Real-time three-dimensional micro-imaging of liquid front in spreading sessile droplet under transient spreading states.

Dynamic wetting behaviors of sessile droplets on substrates play crucial roles for various industrial chemical processes. In the case of complete wetting, it has been proposed that a precursor film which is nanometer-order thickness and micrometer-order length further expands outside the macroscopic contact line of the sessile droplet. While the time evolution of the precursor film is believed to strikingly affect the macroscopic wetting behavior (e.g., spreading velocity), it had been hard to visualize the three-dimensional shape of the precursor film at the early stage of wetting. Further, although the spreading velocity rapidly decreases upon time at the early stage of the wetting (transient state) and then converged to be a constant at later time (steady state), conventional fluid mechanics theories generally describe only the wetting behavior at the steady state. Therefore, experimental observation of time evolution of the precursor film shape in the transient state is essential to proceed the theories to the next step. Here, a monochromatic laser interference microscope was developed to visualize three-dimensional shape of the wetting front of sessile droplets in real time. By detecting an interference of the laser reflected at the liquid and substrate surfaces, the precursor film was successfully visualized with a time resolution of 20 ms and a thickness resolution of about 3.5 nm at worst. For 10 cSt silicone oil on Si substrate, a 60-nm-thick and 70-µm-long precursor film was observed for 2-10 min after dropping and its spreading velocity which decreased with time was quantitatively analyzed.

Analysis of Cation Composition in Dolomites on the Intact Particles Sampled from Asteroid Ryugu.

Characterization of the elemental distribution of samples with rough surfaces has been strongly desired for the analysis of various natural and artificial materials. Particularly for pristine and rare analytes with micrometer sizes embedded on specimen surfaces, non-invasive and matrix effect-free analysis is required without surface polishing treatment. To satisfy these requirements, we proposed a new method employing the sequential combination of two imaging modalities, i.e., microenergy-dispersive X-ray fluorescence (micro-XRF) and Raman micro-spectroscopy. The applicability of the developed method is tested by the quantitative analysis of cation composition in micrometer-sized carbonate grains on the surfaces of intact particles sampled directly from the asteroid Ryugu. The first step of micro-XRF imaging enabled a quick search for the sparsely scattered and micrometer-sized carbonates by the codistributions of Ca2+ and Mn2+ on the Mg2+- and Fe2+-rich phyllosilicate matrix. The following step of Raman micro-spectroscopy probed the carbonate grains and analyzed their cation composition (Ca2+, Mg2+, and Fe2+ + Mn2+) in a matrix effect-free manner via the systematic Raman shifts of the lattice modes. The carbonates were basically assigned to ferroan dolomite bearing a considerable amount of Fe2+ + Mn2+ at around 10 atom %. These results are in good accordance with the assignments reported by scanning electron microscopy-energy-dispersive X-ray spectroscopy, where the thin-sectioned and surface-polished Ryugu particles were applicable. The proposed method requires neither sectioning nor surface polishing; hence, it can be applied to the remote sensing apparatus on spacecrafts and planetary rovers. Furthermore, the non-invasive and matrix effect-free characterization will provide a reliable analytical tool for quantitative analysis of the elemental distribution on the samples with surface roughness and chemical heterogeneity at a micrometer scale, such as art paintings, traditional crafts with decorated shapes, as well as sands and rocks with complex morphologies in nature.

Local elasticity evaluation of acid-denatured collagen by photoacoustic spectroscopy.

While there are various analytical methods for elasticity evaluation, those with micrometer-order spatial resolution are still under developing. As some of biological tissues such as capillary vessels and cochlea are very small and/or highly heterogeneous, development of analytical techniques with such high spatial resolution has been desired for biological and medical purposes. Especially, the elasticity of capillary vessels (several micrometer in diameter) would be an important indicator to find out early diseases. To measure the local elasticity for such small and/or heterogeneous samples, we have proposed an approach based on a temporal waveform of photoacoustic (PA) signal, i.e., time-domain PA. As the time-domain PA contains both the vibrating frequency and the sound propagation time after the excitation, it provides the information on the local elasticity (from the frequency) at a specific depth (from the propagation time) of samples. In the present study, the signal from collagen sheets were obtained and analyzed as models of blood vessel walls and scaffolds for regenerative medicine. In contrast to previous studies using the agarose gel which showed a single frequency peak, the signal from the collagen sheets was mainly composed of two frequency peaks, assignable to surface and bulk vibration. Further, the bulk vibration was found to sensitively reflect the elasticity of the samples. Since the PA effect can be induced only at the position where the light absorber exists, the analytical method proposed here would allow us to measure the local elasticity and its spatial distribution in blood vessels and other tissues.

Real-time observation of the precursor film for low viscosity liquids spreading on solid substrates by a customized differential laser interference microscopy.

While static wettability is well treated with Young's equation via its static contact angle, theoretical analyses for wetting dynamics are not yet reaching consensus due to a singularity of the spreading forces worked at the vapor/liquid/solid contact line. One plausible explanation to overcome the singularity problem is that there is a so-called precursor film spreading outside the apparent contact line. After its first finding in 1919, many researchers have attempted to visualize its shape. However, because its length and thickness are as small as micrometer and nanometer-order, respectively, its visualization still remains a challenging issue especially for low-viscosity liquids. In the present study, we developed a differential laser interference microscope, which has a thickness resolution of approximately 2 nm at the best, and applied it to the wetting front of 10 cSt of silicone oil spreading on a silicon wafer with an almost constant spreading velocity. As a result, the precursor film of 14 µm long and 108 nm thick was clearly visualized. While the macro contact line has a finite advancing contact angle of 4.0°, the gradient of the precursor film surface gradually decreased and converged to ~ 0.1° at the micro-contact angle. The shape of the precursor film was independent of the time after the dropping for the range of 600 s ± 10%, which is consistent to theoretical estimation. The present study demonstrated that our interferometer simultaneously achieved nanometer thickness resolutions, micrometer in-plane spatial resolution, and at least a millisecond temporal resolution with a simple optical setup.

New analytical method for the evaluation of heterogeneity in cation compositions of dolomites by micro-XRF and Raman spectroscopies.

Dolomite (CaMg(CO3)2) is an abundant carbonate mineral contained in sedimentary rocks and plays significant roles in water and carbon cycle in geo/cosmochemical environments. Since the cation compositions of carbonates are sensitive to the aqueous environment where they were precipitated and persisted, quantitative analysis of their cation compositions provides valuable information on the aqueous environments and their changes. The difficulty for the analysis of natural dolomite is that Mg2+ is continuously substituted by Fe2+ or Mn2+, and hence they sometimes possess micrometer-scale heterogeneity. Such heterogeneity carries quite important information on the gradual changes in aqueous environments due to changes in thermodynamic conditions and/or aqueous chemical compositions. In the present study, we explored a new quantitative scale to assess such heterogeneity of cation composition in natural dolomite and ferroan dolomite by combining X-ray fluorescence (XRF) and Raman spectroscopy. While the Fe + Mn content differed spot-by-spot, it was found that the Raman wavenumber and Fe + Mn content linearly correlated with each other. Since the spatial resolution of micro-Raman spectroscopy is as high as 1 µm, it does not require vacuum conditions, and is free from so-called matrix effect faced in other methods utilizing X-Rays and electron beams, the proposed qualitative analytical scale can provide a useful tool to assess the cation compositions in dolomites found in nature.

Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy.

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface-selective vibrational (χ(2)) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the χ(2) (Imχ(2)) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt-concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pKa value (4.8 ± 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Imχ(2) spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

Non-destructive estimation of the cation composition of natural carbonates by micro-Raman spectroscopy.

Carbonates play a crucial role in the water and carbon cycles of both geochemical and cosmochemical environments. As carbonates do not exist homogeneously with other minerals in rocks and sands of various sizes, an analytical method that simultaneously satisfies non-destructivity and high spatial resolution has been desired. Further, the ability of semi-quantitative analysis with carbonates-selectivity and without any pre-treatments is added, for its applicability would be extended to remote sensing for deep sea and outer spaces. Here, we focused on the application of micro-Raman spectroscopy, where the vibrational wavenumbers of the translational (T) and librational (L) modes of carbonates are sensitively related to their cation composition. By comparing the semi-quantitative information obtained by X-ray fluorescence spectroscopy, it was found that these vibrational wavenumbers are approximately linearly related to the cation composition. Consequently, a conversion matrix was proposed to estimate the cation composition from the T and L mode vibrational wavenumbers. This method is universally applicable to any cation composition in carbonates, with no background information on the analyte required. To improve the accuracy, conversion matrices were further optimized to three solid-solution series of carbonates. It is worth noting that the proposed conversion matrices are free from matrix effects and do not depend on the total amount of carbonate in a sample. Therefore, the proposed method provides a useful analytical basis for remote sensing of the cation composition of carbonates, both in terrestrial and extra-terrestrial environments.

Micro-Raman spectroscopic analysis on natural carbonates: linear relations found via biaxial plotting of peak frequencies for cation substituted species.

Carbonates are ubiquitous minerals carrying important information on aqueous environments where they precipitated on the Earth and space. While their ideal chemical formulae are denoted as simple as MCO3 or M1M2(CO3)2 (M: metal cations), natural carbonates generally form solid-solution series and their compositions deviate from the ideal formulae. Since their cation composition due to the substitution provides a sensitive indicator for chemical and thermodynamic environments of aqueous solutions where they precipitated, their composition analysis has been widely carried out from the environmental/geochemical/astrochemical aspects. However, in widely used back-scattered electron and energy dispersion X-Ray analyses, samples should be generally sliced and/or their surface be polished prior to the measurements. For analyzing rare samples with small sizes, such as ones sampled from deep-sea and/or meteorites and asteroids, a non-destructive method without any pretreatments has been strongly desired. Here, a novel analytical method for discriminating various carbonates with Raman micro-spectroscopy is demonstrated, showing that the biaxial plot of the peak frequencies of their lattice modes linearly moves upon partial substitution of the cations. The cation substitution leads to linear movement in the biaxial map, and the slopes of the movement were different for Mg2+-Fe2+ and Mn2+-Fe2+ substitutions. This finding suggests that the micro-Raman analysis would be a non-destructive analytical method for evaluating the relative amount of Mg2+, Fe2+, and Mn2+ in dolomite-ankerite-kutnohorite solid-solution series, as well as Mg2+/Fe2+ ratio for magnesite-breunnerite-siderite. It would be helpful for analyzing the present and past terrestrial and cosmochemical environments.

A hydrogen-bonding structure in self-formed nanodroplets of water adsorbed on amorphous silica revealed via surface-selective vibrational spectroscopy.

Water adsorption onto a material surface is known to change macroscopic surface properties such as wettability and friction coefficient. While the role of the adsorbed water has been discussed for a long time, the interfacial structure of the adsorbed water has not been fully recognized in many cases. In this study, the hydration structure of water adsorbed on a vapor/silica interface at room temperature was studied via heterodyne-detected vibrational sum-frequency generation spectroscopy. The vibrational spectra of the interfacial molecules obtained here were different from those estimated via conventional sum-frequency generation spectroscopy. Interestingly, our results suggest that, at low humidity, the adsorbed water on silica forms nanodroplets instead of a uniform film. Because no silanol group was found to be hydrogen-bonding free, it was concluded that water molecules gather around the silanol group to form strongly hydrogen-bonded droplets. At high humidity, while the adsorbed water partially behaves like a bulk liquid, deprotonation of the silanol was not observed, unlike the case of silica surfaces in contact with bulk liquid water.

Characterization of the Elasticity of Small Objects Buried in Media Based on the Measurements of Photoacoustic Temporal Waveforms.

Since the elasticity of biological tissues is related to their pathological states, the development of new methods allowing for non-invasive measurements of the elasticity has been desired in the medical field. We present a characterization of the elasticity of objects buried in media from the temporal waveforms of photoacoustic signals. As the increment in Young's moduli of the objects, the frequency corresponding to the gravitational center of the power spectra obtained by the Fourier-transformation of the waveforms is increased. In our experiment configuration, the elasticity of buried objects is able to be identified up to about 1 MPa of Young's modulus from the frequency. These results suggest that measurements on the temporal waveforms of photoacoustic signals and the resultant power spectra would provide a useful method for evaluating the elasticity of deeply-situated microscopic pathological lesions, such as stage 0 or 1 mammary gland cancer, which is difficult by conventional ultrasound elastography.

Structure of water and polymer at the buried polymer/water interface unveiled using heterodyne-detected vibrational sum frequency generation.

The structure of the prototypical acrylic polymer (poly(methyl methacrylate): PMMA)/water interface is elucidated at the molecular level using heterodyne-detected sum-frequency generation. Two distinct OH groups of interfacial water are found at the interface: one forms hydrogen bonds with the carbonyl group and the other weakly interacts with the ester methyl group of the polymer surface.

The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy.

Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ(2) spectrum. The obtained Imχ(2) spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ(2) spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M.

Change of the isoelectric point of hemoglobin at the air/water interface probed by the orientational flip-flop of water molecules.

Elucidation of the molecular mechanisms of protein adsorption is of essential importance for further development of biotechnology. Here, we use interface-selective nonlinear vibrational spectroscopy to investigate protein charge at the air/water interface by probing the orientation of interfacial water molecules. We measured the Im χ(2) spectra of hemoglobin, myoglobin, serum albumin and lysozyme at the air/water interface in the CH and OH stretching regions using heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy, and we deduced the isoelectric point of the protein by monitoring the orientational flip-flop of water molecules at the interface. Strikingly, our measurements indicate that the isoelectric point of hemoglobin is significantly lowered (by about one pH unit) at the air/water interface compared to that in the bulk. This can be predominantly attributed to the modifications of the protein structure at the air/water interface. Our results also suggest that a similar mechanism accounts for the modification of myoglobin charge at the air/water interface. This effect has not been reported for other model proteins at interfaces probed by conventional VSFG techniques, and it emphasizes the importance of the structural modifications of proteins at the interface, which can drastically affect their charge profiles in a protein-specific manner. The direct experimental approach using HD-VSFG can unveil the changes of the isoelectric point of adsorbed proteins at various interfaces, which is of major relevance to many biological applications and sheds new light on the effect of interfaces on protein charge.

Hydrogen-bonding interactions of uric acid complexes with water/melamine by mid-infrared spectroscopy.

Hydrogen (H)-bonding interactions of uric acid (UA) with water have been investigated via IR-UV double resonance measurements in the mid-IR region. Comparison of the present results with those obtained previously in the near-IR region enables us to examine microscopic hydration effects that are specific to the H-bonding acceptor sites of UA. It is shown that hydration of the C8O site promotes the mode coupling of this stretch with the C2O stretch. The occurrence of this coupling is manifested in the IR intensity pattern, in which the transition associated with the in-phase contribution C8O + C2O is significantly suppressed, whereas the corresponding out-of-phase contribution gives rise to a strong peak. We also measured the mid-IR spectra of the 1 : 1 complex formed between UA and melamine (MEL) and carried out a structural analysis using the spectroscopic signature of the H-bonding derived from the result of the monohydrated cluster. It is shown that the complex possesses a triple H-bonding structure with the C2O acceptor site of UA H-bonded to MEL. Furthermore, the IR-depleted UV spectroscopy technique was employed in order to ascertain whether other structural isomers are present in the probe UV spectra.

Ab initio studies on the photophysics of uric acid and its monohydrates: role of the water molecule.

The photophysical behavior of three lowest-energy tautomers of uric acid and seven most stable isomers of uric acid monohydrate is comprehensively studied by ab initio calculations. Ground-state energies are calculated with the CCSD(T) method, while excitation and ionization energies as well as excited-state potential energy profiles of photoinduced processes are calculated with the CC2 method. For the (1)ππ* state, it is found that the excitation energy of the monohydrate cluster is significantly lower than that of isolated uric acid when the water molecule is hydrogen-bonded at a specific carbonyl group. The calculated excited-state potential energy profiles suggest that some monohydrate isomers can undergo a migration of the water molecule from one site to another site in the (1)ππ* state with a small energy barrier. It is also found for both uric acid and its monohydrate that nonradiative decay via the NH bond dissociation in the (1)πσ* state is likely to occur at higher excitation energies. On the basis of the computational results, possible mechanisms for the absence of specific isomers of uric acid monohydrate from the resonant two-photon ionization spectrum are discussed.

Controlling Glycosyl Bond Conformation of Guanine Nucleosides: Stabilization of the anti Conformer in 5'-O-Ethylguanosine.

Nucleosides that consist of base and sugar moieties can adopt two main conformations, syn and anti, about the glycosidic bond. We have investigated the conformational properties of guanine nucleosides in the gas phase by using laser desorption combined with IR-UV double resonance spectroscopy. In guanosine, syn conformation is preferred as a result of internal hydrogen bonding between the 5'-OH group of the sugar and the N3 site of the guanine moiety. We have therefore employed a chemically modified nucleoside 5'-O-ethylguanosine, in which possible glycosyl bond conformations are restricted upon ethylation of the 5'-OH group. The result shows that anti conformer is stabilized by the formation of hydrogen bonding involving the 2'-OH group.

Structural identification of uric acid and its monohydrates by IR-UV double resonance spectroscopy.

We have employed IR-UV double resonance spectroscopy to identify the tautomeric and isomeric structures of uric acid and its monohydrated clusters which are produced by the techniques of laser-desorption and supersonic-jet cooling. The IR spectrum obtained for bare uric acid exhibits four distinct NH stretching transitions assignable to those of the most stable triketo form. We have also observed the two most stable monohydrated clusters, each with uric acid in the triketo form and water bonded to either the N3H or N9H site. It is demonstrated that the R2PI spectra of these monohydrates can be separated by using the IR-purified spectroscopic method.

Microhydration of the guanine-guanine and guanine-cytosine base pairs.

Monohydration structures of the guanine-guanine and guanine-cytosine base pairs have been elucidated by IR-UV double resonance spectroscopy combined with ab initio calculations. The systems studied consist of the homodimer of 9-methylguanine and the heterodimer of 9-methylguanine and 1-methylcytosine in which the methyl group is introduced to mimic the presence of the sugar-phosphate backbone and to block specific tautomerization. The monohydrate of the homodimer is identified as that of the most stable symmetric structure formed by the keto tautomers of guanine, which demonstrates that the base pair structure is not influenced by the hydration. It is also shown that at least two structural isomers, one of which retains the Watson-Crick GC pair structure, contribute the monohydrated cluster of the heterodimer. Although stacked base pairs are suggested to be significantly stabilized by the addition of water, the result shows no clear indication for the presence of stacked monohydrates in either homodimer or heterodimer case.

Hydration structures of 2'-deoxyguanosine studied by IR-UV double resonance spectroscopy: comparison with guanosine.

Infrared spectra of mono- and dihydrated clusters of 2'-deoxyguanosine (2'-dGs), which are formed by laser desorption combined with supersonic jet-cooling, have been measured by the technique of IR-UV double resonance spectroscopy. The structures of these hydrates are compared with those reported for guanosine (Gs) to elucidate the importance of the 2'-hydroxy group in the hydration. It is shown that monohydrated structures observed for 2'-dGs are similar to those of Gs, indicating no significant influence by the absence of 2'-OH group. For the dihydrated cluster, two structural isomers are identified and assigned to the dihydrates with the guanine moiety is in the keto form, which are consistent with the lowest-energy structures derived from theoretical calculations. Comparison with the results for Gs suggests that the presence of 2'-OH group leads to the stabilization of specific dihydrate structures involving the sugar group.

IR-UV double resonance spectroscopy of the hydrated clusters of guanosine and 9-methylguanine: evidence for hydration structures involving the sugar group.

Infrared spectra of mono- and dihydrated clusters of guanosine (Gs) formed by laser desorption combined with supersonic jet cooling have been measured by the technique of IR-UV double resonance spectroscopy. The results are compared with those of 9-methylguanine (9MG), in which the sugar group of Gs is replaced with a methyl group, to elucidate the importance of the sugar group in the hydration structures. It is shown that the UV spectrum observed for the monohydrated cluster of Gs is composed of multiple structural isomers of larger stabilities. One of the monohydrates is identified to possess a hydration structure involving the 5'-OH group of the sugar and the amino group of the guanine moiety. The IR spectrum of the dihydrated clusters reveals that the 2'-OH group is significantly influenced by the addition of the second water, which suggests the possibility of specific dihydrate structures for Gs.