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Register a new userTilting behavior of lamellar ice tip during unidirectional freezing of aqueous solutions
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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 which plays a significant role in various domains. However, tilted growth of ice via transition from coplanar to non-coplanar growth in directional solidification has been paid little attention in previous studies and there is misleading explanation of the formation of tilted lamellar ice. Here, we in-situ investigated the variations of tilting behavior of lamellar ice tip under different conditions within a single ice crystal with 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 and enrich our understanding in pattern formation of ice.
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Ice growth from liquid phase has been extensively investigated in various conditions, especially for ice freely grown in undercooled water and aqueous solutions. Although unidirectional ice growth plays a significant role in sea ice and freeze casting, the detailed pattern formation of unidirectionally grown ice in an aqueous solution remains elusive. For the first time, we in situ proved a crossover from lamellar to spongy ice morphologies of a single ice crystal via unidirectional freezing of an aqueous solution. The spongy ice morphology originates from the intersect of tilted lamellar ice and is observed in a single ice crystal, which is intrinsically different from the competitive growth of bi-crystal composed of two differently orientated grains in directional solidification. These results provide a complete physical picture of unidirectionally grown ice from aqueous solution and are believed to promote our understanding of various pattern of ice in many relevant domains where pattern formation of ice crystal is vital.
Freezing of polymer solutions has been extensively investigated from many aspects, especially the complex pattern formation. The cell/dendrite micro-structures are believed to be in the type of diffusion-induced M-S instability. However, the presence of polymer as an impurity in water is significantly different from that of small ions. The quantitative transient investigation on directional freezing of polymer solutions remains lack due to some challenges. For the first time, we observed the planar instability behaviors during unidirectional freezing of a polymer solution together with a typical ionic solution with manipulated ice orientation, and their pattern formation of S/L interface morphology as a function of time in the transient planar instability process has been revealed and compared to each other. It is found with real-time observation that the polymer solution exhibits a global instability mode instead of a local instability mode that is common in ionic solution during planar instability. W-L model was applied to quantitatively address the variation of solute recoil of ionic/polymer solution. And it is found that the W-L model can only reproduce the solute recoil of ionic solution instead of polymer solution, which indicates the complex physics behind freezing of a polymer solution. The paper provides a spectacular contrast of directional freezing process between polymer solution and ionic solution and is believed to promote relevant investigations in terms of the theoretical approach to describing the freezing behavior of polymer solution.
We report on a detailed characterization of complex dielectric response of Na-DNA aqueous solutions by means of low-frequency dielectric spectroscopy (40 Hz - 110 MHz). Results reveal two broad relaxation modes of strength 20<Deltaepsilon_LF<100 and 5<Deltaepsilon_HF<20, centered at 0.5 kHz< u_LF<70 kHz and 0.1 MHz< u_HF<15 MHz. The characteristic length scale of the LF process, 50<L_LF<750nm, scales with DNA concentration as c_DNA^{-0.29pm0.04} and is independent of the ionic strength in the low added salt regime. Conversely, the measured length scale of the LF process does not vary with DNA concentration but depends on the ionic strength of the added salt as I_s^{-1} in the high added salt regime. On the other hand, the characteristic length scale of the HF process, 3<L_HF<50 nm, varyes with DNA concentration as c_DNA^{-0.5} for intermediate and large DNA concentrations. At low DNA concentrations and in the low added salt limit the characteristic length scale of the HF process scales as c_DNA^{-0.33}. We put these results in perspective regarding the integrity of the double stranded form of DNA at low salt conditions as well as regarding the role of different types of counterions in different regimes of dielectric dispersion. We argue that the free DNA counterions are primarily active in the HF relaxation, while the condensed counterions play a role only in the LF relaxation. We also suggest theoretical interpretations for all these length scales in the whole regime of DNA and salt concentrations and discuss their ramifications and limitations.
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, we in-situ observed the unidirectional growth of lamellar sea ice with well-manipulated ice crystal orientation and visualized tip undercooling of sea ice. A semi-empirical model was proposed to quantitatively address the variation of tip undercooling with growth velocity and salinity and compared with a very recent analytical model. With the real-time observation, interesting phenomena of doublon tip in cellular ice growth and growth direction shift of ice dendritic tip were discovered for the first time, which are attributed to the complex solutal diffusion and anisotropic interface kinetics in sea ice growth. The quantitative experiment provides a clear micro scenario of sea ice growth, and will promote relevant investigations of sea ice in terms of the theoretical approach to describing the diffusion field around faceted ice dendritic tip.
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 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 solid/liquid interface morphology evolution of directionally solidified single crystal ice with its C-axis (optical axis) perpendicular to directions of both thermal gradient and incident light in poly (vinyl alcohol, PVA) solutions. Misty 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 exert significant impact on future design and optimization in porous biomaterials.
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