Biomedical subjects
Publications and source records attributed to O M Braun.
We show that the transition from smooth sliding to stick-slip motion in a single planar frictional junction always takes place at an atomic-scale relative velocity of the substrates.
Rolling friction at a microscopic scale is studied with the help of a simple two-dimensional model. Molecular dynamics simulations show that rolling of spherical lubricant molecules exists only for concentrations lower than the concentration of a close-packed layer. At concentrations higher than a critical one due to jamming of lubricant molecules the rolling of nearest neighboring molecules is hindered. An optimal concentration exists which provides the minimum of kinetic friction. Methods for avoiding jamming and increasing the range of operation of rolling mechanism of friction are discussed.
We study a chain of harmonically interacting atoms confined between two sinusoidal substrate potentials, when the top substrate is driven through an attached spring with a constant velocity. This system is characterized by three inherent length scales and closely related to physical situations with confined lubricant films. We show that, contrary to the standard Frenkel-Kontorova model, the most favorable sliding regime is achieved by choosing chain-substrate incommensurabilities belonging to the class of cubic irrational numbers (e.g., the spiral mean). At large chain stiffness, the well known golden mean incommensurability reveals a very regular time-periodic dynamics with always higher kinetic friction values with respect to the spiral mean case.
We propose a model for a one-dimensional chain of interacting particles in an external periodic potential. In this model the particles have a complex structure treated in a mean-field fashion: particle collisions are inelastic and also each particle is considered as having its own thermostat. We derived the Fokker-Planck equation for this model and demonstrated that the model has a truly equilibrium ground state. When an external dc force is applied to the atoms, the model exhibits a hysteresis even at high temperatures due to the clustering of atoms with the same velocity. Another effect of clustering is phase separation in the steady state when the system splits into regions of immobile atoms ("traffic jams") and regions of running atoms.
We propose a two-dimensional lattice-gas (2D LG) model where atoms may be in two different states: the immobile state, in which they jump as usual in the LG model, and the running state, in which the atoms always jump in the driving direction. The model demonstrates a typical behavior of "traffic-jam" models: the system splits into domains of immobile atoms (jams) and running atoms. We considered four variants of the 2D LG model, namely the multilane and truly 2D models, each with "passive" and "active" atomic jumps. The model has the steady state with a power law distribution of jam sizes characterized by a universal exponent 3/2. The phase diagram of the model shows that the mobility of the 2D system is lower than the mobility of the 1D model due to the spreading of jams in the direction transverse to the driving direction.
We study the nonlinear dc response of a two-dimensional underdamped system of interacting atoms subject to an isotropic periodic external potential with triangular symmetry. When driving force increases, the system transfers from a disorder locked state to an ordered sliding state corresponding to a moving crystal. By varying the values of the effective elastic constant, damping, and temperature, we found different scenarios and intermediate phases during the ordering transition. For a soft atomic layer, the system passes through a plastic-channel regime that appears as a steady-state regime at higher values of the damping coefficient. For high values of the effective elastic constant, when the atomic layer is stiff, the intermediate plastic phase corresponds to a traffic-jam regime with immobile islands in the sea of running atoms. At a high driving of the stiff layer, a solitonlike elastic flow of atoms has been observed.
A thin lubricant film confined between two substrates in moving contact is studied using Langevin molecular dynamics with the coordinate- and velocity-dependent damping coefficient. It is shown that an optimal choice of the interaction within the lubricant can lead to minimal kinetic friction as well as to low critical velocity of the stick-slip to smooth-sliding transition. The strength of this interaction should be high enough (relative to the strength of the interaction of lubricant atoms with the substrates) so that the lubricant remains in a solid state during sliding. At the same time, the strength of the interaction should not be too high, in order to allow annealing of defects in the lubricant at slips.
The mobility and the diffusivity of a dimer (two atoms coupled by an elastic spring) in a periodic substrate potential under the action of the dc and ac external forces are studied. It is shown that the dimer diffusivity may be strongly enhanced due to driving.
Molecular dynamics is used to investigate the melting of a thin lubricant film confined between two crystalline surfaces. The dynamics of the film is significantly affected by the substrate, both in the solid and in the molten phases. The solid phase, able to sustain shear stress, shows, however, large diffusional motions of the atoms. The melting temperature depends strongly on the confinement. A phenomenological microscopic theory, based on the Lindemann criterion, is proposed to explain this effect.
Multiple and supersonic topological excitations (kinks) driven by an external dc force in the Frenkel-Kontorova model (a chain of atoms subjected to a periodic substrate potential) with the exponential interatomic interaction are studied with the help of numerical simulation. The simulation results are interpreted in terms of dynamics of two limiting cases, the exactly integrable sine-Gordon equation and the Toda chain. The stability of driven kinks and scenarios of their destruction are described for a wide range of model parameters.
We analyze a probability of atomic jumps for more than one lattice spacing in activated surface diffusion. First, we studied a role of coupling between the x and y degrees of freedom for the diffusion in a two-dimensional substrate potential. Simulation results show that in the underdamped limit the average jump length scales with the damping coefficient eta as proportional, variant eta(-sigma(lambda)) with 1/2<or=sigma(lambda) less, similar 2/3, so that the diffusion coefficient behaves as D proportional, variant eta(-sigma) with 0<or=sigma less, similar 1/3. Second, we introduced a realistic friction coefficient for the phonon damping mechanism and developed the technique for Langevin equation with a velocity-dependent friction coefficient. The study of diffusion in this model shows that long jumps play an essential role for diffusing atoms of small masses, especially in two limiting cases, in the case of a large Debye frequency of the substrate, when the rate of phonon damping is low, and in the case of a small Debye frequency, when the one-phonon damping mechanism is ineffective.
We present a detailed study of an earthquakelike model that exhibits a "transition" from stick-slip motion to smooth sliding at a velocity of the order of those observed in experiments. This contrasts with the many previous microscopic models in which the transition velocity is many orders of magnitude too large. The results show that experimentally observed smooth sliding at the macroscopic scale must correspond to microscopic-scale stick-slip motion.
Molecular-dynamics study of a thin (one to five layers) lubricant film between two substrates in moving contact are performed using Langevin equations with an external damping coefficient depending on distance and velocity of atoms relative the substrates, motivated by microscopic configurations. They show that the minimal friction coefficient is obtained for the solid-sliding regime. A detailed analysis of the results, the comparison with other microscopic modeling approaches of friction, and the evaluation of quantities that can be compared to experiments, such as the velocity of the transition from stick slip to smooth sliding, are used to discuss the relevance of the microscopic simulations of friction.
We study the nonlinear dc response of a two-dimensional underdamped system of interacting atoms subject to an isotropic periodic external potential with triangular symmetry. We consider various values of the effective elastic constant of the system, two different atomic interaction potentials, and different concentrations of atoms. In the case of a closely packed layer, when its structure is commensurate with the substrate, there is a locked-to-running transition as a function of the driving force, whose mechanism depends on the effective elastic constant. For a low elastic constant, where the layer is weakly coupled, the transition is achieved via the creation of an avalanche of moving particles that leaves a depleted region in its wake. On increasing the effective elastic constant the depleted region becomes less marked and there is a crossover to a scenario in which an island of moving particles nucleates the transition. In the case of a partially filled atomic layer, several dynamical phase transitions between states with different atomic mobility are observed. The mobility of atoms as a function of the external force can vary nonmonotonically with increasing force. For the case of a small external damping, the system can be trapped at a large force in an immobile metastable state, thus demonstrating a "fuse-safety device" on an atomic scale.
Diffusion of a particle in the N-dimensional external potential which is periodic in one dimension and unbounded in the other N-1 dimensions is investigated. We find an analytical expression for the overdamped diffusion and study numerically the cases of moderate and low damping. We show that in the underdamped limit, the multidimensional effects lead to a reduction (compared to the one-dimensional motion) in the jump lengths between subsequent trappings of the atom at the minima of the external periodic potential. As an application, we consider the diffusion of a dimer adsorbed on the crystal surface.
Epithelioid angiomyolipoma is a recently recognized tumor entity, of which 9 tumors are documented in the literature. We report a case that occurred in a 19 years old patient. The tumor consisted of epithelioid cells ranging from medium sized and polygonal to giant cells with prominent nucleoli. Remarkably, there were many multinucleated giant cells. Hemorrhage, necrosis, and clusters of foamy macrophages were also present. Obvious elements of typical angiomyolipoma, especially fatty tissue was not found. Flow cytometry revealed diploid tumor cells with a high S-phase fraction of 9.7%. Melanoma-associated antigen HMB-45 and CD 68 was detected in the epithelioid and spindle cells. The multinucleated giant cells stained for desmin and KP1, and to a lesser extent for HMB-45 and vimentin. There was no expression of cytokeratin or epithelial membrane antigen. Epithelioid angiomyolipoma often pose problems in diagnosis, particularly with regard to distinction from renal cell carcinoma. Previous reports in the literature suggest that epithelioid angiomyolipoma may have malignant potential.
Combined modality therapy, consisting of radiation, chemotherapy and surgery are used to treat primary tumours aiming to preserve function and increase tumour control. In the present prospective trial 112 patients underwent combined preoperative radio-chemotherapy, 35 patients were treated with combined radio-chemotherapy as only treatment. At a median follow-up of 26 months eight patients (2.8%) have developed an osteo-radionecrosis, which is comparable with data from the literature. When known risk factors are avoided the incidence of osteo-radionecrosis is not increased following combined therapy. The most important factors for development of osteo-radionecrosis following radio-chemotherapy are large tumours and tumour infiltration in the mandible.
Forty-one patients with Stage III and IV squamous cell carcinomas (SCC) of the head and neck were treated preoperatively with mitomycin C and 5-fluorouracil and concomitant radiotherapy. Operation specimen were examined histologically, the percentage of vital tumor cells and devitalized tumor cells were graduated. The grade of regression was classified according to a four-stage scale. Tumor regression was elevated as good (Grades 1, 2) and bad (3, 4) response to combined preoperative therapy. After a follow-up of 18 to 30 months, 14 of 41 patients have experienced a locoregional recurrence; all these patients were bad responders (Grades 3, 4) to preoperative radiochemotherapy. There was a statistically significant correlation between tumor regression grade and probability of survival (P less than 0.001). The authors conclude that the prognosis of patients with pretreated SCC of head and neck depends on the histologic grade of tumor regression.