### Session H33: Focus Session: Friction, Fracture and Deformation II

 Tuesday, March 14, 2006 11:15AM - 11:27AM H33.00001: Effect of Counterface Roughness on Sliding Friction of Alkane Monolayers Kevin Van Workum , Ginger Chateauneuf , Guangtu Gao , David Schall , Judith Harrison Classical molecular dynamics simulations of carbon-based counterfaces sliding against a monolayer of alkane chains are presented. The tribological behavior three carbon-based counterfaces of varying roughness are compared. The distribution of contact forces between individual monolayer chain groups and the counterface shows a strong dependence on the surface roughness of the counterface. Distinctions between contact forces which oppose and those which assist the counterface motion are made and are related to the net friction. A perfectly smooth counterface shows a realitively narrow distribution of contact forces. The width of the distribution of contact forces increases with increasing counterface roughness. The local motion of the alkane chain end-groups is also discussed in terms of the roughness of the sliding counterface. Tuesday, March 14, 2006 11:27AM - 11:39AM H33.00002: Molecular dynamic simulations of the contact between elastic solids and fractal substrates Carlos E. Campana , Martin H. M\"{u}ser The contact mechanics and friction between unlubricated, elastic solids with fractal surfaces is studied using molecular dynamics. Large interfaces could be studied, owing to the use of elastic Green's functions for semi-infinite, elastic lattices. Our simulations confirm the emerging picture that friction and pressure distribution depend mainly on the mean slope of the walls in contacts (unlubricated and chemically passivated solids). Besides, the level of discreteness of the description plays an essential role. This supports another emerging picture, namely that the detailed structure of the very last layer is crucial for friction between two solids. Tuesday, March 14, 2006 11:39AM - 11:51AM H33.00003: Friction and plasticity between self-affine surfaces Binquan Luan , Mark Robbins , Judith Harrison Simulations are used to study the contact area and adhesion between two amorphous solids with self-affine fractal surfaces, and the results are compared to continuum calculations. The friction between non-adhesive surfaces is proportional to load, but the coefficient of friction increases with roughness. The friction is much higher than expected for elasticallly deforming surfaces,$^*$ and substantial plastic deformation is observed. Indeed, friction forces for different surface roughness collapse when plotted against the number of plastic rearrangements per unit sliding distance. Including adhesion leads to an increase in both friction and plasticity. \\ $^*$ M. H. M\"user, L. Wenning, and M. O. Robbins, Phys. Rev. Lett. {\bf 86}, 1295 (2001). Tuesday, March 14, 2006 11:51AM - 12:27PM H33.00004: Superlubricity and atomic-scale energy dissipation in ultrahigh vacuum Invited Speaker: Enrico Gnecco Superlubricity'' has been recently achieved on ionic crystals in ultra-high vacuum in two different ways. First, the normal load exerted by a sharp AFM tip on the crystal surface has been reduced below a critical threshold under extremely low noise conditions, and kept constant while scanning in the usual way. The transition from the stick-slip motion commonly observed on the atomic scale to the superlubricated regime occurs in a continuous way, and can be theoretically described introducing a parameter $\eta$, which is, respectively, larger or smaller than 1 in the two regimes. A comparison with the Tomlinson model allowed us to carefully estimate the contact stiffness and the interaction between tip and surface down to the superlubricated regime [1]. The static'' superlubricity obtained in such way cannot be easily extended to systems of practical interest, like micro- and nano-electromechanical devices. The main problem is the smallness of the applied loads (below 1 nN), which must be maintained for a long time. This obstacle is removed if superlubricity is achieved in a different dynamic'' way. When an $ac$ voltage is applied between the tip and a counterelectrode on the other side of the crystal sample, and the actuation frequency corresponds to a normal resonance of the system, a sistematic decrease of friction to negligible values is also observed. In such case the magnitude of the applied load is not subject to upper limitations. This effect is probably due to a delicate interplay between thermal activation and the fast variation of the tip-surface interaction, as suggested by recent computer simulations. \newline [1] A. Socoliuc, R. Bennewitz, E. Gnecco, and E. Meyer, Phys. Rev. Lett. 92 (2004) 134301 Tuesday, March 14, 2006 12:27PM - 12:39PM H33.00005: Study of nano-scale kinetic friction using vortices in superconductors and charge-density waves A. Maeda , Y. Inoue , H. Kitano , S. Savelev , F. Nori , I. Tskada , S. Okayasu In analogy with the standard macroscopic friction in massive blocks, we present a comparative study of the friction force felt by moving quantized vortices in high-$T_{c}$ superconductors and charge-density waves (CDWs). Using $I-V$ characteristic measurement and a model for this data, our observations: (1) provide a link between friction at the micro- and macroscopic scales; (2) explain the roundness of the static-kinetic friction transition in terms of system sizes (critical-phenomena view) and thermal fluctuations; and (3) explain the crossing of the kinetic friction $F_{k}$ versus velocity $v$ for our pristine (high density of very weak defects) and our columnar-defect-irradiated samples (with lower density of deeper pinning defects). We will also investigate any possibilities of the observation of the scaling relation between the velocity dependence of the kinetic friction and the waiting time dependence of the static friction in these systems. Tuesday, March 14, 2006 12:39PM - 12:51PM H33.00006: Friction Measurements With Dewetted Polymer Droplets: Bridging the Gap in Contact Area Between Lateral Force Microscopy and the Surface Forces Apparatus Andrew B. Croll , Kari Dalnoki-Veress Lateral Force Microscopy (LFM) is an important tool that essentially created the field of nano-tribology some 20 years ago. The interest in this technique stems from its ability to measure true single asperity surface contact. Although tremendously successful, LFM lacks the ability to vary contact area without complicated tip modifications, most of which require modeling to determine contact area. At the other end of the contact area length scale is the Surface Forces Apparatus (SFA). This technique allows for extremely high precision friction measurements, but only on macroscopic lateral length scales. Here we present a simple technique that uses dewetted polystyrene droplets (spherical caps) to overcome these obstacles. Droplets are reversibly attached to conventional atomic force microscope tips, and scanning takes place much as in LFM. The dewetted droplets have the advantage of very flat, well-defined, rigid contact with the substrate, over several orders of magnitude in area ($\sim$10 $\mu$m$^{2}$ -- 10000 $\mu$m$^{2})$. Measurements on an extremely thin poly(dimethyl siloxane) layer yield insight into the complex interplay between viscosity and elasticity in thin polymer lubricants. Tuesday, March 14, 2006 12:51PM - 1:03PM H33.00007: Formation of nanoscale water bridges Elisa Riedo , Robert Szoszkiewicz , Tai-De Li , Jianping Gao , Uzi Landman The water bridges provide stability to sand castles, act as transport channels for dip-pen nanolitography and increase adhesion and friction in micro- and nano- devices such as MEMS. The kinetics of capillary~condensation and growth at the nanoscale is studied here using friction force~microscopy and molecular dynamics calculations. At 40{\%} relative humidity we find that the meniscus~nucleation times increase from 0.7 ms up to 4.2 ms when the~temperature decreases from 332 K to 299 K. The nucleation times grow~exponentially with the inverse temperature 1/T obeying an Arrhenius~law. We obtain a nucleation energy barrier of 7.8*10\^{}{\{}-20{\}}$\sim$J~and an attempt frequency ranging between 4-250$\sim$GHz, in excellent~agreement with theoretical predictions. These results provide direct~experimental evidence that capillary condensation is a thermally~activated phenomenon. Tuesday, March 14, 2006 1:03PM - 1:15PM H33.00008: The effect of packing density on self-assembled monolayer friction: Investigation of frictional contrast between OTS phase-separated regions Erin Flater , Robert Carpick Motivated by the lack of fundamental understanding of friction, and that friction and wear are major limiting factors for surface micromachined devices, we use atomic force microscopy (AFM) to determine the nanoscale frictional properties of alkylsilane monolayers commonly used in these microscale devices to reduce adhesion and friction. Quantitative nanoscale single asperity measurements of friction and contact stiffness are performed using monolayer-terminated AFM tips on monolayer-terminated silicon. By comparing the two structural phases present in octadecyltrichrolosilane (OTS) monolayers, we observe that friction depends on the local molecular packing density. The liquid condensed phase shows measurably lower friction at low loads than the liquid expanded phase, demonstrating that lower friction is associated with higher molecular packing density. However, the phases exhibit the same frictional response at higher loads, suggesting that compressed forms of both phases are structurally and tribologically equivalent. We discuss these results in terms of stress-induced molecular conformational changes in the confined interface. We acknowledge our collaborators: W. Robert Ashurst at Auburn University, who provided the OTS samples, and Maarten P. de Boer and Alex D. Corwin at Sandia National Laboratories. Tuesday, March 14, 2006 1:15PM - 1:27PM H33.00009: Coverage dependence of 1-propanol adsorption on the Si(001) surface and fragmentation dynamics Jian-Ge Zhou , Frank Hagelberg The chemisorption of organic molecules on silicon surfaces is a highly topical subject of current research, both experimental and computational. This interest may be ascribed to both the fundamental nature of this problem, involving the interaction between finite units and periodic substrates, but also to its relevance to various areas of recent technology, such as insulator films, nanolithography, chemical and biological sensors, and molecular electronics. The organic layers are formed by depositing organic compounds on the semiconductor surface. In order to optimize this process, the understanding of the interaction between the surface and the organic species is crucial. The geometric, electronic, energetic, and dynamic properties of 1-propanol absorbed on the Si(001)-(2x1) surface are studied from first principles by use of a slab approach. The 1-propanol molecule initially interacts with the Si surface through formation of a dative bond, subsequently the physisorbed 1-propanol molecule reacts with the surface by cleavage of the O-H bond, and the Si(001)-(2x1) surface undergoes further reconstruction as a result of the adsorption of the organic species. The band structure and density of states (DOS) are first analyzed for this system. The band gap of the Si-1-propanol film increases as the coverage level is enhanced. Good agreement is found with available experimental data. Tuesday, March 14, 2006 1:27PM - 1:39PM H33.00010: Buckling Instability In Bilayer Membranes A. Concha , J. McIver , P. Mellado , R.L. Leheny We have analyzed the patterns generated when a stress field is introduced to the inner layer of a cylindrically symmetric bilayer membrane through a combination of analytic calculations, numerical simulations, and experiments. A wrinkled structure forms that we explain in terms of a competition between bending and stretching energies under a suitable geometrical constraint. The wavelength, $\lambda$, at the onset of the instability is found theoretically and experimentally to be $\lambda=\pi\sqrt{8B/T_{c}}$, where $T_{c}$ is the critical tension of the inner layer marking the onset of the instability, and $B$ is the bending modulus of the membrane. We have also investigated the formation of pseudo-fractal structures that emerge beyond the onset of the instability. We further explain the existence of defects in the regular pattern as a consequence of multiple metastable states in the effective potential that describes this system.