(L. Regev).Keywords: Calcium carbonate, infrared spectroscopy, pyrotechnology, diagenesis, ash, plaster AbstractInfrared spectrometry is a well established method for the identification of minerals. Due to its simplicity and the short time required to obtain a result, it can be practiced on-site during excavation using portable infrared spectrometers. However, the identification of a mineral may not be sufficient. For example, a lime plaster floor and a crushed chalk surface have similar appearance and are composed of the same mineralcalcite. Here we exploit differences in the infrared spectra of geogenic, biogenic and pyrogenic calcites for the identification of each calcite type. The infrared calcite spectrum has three characteristic peaks in the region of 400-4000 cm -1 , designated ν 2 , ν 3 , and ν 4 . Manuscript Click here to view linked References2 When a calcite sample is ground, as part of the measurement preparation procedure, some grinding dependent changes will be revealed in the infrared spectrum. With additional grinding, the ν 3 peak narrows and the heights of the ν 2 and ν 4 peaks decrease, when both are normalized to the ν 3 height. By plotting the normalized heights of the ν 2 versus the ν 4 of several grindings of the same sample, a characteristic trend line is formed for each calcite type. The trend lines of geogenic calcites have the mildest slopes and highest ν 4values when compared to pyrogenic calcites, which can be further divided to ash and plaster/mortar samples. This method can assist in the differentiation of the various calcites, including insights on homogeneity and preservation state of the calcitic materials in question.
Perfect crystallinity is defined as three-dimensional, atomic-level periodic order in a material.[1] The degree of crystallinity can be strongly affected by the specimen's formation process and often plays an important role in its resulting chemical and mechanical properties.Calcium carbonate (CaCO 3 ), for example, exhibits a wide range of gradually changing crystallinity. The two extremes of this range are single-crystalline calcite, which exhibits a single periodic order across macroscopic distances, and amorphous calcium carbonate (ACC), [2] which exhibits a degree of short-range order but a lack of registry between adjacent local units that destroys all long-range periodic order.In biogenic, geogenic, and anthropogenic materials, variations in crystallinity and local atomic order have each provided valuable insights into material formation pathways.[3]Extracting useful and reliable material structure information, however, usually requires careful and time intensive sample preparation and often highly specialized equipment. Here,
We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of-but distinct from-two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns. DOI: 10.1103/PhysRevE.77.050402 PACS number͑s͒: 82.70.Dd, 64.70.pv, 64.75.Yz The self-assembly of colloidal microspheres has been used to address the fundamental questions of how materials crystallize ͓1-6͔ or fail to crystallize ͓7-9͔. Micrometer-scale colloidal crystals can be used as a template that, using further processing methods, can be used to create photonic materials ͓10-12͔, optical sensors ͓13͔, and antireflection coatings ͓14͔. However, the high density of missing-sphere defects and cracks in photonic crystals produced via self-assembly ͓15͔ remains a serious limitation, and thus the study of colloidal defects ͓16͔ is an active area of research. Spin-coating of colloidal suspensions is the quickest and most reproducible method to make large-area colloidal crystals. While spin-coating has been proposed to fabricate single crystals for photonic applications ͓17,18͔, the symmetric radial optical interference patterns observed are unexpected for single crystals. We find here that spin-coated colloidal films are indeed neither single crystals nor powder polycrystals, but are in fact a unique polycrystal phase. While true singledomain sizes are ϳ10 m, there is orientational correlation on the centimeter scale. Our results demonstrate a novel crystal packing strategy by which long-range orientational order develops in the absence of long-range positional order, reminiscent of two-dimensional colloidal hexatic phases ͓19,20͔, and leading to crack-free crystals. Distinct from colloidal hexatic phases, our polycrystals exhibit centimeterscale orientational order, which arises due to the spinning axis and can be produced with fourfold, sixfold, or mixed symmetries for a range of thicknesses as a function of spin speed. The electrodeposition of magnetic material through colloidal polycrystals demonstrates their feasibility for material templating applications.The standard technique to make large-area close-packed crystals is controlled ͑vertical͒ drying, utilizing capillary forces ͓21-23͔ to direct self-assembly. Other external shear ͓24͔, electric ͓25͔, electrohydrodynamic ͓26,27͔, and gravitational forces ͓28͔ have also been used. Making dried colloidal crystals with these methods is slow, taking from hours to days. Spin-coating has been shown to be a robust technique ͓17,18,29,30͔ to make large-area colloidal crystals in minutes. In this work, we ...
We demonstrate a correlation between how an IR-active vibrational mode responds to temperature changes and how it responds to crystallinity differences. Infrared (IR) spectroscopy was used to track changes in carbonate-related vibrational modes in three different CaCO3 polymorphs (calcite, aragonite, and vaterite) and CaMg(CO3)2 (dolomite) during heating. Of the three characteristic IR-active carbonate modes, the in-plane bending mode (ν4) shows the most pronounced changes with heating in polymorphs that have planar carbonate arrangements (calcite, aragonite, and dolomite). In contrast, this mode is virtually unchanged in vaterite, which has a canted arrangement of carbonate units. We correlate these trends with recent studies that identified the ν4 mode as most susceptible to changes related to crystallinity differences in calcite and amorphous calcium carbonate. Thus, our results suggest that studies of packing arrangements could provide a generalizable approach to identify the most diagnostic vibrational modes for tracking either temperature-dependent or crystallinity-related effects in IR-active solids.
Spin coating is an out-of-equilibrium technique for producing polymer films and colloidal crystals quickly and reproducibly. In this review, we present an overview of theoretical and experimental studies of the spin coating of colloidal suspensions. The dynamics of the spin coating process is discussed first, and we present insights from both theory and experiment. A key difference between spin coating with polymer solutions and with monodisperse colloidal suspensions is the emergence of long range (centimeter scale) orientational correlations in the latter. We discuss experiments in different physical regimes that shed light on the many unusual partially-ordered structures that have long-range orientational order, but no long-range translational order. The nature of these structures can be tailored by adding electric or magnetic fields during the spin coating procedure. These partially-ordered structures can be considered as model systems for studying the fundamentals of poorly crystalline and defect-rich solids, and they can also serve as templates for patterned and/or porous optical and magnetic materials.
We have developed a simple and economical procedure to demonstrate the effects of roughness and wetting fraction on the equilibrium contact angles of liquid droplets on solid surfaces. Contact angles for droplets placed on a rough surface, which wet only a portion of the surface, are larger than the contact angles of droplets formed by condensation of steam, which wet the surface more completely. These contact angle data facilitate assessments of changes in true surface area, due to surface roughening, as well as changes in the fractional contact areas of the water droplets, due to the formation of air pockets between the rough surface and the droplet.
The spin-coating of colloidal suspensions is an inherently nonequilibrium process that gives rise to highly reproducible, but polycrystalline, films with different symmetries depending on experimental parameters. In this study, we explore the transient dynamics of evaporative colloid spin-coating for the first time, via a combination of high-speed imaging, atomic force microscopy, static photography, and scanning electron microscopy. As the wet colloidal film thins and dries, we observe several symmetry transitions, while at the same time remarkably, the thinning rate (in nondimensional time units) collapses to one universal curve for all rotation rates. We correlate static and dynamic measures of crossovers in ordering regimes, and obtain an estimate of the evaporation rate in the late stage of drying. We conclude that the thinning dynamics controls the local volume fraction and stress profiles, which in turn drives the structural transitions. SECTION Macromolecules, Soft Matter C olloidal self-assembly is a facile and promising method for making photonic crystals, 1 but the control of defects is a challenge. Nonequilibrium approaches to colloidal self-assembly are likely crucial to the making of large-area colloidal crystals.2,3 Colloid spin-coating has recently emerged as a highly robust and reproducible nonequilibrium method to make multilayer colloidal crystalline films. [4][5][6] However, in spite of its robustness, the spin-coating route to colloidal crystals is fraught with challenges. The highly uniform structural colors exhibited by these films have been shown to arise from a polycrystal where different crystallites exhibit longrange orientational order. 7 In order to develop strategies to produce crystals with a greater degree of translational order, a deeper understanding of the dynamical mechanisms of structure formation is required.While the dynamics of spin-coating has been studied extensively and quantitatively in simple one-component fluids 8,9 as well as in polymer solutions, 10,11 the study of the dynamics of colloid spin-coating has been limited to numerical studies of thinning rates.12 A recent work demonstrates that spin-coating flows (in the absence of evaporation) control the local stress profiles, and drive crystallization when the Peclet number exceeds a critical value. 6 Two questions that have remained unaddressed;how the dynamics interlinks with the emergence of different symmetries, and the role played by evaporation;are the focus of the experiments reported here.We begin with our observations of the transient dynamics. Using high-speed imaging and the lighting configuration A (Figure 6), we are sensitive to changes in long-range orientational order via the appearance of symmetries in the optical reflections. In all experiments (at varying rotation speeds) common dynamical features are observed in the fluid phase. While the results presented here pertain to experiments using methyl ethyl ketone (MEK) as solvent, similar results were found with acetone as solvent. This phase...
ABSTRACT. The Pre-Pottery Neolithic Β (PPNB) site of Yiftahel, Israel, contains abundant plaster floors. We surveyed the states of preservation of the plasters using an infrared spectroscopic assay that characterizes the extent of disorder of the atoms in the calcite crystal lattice. We identified the 3 best-preserved plaster samples that had disorder signatures most similar to modern plaster. We then studied the surface layers, fine-grained matrices, and large aggregates of these samples using micromorphology, Fourier transform infrared (FTIR) microscopy, stable carbon and radiocarbon concentrations. Even though some of the plaster components have a geogenic appearance in micromorphology slides and in FTIR spectra, the 14 C analyses show that all components were exposed to high temperatures and as a result were equilibrated with the 14 C content of the atmosphere ~ 10,000 yr ago. This implies that the plasters at Yiftahel were produced entirely from heat-altered calcite. We also show that these plasters have undergone significant diagenesis. The plaster component with the most disordered atomic signature, and hence the most similar in this respect to modern plaster, did indeed produce a 14 C date close to the expected age.
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