Nucleation-release model

Theriot, J.A., Mitchison, T.J. (1992). The nucleation-release model of actin filament dynamics in cell motility. Trends Cell Biol. 2,219-222. 

As stated in Section 2.1, there is a waiting period between the time of release of one bubble and the time of nucleation of the next at a given nucleation site. This is the period when the thermal boundary layer is reestablished and when the surface temperature of the heater is reheated to that required for nucleation of the next bubble. To predict the waiting period, Hsu and Graham (1961) proposed a model using an active nucleus cavity of radius rc which has just produced a bubble that eventually departs from the surface and has trapped some residual vapor or gas that serves as a nucleus for a new bubble. When heating the liquid, the temperature of the gas in the nucleus also increases. Thus the bubble embryo is not activated until the surrounding liquid is hotter than the bubble interior, which is at... 

The inhibitoiy effect of citrate anions on the calcification of PHEMA hydrogels has been evaluated in this study. Here the release of citric acid from the hydrogels was designed such that the molar ratios of calcium to citric acid were unfavorable for nucleation of calcium phosphates (30). The study constitutes the first model investigation of the role of citrate anions in prevention of calcification of PHEMA hydrogels. 

Fig. 10. Model of reaction cycle responsible for oscillations. Microtubules (Mt GTP) are formed from active subunits (loaded with GTP, Tu GTP, step 1), GTP is hydrolyzed upon incorporation of the subunits (Mt GDP, step 2), leading to the destabilization of microtubules and eventually to their disassembly into oligomers loaded with GDP (01 GDP) which transiently lock the subunits in an unpolymerizable state. When inactive subunits (Tu - GDP) are released from oligomers (step 4) they can be recharged to Tu GTP (step-5), leading to the reassembly of microtubules. Side reactions are the dissolution of rings into oligomers and subunits (observed with microtubule protein just after the T-jump and responsible for the undershoot, see Fig. 3, 4), and the nucleation during the first round of assembly. From [15]...

Recently, there has been growing evidence that metastable pits have effects both in time [6, 8] and space [9] on each other. As indicated in Fig. 8.1C and D, it is believed that active pits release aggressive ions which weaken the protective oxide layer. Thus, the nucleation rate of new metastable pits is enhanced in the vicinity of active pits. Based on these assumptions, a two-dimensional stochastic reaction-diffusion model was developed by A. Mikhailov, J. Hudson, and coworkers [10]. Four variables play an important role in this model ... 

In the present model, the entry to the normal state is initiated by a flux jump in which sufficient energy is released to raise a length of wire above its critical temperature, thus nucleating a normal zone. 

Nucleation involves activation of dormant nuclei in the amorphous phase upon release in the solution phase via gel dissolution [109]. The dormant nuclei are located near the surface of the gel particles, thus becoming activated in the early phase of the gel dissolution/growth process [110], For ZSM-5 synthesis, gel dissolution and nucleation were experimentally proven and successfully modeled to be interfacial phenomena at the gel/solution boundary [111]. 

The criteria for cavitation in polymers modified with rubbers were modeled by Lazzeri and Bucknall (Bucknall et al. 1994 Lazzeri and Bucknall 1993, 1995). They are based on energy release rate principles similar to those used in fracture mechanics. Void nucleation and expansion in elastomer particles are accompanied by the formation of a new surface, significant stretching of the surrounding layers of elastomer, and the stress relaxatimi in the adjacent matrix. All of these are driven by... 

The scheme of reducing the number of variables by introducing delays into a model can be used with systems that involve physical as well as chemical processes. When formic acid is added to concentrated sulfuric acid, the formic acid loses water, and carbon monoxide is produced. Under certain conditions, the CO gas bubbles are released from the solution in an oscillatory fashion (Morgan, 1916). The first successful efforts to model this phenomenon (Smith and Noyes, 1983) take explicit account of the process by which CO nucleates and forms... 

MELCOR incorporates TRAPMELT models to treat the behavior of vapors released from the core. It includes nucleation in the gas volume condensation onto surfaces, such as on suspended aerosols and structures. Retention by water pools is evaluated by SPARC. MELCOR considers surface heating by decay energy and revaporization of previously deposited materials. This phenomenon could be of importance later in the accident, when support plates degrade and collapse into the water pools present in the bottom of the vessel releasing steam. 

The mixing process is, of course, irreversible, and is accompanied by an irreversible increase in entropy. Mixing with evaporation is particularly difficult to model because, in addition to the thermal mixing and release of thermal overfill, there is the compositional heat of mixing with irreversible entropy production which is sig-nificantiy path dependent. A considerable volume of vapour is produced by the usual positive heat of compositional mixing and by thermal contact between colder and hotter components (no homogeneous nucleate boiling has been observed) before the final equilibrium state of the mixture is achieved. 

---