesion force. Estimation of the distance between the particles and surfaces has been made based on the Lifshitz-van der Waals constant derived from low frequency dielectric measurements. There was a decrease in distance of separation with increase in press-on force for both sets of particles. The contact between angular lactose monohydrate particles and a lactose monohydrate surface can be increased by more than that between irregular or spherical salmeterol xinafoate particles and their equivalent compact surface.     

Assessment of adhesion and autoadhesion forces between particles and surfaces. Part II. The investigation of adhesion phenomena of Salmeterol Xinafoate and lactose monohydrate particles in particle-on-particle and particle-on-surface contact

The centrifuge technique has been used to compare particle-on-particle and particle-on-surface adhesion with Salmeterol Xinafoate particles on single lactose particles or compacted lactose surfaces. The results from particle-on-particle and particle-on-surface adhesion measurements are not equivalent in terms of median adhesion force. The assessment of adhesion using either particle-on-particle or particle-on-surface systems depends on the process on which information is required: e.g. mixing problems are better investigated by applying the particle-on-particle technique, whereas surface transport problems such as powder compaction or flow can be more appropriately studied using particle-on-surface measurements. Taking autoadhesion measurements from the previous study into account, adhesion and autoadhesion forces were found to allow the prediction and choice of mixture components for powder blends. The results suggest that the homogeneity of such a powder blend depends both on thermodynamic properties and on the adhesion and autoadhesion properties of the single components.     

Continuum contact models for coupled adhesion and friction

We develop two new continuum contact models for coupled adhesion and friction, and discuss them in the context of existing models proposed in the literature. Our new models are able to describe sliding friction even under tensile normal forces, which seems reasonable for certain adhesion mechanisms. In contrast, existing continuum models for combined adhesion and friction typically include sliding friction only if local contact stresses are compressive. Although such models work well for structures with sufficiently strong local compression, they fail to capture sliding friction for soft and compliant systems (like adhesive pads), for which the resistance to bending is low. This can be overcome with our new models. For further motivation, we additionally present experimental results for the onset of sliding of a smooth glass plate on a smooth elastomer cap under low normal loads. As shown, the findings from these experiments agree well with the results from our models. In this paper we focus on the motivation and derivation of our continuum contact models, and provide a corresponding literature survey. Their implementation in a nonlinear finite element framework as well as the algorithmic treatment of adhesion and friction will be discussed in future work.     

磨光花岗岩表面化学改性与摩擦力改变的相依性

为了在不改变花岗岩磨光表面外观的同时,提高见水就滑的花岗岩表面摩擦力,增加光滑硬表面的防滑能力,利用无色无味水溶性的复合有机硅与主要成分是硅酸盐的花岗岩磨光表面反应,使花岗岩的表面润湿性、抗冻融性、表面电性质、微观形貌发生变化的同时,其磨光表面的摩擦力也随之变化。     

Surface force-induced bonding at metal multiasperity contacts

Adhesion at multiasperity contacts of clean metal surfaces obtained by rupture in an ultrahigh vacuum and by vacuum deposition of thin metal films onto flat surfaces has been investigated. Indium and lead were chosen in this study because their high surface energy to hardness ratios allowed us to expect a marked surface force effect in adhesion. In the region of small applied loads, the adhesion coefficient varied from ~1 to ~200 while surface roughness R_a varied from ~120 to ~0.01 μm. In the case of smooth surfaces (R_a = 0.01-0.03 μm), complete bonding of the contacting surfaces at room temperature and under small applied loads (0.1-0.5 MPa, which is much smaller than the yield stress of metals in contact) occurred. This effect is considered to be evidence of a surface force contribution to adhesion. Such effect appeared to be useful in bonding non-metallic materials by the use of thin metal films as an adhesion-promoting interlayer.     

Dispersive forces of particle–surface interactions: direct AFM measurements and modelling

Fundamentals of particle–particle interaction are of great interest in agglomeration processes. Particle adhesion depends on dispersive forces (van der Waals force), local chemical bindings, Coulomb force and capillary attractions. Additionally, surface properties like roughness, adsorption layers and surface chemistry strongly affect adhesion forces. van der Waals interactions are poorly understood because popular ab initio force calculations for molecules like density functional theory (DFT) often do not lead to proper results. van der Waals forces are difficult to measure directly. We present direct measurements of particle–particle and particle–surface interactions in the gas phase carried out with an atomic force microscope (AFM). Special emphasis is given to a proper statistical treatment of the data. For modelling of particle adhesion, we use computer-assisted empirical potential methods. Parameters like adsorbed water and surface roughness are considered. We extract parameters for weak interactions from the Lifshitz theory and gas adsorption data. Adsorbing molecules can be used as probes to measure dispersive forces. Studying surface and particle properties combined with computer-assisted modelling is a basic requisite to reach the aim of predicting particle–particle interactions in industrial processes.     

接地特性对轮胎纯侧偏六分力影响的数值研究

从5种典型接地特性(蝴蝶形、轻微蝴蝶蝶形、近似矩形、胶囊形和椭圆形,编号为a—e型)出发,考虑摩擦因数随接地压力变化,运用数值仿真方法,针对接地特性对轮胎纯侧偏工况下的六分力影响开展研究。结果显示,考虑摩擦因数对接地压力的依赖关系才能体现出接地特性对侧向力的影响,接地特性为d型的轮胎侧向力较大,侧倾力矩和回正力矩由大到小的顺序为e,d,c,b,a型,侧倾力矩受接地特性影响最为显著,可为轮胎设计和车-胎配套分析提供指导。     

Modeling the micro-scale static friction coefficient of the MEMS silicon surfaces affected by Ag and Au deposition using the thermal evaporation method

In different macro-scale applications, regarding what should be done, many of the prepared surfaces are immediately used after machining with no limitation. Since the micro-electromechanical applications have different conditions, their different properties, such as friction and adherence, must be checked. In the present study, using the thermal evaporation method, the silver and gold layers with different thicknesses were deposited on the silicon surfaces widely used in micromechanical systems (e.g. micro-assembly). Thereafter, the effects of the deposition on the geometrical properties of the surfaces were studied. Also, the effects of the deposition on the adherence and friction properties of the surfaces were examined by modeling the geometrical condition. To this end, the height function distribution was used to exploit the surface friction model in conjunction with the DMT surface adhesion model. The results show the extent to which gold affects the adhesion and friction properties of the coated surfaces.     

The effect of cohesive forces on the fluidization of aeratable powders

The effects of cohesive forces of van der Waals type in the fluidization/defluidization of aeratable type A powders in the Geldart classification are numerically investigated. The effects of friction and particle‐size distribution (PSD) on some design‐significant parameters, such as minimum fluidization and bubbling velocities, are also investigated. For these types of particles, cohesive forces are observed as necessary to fully exhibit the role friction plays in commonly observed phenomena, such as pressure overshoot and hysteresis around minimum fluidization. This study also shows that a full‐experimental PSD consisting of a dozen particle sizes may be sufficiently represented by a few particle diameters. Reducing the number of particle types may benefit the continuum approach, which is based on the kinetic theory of granular flow, by reducing computational expense, while still maintaining the accuracy of the predictions. Published 2013 American Institute of Chemical Engineers AIChE J 60: 473–484, 2014     

Improving the adhesion between epoxy coatings and aluminium substrates

Two different methods were followed to improve the adhesion and durability of the adhesion of a commonly used epoxy coating on an aluminium substrate. The first method was by application of a thin polymeric layer, having a thickness of around 10 nm, on the aluminium substrate prior to application of the epoxy coating. The functional groups in the polymers were chosen so as to be capable of chemisorption to the oxide surface and should also to be capable of being involved in the curing reaction of the epoxy resin. These polymers were poly(acrylic acid) (PAA), poly(ethylene-alt-maleic anhydride) (PEMah) and poly(vinyl phosphonic acid) (PvPA). An investigation of the interphasial region between the epoxy coating and the aluminium substrate in the final cured system showed that the polymeric layers were indeed involved in the curing reaction with the epoxy.

For the poly(ethylene-alt-maleic anhydride)-based system, this resulted in the formation of a cured, mixed poly(ethylene-alt-maleic anhydride)/epoxy interphasial region between coating and substrate while for the two other polymers, a weakly cured interphasial region was formed. The second method of adhesion and durability improvement was by hydration of the aluminium substrates, performed by immersion in boiling water. This procedure results in the formation of a porous pseudoboehmite oxyhydroxide layer. The epoxy coating was found to be capable of fully penetrating into the layer. The adhesion of the epoxy coatings was tested initially and after exposure to 40 °C water and 40 °C 5% acetic acid. The poly(ethylene-alt-maleic anhydride)-based system resulted in a very good initial adhesion and durability in presence of water for the epoxy coating, while the systems based on the other two polymers did not. The pseudoboehmite-based system also resulted in very good initial adhesion and durability in the presence of water. None of the improved systems were, however, found to be able to withstand 40 °C 5% acetic acid and showed severe corrosion underneath the epoxy coating.     

Relationship between surface energy and adhesion strength in polyethylene—paper composites

This work reports adhesion behaviour of polyethylene on paper, and deals with the surface energy of the materials involved in the manufacture of these composites, and its influence on the adhesion strength, at constant roughness, for the paper substrates. The surface energy of different papers treated with various sizing agents was determined by measuring contact angles according to the Owens-Wendt method. The peeling energy was shown to follow a linear relationship versus the reversible energy of adhesion. This result is explained by the fact that rupture takes place at the interface and that the size of the defect at the interface depends on the spreading coefficient. Corona treatment, applied to strongly sized papers before making the composites, restored the adhesion strength to its original range of values, again demonstrating the thermodynamic character of adhesion in thermoplastic-paper composites.     

CuO thin films produced for improving the adhesion between Cu and Al2O3 foils in a direct bonded copper (DBC) process

The direct bonded copper (DBC) process was carried out between Cu and Al₂O3 foils and CuO thin films were grown on the surface of Cu foils to reduce the defects produced by the DBC on the surface. CuO thin films were synthesized using a magnetron sputtering system, employing a target of Cu with 99.99% of purity and substrates of Cu foils. The discharge atmosphere for the films growth was (Ar + O₂). Once the coatings were grown, coated and uncoated Cu foils were joined at both sides (one on the top and the other at the button) of the alumina foil using the traditional direct bonded process. The Atomic concentration, chemical composition and bonding configuration of both cases were studied by X-ray photoelectron spectroscopy (XPS), finding Metallic Cu, Cu₂O and Cu–O bonds; furthermore, the atomic concentration analysis showed that coated Cu foil exhibited lower oxygen percentage, compared with uncoated one. The study of the surface defects was carried out using scanning electron microscopy (SEM) showing that the Al₂O3 ceramic was better pasted with the Cu foil including the CuO thin film.     

On the relationship between porosity and interparticle forces

This paper presents an attempt to quantify the relationship between porosity and interparticle forces for mono-sized spheres. Two systems are considered: the packing of wet coarse spheres where the dominant interparticle force is the capillary force, and the packing of dry fine spheres where the dominant force is the van der Waals force. The interrelationships between porosity, capillary force and liquid content are first discussed based on the well-established theories and experimental observations. The resultant relationship between porosity and capillary force is then applied to the packing of fine particles to quantify the van der Waals force in a packing. A generalised relationship between porosity and interparticle forces results as an extension of this analysis. The usefulness of this relationship is finally demonstrated in depicting the fundamentals governing the relationship between porosity and particle size.     

Measurement of the adhesion and removal forces of submicrometer particles on silicon substrates

In this study, the magnitude of the applied removal force and its relationship to the theoretical adhesion force is determined. The removal is done through the application of known hydrodynamic drag and lift forces on submicrometer particles. The hydrodynamic removal forces are then correlated with the removal percentage and the adhesion forces. A useful correlation that can be used to determine the adhesion force from the known applied removal force and the removal percentage is presented. Below 90% removal, the data indicate a linear relationship between the removal force and the removal percentage. The effects of time on the adhesion force and particle deformation are also presented.     

Relationship between the adhesion properties of UV-curable alumina suspensions and the functionalities and structures of UV-curable acrylate monomers for DLP-based ceramic stereolithography

Digital light processing (DLP)-based ceramic stereolithography has attracted significant attentions due to the high printing speed and high dimensional accuracy of DLP printers. However, undesired dropping of unfinished ceramic parts during printing, owing to inadequate adhesion between the first cured layer and the substrate of the building platform, still remains a challenge. In this study, the relationship between the adhesion properties of ultraviolet (UV)-curable alumina (α-Al₂O₃) suspensions and the functionalities and structures of UV-curable acrylate monomers was investigated. With an increase in the proportions of monofunctional monomers, the adhesion abilities of UV-curable alumina suspensions enhanced because of reduced volume shrinkage, however, inferior curing performances were observed due to a decrease in the double bond densities. Furthermore, the large-volume branched chain structures in monofunctional monomers and ethyoxyl groups in polyfunctional monomers effectively decreased the volume contraction, improving the adhesion performances of UV-curable alumina suspensions and facilitating the conversion of double bonds to provide excellent curing properties, further guaranteeing strong adhesion of these suspensions to the substrate.     

Analysis of structural,morphological alterations,wettability characteristics and adhesion to enamel after various surface conditioning methods

Minimal invasive dental reconstructions and orthodontic appliances are bonded to enamel without removing the enamel with rotating instruments but the top layer of enamel may be partially aprismatic and impair adhesion. The objectives of this study were to investigate the effect of mechanical surface conditioning methods for removing enamel on its structural, morphological alterations, wettability characteristics, and adhesion of resin-based cement to the conditioned surfaces. Maxillary human incisors (N = 40, n_quadrant = 160) were obtained and coronal sections were embedded in acrylic with their labial surfaces exposed. The teeth were randomly divided into four groups and the enamel surface of each tooth was divided into four quadrants. The surfaces were conditioned in a clockwise manner by one of the following methods: (1) Non-conditioned enamel acted as the control group (C); (2) Silicone-coated disk (Sof-Lex disc, Black, 3 M ESPE) (SD); (3) Diamond bur at slow speed (DB) and (4) Airborne particle abrasion (50 μm Al₂O₃, 2 bar, 5 s) (AA). Surface roughness was measured at each quadrant using a non-contact digital profilometer and contact angle measurements were performed using a goniometer. Enamel surfaces were then etched with 37% H₃PO₄ for 60 s and roughness and wettability measurements were repeated. The enamel surfaces in each quadrant received resin composite luting cement (Variolink II, Ivoclar Vivadent) incrementally in a polyethylene mold (diameter: 1 mm²; height: 4 mm) and photopolymerized. The specimens were stored in distilled water for 24 h at 37 °C until the testing procedures and then shear force was applied to the adhesive interface until failure occurred in a Universal Testing Machine (0.5 mm/min). Microshear bond (μSBS) was calculated by dividing the maximum load (N) by the bonding surface area of the resin cement. Representative enamel surfaces were analyzed under the scanning electron microscope (SEM) (x5000) to assess the surface morphology. Failure types were analyzed using optical microscope and SEM. Data (MPa) were analyzed using one-way ANOVA and Tukey`s test for each parameter and Linear model for group comparisons (α = 0.05). Surface conditioning method significantly affected the adhesion results (p < 0.001), surface roughness (p = 0.017), and contact angle (p < 0.001). Interaction terms were significant (p > 0.05). AA (338 ± 182) created significantly higher surface roughness compared to SD (308 ± 180) and DB (242 ± 197) (p < 0.05). After etching with 37% H₃PO₄, DB (307 ± 223) resulted in significantly lower roughness than those of SD (385 ± 173) and AA (414 ± 193) (p < 0.05). AA (40 ± 11) delivered significantly lower contact angle compared to those of SD (61 ± 9) and DB (59 ± 10). After etching with 37% H₃PO₄, AA (42 ± 10) and DB (50 ± 10) presented the lowest contact angle (p < 0.05). Mean μSBS results (MPa) showed significant difference between the experimental groups (p = 0.011) and were in descending order as follows: DB (20 ± 8)^a?a b < C (12 ± 5)^b. Failure types were predominantly mixed failure type between the enamel and the resin cement with more than half of the resin remained on the enamel surface (32 to 33 out of 40) in all groups. Cohesive failure in the enamel was not observed in any of the groups. SEM analysis showed that AA group leaves abundant particles on the enamel surface and after DB and AA, etching could not remove the particles completely and expose the enamel prisms.     

Effect of wax on the adhesion strength between acrylic urethane film and chromated steel sheet,and the friction coefficient of the film

The effect of wax in an acrylic urethane (AU) film on the adhesion strength between AU film and steel sheet treated with Zn-Ni and chromium was investigated by varying the amount of wax using a pull-off test. The adhesion strength decreased as the amount of wax increased. Since the wax in the AU film can react with the reactive functional groups of imine ester (hardener), the cohesive strength of the AU film can be reduced. The distribution of wax in the AU film on a polar and a nonpolar substrate, whose contact angles with water were 30° and 75°, respectively, was examined using contact angle measurements and X-ray photoelectron spectroscopy (XPS). The distribution of wax in the AU film was greatly affected by the degree of polarity of the substrate. In the case of the nonpolar substrate, the wax in the AU film migrated towards the film/nonpolar substrate interface to form a weak boundary layer, where the failure occurred. In the case of the polar substrate, the amount of wax at the film/polar substrate interface was much smaller than that at the film/nonpolar substrate interface and increased linearly with increasing wax content in the AU film. The friction coefficient of the AU film against chromated steel sheet was also measured with varying amounts of imine-ester and additives such as wax and colloidal silica. The friction coefficient appears to be influenced only by the amount of wax.