Ostwald-ripening phenomenon

The evolution of spin-lattice relaxation times as a function of time was measured for the tricaprin/tristearin 50 50 (w/w) mixture at 40°C. At this temperature, tricaprin was liquid and tristearin was in the P form (checked by XRD measurements, results not shown). The Solid Fat Content (SFC) was 60%. The small deviation from theory (50%) was due to the presence of co-crystals which delay the melting of triceqjrin crystals. It is already known that the average crystal size increases as a fimction of time via the Ostwald Ripening phenomenon according to a power law model. The evolution of T for the present system is shown in Figure 1. 

From of our model, which is greatly simplified, it is clear that the presence of a small quantity of an insoluble solute in a water-in-oU dispersion (or an insoluble vapor in the case of a foam) can prevent the destabUizing effect of the Ostwald ripening phenomenon. It appears that with changes to the radii of two bubbles, a difference in salt concentration on the order of 0.0016% to 0.0004% for a radii of 100 to 400 nm is sufficient to stop Oswald ripening (when Y = lOmN/m). 

Ostwald ripening chem Solution-crystallizer phenomenon in which small crystals, more soluble than large ones, dissolve and reprecipitate onto larger particles. ost, valt rTp-a-nii) ... 

An important example of this phenomenon is to be found in the ageing of colloidal dispersions (often referred to as Ostwald ripening). In any dispersion there exists a dynamic equilibrium whereby the rates of dissolution and deposition of the dispersed phase balance in order that saturation solubility of the dispersed material in the dispersion medium be maintained. In a polydispersed sol the smaller particles will have a greater solubility than the larger particles and so will tend to dissolve, while the larger particles will tend to grow at their expense. In... 

Ostwald ripening, the disappearance of very small crystals because of an increase in solubility, has been discussed in Sections 4.2.1.2 and 4.3.1.7 Despite the fact that the concept applies most directly to particles <1 micron in size, it has considerable commercial signilicance because of its participation in the aging of seed to increase the size, narrow the size distribution, and improve the crystallinity of particles in a crystal growth operation. Mullin (2001, pp. 320-322) contains an excellent discussion of this phenomenon. 

Figure 28 clearly shows the importance of diffusion within a chemisorbed layer to surface reaction processes (Leibsle and Bowker, in prep.). In this series of STM images the surface methoxy species on Cu(110) is decomposing (evidenced by the loss of total area of methoxy islands), but diffusion is taking place between islands since big islands get bigger at the expense of smaller ones, which eventually disappear. This kind of diffusion phenomenon can be classified as surface mediated Ostwald ripening. 

In case of a constant contact of emulsion droplets, which is typical of highly concentrated systems, diffusive transfer (isothermal distillation, recondensation, Ostwald ripening) is of great importance, along with coalescence. This phenomenon has been extensively studied in [46- 7]. 

Patterns usually appear due to the instability of a uniform state. However, such an instability does not necessarily lead to pattern formation. Let us consider, e.g., phase separation of a van-der-Waals fluid near the critical point Tc. For T > Tc, there exists only one phase, while for T < Tc, there exist two stable phases, corresponding to gas and liquid, and an unstable phase whose density is intermediate between those of the gas and the liquid. When an initially uniform fluid is cooled below Tc, the unstable phase is destroyed, and in the beginning one observes a mixture of stable-phase domains, i.e. hquid droplets and gas bubbles, which can be considered as a disordered pattern. However, the domain size of each phase grows with time (this phenomenon is called Ostwald ripening or coarsening). Finally, one observes a full separation of phases a liquid layer is formed in the bottom part of the cavity, and a gas layer at the top. Thus, the instabihty of a certain uniform state is not sufficient for getting stable patterns. Below we formulate some mathematical models that describe both phenomena, domain coarsening and pattern formation. 

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