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 Situ QXAFS Studies on the Dynamic Coalescence and Dispersion Processes of Pd in USY Zeolite [30]... 

From Equation 13.1, the existence of a critical value of the Weber number emerges, and then a critical particle size for the dispersed phase below which there is no particle deformation. Furthermore, if only physical interactions exist between components, the remaining dynamic coalescence mechanism could change on further processing steps such as a nonstable postprocessing morphology. 

Good models for bubble swarm dynamics, coalescence and breakup rates, interfacial area transport, and bubble size distributions must be based on real physical phenomena. Bubbly flow instability, for example, has typically been treated as a... 

Microrheology considers only individual drops in an infinite sea of the matrix fluid. At concentrations with < )> 0.005, the coalescence effects must be taken into account. Coalescence can be driven either by the thermodynamics (i.e. minimization of the interfacial energy), or by flow (shear coalescence). During compounding the latter type dominates. It has been shown that the dynamic coalescence increases with and thus at equilibrium between dispersion and coalescence the drop diameter can be expressed as [7] ... 

The specific surface, a, is also relatively insensitive to the duid dynamics, especially in low viscosity broths. On the other hand, it is quite sensitive to the composition of the duid, especially to the presence of substances which inhibit coalescence. In the presence of coalescence inhibitors, the Sauter mean bubble size, is significantly smaller (24), and, especially in stirred bioreactors, bubbles very easily circulate with the broth. This leads to a large hold-up, ie, increased volume fraction of gas phase, 8. Sp, and a are all related... 

During the formation of a spray, its properties vary with time and location. Depending on the atomizing system and operating conditions, variations can result from droplet dispersion, acceleration, deceleration, coUision, coalescence, secondary breakup, evaporation, entrainment, oxidation, and solidification. Therefore, it may be extremely difficult to identify the dominant physical processes that control the spray dynamics and configuration. 

Foams Two excellent reviews (Shedlovsky, op. cit. Lemlich, op. cit.) covering the literature pertinent to foams have been published. A foam is formed when bubbles rise to the surface of a liquid and persist for a while without coalescence with one another or without rupture into the vapor space. The formation of foam, then, consists simply of the formation, rise, and aggregation of bubbles in a hquid in which foam can exist. The hfe of foams varies over many magnitudes—from seconds to years—but in general is finite. Maintenance of a foam, therefore, is a dynamic phenomenon. 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

Homoleptic complexes have been obtained also with tetrakis(l-pyrazolyl)borates, e.g., [ B(pz)4 2Cd] and [ B(3-Mepz)4 2Cd] (both P2 jc, Z = 2) in both compounds structure analyses the ligands have been shown to coordinate trihapto, i.e., with one pz ring free. In both cases Cd has a distorted octahedral environment, with averaged structural data very similar to those for the tris(l-pyrazolylhydridoborate complexes.201 Variable-temperature 3H NMR studies of these and of mixed complexes with tris- and bis(l-pyrazolylhydridoborates indicate fluxional behavior (coalescence temperatures and barriers for the dynamic processes are given). 

---