Dry Sliding Contact Between Rough Surfaces at the Atomistic Scale

Although, a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws have long since described the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Some current research suggests that a linear dependence of friction on the real contact area is observed at the atomistic level, but only for specific cases (indentors and rigid substrates). Because continuum models are not applicable at the atomic scale, other modeling techniques (such as molecular dynamics simulations) are necessary to elucidate the physics of friction at the small scale. We use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load. We find that friction increases with roughness. Also all sliding cases show considerable surface flattening, reducing the friction close to zero after repetitive sliding. This questions the current view of (static) roughness at the atomistic scale, and possibly indicates that the macroscopic laws of friction break down several orders of magnitude before reaching the atomic scal

The effect of loading on surface roughness at the atomistic level

One of the key points to better understand the origins of friction is to know how two surfaces in contact adhere to one another. In this paper we present molecular dynamics (MD) simulations of two aluminium bodies in contact, exposed to a range of normal loads. The contact surfaces of both aluminium bodies have a self-affine fractal roughness, but the exact roughness varies from simulation to simulation. Both bodies are allowed to have an adhesive interaction and are fully deformable. Tracking important contact parameters (such as contact area, number of contact clusters, and contact pressure) during a simulation is challenging. We propose an algorithm (embedded within a parallel MD code) which is capable of accessing these contact statistics. As expected, our results show that contact area is increasing in proportion with applied load, and that a higher roughness reduces contact area. Contact pressure distributions are compared to theoretical models, and we show that they are shifted into the tensile regime due to the inclusion of adhesion in our mode

On the Propagation of Slip Fronts at Frictional Interfaces

The dynamic initiation of sliding at planar interfaces between deformable and rigid solids is studied with particular focus on the speed of the slip front. Recent experimental results showed a close relation between this speed and the local ratio of shear to normal stress measured before slip occurs (static stress ratio). Using a two-dimensional finite element model, we demonstrate, however, that fronts propagating in different directions do not have the same dynamics under similar stress conditions. A lack of correlation is also observed between accelerating and decelerating slip fronts. These effects cannot be entirely associated with static local stresses but call for a dynamic description. Considering a dynamic stress ratio (measured in front of the slip tip) instead of a static one reduces the above-mentioned inconsistencies. However, the effects of the direction and acceleration are still present. To overcome this, we propose an energetic criterion that uniquely associates, independently on the direction of propagation and its acceleration, the slip front velocity with the relative rise of the energy density at the slip ti

Coarse Grained Molecular Dynamics Simulations of Transmembrane Protein-Lipid Systems

Many biological cellular processes occur at the micro- or millisecond time scale. With traditional all-atom molecular modeling techniques it is difficult to investigate the dynamics of long time scales or large systems, such as protein aggregation or activation. Coarse graining (CG) can be used to reduce the number of degrees of freedom in such a system, and reduce the computational complexity. In this paper the first version of a coarse grained model for transmembrane proteins is presented. This model differs from other coarse grained protein models due to the introduction of a novel angle potential as well as a hydrogen bonding potential. These new potentials are used to stabilize the backbone. The model has been validated by investigating the adaptation of the hydrophobic mismatch induced by the insertion of WALP-peptides into a lipid membrane, showing that the first step in the adaptation is an increase in the membrane thickness, followed by a tilting of the peptide

Withdrawal of unnecessary antidepressant medication:a randomised controlled trial in primary care

Background: Antidepressant use has increased exponentially in recent decades, mostly due to long continuation.Aim: To assess the effectiveness of a tailored recommendation to withdraw antidepressant treatment.Design &amp; setting: Randomised controlled trial in primary care (PANDA study) in the Netherlands.Method: Long-term antidepressant users (≥9 months) were selected from GPs prescription databases. Patients were diagnosed with the Composite International Diagnostic Interview (CIDI). Long-term users without indication for maintenance treatment (overtreatment) were selected. The intervention consisted of disclosure of the current psychiatric diagnosis combined with a tailored treatment recommendation. Patients were followed for 12 months.Results: The study included 146 participants from 45 family practices. Of the 70 patients in the intervention group, 34 (49%) did not comply with the advice to stop their antidepressant medication. Of the 36 (51%) patients who agreed to try, only 4 (6%) succeeded. These figures were consistent with the control group, where 6 (8%) of the 76 patients discontinued antidepressant use successfully. Patients who were recommended to discontinue their antidepressant medication reported a higher relapse rate than the control group (26% versus 13%, P = 0.05).Conclusion: Changing inappropriate long-term antidepressant use is difficult.</p

Dry sliding contact between rough surfaces at the atomistic scale

Although, a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws have long since described the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Some current research suggests that a linear dependence of friction on the real contact area is observed at the atomistic level, but only for specific cases (indentors and rigid substrates). Because continuum models are not applicable at the atomic scale, other modeling techniques (such as molecular dynamics simulations) are necessary to elucidate the physics of friction at the small scale. We use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load. We find that friction increases with roughness. Also all sliding cases show considerable surface flattening, reducing the friction close to zero after repetitive sliding. This questions the current view of (static) roughness at the atomistic scale, and possibly indicates that the macroscopic laws of friction break down several orders of magnitude before reaching the atomic scale

Tip dependence of three-dimensional scanning force microscopy images of calcite-water interfaces investigated by simulation and experiments

In this study, we have investigated the influence of the tip on the three-dimensional scanning force microscopy (3D-SFM) images of calcite-water interfaces by experiments and simulations. We calculated 3D force images by simulations with the solvent tip approximation (STA), Ca, CO(3)and OH tip models. For all the 3D images, thezprofiles at the surface Ca and CO(3)sites alternately show oscillatory peaks corresponding to the hydration layers. However, the peak heights and spacings become larger when the mechanical stability of the tip becomes higher. For analyzing thexyslices of the 3D force images, we developed the extended STA (E-STA) model which allowed us to reveal the strong correlation between the hydration structure just under the tip and the atomic-scale force contrasts. Based on these understandings on the image features showing the strong tip dependence, we developed a method for objectively estimating the similarity between 3D force images. With this method, we compared the simulated images with the three experimentally obtained ones. Among them, two images showed a relatively high similarity with the image obtained by the simulation with the Ca or the CO(3)tip model. Based on these agreements, we characterized the hydration structure and mechanical stability of the experimentally used tips. The understanding and methodology presented here should help us to derive accurate information on the tip and the interfacial structure from experimentally obtained 3D-SFM images.Peer reviewe

Wave packet propagation by the Faber polynomial approximation in electrodynamics of passive media

Maxwell's equations for propagation of electromagnetic waves in dispersive and absorptive (passive) media are represented in the form of the Schr\"odinger equation $i\partial \Psi/\partial t = {H}\Psi$, where ${H}$ is a linear differential operator (Hamiltonian) acting on a multi-dimensional vector $\Psi$ composed of the electromagnetic fields and auxiliary matter fields describing the medium response. In this representation, the initial value problem is solved by applying the fundamental solution $\exp(-itH)$ to the initial field configuration. The Faber polynomial approximation of the fundamental solution is used to develop a numerical algorithm for propagation of broad band wave packets in passive media. The action of the Hamiltonian on the wave function $\Psi$ is approximated by the Fourier grid pseudospectral method. The algorithm is global in time, meaning that the entire propagation can be carried out in just a few time steps. A typical time step is much larger than that in finite differencing schemes, $\Delta t_F \gg \|H\|^{-1}$. The accuracy and stability of the algorithm is analyzed. The Faber propagation method is compared with the Lanczos-Arnoldi propagation method with an example of scattering of broad band laser pulses on a periodic grating made of a dielectric whose dispersive properties are described by the Rocard-Powels-Debye model. The Faber algorithm is shown to be more efficient. The Courant limit for time stepping, $\Delta t_C \sim \|H\|^{-1}$, is exceeded at least in 3000 times in the Faber propagation scheme.Comment: Latex, 17 pages, 4 figures (separate png files); to appear in J. Comput. Phy

Behavioural Activation versus Treatment as Usual for Depressed Older Adults in Primary Care:A Pragmatic Cluster-Randomised Controlled Trial

Introduction: Effective non-pharmacological treatment options for depression in older adults are lacking.Objective: The effectiveness of behavioural activation (BA) by mental health nurses (MHNs) for depressed older adults in primary care compared with treatment as usual (TAU) was evaluated.Methods: In this multicentre cluster-randomised controlled trial, 59 primary care centres (PCCs) were randomised to BA and TAU. Consenting older (=65 years) adults (n = 161) with clinically relevant symptoms of depression (PHQ-9 = 10) participated. Interventions were an 8-week individual MHN-led BA programme and unrestricted TAU in which general practitioners followed national guidelines. The primary outcome was self-reported depression (QIDS-SR16) at 9 weeks and 3, 6, 9, and 12-month follow-up.Results: Data of 96 participants from 21 PCCs in BA and 65 participants from 16 PCCs in TAU, recruited between July 4, 2016, and September 21, 2020, were included in the intention-to-treat analyses. At post-treatment, BA participants reported significantly lower severity of depressive symptoms than TAU participants (QIDS-SR16 difference = -2.77, 95% CI = -4.19 to -1.35), p &lt; 0.001; between-group effect size = 0.90; 95% CI = 0.42-1.38). This difference persisted up to the 3-month follow-up (QIDS-SR16 difference = -1.53, 95% CI = -2.81 to -0.26, p = 0.02; between-group effect size = 0.50; 95% CI = 0.07-0.92) but not up to the 12-month follow-up [QIDS-SR16 difference = -0.89 (-2.49 to 0.71)], p = 0.28; between-group effect size = 0.29 (95% CI = -0.82 to 0.24).Conclusions: BA led to a greater symptom reduction of depressive symptoms in older adults, compared to TAU in primary care, at post-treatment and 3-month follow-up, but not at 6- to 12-month follow-up.</p