Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L17: Focus Session: Friction and Adhesion |
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Sponsoring Units: GSNP Chair: Gianpietro Moras, Fraunhofer IWM Room: 402 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L17.00001: Probing locally the onset of slippage at a model multi-contact interface Victor Romero, Elie Wandersman, George Debr\'egeas, Alexis Prevost We investigate the interfacial dynamics in a frictional joint between a micro-patterned elastomer and a smooth glass slide in the stick-slip regime. A novel technique is developed to decorate the surface of PDMS blocks with thousands of spherical caps (100 $\mu$m in curvature) whose positions and heights are controlled at $\mu$m scale. Such samples are rubbed against bare glass slides while the macroscopic normal and shear loads are monitored. The use of model spherical asperities provides a direct access to the local normal and shear stress within the frictional joint through optical tracking of the micro-contacts area and in-plane displacement. This method allows us to evidence the existence of one or more slip waves propagating inward from the contact edge right before the onset of slip events. The wave front is found to propagate normally to the iso-pressure contour lines at a velocity proportional to the macroscopic (imposed) shear rate. A simple quasi-static model of the multi-contact interface is derived that qualitatively accounts for the observed dynamics of these slip precursors. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L17.00002: Depinning transition and 2D superlubricity in incommensurate colloidal monolayers Davide Mandelli, Andrea Vanossi, Nicola Manini, Erio Tosatti Colloidal monolayers sliding over periodic corrugated potential are highly tunable systems allowing to visualize the dynamics between crystalline surfaces [1]. Based on molecular dynamics, Vanossi and coworkers [2] reproduced the main experimental results and explored the potential impact of colloid sliding in nanotribology. The degree of interface commensurability was found to play a major role in determining the frictional properties, the static friction force Fs becoming vanishingly small in incommensurate geometries for weak corrugation U0.Lead by this result,here we systematically investigate the possibility to observe a 2D Aubry-like transition [3] from a superlubric state to a pinned state for increasing U0. By using a reliable protocol, we generate annealed configurations at different values of U0 for an underdense monolayer. We find Fs to be vanishingly small up to a critical corrugation Uc coinciding with an abrupt structural transition in the ground state configuration. Similarly to what is observed in the Frenkel Kontorova model,this transition is characterized by a significant decrease in the number of particles sampling regions near the maxima of the substrate potential.\\[4pt] [1] T. Bohlein, Nat. Mat.,11,126; [2] A. Vanossi, PNAS USA,109,16426; [3] S. Aubry, Phys.D,8,381. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L17.00003: Does rotational melting make molecular surfaces more slippery? Andrea Benassi, Carlo Pignedoli, Daniele Passerone, Andrea Vanossi, Erio Tosatti Crystals made up of spherical, weakly interacting molecules generally exhibit a phase transition between a low temperature ordered phase and a plastic phase, where the rotational order is thermally lost. In C$_{60}$ fullerene, the transition takes place at T$_r$=260K in bulk, initiating at a lower temperature at a (111)surface. We explore by MD simulations whether a slider should experience a change of friction on that surface in correspondence with the phase transition. Modeling the slider as a C$_{60}$ flake attached to a sliding tip, we obtain a response dependent on the orientation and the angular compliance of the flake. An orientation angle commensurate with the C$_{60}$ surface yields a large adhesion and friction, both dropping by only about 20\% at the plastic transition. An incommensurate angle yields both adhesion and friction a factor 2 smaller and relatively unaffected by the transition. Finally, a sliding flake with an incommensurate angle but a compliant orientation offers the possibility of a very different sliding behavior, remaining incommensurate with very low adhesion/friction above T$_r$, but jumping to a commensurate angle with high adhesion/friction below T$_r$. This third possibility might have been realized in the AFM experiment by Liang et al.(PRL 2003). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L17.00004: Tribo-induced melting transitions and internal friction at magnetic and nonmagnetic asperity contacts Jacqueline Krim, Liming Pan, Keeley Stevens We report a study of tribo-induced nanoscale surface melting mechanisms that employs a combined QCM-STM technique [1] for a range of Au and Au-Ni alloys with varying compositional percentages and phases [2]. A transition from solid-solid to solid-``liquid like'' contact was observed for most samples at sufficiently high asperity sliding speeds. Pure gold, solid-solution and two-phase Au-Ni (20 at.{\%} Ni) alloys were compared [3]. Samples with 5-20{\%} nickel alloyed with gold were deposited as a homogenous solid-solution or as a two-phase FCC solid through the modification of annealing procedures. The solid solution is known to be paramagnetic for concentrations below 35{\%} Ni while the two phase solid maintains domains of ferromagnetism within bulk gold. A ``flexing'' effect associated with the application of an external magnetic field on the two-phase alloy samples illuminates physical mechanisms that correlate with the observed tribo-induced melting temperatures [4].\\[4pt] [1] B. D. Dawson, S. M. Lee, and J. Krim, Phys. Rev. Lett. 103, 205502 (2009).\\[0pt] [2] L. Pan, Ph.D. Thesis, North Carolina State University (2011).\\[0pt] [3] Zhenyin Yang; Lichtenwalner, D.J.; Morris, A.S.; Krim, J.; Kingon, A.I, Journal of Microelectromechanical Systems, April 2009, Volume: 18 Issue:2, 287-295.\\[0pt] [4] K. Stevens, L. Pan and J. Krim, (2014) submitted [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L17.00005: Non-intrusive measurements of frictional forces between micro-spheres and flat surfaces Wei-Hsun Lin, Chiara Daraio We report a novel, optical pump-probe experimental setup to study micro-friction phenomena between micro-particles and a flat surface. We present a case study of stainless steel microspheres, of diameter near 250$\mu$m, in contact with different surfaces of variable roughness. In these experiments, the contact area between the particles and the substrates is only a few nanometers wide. To excite the particles, we deliver an impulse using a pulsed, high-power laser. The reaction force resulting from the surface ablation induced by the laser imparts a controlled initial velocity to the target particle. This initial velocity can be varied between 10$^{-5}$ to 1 m/s. We investigate the vibrating and rolling motions of the micro-particles by detecting their velocity and displacement with a laser vibrometer and a high-speed microscope camera. We calculate the effective Hamaker constant from the vibrating motion of a particle, and study its relation to the substrate's surface roughness. We analyze the relation between rolling friction and the minimum momentum required to break surface bonding forces. This non-contact and non-intrusive technique could be employed to study a variety of contact and tribology problems at the microscale. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L17.00006: Exploring field effects on ionic liquid boundary lubrication Rosario Capozza, Andrea Benassi, Andrea Vanossi, Erio Tosatti Ionic liquids, organic salts that are liquid at room temperature, are of great physical as well as of technological interest. Their adhesion properties to solid surfaces under pressure suggests their use as boundary lubricants. One potentially interesting feature would be the possibility that electrical charging of the solid plates or more generally an applied static or dynamic electric field could modify the nearby perpendicular and parallel ordering of ions, and in turn also modify the sliding friction. While these effects have just begun to be pursued by experimental groups, we have undertaken molecular dynamics simulations aimed at exploring some of these questions. Preliminary results obtained using very simple molten salt boundary lubrication models will be presented and discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L17.00007: Friction from formation and rupture of molecular contacts Invited Speaker: Michael Urbakh Frictional motion plays a central role in diverse systems and phenomena that span vast ranges of scales, from the nanometer contacts inherent in micro- and nanomachines and biological molecular motors to the geophysical scales characteristic for earthquakes. Despite the practical and fundamental importance of friction and the growing efforts in the field, many key aspects of dynamics of friction are still not well understood. One of the main difficulties in understanding and predicting frictional response is the complexity of highly non-equilibrium processes going on in any tribological contact which include detachment and re-attachment of multiple microscopic contacts (bonds) between the surfaces in relative motion while still in contact. In this lecture I will discuss microscopic models which establish relationships between the dynamics of formation and rupture of individual contacts and frictional phenomena. First, I will introduce a phenomenological model that describes friction through thermally activated rupture and formation of molecular contacts. Then, I will focus on a microscopic model that includes the effect of thermally activated jumps of the surface atoms between the sliding surfaces on nanoscopic friction. I will show that the proposed models explain a nonmonotonic dependence of friction on temperature, which has been observed in recent friction force microscopy experiments for different material classes. These models offer a new conceptual framework to describe the dynamics of nanoscale friction. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L17.00008: Effect of Surface Roughness on Nanoparticle Adhesion Zhen Cao, Andrey Dobrynin, Jan-Michael Carrillo, Andrew Oyer, Mark Stevens We study effect of surface roughness on adhesion of soft nanoparticles. Using molecular dynamics simulations we obtained deformation of nanoparticles and their effective contact area with substrates as a function of nanoparticle crosslinking density, surface energy, work of adhesion, and surface roughness. We modeled adhesion of nanoparticles on substrates with periodic patterns (1-D stripes and 2-D square lattice of posts) and with random height distribution. Our simulations show that the JKR-like model can be applied to describe adhesion of strongly crosslinked large nanoparticles, while for the weakly crosslinked nanoparticles, that undergo large deformations, the change of surface energy should be included to account for nanoparticle shape deformation. We propose a simple scaling model which shows that equilibrium shape of nanoparticle is a result of fine interplay between nanoparticle surface energy, elastic energy and work of adhesion to the substrate. The predictions of the scaling model are in a very good agreement with simulation results. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L17.00009: When are rough surfaces sticky? Lars Pastewka, Mark Robbins Van-der Waals interactions operate between all surfaces and are strong enough to hold 1000kg per square centimeter. Yet, few surfaces are adhesive. This discrepancy between atomic and macroscopic forces is due to roughness and has been dubbed the adhesion paradox. To quantify this behavior, we carried out molecular statics and continuum simulations of the contact area, stiffness and adhesion between rigid, randomly rough surfaces and elastic substrates. The surfaces are self-affine with Hurst exponent 0.3 to 0.8 and different short and long wavelength cutoffs. The rms surface slope and the range and strength of the adhesive potential are also varied. For parameters typical of most solids, the effect of adhesion decreases as the ratio of long to short wavelength cutoff increases. In particular, the pull-off force decreases to zero and the area of contact A becomes linear in the applied load L. A simple scaling argument is developed that describes the increase in the ratio A/L with increasing adhesion and a corresponding increase in the contact stiffness. The argument predicts a crossover to finite contact area at zero load when surfaces are exceptionally smooth or the ratio of surface tension to bulk modulus is unusually large, as for elastomers. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L17.00010: Contact of rough surfaces at the atomic scale Tristan Sharp, Lars Pastewka, Mark O. Robbins Roughness on solid surfaces typically causes contacting solids to remain microscopically separated, with important consequences for sealing, adhesion, and friction. Continuum theory and continuum simulations of rough contact have greatly contributed to a statistical understanding of these processes. However, continuum treatments neglect atomic-scale geometry at the surface, thereby making an uncontrolled approximation. Here, we perform molecular dynamics simulations of rough surfaces to test the consequences of atomic-scale features. We focus on atomic plasticity and atomic steps that form terraces on rough surfaces of crystalline solids. We find that the atomic features treated here do not dramatically alter the large-scale solid deformations predicted by continuum calculations. However, different behavior emerges at small scales. Continuum treatments underestimate the number of atoms with very low and very high stress. Step edges concentrate stress and change the small-scale morphology of contact patches. A new statistical quantity, the characteristic ratio of step height to step width, is found to be useful when extending continuum theory to treat atomic-scale steps. These results are discussed in context of the recent scaling theory of Persson. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L17.00011: Friction in the peeling test Suomi Ponce, Jose Bico, Benoit Roman Peeling tests are commonly used to probe adhesives. We are interested in the adhesion of soft elastomers on rigid substrates through van der Waals interactions. Such elastomers are prone to slide when sheared and exhibit a characteristic friction stress. We show how the classical relation from Kendall fails to predict the peeling force for such systems for low values of the peeling angle (the actual force being largely underestimated). We propose an implement to Kendall's approach that accounts for friction. In the limit of zero angle this description provides a maximum force proportional to the adhered area in agreement with our experimental observations. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L17.00012: Strong dynamical effects during stick-slip adhesive peeling Marie-Julie Dalbe, Stephane Santucci, Loic Vanel, Pierre-Philippe Cortet We consider the classical problem of the stick-slip dynamics observed when peeling an adhesive tape at a constant velocity. From fast imaging recordings, we extract the dependencies of the stick and slip phases durations with the imposed peeling velocity and peeled ribbon length. Predictions of Maugis and Barquins [in Adhesion 12, edited by K.W. Allen, Elsevier ASP, London, 1988, pp. 205-222] based on a quasistatic assumption succeed to describe quantitatively our measurements of the stick phase duration. Such model however fails to predict the full stick-slip cycle duration, revealing strong dynamical effects during the slip phase.\\[4pt] Dalbe et al., Soft Matter, (2013), DOI : 10.1039/c3sm51918j. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L17.00013: Stick-slip friction and ageing in Velcro$^{\mbox{\textregistered }}$ Lisa Mariani, Paul Angiolillo The mesoscopic hook and loop system of Velcro$^{\mbox{\textregistered }}$ provides a model of stick-slip friction that exhibits behavior reminiscent of results seen in nanoscale model systems. The friction is linearly dependent on contact area and independent of driving velocity. Morever, there is a power law dependence of the friction on loading, with exponent between 1/4 and 1/3. Furthermore, the evolution of stick-slip to more smooth sliding, as controlled by contact area, is also noted. These transition predictions follow power law profiles, as well, with respect to increasing contact area. Thus, the hook-and-loop system shows to be a good mesoscale model system of stick-slip friction and provides a link between nanoscale and macroscale friction. Through an investigation into the ageing of the hooks in the system, the data suggests that the hooks age during the shearing regime and take a characteristic time to return to initial attachment strength. Additionally, there does not appear to be a significant affect of ageing on the kinetic friction experienced by the system. [Preview Abstract] |
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