Crystal templating through liquid–liquid phase separation

Abstract

Controlled induction of crystal nucleation is a highly desirable but elusive goal. Attempts to speed up crystallization, such as high super saturation or working near a liquid-liquid critical point, always lead to irregular and uncontrollable crystal growth. Here, we show that under highly nonequilibrium conditions of spinodal decomposition, water crystals grow as thin wires in a template-less formation of "Haareis". This suggests that such nonequilibrium conditions may be employed more widely as a mechanism for crystal growth control.The physical chemistry of crystal nucleation is of great fundamental and practical importance but is yet poorly understood. It is therefore one of the grand challenges on the border between physics, chemistry, and chemical engineering. Crystal nucleation in melt or solution is typically described by Gibbs's classical nucleation theory based on the competition between the free energy of solidification and the free energy due to the presence of the interface. 1, 2 The latter results in a barrier to crystallization and hence super-saturation and a metastable nonequilibrium state. Thermodynamic fluctuations then lead to pre-nucleation sites, the majority of which will redissolve. 3 Occasionally, a nucleus will grow big enough to overcome the barrier (a critical nucleus) and continue to grow. Only at considerable super-saturation will the energy barrier disappear, at which point homogeneous nucleation will occur.As a result, crystal nucleation is generally a rare process that is difficult to study either experimentally or even through computer simulation. In addition, Ostwald's rule of stages suggests that there are intermediate metastable states critical to the understanding of the path and thermodynamics of nucleation. Such metastable states are typically too rare or short-lived to be observed.However, recent work by Gebauer and others has shown that in some cases (such as the nucleation of carbonates from aqueous solution 4-7 ) solute clusters may form that aggregate into amorphous clusters, which then transform into crystal nuclei. 4, 7-9 Such non-classical nucleation theories do not require a "critical nucleus". These theories appear to, but may not necessarily, 10 be counter to thermodynamic theory. Interestingly, a number of light scattering studies of solutions have shown anomalous clustering in solution suggesting that the effect might be more general. 11,12 In the 1990s, Frenkel introduced the concept of the enhancement of crystal nucleation due to the presence of liquid-liquid critical points. 13 Such a critical point would induce concentration fluctuations that would give rise to droplets of so-called "dense fluid" in which the nucleation probability would be greatly enhanced. 7,[14][15][16] Thus, in this scheme the nucleation mechanism is not changed (it could be classical or non-classical) other than to provide an environment with an increased concentration. Although Frenkel's theory was developed for protein crystallization, it is now widely used in chemical-engineering d...

Frustration of crystallisation by a liquid–crystal phase

“…Polarisation microscopy images and videos were recorded using a Nikon Eclipse 50i microscope 33 . Confocal Raman microscopy experiments were performed using a Horiba LabRAM HR confocal microscope system.…”

Control over phase separation and nucleation using a optical-tweezing potential

“…All these mixtures have upper consolute temperatures near room temperature. Samples were held in a cryogenically cooled stage mounted in a modified microscope, 26,43,44 which allowed a 785-nm (<200 mW) or 1047-nm (<8 W) laser beam to be focused in the sample.…”

“…23,34,35 Could liquid-liquid phase separation, oiling out, pre-nucleation clusters, and laser-induced nucleation all be aspects of the same phenomenon? 26,36,37 On approaching a liquid-liquid demixing critical point, concentration fluctuations increase. As a result, it should be easier for an external "force", such as optical tweezing, to manipulate the concentration locally.…”

“…A number of recent experiments have called into question the validity of classical Gibbs nucleation theory and invoked the presence of so-called pre-nucleation clusters. [24][25][26] Although there have been experimental studies reporting pre-nucleation clusters, 27,28 they are still considered controversial. 29,30 1 a :i':…”

Using optical tweezing to control phase separation and nucleation near a liquid–liquid critical point

“…The metastable critical point idea found some traction in the chemical engineering community where it was related to the phenomenon of ''oiling out'' without too much detailed theoretical analysis. 23,[32][33][34] Could liquid-liquid phase separation, oiling out, pre-nucleation clusters, and laser-induced nucleation all be aspects of the same phenomenon? 26,35,36 On approaching a liquid-liquid demixing critical point, concentration fluctuations increase.…”