The effect of self-affine roughness on solid contact is examined with molecular dynamics and continuum calculations. The contact area and normal and lateral stiffnesses rise linearly with the applied load, and the load rises exponentially with decreasing separation between surfaces. Results for a wide range of roughnesses, system sizes and Poisson ratios can be collapsed using Persson's contact theory for continuous elastic media. The atomic scale response at the interface between solids has little affect on the area or normal stiffness, but can greatly reduce the lateral stiffness. The scaling of this effect with system size and roughness is discussed. PACS numbers: 46.55.+d, 62.20.Qp, 81.40.Pq The presence of roughness on a wide range of length scales has profound effects on contact and friction between experimental surfaces. Under a broad range of conditions [1][2][3][4][5][6], the area of intimate contact between rough surfaces A c is orders of magnitude smaller than the apparent surface area A 0 . As discussed below, this provides the most common explanation for Amontons' laws that friction is proportional to load and independent of A 0 . Because A c is small, the interfacial region is very compliant. In a range of applications the interfacial compliance can significantly reduce the stiffness of macroscopic joints formed by holding two components together under pressure [1,7].In this paper, we examine the effect of surface roughness on the normal and lateral stiffness of contacts between elastic solids using molecular dynamics (MD) and continuum calculations. The results provide a numerical test of recent continuum theories [8,9] and their applicability to real solids. The contact area and normal stiffness approach continuum predictions rapidly as system size increases. Continuum theory also captures the internal deformations in the solid under tangential load, but the total lateral stiffness may be greatly reduced by atomic scale displacements between contacting atoms on the opposing surfaces. This makes it a sensitive probe of the forces underlying friction and may help to explain unexpectedly small experimental results [10].The topography of many surfaces can be described as a self-affine fractal [2,11]. Over a wide range of lengths, the root mean squared (rms) change in height dh over a lateral distance ℓ scales as a power law: dh ∼ ℓ H , where the roughness or Hurst exponent H is typically between 0.5 and 0.9. Greenwood and Williamson (GW) considered the peaks of rough landscapes as independent asperities and found that A c rose linearly with normal load F N for nonadhesive surfaces [2]. This explains Amontons's laws if there is a constant shear stress at the interface. A linear scaling of area with load is also obtained from Persson's scaling theory, which includes elastic coupling between contacts approximately [12,13].Dimensional analysis implies that the linear relation between load and area must have the form( 1) where a modulus like the contact modulus E ′ is the only dimensional quantity char...

Geometrically imposed force cancellations lead to ultra-low friction between rigid incommensurate crystalline asperities. Elastic deformations may avert this cancellation but are difficult to treat analytically in finite and 3D systems. We use atomic-scale simulations to show that elasticity affects the friction only after the contact radius a exceeds a characteristic length set by the core width of interfacial dislocations bcore. As a increases past bcore, the frictional stress for both incommensurate and commensurate surfaces decreases to a constant value. This plateau corresponds to a Peierls stress that drops exponentially with increasing bcore but remains finite.

Modeling interfacial phenomena often requires both a detailed atomistic description of surface interactions and accurate calculations of long-range deformations in the substrate. The latter can be efficiently obtained using an elastic Green's function if substrate deformations are small. We present a general formulation for rapidly computing the Green's function for a planar surface given the interatomic interactions and coupling the Green's function to explicit atoms. The approach is fast, avoids ghost forces and is not limited to nearest-neighbor interactions. The full system comprising explicit interfacial atoms and an elastic substrate is described by a single Hamiltonian and interactions in the substrate are treated exactly up to harmonic order. This concurrent multi-scale coupling provides simple, seamless elastic boundary conditions for atomistic simulations where near-surface deformations occur, such as nanoindentation, contact, friction, or fracture. Applications to surface relaxation and contact are used to test and illustrate the approach.

In polycrystalline materials, grain boundaries are sites of enhanced atomic motion, but the complexity of the atomic structures within a grain boundary network makes it difficult to link the structure and atomic dynamics. Here, we use a machine learning technique to establish a connection between local structure and dynamics of these materials. Following previous work on bulk glassy materials, we define a purely structural quantity (softness) that captures the propensity of an atom to rearrange. This approach correctly identifies crystalline regions, stacking faults, and twin boundaries as having low likelihood of atomic rearrangements while finding a large variability within high-energy grain boundaries. As has been found in glasses, the probability that atoms of a given softness will rearrange is nearly Arrhenius. This indicates a well-defined energy barrier as well as a well-defined prefactor for the Arrhenius form for atoms of a given softness. The decrease in the prefactor for low-softness atoms indicates that variations in entropy exhibit a dominant influence on the atomic dynamics in grain boundaries.

Abstract. Optical coherence tomography ͑OCT͒ is an evolving noninvasive imaging modality and has been used to image the larynx during surgical endoscopy. The design of an OCT sampling device capable of capturing images of the human larynx during a typical office based laryngoscopy examination is discussed. Both patient's and physician's movements were addressed. In vivo OCT imaging of the human larynx is demonstrated. Though the long focal length limits the lateral resolution of the image, the basement membrane can still be readily distinguished. Office-based OCT has the potential to guide surgical biopsies, direct therapy, and monitor disease. This is a promising imaging modality to study the larynx. © 2006 Society of Photo-Optical Instrumentation Engineers.