K. Komvopoulos

CONTACT ANALYSIS OF ELASTIC-PLASTIC FRACTAL SURFACES

Rough surfaces are characterized by fractal geometry using a modified two-variable Weierstrass–Mandelbrot function. The developed algorithm yields three-dimensional fractal surface topographies representative of engineering rough surfaces. This surface model is incorporated into an elastic-plastic contact mechanics analysis of two approaching rough surfaces. Closed form solutions for the elastic and plastic components of the total normal force and real contact area are derived in terms of fractal parameters, material properties, and mean surface separation distance. The effects of surface topography parameters and material properties on the total deformation force are investigated by comparing results from two- and three-dimensional contact analyses and elastic and elastic-perfectly plastic material behaviors. For normal contact of elastic-perfectly plastic silica surfaces and range of surface interference examined, the interfacial force is predominantly elastic and the real contact area is approximately ...

Surface engineering and microtribology for microelectromechanical systems

Micromachines, particularly surface micromachines, often include smooth and chemically active surfaces. Because the kinetic energies, start-up forces and torques involved in their operation, and hence available to overcome retarding forces, are necessarily small, surface effects will be critical whenever micromachine contact occurs, whether unintentionally or as part of normal operation. Consequently, basic knowledge of the surface topography characteristics and microscale tribological phenomena arising at micromachine interfaces is of paramount importance to the reliability and robustness of microelectromechanical systems (MEMS). In view of the very small masses and tight tolerances, smooth surfaces, and light loads, characterization of chemomechanical surface interactions must be performed at the microscale. Microtribology is an emerging field dealing with friction and wear phenomena occurring at micrometer scales. Recent developments in this field have begun to lend valuable insight into surface interaction and material property characterization on scales relevant to MEMS. In this publication, the emphasis is on the analysis of various surface micromechanisms, such as solid bridging, liquid meniscus formation, van der Waals force, and electrostatic charging, and the significance of surface roughness and material properties. Experimental and theoretical results for the stiffness of silicon micromachines needed to overcome stiction are compared for different surface roughnesses. The fabrication, basic operation features, and measuring capabilities of microstructures designed specifically to perform microscale tribotesting under controlled conditions are presented. The efficacy of different surface engineering techniques, such as formation of standoff bumps on one of the countersurfaces, roughening (texturing), and surface chemistry modification (e.g. self-assembled monolayers), to reduce high adhesion forces at MEMS interfaces is interpreted in light of analytical and experimental results. It is demonstrated that surface engineering and modification of physicochemical surface properties are effective means of enhancing the reliability and performance of microsystems.

The effect of release-etch processing on surface microstructure stiction

A passive polysilicon microstructure is described which has been used to evaluate the stiction or unwanted adhesion occurring after release etch, rinse and dry processing. The results are interpreted in terms of particle adhesion theory. A residue dissolved in the water and redeposited during drying is responsible for one form of adhesion, by solid bridging. A reaction at silicon surfaces immersed in water is suggested as the source of this residue. Formation of a chemical oxide layer to protect the silicon surfaces alleviates adhesion due to this mechanism. Use of a hydrophobic, low surface energy coating that is applied as part of the rinse process is also described.<<ETX>>

Electrical contact resistance theory for conductive rough surfaces

A general electrical contact resistance (ECR) theory for conductive rough surfaces was derived from first principles. The analysis is based on fractal geometry for the surface topography description, elastic-plastic deformation of contacting asperities, and size-dependent constriction resistance of microcontacts. Relations for the ECR in terms of contact load and apparent contact area are obtained for isotropic, homogeneous, conductive surfaces with known material properties and surface topography. Useful design guidelines for electrical contacts are extracted from the numerical results. A general relation between the dimensionless real contact area and the dimensionless ECR is introduced for lightly loaded contacts that depends only on the electron mean free path. Approaches for determining the surface roughness, material properties, and real contact area are discussed in the context of relatively simple ECR measurements.

Adhesion and friction forces in microelectromechanical systems: mechanisms, measurement, surface modification techniques, and adhesion theory

Microscopic devices capable of performing sensing, actuation, and control tasks, known as microelectromechanical systems (MEMS), are expected to lead to new technologies with profound impact on science and engineering. The multidisciplinary nature of these miniaturized devices has necessitated the integration of basic knowledge of various mechanical, electrical, chemical, and thermal phenomena encountered at the microscale, and the discovery of novel methods for fabricating versatile micromachines. As the growth of MEMS accelerates, reliability and long-term durability issues are expected to assume even greater importance. In view of the low stiffness and large surface-to-volume ratio of micromachine devices, high interfacial attractive (adhesion) forces often lead to permanent surface attachment, a phenomenon known as stiction. Representative examples where surface interaction influences micromachine functionality are presented to elucidate the importance of adhesion and friction forces at the microdevice level, followed by a description of special microstructures for static and dynamic friction testing under conditions typical of MEMS devices. Surface texturing, fabrication of stand-off microfeatures, and alteration of the surface chemical behavior by adsorption of low surface energy self-assembled monolayers are shown to be effective surface modification techniques for controlling adhesion and friction at MEMS interfaces. Simulation results from a generalized adhesion theory are used to interpret the contribution of capillary, van der Waals, and electrostatic attractive forces to the total interfacial force. The theory is based on the surface topography description by fractal geometry and uses elastic-plastic constitutive models for the deformation of asperity microcontacts derived from finite element simulation results. Analytical results demonstrate the profound effect of surface roughness on the magnitude of adhesion and repulsive forces at polysilicon interfaces.

Directional adhesion of gecko-inspired angled microfiber arrays

Arrays of angled microfibers with a gecko-inspired structure were fabricated from a stiff thermoplastic polymer (polypropylene) with elastic properties similar to those of β-keratin of natural setae. Friction experiments demonstrated that this fibrillar polymer surface exhibits directional adhesion. Sliding of clean glass surfaces against and along the microfiber direction without applying an external normal force produced an apparent shear stress of 0.1 and 4.5 N/cm2, respectively. This directional adhesion is interpreted in the context of a nonlinear elastic bending model of an angled beam. Shearing and normal contact experiments yielded further evidence of the anisotropic adhesion of the fibrillar polymer and revealed the occurrence of a pull-off (adhesive) force at the instant of surface detachment, unlike vertically aligned microfiber arrays of the same material that exhibited a zero pull-off force. The results of this study provide impetus for the design of gecko-inspired adhesives with angled struc...

Femtosecond laser aperturless near-field nanomachining of metals assisted by scanning probe microscopy

Ultrashort pulsed-laser radiation is an effective method for precision materials processing and surface nano-/micromodification because of minimal thermal and mechanical damage. This study demonstrates that controllable surface nanomachining can be achieved by femtosecond laser pulses through local field enhancement in the near-field of a sharp probe tip. Nanomachining of thin gold films was accomplished by coupling 800-nm femtosecond laser radiation with a silicon tip in ambient air. Finite-difference time-domain numerical predictions of the spatial distribution of the laser field intensity beneath the tip confirmed that the observed high spatial resolution is due to the enhancement of the local electric field. Possible structuring mechanisms and factors affecting this process are discussed. The present process provides an intriguing means for massive nanofabrication due to the flexibility in the substrate material selection, high spatial resolution of ∼10 nm (not possible with standard nanomachining techniques), and fast processing rates achievable through simultaneous irradiation of multiarray tips.

Three-Dimensional Contact Analysis of Elastic-Plastic Layered Media With Fractal Surface Topographies

Three-dimensional rough surfaces were generated using a modified two-variable Weierstrass-Mandelbrot function with fractal parameters determined from real surface images. The number and size of truncated asperities were assumed to follow power-law relations. A finite element model of a rigid sphere in normal contact with a semi-infinite elastic-plastic homogeneous medium was used to obtain a constitutive relation between the mean contact pressure, real contact area, and corresponding representative strain. The contact model was extended to layered media by modifying the constitutive equation of the homogeneous medium to include the effects of the mechanical properties of the layer and substrate materials and the layer thickness. Finite element simulations of an elastic-plastic layered medium indented by a rigid sphere validated the correctness of the modified contact model. Numerical results for the contact load and real contact area are presented for real surface topographies resembling those of magnetic recording heads and smooth rigid disks. The model yields insight into the evolution of elastic, elasticplastic, and fully plastic deformation at the contact interface in terms of the maximum local surface interference. The dependence of the contact load and real contact area on the fractal parameters and the carbon overcoat thickness is interpreted in light of simulation results obtained for a tri-pad picoslider in contact with a smooth thin-film hard disk.

Cell-Shape Regulation of Smooth Muscle Cell Proliferation

Vascular smooth muscle cells (SMCs) play an important role in vascular remodeling. Heterogeneity and phenotypic changes in SMCs are usually accompanied by a morphological difference, i.e., elongated/spindle-like versus spread-out or epithelioid/rhomboid cell shapes. However, it is not known whether the cell shape directly regulates SMC proliferation, and what the underlying mechanisms are. In this study, microgrooves and micropatterned matrix islands were used to engineer the cell shape and investigate the associated biophysical and biological mechanisms. Compared to spread-out SMCs on nonpatterned surfaces, SMCs on micropatterned surfaces demonstrated elongated morphology, significantly lower cell and nucleus shape indexes, less spreading, a lower proliferation rate, and a similar response (but to a lesser extent) to platelet-derived growth factor, transforming growth factor-beta, and mechanical stretching. DNA microarray profiling revealed a lower expression of neuron-derived orphan receptor-1 (NOR-1) in elongated SMCs. Knocking down NOR-1 suppressed DNA synthesis in SMCs, suggesting that NOR-1 is a mediator of cell elongation effects. Regulation of DNA synthesis in SMCs by the cell shape alone and a decrease in DNA synthesis in the case of small cell spreading area were achieved by micropatterning SMCs on matrix islands of different shapes and spreading areas. Changes in the cell shape also affected the nucleus shape, whereas variations in the cell spreading area modulated the nucleus volume, indicating a possible link between nucleus morphology (both shape and volume) and DNA synthesis. The findings of this investigation provide insight into cell shape effects on cell structure and proliferation, and have direct implications for vascular pathophysiology.

A Fractal Analysis of Stiction in Microelectromechanical Systems

The strong adherence (stiction) of adjacent surfaces is a major design concern in microelectromechanical systems (MEMS). Advances in micromachine technology greatly depend on basic understanding of microscale stiction phenomena. An analysis of the different stiction micromechanisms and the elastic deformation of asperities at MEMS interfaces is developed using a two-dimensional fractal description of the surface topography. The fractal contact model is scale independent since it is based on parameters invariant of the sample area size and resolution of measuring instrument. The influence of surface roughness, relative humidity, applied voltage, and material properties on the contributions of the van der Waals, electrostatic, and capillary forces to the total stiction force is analyzed in light of simulation results. It is shown that the effects of surface roughness and applied voltage on the maximum stiction force are significantly more pronounced than that of material properties. Results for the critical pull-off stiffness versus surface roughness are presented for different material properties and microstructure stand-free surface spacings. The present analysis can be used to determine the minimum stiffness of microdevices required to prevent stiction in terms of surface roughness, apparent contact area, relative humidity, applied voltage, and material properties.