Molecular-dynamics simulations of cluster–surface collisions: Emission of large fragments

Dynamics and Kinetics of Methanol-Graphite Interactions at Low Surface Coverage

The processes of molecular clustering, condensation, nucleation, and growth of bulk materials on surfaces, represent a spectrum of vapor-surface interactions that are important to a range of physical phenomena. Here, we describe studies of the initial stages of methanol condensation on graphite, which is a simple model system where gas-surface interactions can be described in detail using combined experimental and theoretical methods. Experimental molecular beam methods and computational molecular dynamics simulations are used to investigate collision dynamics and surface accommodation of methanol molecules and clusters at temperatures from 160 K to 240 K. Both single molecules and methanol clusters efficiently trap on graphite, and even in rarified systems methanol-methanol interactions quickly become important. A kinetic model is developed to simulate the observed behavior, including the residence time of trapped molecules and the quantified Arrhenius kinetics. Trapped molecules are concluded to rapidly diffuse on the surface to find and/or form clusters already at surface coverages below 10^-6 monolayers. Conversely, clusters that undergo surface collisions fragment and subsequently lose more loosely bound molecules. Thus, the surface mediates molecular collisions in a manner that minimizes the importance of initial cluster size, but highlights a strong sensitivity to surface diffusion and intermolecular interactions between the hydrogen bonded molecules.

Molecular Dynamics Simulation of Water Nanodroplet Bounce Back from Flat and Nanopillared Surface

Molecular dynamics simulations of impinging nanodroplets were performed to study the bounce-back condition for flat and nanopillared surfaces. We found that the bounce-back condition can be closely related to the degree of droplet deformation upon collision with the solid surface. When the droplets have little or small deformation, the bounce-back condition solely depends on the hydrophobicity of the surface. On the other hand, when the droplet deformation is large, the impinging velocity dependence of the bounce-back condition becomes stronger due to the increase of the liquid-vapor interfacial area of colliding droplet, which is proportional to the liquid-vapor surface energy. The impinging droplet simulations with nanopillared hydrophobic surfaces were also performed. The contribution of droplet deformation in this case is relatively small because the surface hydrophobicity is enhanced due to the existence of pillars. Finally, we find that the maximum spreading diameter of the impinging droplets exhibits a consistent trend, in terms of the Weber number dependence, as the experimental measurements with macrodroplets.

On the detection of mesospheric meteoric smoke particles embedded in noctilucent cloud particles with rocket-borne dust probes

Mesospheric nanoparticles in the forms of water ice particles and meteoric smoke particles (MSPs) exist in the middle atmosphere where they often play a decisive role in cloud formation and in chemical processes. Direct in situ observations of mesospheric nanoparticles have been made possible by rocket probes developed during the last two decades. Although progress has been made in mapping properties such as electric charge, sizes, and interaction with the plasma and neutral gas, more observations are needed on the size distribution, chemical content, and structure of the MSP to determine their role in cloud formation and chemistry in the mesosphere and stratosphere. We here present the result of a detailed analysis of the performance of a new dust probe MUltiple Dust Detector (MUDD) [O. Havnes et al., J. Atmos Soll.-Terr. Phys. 118, 190 (2014); O. Havenes et al., ibid. (in press)], which should give information of the size distribution of MSP by fragmenting impacting ice particles and releasing a fraction of the MSP which most probably are embedded in them [O. Havnes and L. I. Naesheim, Ann. Geophys. 25, 623 (2007); M. E. Hervig et al., J. Atmos. Sol.-Terr. Phys. 84-85, 1 (2012)]. We first determine the electric field structure and neutral gas condition in the interior of the probe and from this compute, the dynamics and current contribution of the charged fragments to the currents measured as the probe scans the fragment energy. For the single MUDD probe flown in July 2011 on the PHOCUS payload, we find that the fragment currents at the three retarding potentials for MUDD of 0, 10, and 20 V correspond to fragment sizes of ≳0.6 nm, >1.5 nm, and >1.8 nm if the fragments have a negative unit charge. We also discuss the optimum choice of retarding potentials in future flights of MUDD probes. By launching 2 to 3 mechanically identical MUDD probes but with different retarding potentials, we will obtain a much more detailed and reliable fragment (MSP) size distribution.

A comparative study of cluster-surface collisions: Molecular-dynamics simulations of (H2O)1000 and (SO2)1000

A classical molecular-dynamics study of (H2O)1000 and (SO2)1000 clusters impacting with velocities between 6 x 10(2) and 8 x 10(3) ms at normal incidence on a repulsive target is presented. Using the ratio of total kinetic energy to total binding energy of the cluster as a scaling parameter, a general description of the fragmentation dynamics as well as the final fragment size distributions is achieved for the different systems. With increasing ratio, the angular distribution of the emitted monomers rapidly shifts from isotropic to anisotropic. At the highest investigated velocities, a tendency to recover more isotropic distributions is observed. Comparable transient compression of the impacting cluster is reached, on the other hand, for the same, unscaled collision velocities in both systems. For both H2O and SO2 the obtained internal temperatures of the cluster fragments are found to be independent of impact energy and close to the boiling temperature of the respective systems.

Comparison of classical and quantum dynamics for collinear cluster scattering

The collinear dynamics of a cluster of four particles colliding with a fixed particle representing a surface is investigated using a four-dimensional wave packet approach. The properties of the system are chosen to resemble a water cluster interacting with graphite, but a deeper surface-particle potential is also considered causing significant dissociation of the cluster. Having four different product arrangement channels the system is quantum mechanically demanding but still manageable. The dynamical richness makes it a suitable benchmark system for evaluation of classical and quantum/classical schemes. The average energy transferred to the cluster and the three dissociation probabilities are presented as function of the initial state of the cluster. In addition to wave packet data, results obtained using quasiclassical as well as Wigner sampled classical trajectories are presented. The main conclusion is that classical mechanics can describe the dynamics of the system in a very satisfactory way. Including zero-point energy in the classical simulations is particularly important for a good description of dissociation but less important for energy transfer.