An improved meniscus surface model for contacting rough surfaces

Based on the Extended-Maugis-Dugdale (EMD) elastic theory, a single asperity capillary meniscus model considering asperity deformation due to both contact and adhesive forces was developed. Specifically included in the single asperity meniscus model was the solid surface interaction inside the contact area. Subsequently, the single asperity model was coupled with a statistical roughness surface model to develop an improved meniscus surface model applicable to a wide-range of humidity levels and adhesion parameter values. Simulations were performed using typical surfaces found in microelectromechanical systems (MEMS) and magnetic storage hard disk drives to examine the effects of surface roughness and relative humidity. It was found that smoother surfaces give rise to higher adhesive and pull-off forces, and at higher relative humidity levels, the capillary force governs the adhesive behavior. As the humidity decreases, the solid surface interaction increases and needs to be included in the total meniscus adhesion. By integrating the adhesive force-displacement curves, the adhesion energy per unit area was calculated for MEMS surfaces and favorably compared with reported experimental data.     

Adhesive transition from noncontacting to contacting elastic spheres: extension of the Maugis-Dugdale model

In this paper, an adhesion model for spherical noncontact is proposed based on the Maugis-Dugdale (MD) adhesive contact model. The proposed noncontact model is combined with the MD contact model, thus providing a full range adhesion model with analytical transition from noncontacting to contacting asperity geometry. The proposed model is favorably compared with the full range improved DMT model for low surface energy values. The transition process from noncontact to contact and the adhesion instability that occurs during this transition are also investigated. It is found that jump-off points are different for displacement control and force control. Moreover, under displacement control, jump-on and jump-off points are different when the adhesion parameter lambda is greater than 0.95, whereas they are identical for lambda<0.95. By curve fitting a relationship between the critical approach under displacement and force control separately and the adhesion parameter lambda, approach prediction equations for jump-on and jump-off under different adhesion levels were obtained.     

Correction of random surface roughness on colloidal probes in measuring adhesion

Atomic force microscopes (AFM) are commonly used to measure adhesion at nanoscale between two surfaces. To avoid uncertainties in the contact areas between the tip and the surface, colloidal probes have been used for adhesion measurements. We measured adhesion between glass spheres and silicon (100) surface using colloidal probes of different radii under controlled conditions (relative humidity of < 3%, temperature of 25 +/- 1 degrees C). Results showed that the adhesion forces did not correlate with the radii of the spheres as suggested by elastic contact mechanics theories. Surface roughness and random surface features were found on the surfaces of the colloidal probes. We evaluated various roughness parameters, Rumpf and Rabinovich models, and a load-bearing area correction model in an attempt to correct for the roughness effects on adhesion, but the results were unsatisfactory. We developed a new multiscale contact model taking into account elastic as well as plastic deformation in a successive contacting mode. The new model was able to correct for most of the surface roughness features except for surface ridges with sharp angular features, limited by the spherical asperity assumption made in the model.     

Wettability characterization of transparent MgF<Subscript>2</Subscript> nanoparticle coatings with SiO<Subscript>2</Subscript> binder covered with fluoroalkylsilane self-assembled monolayers

SiO₂-added MgF₂ nanoparticle coatings with various surface roughness properties were formed on silica-glass substrates from autoclaved sols prepared at 100–180 °C. The samples were exposed to fluoro-alkyl silane (FAS) vapor to give hydrophobicity. All nanoparticle samples before FAS treatment had transmittances higher than 93% and such values were preserved even after FAS treatment. We examined root mean square roughnesses of the nanoparticle coatings with a Scanning Probe Microscope. We also examined their static and dynamic wettabilities with a contact angle meter and calculated their adhesive energies and surface free energies (SFEs). The surface roughness of the nanoparticle coating increased with the increase of the autoclave temperature. In addition, higher autoclave temperature caused increases in the sliding angle and decreases in the SFE. Interestingly, the higher the contact angle was, the larger the sliding angle was, although smaller sliding angle was expected with a larger contact angle.     

Self-affine roughness influence on the friction coefficient for rubbers onto solid surfaces

In this paper we investigate the influence of self-affine roughness on the friction coefficient mu(f) of a rubber body under incomplete contact onto a solid surface. The roughness is characterized by the rms amplitude w, the correlation length xi, and the roughness exponent H. It is shown that with increasing surface roughening at short and/or long length scales (decreasing H and/or increasing ratio w/xi, respectively), the maximum of the friction coefficient mu(f) shifts to lower sliding velocities. The latter occurs only for conditions of incomplete contact for small contact length scales lambda (xi).     

Relationship between the dynamic softening transition and wet sliding friction of elastomer compounds

For properly chosen elastomer compounds, thermorheological characterization is combined with an examination of the variation of the wet sliding friction with temperature. A conceptual argument leads to the assumption that the wet sliding friction should maximize at the energy dissipation peak associated with the dynamic softening transition at a characteristic frequency determined by the sliding speed and the effective smallest surface asperity scale. The dynamic softening transition is characterized with the peak in tan δ/G′ⁿ, where tan δ is the loss tangent, G′ is the elastic modulus, and n is a constant between 0 and 1. The William–Landel–Ferry transform is uncritically applied for extrapolating the position of the peak in tan δ/G′ⁿ at high frequencies. Even based on the criterion of tan δ, the results obtained on a concrete surface indicate that the effective smallest asperity scale is of order of 100 μm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2467–2478, 2004     

An approximate model for the adhesive contact of rough viscoelastic surfaces

Surface roughness is known to easily suppress the adhesion of elastic surfaces. Here, a simple model for the contact of viscoelastic rough surfaces with significant levels of adhesion is presented. This approach is derived from our previous model (Barthel, E.; Haiat, G. Langmuir 2002, 18, 9362) for the adhesive contact of viscoelastic spheres. For simplicity, a simple loading/unloading history (infinitely fast loading and constant pull-out velocity) is assumed. The model provides approximate analytical expressions for the asperity response and exhibits the full viscoelastic adhesive contact phenomenology such as stress relaxation inside the contact zone and creep at the contact edges. Combining this model with a Greenwood-Williamson statistical modeling of rough surfaces, we propose a quantitative assessment of the adhesion to rough viscoelastic surfaces. We show that moderate viscoelasticity efficiently restores adhesion on rough surfaces over a wide dynamic range.     

Roughness and hydrophobicity studies of nanofiltration membranes using different modes of AFM

Determination of the surface roughness by AFM is crucial to the study of particle fouling in nanofiltration. It is, however, very difficult to compare the different roughness values reported in the literature because of a lack in uniformity in the methods applied to determine surface roughness. AFM is used in both noncontact mode and tapping mode; moreover, the size of the scan area is highly variable. This study compares, for six different nanofiltration membranes (UTC-20, N30F, Desal 51HL, Desal 5DL, NTR7450, NF-PES-10), noncontact mode AFM with tapping mode AFM for several sizes of the scan area. Although the absolute roughness values are different for noncontact AFM and tapping mode AFM, no difference is found between the two modes of AFM in ranking the nanofiltration membranes with respect to their surface roughness. NTR 7450 and NF-PES-10 are the smoothest membranes, while the roughest surface can be found with Desal 51HL and Desal 5DL. UTC-20 and N30F are characterized by an intermediate roughness value. An increase in roughness with increasing scan area is observed for both AFM modes. Larger differences between the roughnesses of the membranes are obtained with tapping mode AFM because of the tapping of the tip on the surface. Phase imaging is an extension of tapping mode AFM, measuring the phase shift between the cantilever oscillation and the oscillation of the piezo driver. This phase shift reflects the interaction between the cantilever and the membrane surface. A comparison with contact angle measurements proves that a small phase shift corresponds to a large contact angle, representing a hydrophobic membrane surface.     

Friction and wear behavior of PI and PTFE composites for ultrasonic motors

High‐performance polymer‐based frictional materials have become increasingly important to improve the mechanical output properties of ultrasonic motors. This study discussed the friction and wear behavior of 2 dominating frictional materials of polymer composites for ultrasonic motors, polyimide (PI), and polytetrafluoroethylene (PTFE) filled by aramid fibers (AF) and molybdenum disulfide (MoS₂). To explore the wear mechanisms, the tribo‐pair contact stress was theoretically characterized, and the interface temperature rise was numerically predicted. The predictions showed that the flash temperature on asperity tips could reach the glass transition temperature of the polymer materials. The experimental results indicated that the contact stress and sliding speed have a small effect on the friction of the PI composite but influence considerably the friction of the PTFE composite. A higher contact stress brings about a higher specific wear rate, but a higher sliding speed reduces the wear rate. Compared with AF/MoS₂/PTFE, the AF/MoS₂/PI has much better tribological performance under high loads and speeds.     

Theoretical analysis of localized heating in human skin subjected to high voltage pulses

Electroporation, the increase in the permeability of bilayer lipid membranes by the application of high voltage pulses, has the potential to serve as a mechanism for transdermal drug delivery. However, the associated current flow through the skin will increase the skin temperature and might affect nearby epidermal cells, lipid structure or even transported therapeutic molecules. Here, thermal conduction and thermal convection models are used to provide upper and lower bounds on the local temperature rise, as well as the thermal damage, during electroporation from exponential voltage pulses (70 V maximum) with a 1 ms and a 10 ms pulse time constant. The peak temperature rise predicted by the conduction model ranges from 19 degrees C for a 1 ms time constant pulse to 70 degrees C for the 10 ms time constant pulse. The convection (mass transport) model predicts a 18 degrees C peak rise for 1 ms time constant pulses and a 51 degrees C peak rise for a 10 ms time constant pulse. The convection model compares more favorably with previous experimental studies and companion observations of the local temperature rise during electroporation. Therefore, it is expected that skin electroporation can be employed at a level which is able to transport molecules transdermally without causing significant thermal damage to the tissue.     

Complex conductivity of water-saturated packs of glass beads

The low-frequency conductivity response of water-saturated packs of glass beads reflects a combination of two processes. One process corresponds to the polarization of the mineral/water interface coating the surface of the grains. The other process corresponds to the Maxwell-Wagner polarization associated with accumulation of the electrical charges in the pore space of the composite medium. A model of low-frequency conductivity dispersion is proposed. This model is connected to a triple-layer model of electrochemical processes occurring at the surface of silica. This model accounts for the partition of the counterions between the Stern and the diffuse layers. The polarization of the mineral/water interface is modeled by the electrochemical polarization model of Schurr for a spherical grain. We take into account also the DC surface conductivity contribution of protons of the sorbed water and the contribution of the diffuse layer. At the scale of a macroscopic representative elementary volume of the porous material, the electrochemical polarization of a single grain is convoluted with the grain size distribution of the porous material. Finally, the Maxwell-Wagner polarization is modeled using the complex conductivity of a granular porous medium obtained from the differential effective medium theory. The predictions of this model agree well with experimental data of spectral induced polarization. Two peaks are observed at low frequencies in the spectrum of the phase. The first peak corresponds to the distribution of the size of the beads and the second peak is due to the roughness of the grains.     

Multiasperity contact adhesion model for universal asperity height and radius of curvature distributions

A new approach to the multiasperities contact interaction between two surfaces is presented. Each asperity is individually considered with its own different height and radius of curvature. Different materials, such as polyvinylchlorine (PVC) and stainless steel, are used as model systems. For each of the model materials, a set of asperities was generated using Monte Carlo method. Both asperity heights and radii were based on their statistical distributions experimentally obtained. Contact forces were determined for each asperity at a given distance between the two surfaces, while the deformation of each asperity was calculated according to the Johnson-Kendall-Roberts (JKR) or the Derjaguin-Muller-Toporov (DMT) model (depending on the material). The contribution of each asperity to the overall surface was summed, and the overall contact force was determined. The developed method was validated against contact force measurements obtained with atomic force microscopy (AFM).     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Model for solid-liquid and solid-solid friction of rough surfaces with adhesion hysteresis

Mechanisms of energy dissipation during solid-solid and solid-liquid friction are discussed. A conservative van der Waals adhesion force, when combined with surface imperfectness, such as deformation, leads to adhesion hysteresis (AH). When an asperity slides upon a substrate, the substrate is subjected to a loading-unloading cycle, and energy is dissipated due to the AH. Another mechanism, which leads to energy dissipation, involves energy barriers between metastable states due to surface roughness. Both mechanisms are fundamental for sliding and result in both solid-liquid and solid-solid friction.     

Direct observation of internal fluidity in a water droplet during sliding on hydrophobic surfaces

In the current study, we used a high-speed camera system with particle image velocimetry to observe the internal fluidity of water droplets during sliding. The droplets' velocity during sliding was controlled by slipping and rolling motions. On the superhydrophobic surface, with a contact angle of 150 degrees, the droplet fell at high velocity by slipping. However, on a normal hydrophobic surface whose water contact angle was around 100 degrees, both slipping and rolling controlled the droplet's velocity during sliding. In addition, the advancing velocity might be large when the slip velocity is large and the contact area is small.     

Relaxations in thermosets. VI. Effects of crosslinking on sub-Tg relaxations during the curing and aging of epoxide-based thermosets

The dielectric permittivity and loss of diglycidyl ether of bisphenol-A-based thermosets cured with diaminodiphenyl methane and diaminodiphenyl sulfone have been measured over a temperature range 77–400 K after curing or aging for a predetermined duration. Of the two sub-T_g relaxations, the height of the γ relaxation peak monotonically decreases during both the cure and postcure periods, and the height of the β relaxation peak first increases to a maximum value and then decreases. This decrease is attributed to physical aging effects. The height of the α-relaxation peak decreases. The γ- and β-relaxation peaks become increasingly separated in temperature. A concept of accumulated equivalent curing time which is based upon known chemical kinetics has been introduced for use in both theoretical and practical aspects of the study of thermosets. It is shown that substantial curing of the sample occurs during its slow heating to the curing temperature. The use of this concept in the curing of thermosets is illustrated. A procedure for the analysis of the distribution of relaxation times from a set of results limited in both frequency and temperature range is described. The distribution parameter is 0.20 and 0.16 for the γ and β process, respectively, and remains constant with postcuring and physical aging. The distribution parameter for the α process decreases from 0.60 to 0.36 on curing.     

Boundary slip and wetting properties of interfaces: correlation of the contact angle with the slip length

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.     

Highly Hydrophobic Conducting Nanocomposites Based on a Fluoropolymer with Carbon Nanotubes

A procedure was suggested for preparing highly hydrophobic conducting coatings based on fluoropolymers with carbon nanotubes of two types: Taunit-MD and carbon nanotubes functionalized with alkyl groups. The surface resistance, contact angle, sliding angle, and surface roughness were measured; structural features of the nanocomposites were studied. The properties of the coatings obtained depend on the concentration and type of the carbon nanotubes used. Introduction of functionalized carbon nanotubes into a fluoropolymer matrix allows preparation of coatings with higher values of the sliding angle and electrical resistance. The contact angle and sliding angle depend on the surface roughness and structure in different fashions.     

Adhesion between Nanoscale Rough Surfaces

In this investigation, the adhesion between particles and plates with root-mean-square, rms, surface roughness of 0.17-10.5 nm was measured by atomic force microscopy. Measurements obtained with particles both larger and smaller than the surface asperities are presented. Results indicate adhesion force decreases sharply with increasing surface roughness in the nanometer scale (<2 nm), followed by a gradual and slow decrease with further increase in roughness. Existing models were found to significantly underestimate adhesion force. Hence, a new model based on a geometry that considers both the height and breadth of asperities yielding an increased asperity radius compared to previous approaches, as detailed in Part I of this series, is applied using both van der Waals and elastic deformation/work of adhesion based approaches. For the system studied in this investigation, the adhesion forces predicted by the proposed model are considerably more accurate than those predicted by past models. Copyright 2000 Academic Press.     

Effect of surface mobility on the particle sliding along a bubble or a solid sphere

The sliding velocity of glass beads on a spherical surface, made either of an air bubble or of a glass sphere held stationary, is measured to investigate the effect of surface mobility on the particle sliding velocity. The sliding process is recorded with a digital camera and analyzed frame by frame. The sliding glass bead was found to accelerate with increasing angular position on the collector's surface. It reaches a maximum velocity at an angular position of about 100 degrees and then, under certain conditions, the glass bead leaves the surface of the collector. The sliding velocity of the glass bead depends strongly on the surface mobility of a bubble, decreasing with decreasing surface mobility. By a mobile surface we mean one which cannot set up resistive forces to an applied stress on the surface. The sliding velocity on a rigid surface, such as a glass sphere, is much lower than that on a mobile bubble surface. The sliding velocity can be described through a modified Stokes equation. A numerical factor in the modified Stokes equation is determined by fitting the experimental data and is found to increase with decreasing surface mobility. Hydrophobic glass beads sliding on a hydrophobic glass sphere were found to stick at the point of impact without sliding if the initial angular position of the impact is less than some specific angle, which is defined as the critical sticking angle. The sticking of the glass beads can be attributed to the capillary contracting force created by the formation of a cavity due to spontaneous receding of the nonwetting liquid from the contact zone. The relationship between the critical sticking angle and the particle size is established based on the Yushchenko [J. Colloid Interface Sci. 96 (1983) 307] analysis.