Tip splitting in dendritic growth of ice crystals

Spatiotemporal Control of Ice Crystallization in Supercooled Water via an Ultrashort Laser Impulse

Focused irradiation with ultrashort laser pulses realized the fine spatiotemporal control of ice crystallization in supercooled water. An effective multiphoton excitation at the laser focus generated shockwaves and bubbles, which acted as an impulse for inducing ice crystal nucleation. The impulse that was localized close to the laser focus and accompanied by a small temperature elevation allowed the precise position control of ice crystallization and its observation with spatiotemporal resolution of micrometers and microseconds using a microscope. To verify the versatility of this laser method, we also applied it using various aqueous systems (e.g., plant extracts). The systematic study of crystallization probability revealed that laser-induced cavitation bubbles play a crucial role in inducing ice crystal nucleation. This method can be used as a tool for studying ice crystallization dynamics in various natural and biological phenomena.

In-situ comparison of interface instability of basal and edge planes during unidirectional growth of sea ice

The evolutions of water/ice interface with distinct ice orientations are one of the central problems in sea ice growth. Some misunderstandings prevailed in previous hypothesis on morphological evolution of sea ice, which need the validation with carefully designed experiments. Up to now, there is no in-situ comparison of unidirectional freezing behavior between distinct ice orientations, which makes the previous hypothesis on sea ice growth skeptical about its physical basis. The aim of this paper is to provide an in-situ comparison of unidirectional growth behavior of lamellar sea ice between basal and edge planes. Our experiment is realized by a delicate design of parallel freezing samples with two specified ice orientations in a modeled sea water under an imposed thermal gradient. The planar instability as well as the unidirectional cellular/dendritic growth patterns of water/ice interfaces of basal and edge plane ice are compared. It is found that the instability grows faster for edge plane ice, which is addressed by an incubation time coupled with the surface tension anisotropy. In addition, two types of tilted growth patterns of sea ice after planar instability are observed simultaneously, which are suggested to stem from distinct physical origins and provide a new insight into the tilted growth of cellular/dendritic ice. Collectively, these experimental results are suggested to promote our understanding of morphological evolution of unidirectional growth of sea ice, as well as its diverse relevance to many other areas involving ice growth.

Waste sludge disintegration, methanogenesis and final disposal via various pretreatments: Comparison of performance and effectiveness

This study compared the three wastewater pretreatments of ozonation, Fe²⁺-S₂O₈ ^2- and freeze-thawing (F/T) in the disintegration, anaerobic digestion (AD) and final disposal of the sludge. The F/T pretreatment increased the dewaterability and settleability of the sludge by 7.8% and 47.1%, respectively. The ozonation pretreatment formed more volatile fatty acids (VFAs), with a peak value of 320.82 mg SCOD/L and controlled the release of sulfides. The Fe²⁺-S₂O₈ ^2- pretreatment removed heavy metals through the absorption and flocculation of ferric particles formed in-situ. During the anaerobic digestion of the sludge, the ozonation pretreatment accelerated the hydrolysis rate (k) rather than the biochemical methane potential (B₀) of the sludge due to the high VFA content in the supernatant. Comparatively, the F/T pretreatment facilitated the B₀ with great economic efficiency by enhancing the solubilisation of the sludge. Although Fe²⁺-S₂O₈ ^2- pretreatment decreased the methane production, the ferric particle was a unique advantage in the disintegration and harmless disposal of the sludge. The digested sludge had more VFAs after ozonation pretreatment, which contributed to the recycling of carbon. In addition, the lower sludge volume could save the expense of transportation and disposal by ozonation pretreatment. Different pretreatments had different characteristics. The comparative study provided information allowing the selection of the type of pretreatment to achieve different objectives of the treatment and disposal of sludge.

Quantitative determination of tip undercooling of faceted sea ice within situexperiments

Sea ice growth with lamellar microstructure containing brine channels has been extensively investigated. However, the quantitative growth information of sea ice remains lack due to the uncontrolled crystalline orientation in previous investigations. For the first time, wein situobserved the unidirectional growth of lamellar sea ice with well-manipulated ice crystal orientation and visualized tip undercooling of lamellar sea ice. With the real-time observation, interesting phenomena of doublon tip in cellular ice growth and growth direction shift of unidirectionally grown ice tip are discovered for the first time, which are attributed to the complex solutal diffusion and anisotropic interface kinetics in ice growth. The quantitative experiments provide a clear micro scenario of sea ice growth, and are suggested to promote relevant investigations of sea ice.

Experimental study on treating landfill sludge by preconditioning combined with vacuum preloading: Effects of freeze-thaw and FeCl3 preconditioning

The deep dewatering of landfill sludge (LS) mainly uses the methods of chemical preconditioning and mechanical dewatering, which is easy to cause environmental pollution and is not conducive to the subsequent recycling treatment of sludge. To find a more environment-friendly and efficient method for LS's deep dewatering and volume reduction, an in-situ sludge treatment method combining freeze-thaw (F/T) preconditioning and vacuum preloading was proposed. Firstly, the F/T test of LS was carried out to explore the optimum freezing temperature. FeCl₃, the most widely used agent, was selected as the chemical preconditioning. Then carry out vacuum preloading model box test. The data were compared after the test. The mechanisms of the two different sludge preconditioning methods on the LS's consolidation were analyzed. The results show that: after freezing, the specific resistance of LS decreases obviously, the overall particle size increases, the content of small particles decreases. Too fast freezing rate is not conducive to the LS's dewatering. After preconditioning (F/T and FeCl₃) combined with vacuum preloading, the volume reduction ratio was 57.1% and 41.1% respectively, the water content was reduced from 73.4% to 53.7% and 58.1%, and the unconfined compressive strength(UCS) was improved from 15.5 kPa to 50.9 kPa and 77.3 kPa. The total water discharge, drainage rate, volume reduction, and water content of freeze-thaw preconditioned LS are better than FeCl₃ preconditioned, while FeCl₃ preconditioned LS has higher UCS. F/T can aggregate small sludge particles but the acidification and hydrolysis of FeCl₃ always produce small particle, which is not conducive to the consolidation of LS during vacuum preloading.

Labyrinth ice pattern formation induced by near-infrared irradiation

Patterns are broad phenomena that relate to biology, chemistry, and physics. The dendritic growth of crystals is the most well-known ice pattern formation process. Tyndall figures are water-melting patterns that occur when ice absorbs light and becomes superheated. Here, we report a previously undescribed ice and water pattern formation process induced by near-infrared irradiation that heats one phase more than the other in a two-phase system. The pattern formed during the irradiation of ice crystals tens of micrometers thick in solution near equilibrium. Dynamic holes and a microchannel labyrinth then formed in specific regions and were characterized by a typical distance between melted points. We concluded that the differential absorption of water and ice was the driving force for the pattern formation. Heating ice by laser absorption might be useful in applications such as the cryopreservation of biological samples.

Direct calculation of solid-liquid interfacial free energy for molecular systems: TIP4P ice-water interface

By extending the cleaving method to molecular systems, we perform direct calculations of the ice Ih-water interfacial free energy for the TIP4P model. The values for the basal, prism, and {112[over]0} faces are 23.3+/-0.8 mJ m{-2}, 23.6+/-1.0 mJ m{-2}, and 24.7+/-0.8 mJ m{-2}, respectively. The closeness of these values implies a minimal role of thermodynamic factors in the anisotropic growth of ice crystals. These results are about 20% lower than the best experimental estimates. However, the Turnbull coefficient is about 50% higher than for real water, indicating a possible limitation of the TIP4P model in describing freezing.

Dendrite engineering on xenon crystals

The experimental work presented focuses on transient growth, morphological transitions, and control of xenon dendrites. Dendritic free growth is perturbed by two different mechanisms: Shaking and heating up to the melting temperature. Spontaneous and metastable multitip configurations are stabilized, coarsening is reduced, leading to a denser sidebranch growth, and a periodic tip splitting is found during perturbation by shaking. On the other hand, heating leads to controlled sidebranching and characteristic transitions of the tip shape. A deterministic behavior is found besides the random-noise-driven growth. The existence of a limit cycle is supported by the findings. Together the two perturbation mechanisms allow a "dendrite engineering"--i.e., a reproducible controlling of the crystal shape during its growth. The tip splitting for dendritic free growth is found not to be a splitting of the tip in two; rather, the respective growth velocities of the main tip and the fins change. The latter then surpass the main tip and develop into new tips. The occurrence of three- and four-tip configurations is explained with this mechanism. Finite-element calculations of the heat flow and the convective flow in the growth vessel show that the idea of a single axisymmetric toroidal convection roll across the whole growth vessel has to be dropped. The main effect of convection under Earth's gravity is the compression of the diffusive temperature field around the downward-growing tip. A model to explain the symmetry of dendritic crystals--e.g., snow crystals--is developed, based on the interaction of crystal shape and heat flow in the crystal.

Morphological instability in freezing colloidal suspensions

We present a linear stability analysis of a planar ice interface during unidirectional solidification of a hard-sphere colloidal suspension. We find that the interface can become unstable due to constitutional supercooling, yielding a new mechanism for pattern formation in colloidal systems. The interfacial stability is shown to depend strongly on the size and concentration of the particles. Increasing the particle radius tends to stabilize the interface, while increasing the concentration has a destabilizing effect. Additional effects that may influence the stability and morphology of such a system are described.

Measurements of the three-dimensional shape of ice crystals in supercooled water

Experimentally grown ice crystals from ultrapure supercooled water are imaged by means of Mach-Zehnder interferometry. By analyzing the fringe patterns the phase information and thus the three-dimensional shape of the ice crystals is recovered quantitatively. The integral parameters height of the basal plane, volume, and surface of the crystals are measured as a function of time and supercooling. It is found that all measured parameters follow a power law as a function of time and the exponents are found to be independent of the supercooling. The shape transition from the prismatic to the basal face along the main growth direction of the ice dendrites as a function of the distance from the tip is found to be a power law as well. Our findings support the validity of universal growth laws in pattern forming systems.

Calculation of solid-liquid interfacial free energy: A classical nucleation theory based approach

We present a simple approach to calculate the solid-liquid interfacial free energy. This new method is based on the classical nucleation theory. Using the molecular dynamics simulation, we employ spherical crystal nuclei embedded in the supercooled liquids to create an ideal model of a homogeneous nucleation. The interfacial free energy is extracted by fitting the relation between the critical nucleus size and the reciprocal of the critical undercooling temperature. The orientationally averaged interfacial free energy is found to be 0.302+/-0.002 (in standard LJ unit). The temperature dependence of the interfacial free energy is also obtained in this work. We find that the interfacial free energy increases slightly with increasing temperature. The positive temperature coefficient of the interfacial free energy is in qualitative agreement with Spaepen's analysis [Solid State Phys. 47, FS181 (1994)] and Turnbull's empirical estimation [J. Appl. Phys. 21, 1022 (1950)].

Calculation of the crystal-melt interfacial free energy of succinonitrile from molecular simulation

The crystal-metal interfacial free energy for a six-site model of succinonitrile [N triple bond C-(CH(2))(2)-C triple bond N] has been calculated using molecular-dynamics simulation from the power spectrum of capillary fluctuations in interface position. The orientationally averaged magnitude of the interfacial free energy is determined to be (7.0+/-0.4)x10(-3) J m(-2). This value is in agreement (within the error bars) with the experimental value [(7.9+/-0.8)x10(-3) J m(-2)] of Marasli et al. [J. Cryst. Growth 247, 613 (2003)], but is about 20% lower than the earlier experimental value [(8.9+/-0.5)x10(-3) J m(-2)] obtained by Schaefer et al. [Philos. Mag. 32, 725 (1975)]. In agreement with the experiment, the calculated anisotropy of the interfacial free energy of this body-centered-cubic material is small. In addition, the Turnbull coefficient from our simulation is also in agreement with the experiment. This work demonstrates that the capillary fluctuation method of Hoyt et al. [Phys. Rev. Lett. 86, 5530 (2001)] can be successfully applied to determine the crystal-melt interfacial free energy of molecular materials.