Relation between the cleanability of bare or polysiloxane-coated stainless steels and their water contact angle hysteresis

Cleaning of bare or coated stainless steel surfaces is investigated using some specific techniques for both particulate soil and oil removal. Particulate soil is removed from the surface by a water drop sliding, whereas oil is eliminated by shear flow of a commercial detergent. The cleanability performance is found to depend both on surface energy and topography. In general, the water contact angle hysteresis, which itself is related to the advancing contact angle and the surface roughness, is found to be an appropriate criterion for characterizing the cleaning performance. This finding is discussed in terms of retention and removal forces during the cleaning process and could provide in the future a criterion for material selection for industrial use of stainless steel surfaces.     

Effects of Electrostatic and Capillary Forces and Surface Deformation on Particle Detachment in Turbulent Flows

In this work the rolling detachment of particles from surfaces in the presence of electrostatic and capillary forces based on the maximum adhesion resistance was studied. The effective thermodynamic work of adhesion, including the effects of electrostatic and capillary forces, was used in the analysis. The Johnson, Kendall and Roberts (JKR) and Derjaguin, Muller and Toporov (DMT) models for elastic interface deformations and the Maugis–Pollock model for plastic deformation were extended to include the effect of electrostatic and capillary forces. Under turbulent flow conditions, the criteria for incipient rolling detachment were evaluated. The turbulence burst model was used to evaluate the airflow velocity near the substrate. The critical shear velocities for removal of particles of different sizes were evaluated, and the results were compared with those without electrostatic and capillary forces. The model predictions were compared with the available experimental data and good agreement was observed.     

Detachment of rough particles with electrostatic attraction from surfaces in turbulent flows

The detachment of particles with coarse and fine roughnesses from surfaces in a turbulent boundary layer flow including electrostatic effects is studied. It is assumed that the real area of contact is determined by elastic deformation of asperities, and the effect of topographic properties of surfaces is included. The Johnson-Kendall-Roberts (JKR) adhesion model is used for analyzing the behavior of individual asperities. For an average Boltzmann charge distribution, the saturation charge condition as well as a fixed charge per unit mass, the Coulomb, the image, the dielectrophoretic, and the polarization forces acting on the particle in the presence of an imposed electric field are evaluated. The theories of rolling and sliding detachment are used to study the onset of removal of bumpy particles and those with fine roughness from plane surfaces. The hydrodynamic forces and torques acting on the particle attached to a wall, along with the adhesion force for the particle, are used in the model development. The minimum critical shear velocities needed to detach particles of different sizes from plane surfaces in the presence of an applied electric field are evaluated and discussed. The electric detachment of the particles is also studied and the field strength needed for particle removal is determined. It is shown that the surface charge distribution significantly affects the removal of particles from surfaces.     

Effects of capillary force and surface deformation on particle removal in turbulent flows

A new rolling detachment model for particle removal in the presence of capillary forces based on the maximum adhesion resistance was developed. The new model uses an effective thermodynamic work of adhesion model that includes the effects of capillary forces generated by the formation of liquid meniscus at the interface. The JKR and DMT models for elastic particle and surface deformations and the Maugis and Pollock model for the plastic deformation were extended to include the effect of capillary forces. Under turbulent flow conditions, the criteria for incipient rolling detachments were evaluated. The turbulence burst model was used to evaluate the air velocity near the substrate. The critical shear velocities for resuspension of particles of different sizes were evaluated and the results were compared with those without capillary force. The model predictions were compared with the available experimental data and good agreement was found.     

Particle Removal in Linear Shear Flow: Model Prediction and Experimental Validation

In many industrial processes, particle contamination is becoming a major issue. Particle detachment from surfaces can be detrimental, e.g., during lithographic processing. During cleaning, however, detachment of particles is aimed for. However, until recently, only little was known on the mechanism of particle detachment due to flowing gasses. In high throughput applications, large gas velocities are likely to occur at certain locations in the system. It is important to test particle behavior experimentally under all conditions that may arise. Therefore, the aim of this study is to be able to predict the risk of particle detachment by modeling. For this purpose, particle–surface interaction is studied for micrometer-sized particles. Based on the particle Reynolds number, critical particle diameters were determined for which the flow-induced forces on the particles (drag and lift forces) are larger than the attractive forces between the particle and the surface (van der Waals force). Among the different possible particle motions (lift, sliding and rotation), particle rotation turns out to be the mechanism responsible for particle removal. A critical particle diameter was defined for which attractive and flow-induced forces are equal. Calculated values of the critical particle diameter agree with the experimental results within a few micrometers. This removal mechanism model can thus be used to calculate the cleaning efficiency of a flow, and for determining the probability of unwanted detachment of particles from surfaces in ultra-clean production or processing environments.     

PARTICLE REMOVAL MECHANISMS IN CRYOGENIC SURFACE CLEANING

Particle removal mechanisms in cryogenic surface cleaning are examined. The effect of impacting solid carbon dioxide particles and the hydrodynamic forces and torques are included in the model. Sliding detachment models and rolling detachment models in conjunction with the theory of critical moment are used. The critical conditions for removal of particles of different sizes are evaluated. For various carbon dioxide pellet diameters, incoming flow angles, nozzle, and cleaning surface separation distances, the critical shear velocities for particle detachment are evaluated.     

The adhesion of animal cells to surfaces: The measurement of critical surface shear stress permitting attachment or causing detachment

The ‘LH Fowler Cell Adhesion Measurement Module’ was successfully adapted to permit the determination of the critical shear forces involved in both the attachment to and the detachment from glass and plastic surfaces, by mammalian fibroblasts. Variations in the sensitivity of different cell types to critical shear forces were observed, although generally the critical shear forces permitting attachment were in the order of 1000-times less than those causing detachment. Finally alterations to culture conditions indicated that the sensitivity of cell attachment to shear forces is strongly affected outside of optimal conditions.     

Detachment of Droplets in a Fully Developed Turbulent Channel Flow

Liquid aerosols deform and detach from solid surfaces under an external force. It is a familiar phenomenon in many engineering applications. This article experimentally investigates the deformation and detachment of liquid droplets on three different solid surfaces in a fully developed turbulent channel flow. It is shown that the droplets either are compressed or elongate under the turbulent flow. The elongation of the droplet due to the turbulent flow is measured and presented. When the friction velocity of the flow exceeds a critical value, the droplets slide along the surface. The critical friction velocity is found empirically to be inversely proportional to the square root of the contact diameter. The sliding velocity after detachment is also reported. It has been observed by many researchers that, when the external force is gravity or a simple shear flow, the retention force of the droplet is proportional to the difference between the cosines of the receding and advancing contact angles. As the shape of a deformed droplet is much more complex under a turbulent flow, this article discusses the applicability of the same relation to the turbulent channel flow.

Copyright 2014 American Association for Aerosol Research     

Mitigating fines plugging in high pressure/temperature hydrotreaters using an induced-pulsing trickle-bed filtration approach

The fine particles deposition/detachment process under liquid flow shear shock or forced periodic operation conditions in a high pressure/temperature trickle-bed reactor was analyzed theoretically using a dynamic multiphase flow deep-bed filtration model. The model incorporates physical effects associated with detachment of the fine particles from the collector surface as a result of colloidal forces in the case of Brownian particles or by the hydrodynamic forces for non-Brownian particles. An important finding of the work was that for non-colloidal fine particles, forced periodic operation procured improvements (assessed in terms of reduction in specific deposit and pressure drop) in the mitigation of plugging in trickle-bed reactors. However, due to the highest critical shear stress values for fine particles in the colloidal range, forced periodic operation did not substantiate useful practical effect. In the circumstances when Brownian fine particles are involved, reduction of plugging may take place under liquid flow shock conditions.     

Analysis of pulsatile flow and its role on particle removal from surfaces

The use of pulsatile flow for energy efficient particle removal from surfaces is evaluated through modeling calculations. The governing equation for pulsatile flow in a channel between parallel plates with an oscillatory pressure input is solved and wall shear stress, identified as a measure for particle removal, calculated for fixed power input. It is observed that as the frequency of oscillation is increased the average wall shear stress with an oscillatory pressure input is higher than the corresponding steady state value only above a critical frequency. Similar results are obtained for pulsatile flow in a pipe. Explanation for this observation is presented based on how velocity profile changes as a function of frequency and consequently its effect on wall shear stress versus power dissipated. Based on these observations we propose that there is a critical frequency above which an oscillatory pressure input will be energy efficient for particle removal.     

Advancing and receding contact lines on patterned structured surfaces

An experimental investigation of advancing and receding contact lines on patterned surfaces was performed in a controlled environment. Hydrophobic polymers were used to create patterned surfaces to mimic defects and the working fluid was water. Surfaces were prepared with holes or pillars every 200 μm and depth/height from 1 to 11 μm. An optical technique was used to measure contact angle. On smooth (control) surfaces, an advancing or receding contact angle was observed. On the patterned surfaces, pinning and depinning at the defects (holes or pillars, respectively) was observed, with advancing or receding contact lines occurring between these depinning/pinning events. The observed pinning/depinning phenomenon of the contact line was investigated to demonstrate the dynamics of the contact line motion over rough surfaces for a small range of contact line velocity. The competition between the Young unbalanced force and the anchoring forces of the defects is thought to dominate the pinning/depinning process. Stick–slip behaviour of the contact line is observed for larger structures and the results show a strong pinning of the contact line on surfaces with larger defects. The datum contact angle and its deviation were measured and a new concept of scaled energy barrier was calculated for advancing contact lines. This was strongly dependent on defect size. An estimation of the unbalanced Young force per unit length was also made for comparison, which also depended on defect size. This new approach allows new insights into this wetting phenomenon.     

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.     

Relationship between adhesion and friction forces

The Surface Forces Apparatus technique was used to measure the normal (perpendicular) and lateral forces between variously prepared surfaces under both dry and lubricated conditions. 'Normal' forces include the force vs distance functions, F(D), for surfaces separated by thin liquid films as well as the adhesion forces and energies, γ, for two surfaces in adhesional contact. 'Lateral' forces include the static and kinetic friction forces F of the surfaces as they slide past each other at a given separation, D. The results show that very thin liquid films confined between two solid surfaces can sustain both normal and shear forces or stresses. The results further indicate that the normal force, F(D) or γ, may be directly related to the static friction force, F_s, and simple equations are proposed that relate these forces (by 'static' friction force is meant the lateral force that must be applied to initiate motion, but not necessarily to maintain this motion). In contrast, the kinetic friction force, F_k, which is the force that must be continually applied to maintain motion at a given velocity, was found to be related, not to the equilibrium or reversible interaction but to the dissipative or irreversible part of the adhesion or interaction energy during a loading-unloading cycle. There is a high degree of correlation in the way that normal forces and friction forces are affected by changes in applied load or pressure, sliding velocity, loading-unloading rates and temperature. These systematic correlations can be conveniently represented by non-equilibrium 'adhesion' and 'friction' phase diagrams.     

Bumpy Particle Adhesion and Removal in Turbulent Flows Including Electrostatic and Capillary Forces

The effect of electrostatic and capillary forces on bumpy particle adhesion and removal in turbulent flows is studied. We use the JKR theory and account for the increase of adhesion by capillary force. The effects of electrostatic forces and nonlinear hydrodynamic drag are included in the analyses. The criteria for incipient rolling and sliding detachments and electrostatic lifting removal are evaluated. A turbulence burst model is used for evaluating the peak air velocity near the substrate. The critical shear velocities for detaching particles of different sizes under different conditions are evaluated. The electric field strength needed for electrostatic removal of particles with different charges is also estimated. The results are compared with those obtained in the absence of the capillary force. Comparisons of the model predictions with the available experimental data are also presented.     

Bumpy Particle Adhesion and Removal in Turbulent Flows Including Electrostatic and Capillary Forces

The effect of electrostatic and capillary forces on bumpy particle adhesion and removal in turbulent flows is studied. We use the JKR theory and account for the increase of adhesion by capillary force. The effects of electrostatic forces and nonlinear hydrodynamic drag are included in the analyses. The criteria for incipient rolling and sliding detachments and electrostatic lifting removal are evaluated. A turbulence burst model is used for evaluating the peak air velocity near the substrate. The critical shear velocities for detaching particles of different sizes under different conditions are evaluated. The electric field strength needed for electrostatic removal of particles with different charges is also estimated. The results are compared with those obtained in the absence of the capillary force. Comparisons of the model predictions with the available experimental data are also presented.     

Mechanical degradation of semi-dilute polymer solutions in laminar flows

Mechanical degradation of a semi-dilute solution of non-hydrolyzed polyacrylamide was studied under laminar flow conditions through fine capillary systems. Using a multi-pass device and capillary tubes of the same diameter and of various lengths we have shown that mechanical degradation (i) occurs at a critical value of the wall shear rate, chosen as a reference deformation rate, which is slightly higher than that of the appearance of high pressure losses in the entrance region of the capillary tube; (ii) is independent of the capillary tube length; (iii) increases with the number of passes N up to a maximum value for a limiting number of passes N_lim which is a decreasing function of deformation rates but does not depend on capillary length. The amount of degradation is expressed in terms of loss of viscous dissipation in shear and transient elongational flow. This last point is determined by studying the total end pressure loss through the capillary tube as a function of the pass number. The high pressure loss is related to viscous dissipation on macromolecules stretched by rapid converging flow. A comparison between a fresh and a fully degraded solution indicates that the degradation affects shear viscosity much less than viscous dissipation in rapid converging flow which is related to the properties of extended macromolecules. Both experimental results and theoretical interpretation suggest that, in our capillary system, the mechanical degradation occurs in the entrance region of the capillary where macromolecules are stretched and consequently submitted to extensional forces which can overcome the C–C bonds strength.     

A Model for Removal of Compact,Rough, Irregularly Shaped Particles from Surfaces in Turbulent Flows

A model for removal of compact, rough, irregularly shaped particles from surfaces in turbulent flow was developed. Following the approach of our previous bumpy particle model, irregularly shaped particles were modeled as spherical particles with a number of bumps on them. To improve the model, the effect of surface roughness was added to the bumps. Each bump was modeled with large number of asperities and the Johnson-Kendall-Roberts (JKR) adhesion theory was used to model the adhesion and detachment of each bump and asperity in contact with the surface. The total adhesion force for each bump was obtained as the summation of each asperity force in contact with the substrate. To account for the variability observed in the removal of particles, the number of bumps and roughness values of particles are assumed to be random, respectively, with Poisson and log-normal distributions. For particle separation from the surface, the theory of critical moment was used, and the orientation of bumps on the surface was considered when determining the range of shear velocity needed for removal of the irregularly, shaped particles. The effects of particle size, turbulent flow, particle irregularity, and particle surface roughness on detachment and resuspension were studied for different particles and surfaces. Model prediction for removal of rough, irregularly shaped graphite particles from steel substrate was compared with the available experimental data and earlier numerical models, and good agreement was obtained. This study may find application in adhesion and detachment of irregular particles from flooring in indoor and outdoor environments.     

异形竖板上降落液膜破裂特性

在实验研究的基础上,对表面蒸发条件下沿异形竖板的降落液膜破裂特性作了简化理论分析,得出了液膜暂时破裂临界状态与永久破裂临界状态的准数关系式,以预测液膜破裂条件.     

Polymer flow through capillaries of variable diameter

A polystyrene melt has been extruded through successive capillaries arranged to produce converging and diverging flow patterns through the twin orifices. Applied pressure at fixed mass flow rate through the combined dies is equal to the sum of the pressure drops in the single capillaries in both flow modes. The Bagley end correction was found to apply to each die in the sequence. Bagley plots were linear with a particular upper capillary at given apparent shear rate in the lower die. No effect of shear history could be detected on the viscous behavior of the polymer, but preshearing in converging flow produced a slight reduction in die swell.