Cluster dynamic coalescence

Another way to prepare model catalysts (under UHV conditions) is to grow the clusters in gas phase and depose them on the substrate. However, to avoid implantation, fragmentation, and dynamic coalescence, it is necessary to soft-land the clusters. A first possibility is to decrease the kinetic energy of the clusters to less... 

In the case of the particles accommodating amine ligands, a new phenomenon has been evidenced, namely, a dynamic exchange at the NMR timescale between free and coordinated amines. It has been correlated to the TEM and HREM results, which show that, at the early stage of the reaction, the particles display a spherical aspect and a small size (ca. 2-3 nm), and that after a few hours, the particles coalesce into elongated wormlike particles, still constituted of pure, unoxidized hep ruthenium. This NMR observation is particularly interesting since it evidences for these particles a fluxionality similar to that of molecular clusters, which is well documented. The ruthenium nanoparticles contain coordinated mobile surface hydrides, as recently demonstrated by a combination of NMR techniques in solution, gas phase, and in the solid state. ... 

The couple S/S is selected with a specific and intense optical absorption of S or S , so that the electron-transfer reaction can be observed directly. In the early stages of atom coalescence, the redox potentials of the atom and of the smallest clusters are generally far below that of the donor and the transfer from S to the oligomer does not occur. The ion reduction is caused exclusively by solvated electrons and alcohol radicals (Eqs. 2, 8, and 9). The nucleation and coalescence dynamics are thus the same as in the absence of (Eqs. 10 and 11). Beyond a certain critical time, tc, that is large enough to enable the growth of clusters and the increase of their potential above the threshold imposed by the electron donor S , electron transfer from this monitor to the supercritical clusters is allowed (Eq. 32) and detected by the absorbance decay of S (Fig. 6). For n > ny. 

The reactor vessel is usually a stirred tank. The monomer phase is subjected either to turbulent pressure fluctuations or to viscous shear forces, which break it into small droplets that assume a spherical shape under the influence of interfacial tension. These droplets undergo constant collisions (collision rate >1 s ), with some of the collisions resulting in coalescence. Eventually, a dynamic equilibrium is established, leading to a stationary mean particle size. Individual drops do not retain their unique identity, but undergo continuous breakup and coalescence instead. In some cases, an appropriate dispersant can be used to induce the formation of a protective Aim on the droplet surface. As a result, pairs of clusters of drops that tend to coalesce are broken up by the action of the stirrer before the critical coalescence period elapses. A stable state is ultimately reached in which individual drops maintain their identities over prolonged periods of time [247]. 

Coaxial air flow and air dynamically driven devices are supplied by Nisco AG. Biichi Labortechnik also supplies an innovative nozzle that enables the use of encapsulators as an air flow system instead of a vibrating nozzle system, which can be an advantage for encapsulation of clusters or large particles. A coaxial air flow or air-driven systan must be equipped with a pump or mechanism to feed the polymer to the nozzle and two connections, the first to deliver the polymer and the second, which is concentric to the first, to deliver the air stream. The air stream is controlled by an air flow meter and a stirred gelling bath is mandatory to keep the beads separated from each other and avoid coalescence. 

Bubble interaction in swarms is a complex process. Bubble clusters commonly form that coalesce more or less simultaneously into very large bubbles. Recent experiments have revealed some of the details behind this behavior. A bubble contacts another only by following its wake to an overtaking collision. Coalescence or breakup occurs only after the collision, when one bubble is pulled into the near wake of the other. Interaction of three or more bubbles in clusters leads to increased coalescence rates. We have also shown analytically that bubbles do not collide like solid particles, but rather are drawn together by the dynamics of the surrounding fluid. Gravity and fluid acceleration drive bubble motion small-scale turbulence tends to prevent rather than enhance coalescence. 

F. Ding, A. Rosen, K. Bolton, Size dependence of the coalescence and melting of iron clusters a molecular-dynamics study. Phys. Rev. B 70(7), 075416 (2004)... 

It is the author s conviction that in many (turbulent) dispersed multiphase flows—except probably in very dense multiphase flow systems—the origin of mesoscale structures is in the fluid—particle interaction, with a secondary role for particle-particle interaction (coUisions, coalescence, breakup). Clustering of particles is believed to be intimately connected with the chaotic dynamics of fluid accelerations, as particles converge toward each other where and when the divergence of the acceleration field is positive (Goto... 

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