Free interface diffusion, crystallization

Hansen CL, Skordalakes E, Berger JM, Quake SR (2002) A robust and scalable microfluidic metering method that allows protein crystal growth by free interface diffusion. Proc Natl Acad Sci USA 99 16531-16536 Herzig-Marx R, Queeney KT, Rebecca JJ, Schmidt MA, Jensen KF (2004) Infrared spectroscopy for chemically specific sensing in silicon-based microreactors. Anal Chem 76 6476-6483... 

A microfluidic chip has been developed for rapid screening of protein crystallization conditions (Hansen et al., 2002) using the free interface diffusion method. The chip is comprised of a multilayer, silicon elastomer and has 480 valves operated by pressure. The valves are formed at the intersection of two channels separated by a thin membrane. When pressure is applied to the top channel it collapses... 

Hansen, C. L., et ah, A robust and scalable microfluidic metering method that allows protein crystal growth by free interface diffusion. PNAS 2002, 99, 16531— 16536. 

Salemme, F.R., A free interface diffusion technique for the crystallization of proteins for X-ray crystallography. Arch. Bio-chem. Biophys. 1972, 151. 

There are other techniques, however, including microbatch crystallization, where the protein and precipitant are just mixed at the final supersaturation concentration. Free interface diffusion is similar to microbatch but the two components have to diffuse toward each other the concentrations of both protein and precipitant therefore vary with distance from the original interface. In microdialysis, the precipitant solution is allowed to equilibrate with the protein solution through a semipermeable membrane, which permits passage of the precipitant but not the protein (Figure 7). Of these techniques, the first two also lend themselves to automation. 

In addition to the vapor diffusion method described previously, other techniques such as the batch and micro-batch methods, bulk and micro dialysis, free interface diffusion, liquid bridge, and concentration dialysis have also been developed to produce crystals for x-ray diffraction analysis (see McPherson, 1982 and McPherson, 1999). 

Figure 12.20 The free interface diffusion method utilizes a capillary to minimize convective mixing between the protein solution and the second layer of solution containing the crystallization agent (e.g., salt or PEG). The capillary is sealed at both ends with an appropriate adhesive or wax to prevent evaporation of the solution.

Experiments demonstrate that along crystal imperfections such as dislocations, internal interfaces, and free surfaces, diffusion rates can be orders of magnitude faster than in crystals containing only point defects. These line and planar defects provide short-circuit diffusion paths, analogous to high-conductivity paths in electrical systems. Short-circuit diffusion paths can provide the dominant contribution to diffusion in a crystalline material under conditions described in this chapter. 

There are significant differences between the structure of YBCO grain boundaries obtained on Y—ZrOi bi-crystals compared to those on SrTiOs bicrystals. These are caused by the differences in interfacial interactions between the two substrates. One difference is the chemical reaction that takes place at the Y—Zr02 interface but not at the SrTi03 one. The reaction results in the formation of BaZr03 and thus provides an excess of Y and Cu [14.19-14.23, 14.27, 14.28], These species are free to diffuse, and a preferential diffusion path is the boundary. This may influence the physical properties of the YBCO grain boundary since they are sensitive to the exact composition of the boundary. 

Here ig is the prefactor, AU is the activation energy for short-distance diffusion of molecules at interfaces. According to the VFT relaxation mode (6.10), AUlkT oc 1/(T Ty). With the increase of crystallization temperatures, molecular diffusion will be enhanced, and AU will be decreased. On the other hand, AG is the critical free energy barrier for crystal nucleation. For the primary nucleation, AG oc AT. With increase of the crystallization temperatures, AT will become smaller, and AG will rise, accordingly the nucleation rate will be decreased. Thus, at high temperatures, the nucleation rate is mainly dominated by the critical free energy barrier for crystal nucleation, the higher the temperature, the smaller the nucleation rate at low temperatures, the nucleation rate will be mainly dominated by the... 

The weakness of sphemlite boundaries in semi-crystalline polymers can be attributed to two effects. First, as sphemlites evolve, the lowest molar mass fractions in the system will be rejected from the crystal melt interface and will diffuse away, such that they become concentrated where neighbouring sphemlites impinge. Second, the density of crystalline stmctures is necessarily greater than that of the melt from which they form such that, when crystalline stmctures form xmder constant volume conditions, the last regions to solidify will tend to contain increased free volume and inbuilt mechanical stresses. Both of these factors mean that sphemlite boundaries are mechanically and electrically weak. 

It is interesting follow the motion of vacancies/cavities during crystallization. Recent simulations of crystallization in GST suggested that there is cavity diffusion to the crystal/glass interface. This was followed by Ge/Sb diffusion to these sites, aiding the formation of cubic, cavity-free crystallites [37]. Here, the fixed crystalline seed comprises, by definition, 6 vacancies as in c-GST, and we have followed how the other vacancies rearrange in different shells of the growing crystallite. The radial... 

For the short crystal, the radiation flux at the center of the cryslal/melt interface exceeds that at the periphery by a factor of six. At the same time, the distribution of radiation heat flux for the crystal of length 197 mm is nearly uniform and similar to the flux obtained when the crystal surface is diffuse. This effect is mainly related to the specular reflection at the conical part of the crystal (its shoulder) while the contributions of the cylindrical part and the free surface of the melt are less significant. Therefore, it is clear why the effect of the specular reflection depends on the crystal length. The shorter the crystal, the closer is the conical part of the crystal to the crystal/melt interface and the more nonuniform is the radiation heat-flux distribution. 

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