Acoustic crystallization and heterogeneous nucleation

By focusing a high intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around Pm + 4.7 bars (Pm is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by two to three orders of magnitude than what had been previously observed. Our result strongly supports a recent suggestion by Balibar, Mizusaki and Sasaki that, in all previous experiments, solid helium nucleated on impurities.Comment: accepted by Phys. Rev. Let

Phase-field approach to heterogeneous nucleation

We consider the problem of heterogeneous nucleation and growth. The system is described by a phase field model in which the temperature is included through thermal noise. We show that this phase field approach is suitable to describe homogeneous as well as heterogeneous nucleation starting from several general hypotheses. Thus we can investigate the influence of grain boundaries, localized impurities, or any general kind of imperfections in a systematic way. We also put forward the applicability of our model to study other physical situations such as island formation, amorphous crystallization, or recrystallization.Comment: 8 pages including 7 figures. Accepted for publication in Physical Review

Phase-separation phenomena in solutions of poly(2,6-dimethyl-1,4-phenylene oxide). III. Pulse-induced critical scattering of solutions in toluene

For the polymer-solvent system poly(phenylene oxide) in toluene the mechanism and kinetics of crystallization have been studied with the Pulse Induced Critical Scattering technique. It was found that after a delay-time the growth mechanism was diffusion controlled. The delay-time is thought to be connected with the nucleation of the crystallites and it disappeared in the seeded crystallizations studied. After incomplete melting of crystallites the first stages of growth resemble a condensation reaction

The Ultimate Fate of Supercooled Liquids

In recent years it has become widely accepted that a dynamical length scale {\xi}_{\alpha} plays an important role in supercooled liquids near the glass transition. We examine the implications of the interplay between the growing {\xi}_{\alpha} and the size of the crystal nucleus, {\xi}_M, which shrinks on cooling. We argue that at low temperatures where {\xi}_{\alpha} > {\xi}_M a new crystallization mechanism emerges enabling rapid development of a large scale web of sparsely connected crystallinity. Though we predict this web percolates the system at too low a temperature to be easily seen in the laboratory, there are noticeable residual effects near the glass transition that can account for several previously observed unexplained phenomena of deeply supercooled liquids including Fischer clusters, and anomalous crystal growth near T_g

Crystal nucleation in adroplet based microfluidic crystallizer

The study presented in this paper deals with the determination of eflucimibe nucleation rate in a droplet based microfluidic crystallizer. The experimental device allows the storage of up to 2000 monodispersed droplets to get nucleation statistics and crystal growth rates under static conditions. Supersaturation was generated by quenching the droplets down to 273 or 293 K. To determine the nucleation kinetics of eflucimibe, the number of appearing crystals is recorded as a function of time. At low time scale, it was found that eflucimibe in the droplets containing active centers (impurities) crystallizes first and thus yields a rapid initial rate. At higher time scale, once all the droplets containing impurities have crystallized, leaving only the droplets that are free of impurities, the nucleation rate falls allowing the determination of the homogeneous nucleation rate. The crystal–solution interfacial energy found in this system σ=3.12 mJ m−2 is in good agreement with the previously published results. Using the crystalnucleation and the growth rate determined experimentally, simulations were performed using a Monte Carlo method. Even if this method correctly predicts the number of droplets that remains empty during the experiments, it was not possible to predict correctly the number of crystals per drop obtained experimentally. The relationship between the growth and nucleation rates and the resultant number of crystals per drop is likely to be complex and dependent on a number of system parameters. The failure of the model may be attributed either to an overestimation of the crystal growth rate or to an enhancement of the nucleation rate due to the presence of seed crystals

Uniaxial and Mixed Orientations of Poly(ethylene oxide) in Nanoporous Alumina Studied by X-ray Pole Figure Analysis

The orientation of polymers under confinement is a basic, yet not fully understood phenomenon. In this work, the texture of poly(ethylene oxide) (PEO) infiltrated in nanoporous anodic alumina oxide (AAO) templates was investigated by X-ray pole figures. The influence of geometry and crystallization conditions, such as pore diameter, aspect ratio, and cooling rates, was systematically examined. All the samples exhibited a single, volume-dependent crystallization temperature (Tc) at temperatures much lower than that exhibited by bulk PEO, indicating “clean” microdomains without detectable heterogeneous nucleation. An “orientation diagram” was established to account for the experimental observations. Under very high cooling rates (quenching), crystallization of PEO within AAO was nucleation-controlled, adopting a random distribution of crystallites. Under low cooling rates, growth kinetics played a decisive role on the crystal orientation. A relatively faster cooling rate (10 °C/min) and/or smaller pores lead to the * ║ pore axis (n⃗) mode (uniaxial orientation). When the cooling rate was lower (1 °C/min), and/or the pores were larger, a mixed orientation, with a coexistence of * ║ n⃗ and * ║ n⃗ , was observed. The results favor the kinetic model where the fastest growth direction tends to align parallel to the pore axis.This work is supported by the National Natural Science Foundation of China (NSFC, 21873109, 51820105005, 21274156). G. L. is grateful to the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015026). G. L., D. W., and A. J. M. also acknowledge European funding by the RISE BIODEST project (H2020-MSCA-RISE-2017-778092). The authors thank Dr. Zhongkai Yang for assistance with pole figure measurement

A Multiscale Approach for the Characterization and Crystallization of Eflucimibe Polymorphs: from Molecules to Particles

We present in this paper a generic multiscale methodology for the characterization and crystallization of eflucimibe polymorphs. The various characterization techniques used have shown that eflucimibe polymorphism is due to a conformational change of the molecule in the crystal lattice. In addition, the two polymorphs are monotropically related in the temperature range tested and have similar structures and properties (ie. interfacial tension and solubility). Consequently, it was found that for a wide range of operating conditions, the polymorphs may crystallize concomitantly. Induction time measurements and metstable zone width determination allow to infer the origin of the concomitant appearance of the polymorphs. A predominance diagram has been established which allows to perfectly control the crystallization of the desired polymorph. However, even if the stable form can be produced in a reliable way, the crystal suspension went toward a very structured gel-like network which limits the extrapolation process. Based on microscopic observation of the crystallization events performed in a microfluidic crystallizer, we propose a range of operating conditions suitable for the production of the stable form with the desired handling properties