Molecular dynamic simulations for a Xe film sliding on an Ag(111) substrate are performed from the submonolayer through the bilayer regime, which, when compared to both friction and surface resistivity measurements, demonstrate that the friction in this system is dominated by phonon excitations. Slip times are found both by direct calculation of the decay of the center-of-mass velocity, as well as from the decay of the velocity correlation function. Agreement of the slip times from the two methods supports the occurrence of a friction force linear in velocity over a wide velocity range.[S0031-9007 (97)04756-X] PACS numbers: 46.30.PaAlthough tribology, the study of friction and wear, has been of technological interest since ancient times [1,2], the topic continues to rouse interest today [3][4][5][6][7][8][9][10][11][12]. Rapid progress in experimental, theoretical, and computational methods provides new insights into the atomic origins of frictional energy dissipation. When a thin film slides on a metal substrate there exists dissipation of energy via two mechanisms: (i) electronic excitations in the metallic substrate [4,9], and (ii) phonon excitations in the film or in the substrate [10]. The dissipation of energy can be characterized by the slip time t (i.e., the time it takes for the film's speed to fall to 1͞e of its original value, assuming it is stopped by friction) or equivalently by a damping coefficient h ϳ 1͞t.In this Letter we study the phonon contribution to friction for Xe films sliding along an Ag(111) substrate using molecular dynamics simulations. It is of great interest to determine the relative contributions of the phonons and electrons to friction, since to date it is not clear which is dominant. To this end, we compare our results with the slip time versus coverage data reported by Daly and Krim [6], and with the electrical resistivity versus coverage data of Dayo and Krim [7]. The results of this comparison provide convincing evidence that phonon excitations make a dominant contribution to the friction for this system.We determine the slip time as a function of coverage, defined as the number of atoms in the film per unit area. We treat a range of film coverages, from submonolayer to bilayer. The slip time is determined by two methods. In the first method, an initial center-of-mass velocity V 0 is produced by an external force exerted on the film for t , 0. The external force is turned off for t . 0, and t is determined by the resulting velocity decay V 0 e 2t͞t . In the second method, no external force is applied. The slip time is determined by the behavior of the thermal equilibrium autocorrelation for the center-of-mass velocity as a function of time. The autocorrelation function is related to the linear response of the system to a small perturbation by the fluctuation-dissipation theorem [12,13], which holds in the limit of zero applied force and, hence, zero velocity. This method is the only one which is valid at the 1 cm͞s velocities appropriate for experiments using the quartz crystal...

It is well known that molecules feel a long-ranged attractive force to metals which arises from the interaction of various molecular charges with their metal-induced ''image charges.'' Here we calculate ͑in this image charge model͒ the friction force coefficients for molecules moving near a metal. We consider the cases of ions, polar molecules, and spherical atoms.

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The aim of the present work is to synthesize, characterize, and test self-assembled anisotropic or Janus particles designed to load anticancer drugs for lung cancer treatment by inhalation. The particles were synthesized using binary mixtures of biodegradable and biocompatible materials. The particles did not demonstrate cyto- and genotoxic effects. Janus particles were internalized by cancer cells and accumulated both in the cytoplasm and nuclei. After inhalation delivery, nanoparticles accumulated preferentially in the lungs of mice and retained there for at least 24 h. Two drugs or other biologically active components with substantially different aqueous solubility can be simultaneously loaded in two-phases (polymer-lipid) of these nanoparticles. In the present proof-of-concept investigation, the particles were loaded with two anticancer drugs: doxorubicin and curcumin as model anticancer drugs with relatively high and low aqueous solubility, respectively. However, there are no obstacles for loading any hydrophobic or hydrophilic chemical agents. Nanoparticles with dual load were used for their local inhalation delivery directly to the lungs of mice with orthotopic model of human lung cancer. In vivo experiments showed that the selected nanoparticles with two anticancer drugs with different mechanisms of action prevented progression of lung tumors. It should be stressed that anticancer effects of the combined treatment with two anticancer drugs loaded in the same nanoparticle significantly exceeded the effect of either drug loaded in similar nanoparticles alone.

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