Particle Adhesion to Drops

The adhesion of spheroidal particles to spherical drops is calculated and discussed in terms of an equilibrium-penetration index. The present study emphasizes the case of particles that are sufficiently large to affect the drop volume upon penetration. It is shown that the more elongated the particles, the steeper the dependence of the penetration index on the contact angle. The effect of line tension on nanoscale particles is considered. Positive line tensions increase the steepness of the dependence of penetration index on contact angle whereas negative line tensions decrease this dependence. In addition, the energy barrier caused by positive line tensions is presented and discussed.     

Adhesion Moment of a Particle on a Wall in Gaseous Environment

If a particle adheres to a wall, it gets flattened. When such a particle gets stressed sideways by an external force, it will start to roll. To get the particle into a rolling motion, a resistive moment must be overcome, the so‐called adhesion moment. An experimental setup was designed to measure the force which must be applied to an adhered particle to trigger a rolling motion in the specimen chamber of an environmental scanning electron microscope (ESEM). From this force, the adhesion moment can be calculated. A detailed explanation of the experiments will be given. The results of the experiments are discussed.     

Time-Dependence of the Adhesion of Micrometer-Size Particles to Substrates: Correlation with Postdeposition Particle Rotation

The time dependence of the detachment force of 7-µm ground polyester particles coated with silica nanoparticles from a ceramer-coated substrate was determined by ultracentrifugation. The detachment force of the particles from the substrate was found to increase with the time since the particle deposition. Scanning electron microscopy (SEM) results show that, following deposition, the particles rotate at approximately the same scale as the observed increase in the detachment force. This suggests that the increase in adhesion may be due to particle rotation from their initial positions obtained upon deposition to a more stable position that results from torques generated by either electrostatic or van der Waals forces acting on the particles.     

Time-Dependence of the Adhesion of Micrometer-Size Particles to Substrates: Correlation with Postdeposition Particle Rotation

The time dependence of the detachment force of 7-µm ground polyester particles coated with silica nanoparticles from a ceramer-coated substrate was determined by ultracentrifugation. The detachment force of the particles from the substrate was found to increase with the time since the particle deposition. Scanning electron microscopy (SEM) results show that, following deposition, the particles rotate at approximately the same scale as the observed increase in the detachment force. This suggests that the increase in adhesion may be due to particle rotation from their initial positions obtained upon deposition to a more stable position that results from torques generated by either electrostatic or van der Waals forces acting on the particles.     

The Effect of Relative Humidity on Particle Adhesion and Removal

The removal of small particles is vital for contamination-free manufacturing. In humid environments liquid can condense between the particle and substrate and give rise to a very large capillary force, which increases the total force of adhesion. The removal and adhesion forces of polystyrene latex (PSL) particles and pigmented coating chips were measured on silicon, polyethylene terephthalate, metallized and polyester coating substrates as a function of humidity. The results indicate that the capillary force is significant at a relative humidity above 50% and dominates at a relative humidity above 70%. At relative humidity below 45%, the electrostatic force becomes significant. The adhesion forces varied depending on the particles and substrates used, but the trend of high adhesion at high and low relative humidity was observed for all PSL particles/substrate systems. The pigmented coating chips/substrate system however, exhibited high adhesion at high relative humidity and low adhesion at low relative humidity.     

The Effect of Relative Humidity on Particle Adhesion and Removal

The removal of small particles is vital for contamination-free manufacturing. In humid environments liquid can condense between the particle and substrate and give rise to a very large capillary force, which increases the total force of adhesion. The removal and adhesion forces of polystyrene latex (PSL) particles and pigmented coating chips were measured on silicon, polyethylene terephthalate, metallized and polyester coating substrates as a function of humidity. The results indicate that the capillary force is significant at a relative humidity above 50% and dominates at a relative humidity above 70%. At relative humidity below 45%, the electrostatic force becomes significant. The adhesion forces varied depending on the particles and substrates used, but the trend of high adhesion at high and low relative humidity was observed for all PSL particles/substrate systems. The pigmented coating chips/substrate system however, exhibited high adhesion at high relative humidity and low adhesion at low relative humidity.     

用于细颗粒分离的水力旋流器的压力特性研究

对用于细颗粒分离的水力旋流器的压力特性 (压力降及压降比 )与流量、分流比、旋数、溢流口和底流口直径及气液比等主要参数之间的关系进行了深入的研究与分析。研究发现 ,水力旋流器内部压力降分别随流量、分流比、旋数及气液比的提高而加大 ,压降比则分别随流量、分流比、旋数的提高而降低。随着溢流口直径的加大 ,水力旋流器的溢流压力降减小 ,而压降比也随之降低 ;随着底流口直径的加大 ,底流压力降减小 ,压降比随之升高。分析可知 ,减少旋流器能耗的有效方法是降低旋数 ,或者减少混合介质中的气液比     

Adhesion of solid particles to gas bubbles. Part 2: Experimental

In slurry bubble columns, the adhesion of solid catalyst particles to bubbles may significantly affect the G-L mass transfer and bubble size distribution. This feature may be exploited in design by modifying the hydrophilic or hydrophobic nature of the particles used. Previously we have proposed a generalised model, describing the adhesion of particles to G-L interface under stagnant conditions. In this work, we studied the adhesion of particles characterised by different degree of hydrophobicity and porosity: non-porous polystyrene and glass beads, unmodified and hydrophobised mesoporous silica, and activated carbon particles. Images recorded at high optical magnification show the particles adhering to gas bubbles individually or as aggregates. In aqueous media, higher liquid surface tension and particle surface hydrophobicity increase the adhesion strength and the tendency of particles to agglomerate, in agreement with the model. The adhesion of non-porous rough-surface particles to gas bubbles can be characterised by the receding contact angle. The advancing contact angle represents better the adhesion of the same particles to liquid droplets. We found that the “effective” contact angle of porous particles is much lower than an “intrinsic” contact angle calculated from the heat of immersion in water, or measured by sessile drop method. An equivalent contact angle derived from the Cassie rule explains the wetting behaviour of particles having the pores filled with liquid.     

Particle Adhesion to Elastomeric Substrates and Elastomeric Substrates with Semi-Rigid Coatings

The force needed to remove micrometer-size polystyrene particles from elastomeric substrates having Young's moduli of 3.8 and 320 MPa was measured using atomic force techniques. It was found that the removal force was approximately an order of magnitude less for the more rigid substrate than for the more compliant substrate. In both cases the removal force was independent of applied load. However, when the more compliant material was overcoated with the stiffer material, the particle removal force was found to increase with increasing pressure, with the limit at low pressure commensurate with the removal force observed for the stiffer substrate and commensurate with the more compliant material at higher pressures. The results are interpreted in terms of the penetration depth of particle asperities into the substrates.     

Adhesion forces of charged particles

Adhesion forces of toner and polymer particles to aluminum substrates were measured by the centrifugal, detachment field and microelectrode detachment field methods, and factors affecting the adhesion forces are discussed. The adhesion forces of toner particles increased with an increase in either particle size or particle charge. The adhesion force of an irregularly shaped toner was larger than that of a spherical toner. The mean adhesion force of polymer particles to aluminum substrates decreased with an increase in surface roughness of the substrates. The CF₄ plasma treatment of the polymer particles shifted their adhesion force distribution in a smaller direction. It was confirmed that the results by the centrifugal and the detachment field methods were in good agreement with each other. The contribution of van der Waals, electrostatic and water bridging forces to the adhesion forces of toner particles are also discussed.     

Adhesion Frequency in Interfacial Dynamics of Copolymers

This paper introduces the concept of adhesion frequency that characterizes the dynamic interaction between a polymer matrix and a solid surface. It is argued that the pressure sensitive bonding of a polymer interface occurs within a critical time scale below which no wetting is established upon contact. The adhesion frequency can be viewed as the inverse of this critical time scale. The bonding kinetics is used to model transient adhesion and take into account the growth of bonding energy with time. The mechanics of the interfacial dynamics is used to derive expressions for the adhesion frequency when inertial, viscous or elastic effects dominate the surface deformation. Transient adhesion effects are also investigated experimentally using non-axisymmetric rotating bits and commercially-available copolymer adhesives. The rotating bits induce an oscillatory motion on the polymer surface. Experiments are conducted with the rotating bits spinning at a wide range of speeds making it possible to obtain the dynamic response of the polymer interface as well as to measure the adhesion frequency for the tested polymers.     

Adhesion Frequency in Interfacial Dynamics of Copolymers

This paper introduces the concept of adhesion frequency that characterizes the dynamic interaction between a polymer matrix and a solid surface. It is argued that the pressure sensitive bonding of a polymer interface occurs within a critical time scale below which no wetting is established upon contact. The adhesion frequency can be viewed as the inverse of this critical time scale. The bonding kinetics is used to model transient adhesion and take into account the growth of bonding energy with time. The mechanics of the interfacial dynamics is used to derive expressions for the adhesion frequency when inertial, viscous or elastic effects dominate the surface deformation. Transient adhesion effects are also investigated experimentally using non-axisymmetric rotating bits and commercially-available copolymer adhesives. The rotating bits induce an oscillatory motion on the polymer surface. Experiments are conducted with the rotating bits spinning at a wide range of speeds making it possible to obtain the dynamic response of the polymer interface as well as to measure the adhesion frequency for the tested polymers.     

SPH方法在剪切流液滴运动规律研究中的应用

阐述了SPH(Smoothed Particle Hydrodynamics)方法的理论基础和计算方法,应用该方法编写程序计算了液滴在剪切流中的运动规律。通过激波和Poiseuille流动这两个验证算例的计算发现,SPH方法能很好地应用于流体运动规律的计算模拟。另外,还对不同的核函数的计算结果进行了比较,分析了影响计算精度的因素,在此基础上,计算模拟了剪切流中单液滴的运动行为,包括变形、破裂行为。     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

Size Distribution of Drops Formed from Nozzles in Air at Velocities below Jetting

The drop size distribution of drops formed from four nozzles (d = 0.5, 1.8, 2.7 and 3.6 mm) were measured for the flow rate range Q = 0.1–1.17 cm³/s and Reynolds number 56–448. Distilled water was used as the dispersed phase and air as the continuous one. The experimental drop size distributions were described satisfactorily by the theoretical upper limit number and volume distributions. The experimental data of minimum and maximum diameters versus the respective Sauter mean diameters gave straight lines with slopes of 0.81 and 1.18, respectively.     

Adhesion and Removal of Particles from Surfaces Under Humidity Controlled Air Stream

The adhesion and the removal of individual micrometer-sized particles on a plane substrate are studied using an air shear flow cell. Laminar isothermal compressible flow characterization enables us to analyze the effect of various parameters such as particle size, air humidity, surface nature and surface charge on the aerodynamic forces required to remove the particles from the substrate. The results show that the increase of humidity (up to a critical value) favors particle removal when particles adhere under strong electrostatic forces on a non-conductive charged substrate. On the contrary, the existence of a capillary force disfavors particle removal beyond this critical humidity. The increase of the humidity disfavors the removal of particles in contact with an uncharged substrate. The results are interpreted in terms of a global adhesion force using a force and torque balance on a single particle in contact with a plane substrate. Moreover, the use of a high-speed video recording system enables us to determine the particle removal mechanisms as a function of the particle Reynolds number.     

Adhesion and Removal of Particles from Surfaces Under Humidity Controlled Air Stream

The adhesion and the removal of individual micrometer-sized particles on a plane substrate are studied using an air shear flow cell. Laminar isothermal compressible flow characterization enables us to analyze the effect of various parameters such as particle size, air humidity, surface nature and surface charge on the aerodynamic forces required to remove the particles from the substrate. The results show that the increase of humidity (up to a critical value) favors particle removal when particles adhere under strong electrostatic forces on a non-conductive charged substrate. On the contrary, the existence of a capillary force disfavors particle removal beyond this critical humidity. The increase of the humidity disfavors the removal of particles in contact with an uncharged substrate. The results are interpreted in terms of a global adhesion force using a force and torque balance on a single particle in contact with a plane substrate. Moreover, the use of a high-speed video recording system enables us to determine the particle removal mechanisms as a function of the particle Reynolds number.     

Adhesion of ultrafine particles—A micromechanical approach

In particle processing and product handling of fine , ultrafine and nanosized particles , the well-known flow problems of dry cohesive powders in process apparatuses or storage and transportation containers include bridging, channelling, widely spread residence time distribution associated with time consolidation or caking effects, chemical conversions and deterioration of bioparticles. Avalanching effects and oscillating mass flow rates in conveyors lead to feeding and dosing problems. Finally, insufficient apparatus and system reliability of powder processing plants are also related to these flow problems. Thus, it is very essential to understand the fundamentals of particle adhesion with respect to product quality assessment and process performance in particle technology.The state-of-the-art in constitutive modelling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single isotropic contact of two smooth spheres is briefly discussed. Then the new models are shown that describe the elastic-plastic force-displacement and moment-angle behaviour of adhesive and frictional contacts.Using the model “stiff particles with soft contacts”, a sphere-sphere interaction of van der Waals forces without any contact deformation describes the “stiff” attractive term. A plate-plate model is used to calculate the “soft” micro-contact flattening and adhesion. Various contact deformation paths for loading, unloading and contact detachment are discussed. Thus, the varying adhesion forces between particles depend directly on this “frozen” irreversible deformation. Thus, the adhesion force is found to be load dependent. Their essential contribution on the tangential force in an elastic-plastic frictional contact with partially sticking within the contact plane and microslip, the rolling resistance and the torque of mobilized frictional contact rotation is shown.