In situ observation the interface undercooling of freezing colloidal suspensions with differential visualization method

Tilting Behavior of Lamellar Ice Tip during Unidirectional Freezing of Aqueous Solutions

Freezing of ice has been largely reported from many aspects, especially its complex pattern formation. Ice grown from liquid phase is usually characteristic of lamellar morphology that plays a significant role in various domains. However, tilted growth of ice via transition from coplanar to noncoplanar in directional solidification has been paid little attention in previous studies and there was a misleading explanation of the formation of tilted lamellar ice. Here, we in situ investigated the variations of tilting behavior of lamellar ice tips under different conditions within a single ice crystal of manipulated orientation via unidirectional freezing of aqueous solutions. It is found that tilted growth of ice tips is sensitive to pulling velocity and solute type. These experimental results reveal intrinsic tilted growth behavior of lamellar ice, which is suggested to enrich our understanding of pattern formation of ice in relevant physical processes.

Thermal regelation of single particles and particle clusters in ice

We investigate the migration by thermal regelation of single particles and clusters of particles surrounded by ice subjected to a temperature gradient. This phenomenon is relevant to the casting of porous materials, to cryopreservation of biological tissue, and to the degradation of paleoclimatic signals held in ice sheets, for example. Using carefully controlled laboratory experiments, we measure the migration rates of single particles and clusters as they approach the freezing front. We find that clusters migrate at a constant rate, while single particles accelerate towards the freezing front. This fundamental difference is attributed to the fact that, during regelation, melt water passes through the interstices of a cluster, limited by its constant permeability, but for a single particle must flow through a thin layer of pre-melted ice whose thickness diverges as the freezing temperature is approached, reducing the viscous resistance to migration. We extend existing theories of particle and cluster migration to include the influences of different thermal conductivities and of latent heat on the local temperature field in and around the particle or cluster. We find that if the specific latent heat is large or the viscous resistance to flow is sufficiently small then the migration rate is determined solely by heat transport.

Advanced Technique for In Situ Raman Spectroscopy Monitoring of the Freezing-Induced Loading Process

The freezing-induced loading (FIL) method is a promising technique for encapsulation of bioactive substances as well as for preparation of nanocomposite materials. A critically important aspect for this method is the remote control of the freezing process. The knowledge of the moment of freezing process ending can allow us to increase the quality of loading and reduce the process duration, thus making this approach more controllable. Herein, we present a photonic technique based on Raman spectroscopy as one of the optimal solutions for remote control of FIL. As a result of our study, the setup for obtaining Raman spectra during the process of liquid vehicle crystallization in suspensions has been developed, which allowed us to analyze the sorption of nanoparticles onto micro- and submicron particles by the FIL method in situ. The main focus of the present work is the in situ Raman spectroscopy monitoring of the crystallization process, including technologically important parameters such as the ice/water interface velocity in water colloids/suspensions and the moment of the final adsorption of the nanoparticles on the microparticles. In contrast to other approaches, Raman spectroscopy allows to directly observe the hydrogen bond formation during crystallization. Additionally, a schematic and a detailed description of the setup are presented here. Thus, the developed technique has a good perspective for scaling up the FIL approach and increasing the area of application of this technology.

Single Ice Crystal Growth with Controlled Orientation during Directional Freezing

Ice growth has attracted great attention for its capability of fabricating hierarchically porous microstructure. However, the formation of tilted lamellar microstructure during freezing needs to be reconsidered due to the limited control of ice orientation with respect to the thermal gradient during in situ observations, which can greatly enrich our insight into architectural control of porous biomaterials. This paper provides an in situ study of the solid/liquid interface morphology evolution of directionally solidified single crystal ice with its C-axis (optical axis) perpendicular to directions of both the thermal gradient and the incident light in poly(vinyl alcohol, PVA) solutions. Multifaceted morphology and V-shaped lamellar morphology were clearly observed in situ for the first time. Quantitative characterizations on lamellar spacing, tilt angle, and tip undercooling of lamellar ice platelets provide a clearer insight into the inherent ice growth habit in polymeric aqueous systems and are suggested to exert significant impact on future design and optimization in porous biomaterials.

Interactions between Nanoparticles and Polymers in the Diffusion Boundary Layer during Freezing Colloidal Suspensions

The present work investigated diffusion interactions between nanoparticles and polymers during freezing colloidal suspensions. The size effects of nanoporous media formed by packed nanoparticles on the diffusion instability of the polymer solution were explored. It is found that small particles under low pulling speeds will obstruct the diffusion of polymers and the corresponding morphology will be banded structures. The intrinsic reason is the inhibited tube-like motion of polymer chains in the nanoporous particle layer. The increased particle size or the decreased solute size will solve the diffusion problem. On the other hand, the small pulling speed constructs an increased length of the particle layer in front of the freezing interface, which presents a longer diffusion path to impede the polymer diffusion. Instead, an increased pulling speed shortens the length of the particle layer so that it is easy for polymers to go through a short porous media. Hence, the diffusion of polymers will control the freezing morphology of the suspension and create dendrites. These results imply that a relatively larger particle size and a moderately higher pulling speed are beneficial for well-developed microstructures in the production of porous ceramics with the freeze-casting method.

External Field Assisted Freeze Casting

Freeze casting under external fields (magnetic, electric, or acoustic) produces porous materials having local, regional, and global microstructural order in specific directions. In freeze casting, porosity is typically formed by the directional solidification of a liquid colloidal suspension. Adding external fields to the process allows for structured nucleation of ice and manipulation of particles during solidification. External control over the distribution of particles is governed by a competition of forces between constitutional supercooling and electromagnetism or acoustic radiation. Here, we review studies that apply external fields to create porous ceramics with different microstructural patterns, gradients, and anisotropic alignments. The resulting materials possess distinct gradient, core–shell, ring, helical, or long-range alignment and enhanced anisotropic mechanical properties.

Interfacial undercooling in solidification of colloidal suspensions: analyses with quantitative measurements

Interfacial undercooling in the complex solidification of colloidal suspensions is of significance and remains a puzzling problem. Two types of interfacial undercooling are supposed to be involved in the freezing of colloidal suspensions, i.e., solute constitutional supercooling (SCS) caused by additives in the solvent and particulate constitutional supercooling (PCS) caused by particles. However, quantitative identification of the interfacial undercooling in the solidification of colloidal suspensions, is still absent; thus, the question of which type of undercooling is dominant in this complex system remains unanswered. Here, we quantitatively measured the static and dynamic interface undercoolings of SCS and PCS in ideal and practical colloidal systems. We show that the interfacial undercooling primarily comes from SCS caused by the additives in the solvent, while PCS is minor. This finding implies that the thermodynamic effect of particles from the PCS is not the fundamental physical mechanism for pattern formation of cellular growth and lamellar structure in the solidification of colloidal suspensions, a general case of ice-templating method. Instead, the patterns in the ice-templating method can be controlled effectively by adjusting the additives.

Interface instability modes in freezing colloidal suspensions: revealed from onset of planar instability

Freezing colloidal suspensions widely exists in nature and industry. Interface instability has attracted much attention for the understandings of the pattern formation in freezing colloidal suspensions. However, the interface instability modes, the origin of the ice banding or ice lamellae, are still unclear. In-situ experimental observation of the onset of interface instability remains absent up to now. Here, by directly imaging the initial transient stage of planar interface instability in directional freezing colloidal suspensions, we proposed three interface instability modes, Mullins-Sekerka instability, global split instability and local split instability. The intrinsic mechanism of the instability modes comes from the competition of the solute boundary layer and the particle boundary layer, which only can be revealed from the initial transient stage of planar instability in directional freezing.