A gel is defi ned as a two-component (solid and liquid), continuous, solid-like material with viscoelastic rheological Crystal Growth of Calcium Carbonate in Hydrogels as a Model of BiomineralizationIn recent years, the prevalence of hydrogel-like organic matrices in biomineralization has gained attention as a route to synthesizing a diverse range of crystalline structures. Here, examples of hydrogels in biological, as well as synthetic, bio-inspired systems are discussed. Particular attention is given to understanding the physical versus chemical effects of a broad range of hydrogel matrices and their role in directing polymorph selectivity and morphological control in the calcium carbonate system. Finally, recent data regarding hydrogel-matrix incorporation into the growing crystals is discussed and a mechanism for the formation of these single-crystal composite materials is presented. Future and is researching crystal growth in gels as a means to form nanocomposites.chains form helices (double or single helices) that subsequently aggregate into three-dimensional (3D) bundles, forming a porous network with fi brous characteristics (Figure 1 a,b). [ 82 , 83 ] Both the gelling and melting temperatures can be tailored by chemical modifi cation such as partial hydroxyethylation. [ 84 ] The mechanical behavior of agarose gels is sensitive to molecular weight and concentration [ 85 ] as well as chemical modifi cation.

Hierarchical porous polymer materials are of increasing importance because of their potential application in catalysis, separation technology, or bioengineering. Examples for their synthesis exist, but there is a need for a facile yet versatile conceptual approach to such hierarchical scaffolds and quantitative characterization of their nonperiodic pore systems. Here, we introduce a synthesis method combining well-established concepts of macroscale spinodal decomposition and nanoscale block copolymer self-assembly with porosity formation on both length scales via rinsing with protic solvents. We used scanning electron microscopy, small-angle x-ray scattering, transmission electron tomography, and nanoscale x-ray computed tomography for quantitative pore-structure characterization. The method was demonstrated for AB- and ABC-type block copolymers, and resulting materials were used as scaffolds for calcite crystal growth.

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Biomineralization strategies include the use of hydrogels to direct the formation of composite, single-crystal-like structures with unique structure-property profiles. Application of similar synthetic approaches to transition-metal oxides has the promise to yield low-temperature routes to hierarchically structured crystals that are optimized for a range of applications. Here, growth of hematite (α-Fe2O3) within a silica hydrogel resulted in hierarchical, mosaic crystals preferentially expressing catalytically active {110} facets, which are absent in solution-grown controls. Quantitative structural and compositional analysis reveals architectural changes that begin with the incorporation of silicon into the hematite lattice and propagate through to the nanoscale domain structure and assembly, leading to microscale morphologies that show improved photocatalytic performance. This work demonstrates the potential of applying bioinspired crystallization techniques to design functional oxides with multiscale architectural features.

Scanning transmission electron microscopy (STEM) allows atomic scale characterization of solid-solid interfaces, but has seen limited applications to solid-liquid interfaces due to the volatility of liquids in the microscope vacuum. Although cryo-electron microscopy is routinely used to characterize hydrated samples stabilized by rapid freezing, sample thinning is required to access the internal interfaces of thicker specimens. Here, we adapt cryo-focused ion beam (FIB) "lift-out," a technique recently developed for biological specimens, to prepare intact internal solid-liquid interfaces for high-resolution structural and chemical analysis by cryo-STEM. To guide the milling process we introduce a label-free in situ method of localizing subsurface structures in suitable materials by energy dispersive X-ray spectroscopy (EDX). Monte Carlo simulations are performed to evaluate the depth-probing capability of the technique, and show good qualitative agreement with experiment. We also detail procedures to produce homogeneously thin lamellae, which enable nanoscale structural, elemental, and chemical analysis of intact solid-liquid interfaces by analytical cryo-STEM. This work demonstrates the potential of cryo-FIB lift-out and cryo-STEM for understanding physical and chemical processes at solid-liquid interfaces.

The mechanics underlying ice–skate friction remain uncertain despite over a century of study. In the 1930s, the theory of self-lubrication from frictional heat supplanted an earlier hypothesis that pressure melting governed skate friction. More recently, researchers have suggested that a layer of abraded wear particles or the presence of quasi-liquid molecular layers on the surface of ice could account for its slipperiness. Here, we assess the dominant hypotheses proposed to govern ice–skate friction and describe experiments conducted in an indoor skating rink aimed to provide observations to test these hypotheses. Our results indicate that the brittle failure of ice under rapid compression plays a strong role. Our observations did not confirm the presence of full-contact water films and are more consistent with the presence of lubricating ice-rich slurries at discontinuous high-pressure zones (HPZs). The presence of ice-rich slurries supporting skates through HPZs merges pressure-melting, abrasion and lubricating films as a unified hypothesis for why skates are so slippery across broad ranges of speeds, temperatures and normal loads. We suggest tribometer experiments to overcome the difficulties of investigating these processes during actual skating trials.

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The use of an inorganic hydrogel as a means to modulate the hierarchical architectures of oxide compounds requires an understanding of the effect of the matrix on intermediate phases. In this work, we report on the crystallization of akaganeite (β-FeOOH), both within a silica hydrogel and from aqueous solution, with a focus on understanding the chemical effects of pH, [Fe 3+ ], and [Cl − ], in concert with the physical effects of the silica hydrogel, on the ultimate formation of hematite (α-Fe 2 O 3 ). A distinct physical consequence of the hydrogel crystallization microenvironment is the stabilization of akaganeite as three-dimensional assemblies, in contrast to the discrete rods that form in solution. Chemically, we find that [Fe 3+ ] affects the size of akaganeite crystals, while [H + ] determines the aspect ratio. We also identify that crystal splitting is correlated to high [Cl − ]. In addition, we demonstrate that planar, branched aggregates of akaganeite rods are favored at high [H + ] and are associated with a pathway to hematite that proceeds through the goethite polymorph (α-FeOOH). With these results, we highlight the physical and chemical variables of the crystallization microenvironment that dictate the structural features of akaganeite crystals and their corresponding hematite forms.