Microspreading studies on rough surfaces by scanning electron microscopy

The use of scanning electron microscopy for direct observation of the effects of surface roughness on the spreading of liquids is described, making it possible to view moving liquid drops at distances less than 1 μm from the advancing contact line. Various surfaces were examined including several with simple forms of roughness which can assist in explaining the behavior of more complex surfaces. Spreading is shown to be highly dependent on the orientation and texture of the roughness; in particular, the presence of sharp edges of step height 0.05 μm are shown to influence spreading significantly. These observations reinforce our previously stated doubts of the significance of conventionally measured macroscopic contact angles.     

Looking at the micro-topography of polished and blasted Ti-based biomaterials using atomic force microscopy and contact angle goniometry

Surface topography of polished and blasted samples of a Ti6Al4V biomaterial has been studied using an atomic force microscope. Surface RMS roughness and surface area have been measured at different scales, from 1 to 50 μm, while at distances below 10 μm the surface RMS roughness in both kinds of samples is not very different, this difference becomes significant at larger scanning sizes. This means that the surface roughness scale that could have a main role in cell adhesion varies depending on the size, shape and flexibility of participating cells. This consideration suggests that in cell–material interaction studies, surface roughness should not be considered as an absolute and independent property of the material, but should be measured at scales in the order of the cell sizes, at least if a microscopic interpretation of the influence of roughness on the adhesion is intended. The microscopic information is contrasted with that coming from a macroscopic approach obtained by contact angle measurements for polar and non-polar liquids whose surface tension is comprised in a broad range. Despite the very large differences of contact angles among liquids for each surface condition, a similar increase for the blasted surface with respect to the polished has been found. Interpretation of these results are in accordance with the microscopic analysis done through the use of a functional roughness parameter, namely the valley fluid retention index, evaluated from the AFM images, which has been shown not to correlate with the RMS roughness, one of the most commonly used roughness parameter.     

Plane hydrodynamic flow in thin free suspended nematic liquid crystal films

Abstract

Radial hydrodynamic flows in free suspended films (with thickness h ≥ 20 μm) of some liquid crystal materials was observed in a narrow temperature range before the line nematic-smectic C transition. The observed flows are explained by a non-linear temperature dependence of the surface tension (Marangoni effect).

Upon thinning the liquid crystal films to a thickness 6 μ < h < 20 μm the hydrodynamic flow changes its character: instead of radial flow with sinking seeding particles we observe two circular plane hydrodynamic flows symmetrical to the film's diameter. The temperature distribution in the free film and the surface tension field are discussed. A model for the established circular flows in thin liquid crystal films is presented.     

Roughness effects of cellulose and paper substrates on water drop impact and recoil

The impact dynamics of water drops on sized and unsized smooth cellulose films and paper surfaces with controlled roughness levels were studied. The objective was to better understand the effect of roughness on the liquid drop impact dynamics on paper surfaces, isolating from the effect chemical heterogeneity. Drop impact in the first few milliseconds were recorded using high-speed CCD camera and the three-phase contact line movement of the water drop was analyzed. Smooth cellulose film surface and rough paper surface showed similar impact dynamics, suggesting that the surface energy plays a more dominant role than surface roughness. Significantly different dynamic contact angles of water drop on the sized and unsized surfaces were observed during drop impact. The Laplace pressure of the curved spreading front pointing to the centre of a spreading drop on these sized cellulose and paper surfaces reduces the three-phase contact line movement, and leads to smaller maximum spreading diameter. Our results suggest that the water drop spreads on the rough surface is most likely via a “roll-over” action rather than “stick and jump” movements.     

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase II_UV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase III_temp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase II_UV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase III_temp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase II_UV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light.     

Oscillating Poiseuille flow in photo-aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^-6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.     

Self-powdering and nonlinear optical domain structures in ferroelastic β′-Gd2(MoO4)3 crystals formed in glass

Ferroelastic β′-Gd₂(MoO₄)₃, (GMO), crystals are formed through the crystallization of 21.25Gd₂O₃–63.75MoO₃–15B₂O₃ glass (mol%), and two scientific curious phenomena are observed. (1) GMO crystals formed in the crystallization break into small pieces with a triangular prism or pyramid shape having a length of 50–500 μm spontaneously during the crystallizations in the inside of an electric furnace, not during the cooling in air after the crystallization. This phenomenon is called “self-powdering phenomenon during crystallization” in this paper. (2) Each self-powdered GMO crystal grain shows a periodic domain structure with different refractive indices, and a spatially periodic second harmonic generation (SHG) depending on the domain structure is observed. It is proposed from polarized micro-Raman scattering spectra and the azimuthal dependence of second harmonic intensities that GMO crystals are oriented in each crystal grain and the orientation of (MoO₄)²⁻ tetrahedra in GMO crystals changes periodically due to spontaneous strains in ferroelastic GMO crystals.     

High sensitive piezoelectric quartz crystal liquid density sensor

<正>We report for the first time a cleavage phenomenon in the resonant peak of a piezoelectric quartz crystal(PQC) in liquid phase.In the presence of a strong longitudinal wave effect,an additional resonant peak appears in the conductance-frequency curve.With gradually increasing liquid density,the additional peak moves from low to high frequency region then disappears.The frequency of the additional resonant peak is sensitive to the change in liquid density.The frequency shift of the additional peak is linear with the liquid density in a given range.For a 5 MHz PQC with a reflection distance of 16 mm for longitudinal wave,the sensitivity to liquid density is 2.61×10~6 Hz g~(-1) cm~3.The overlap between the primary resonant peak and the additional resonant peak causes a decrease in the intensity of the former and an increase in the intensity of the latter.In a combined impedance analysis method,the changes in surface mass loading,density and viscosity of the liquid were monitored simultaneously by a PQC sensor.     

Linear electromechanical effect in a S*c polymer liquid crystal

A linear electromechanical effect, similar to that observed in low-molecular weight S*_c liquid crystals, is reported for a S*_c polymer liquid crystal. A 20 μm thick film of LCP1 (polymer liquid crystal, S*_c, supplied by BDH) was prepared that could be aligned by shearing. An alternating electric field was applied perpendicular and the lateral oscillating motion of the cover glass was measured and analysed. In addition to a linear response, vibrations with the frequency of the applied field, a quadratic response was also observed. The frequency, temperature and applied voltage dependences were measured. The response depends strongly on the alignment. The qualitative analysis of the effects indicates that the oscillations are due to viscous coupling between director reorientation and flow.     

Properties of side chain liquid crystal and amorphous polymers. Applications to non-linear optics

Comb-like liquid-crystalline polymers exhibit many unique properties that challenge not only basic research but also numerous technological opportunities. They combine (partly) the properties of orientation of low molecular weight liquid crystals with the rigidity of polymers. For example, they can be oriented in the mesomorphic state and the structure frozen in a glassy state. These polymers with functionalized pendent groups lead to potential applications in the field of nonlinear optics, or in the domain of electro-optical displays. Other polymers like polysilanes show interesting properties such as photo-conductivity. This paper describes the properties and applications of some new side chain liquid-crystalline polyacrylates and their amorphous copolymers. It also describes the photo-conductive properties of polysilanes and their applications in spatial light modulators with liquid crystals. In the first part of this paper, we describe the properties of liquid crystal copolymers and amorphous polyacrylate copolymers with cyanobiphenyls and/or pendent groups with a large hyperpolarizability. Their different properties are compared with some recent results from the literature. These amorphous copolymers allow one to obtain, after poling in an electric field, high optical non-linear coefficients. We have used these copolymers for the manufacture of an electro-optic modulator working at 1·3 μm in the frequency range of 1 GHz. Applications to second harmonic generation at 1·06 μm are also discussed. In the second part of this paper we describe the photo-conductive properties of polysilanes and the realization and performance of an organic spatial light modulator for optical correlation.     

Frequency response of a quartz crystal microbalance loaded by liquid drops

The frequency response of a quartz crystal microbalance (QCM) in contact with a spreading liquid drop is studied in this paper. An improved model describing the frequency change of the QCM with the shape evolution of the liquid drop with time is proposed based on hydrodynamic analysis, which has not been reported in the literature. It is found that the drop spreading shape, including the base radius and height, has a significant influence on the frequency response of the QCM, resulting in an unexpected increase in the resonant frequency of the QCM. The model shows that the combination of the knowledge about the radial sensitivity of the QCM and the dynamic spreading of the liquid drop is potentially important to optimize the interpretation of the experimental results. The predicted results are verified with experimental results obtained with silicone oil.     

Bubble entrainment with drops

The study reported here was undertaken because recent research on nucleate boiling has implicated vapor entrainment by drops as a mechanism for vapor bubble nucleation. The mechanism has been called secondary nucleation. The purpose of this research was to determine the behavior of entrained air bubbles when a drop of liquid strikes a liquid surface. A liquid drop striking the surface of a pool of the same liquid was found usually to entrain large numbers of small air bubbles. Some of these bubbles are frequently carried rapidly deep into the pool by a vortex ring but many can be deposited in a trail or left floating on the surface. Air bubble entrainment was observed with water and several organic liquids and some differences were noted. Drops with diameters from 200 μm to 4 mm were studied. Sometimes hundreds of bubbles were entrained some with diameters up to 100 μm. These results lend support to the secondary nucleation hypothesis and indicate further research on vapor bubble entrainment under conditions more typical of boiling would be appropriate.     

A thermodynamic interpretation of the behavior of higher fatty alcohols and acids upon the chromatographic paper

An idea was presented of treating the chromatographed substance as a “solute,” and the chromatographic system, composed of the stationary and the mobile phase as a “solvent.” Moreover the concept of “local equilibrium” was introduced, allowing to regard a given chromatographic spot as a “binary solution.” Thus a possibility arose to apply the classical thermodynamic approach, normally used for binary solutions, and namely: μ_i = μ_i + RT ln x_iƒ_i, where μ_i—chemical potential of the “i”-th compound in the solution, μ_i—chemical potential of the pure “i”-th compound, x_i-molar fraction of the “i”-th compound, ƒ_i—its activity coefficient, in a modified form, suitable for the chromatographic purpose.     

A new electrochemical cell adapted to quartz crystal microbalance measurements

As the resonance frequency of an oscillating quartz changes upon the deposition of a given mass to the crystal surface, it can be used as a very sensitive mass measuring device. Despite a growing interest in the use of electrochemical quartz crystal microbalance (EQCM), there is still no commercial available measurement cell which can satisfy all the conditions needed in electrochemical investigations. The design and characteristics of a new electrochemical cell adapted to EQCM measurements are presented. The sensitivity of the microbalance, which is determined by several calibration runs of silver electrodeposition is (183.2±2.9) Hz μg⁻¹ cm². This value, which is close to the theoretical one, confirms the validity of the system we have developed. The calibration procedure and the EQCM using range are then discussed.     

Release of electrolytes from W/O/W double emulsions stabilized by a soluble complex of modified pectin and whey protein isolate

W/O/W double emulsions (DEs) stabilized by charged soluble complexes of whey protein isolate (WPI) and modified pectins were investigated in relation to their stability and the release of two types of electrolytes, NaCl and sodium ascorbate.WPI alone cannot properly stabilize the DEs. The droplet size is relatively large (100 μm) and increases with time. However, addition of modified pectin to form a soluble complex with WPI significantly improved the stability.DEs prepared with two types of oils (medium chain triglycerides (MCT) and R(+)-limonene) were studied by measuring droplet size, creaming, viscosity, and electrolyte release. Irrespective of their very different oil phase nature, both emulsions were stable against coalescence, but R(+)-limonene formed smaller droplets (25 μm) than MCT (35 μm). The electrolyte release rate was significantly higher from the R(+)-limonene that formed DEs with much lower viscosity. R(+)-limonene-DE released 75% of the NaCl after 28 days, while MCT-DE released only 50%. NaCl was released more slowly than sodium ascorbate.Apparently, the release mechanism from R(+)-limonene-DE was found to be “thinning the outer interface and release of the entire inner droplets” while it seems that the release from MCT-DE was slower and “diffusion controlled”.DEs stabilized by WPI/C63 released 12% of the sodium ascorbate after 1 day in milk and remained stable for at least 8 days. However, DEs stabilized with only WPI released about 50% of the sodium ascorbate after 1 day, and phase separated after 8 days.     

XAS and microscopy studies of the uptake and bio-transformation of copper in Larrea tridentata (creosote bush)

Herein we present work directed toward understanding the mechanisms employed by Larrea tridentata (Creosote bush) to uptake and simultaneously defend against the presence of excess copper. The location and nature of copper in the plant have been studied on several length scales: greater than 10 μm (scanning electron microscopy), less than 10 μm (transmission electron microscopy) and atomic level structure and speciation (EXAFS and XANES). Two interesting results are apparent: creosote takes up or adsorbs copper from the soil in the Cu(II) oxidation state and transports it to the leaves where copper is found as Cu(I) and Cu(II). The transport agent appears to be a Cu phytochelatin. Additionally, creosote may be immobilizing and excreting copper via at least two additional mechanisms: storage of metals in vacuoles and excretion of copper into the sticky resinous substance found on the leaf surface. Creosote may also accumulate wind-blown particulates that can easily adhere to the resinous sticky surface of the plant. If, however, the particulates are <10 μm they may enter the leaf by respiration through the plant ‘stomata’ that have openings between 5 μm and 10 μm. As such, creosote may be a natural bio-indicator for airborne particulates that are <10 μm.     

Looking at the micro-topography of polished and blasted Ti-based biomaterials using atomic force microscopy and contact angle goniometry

Surface topography of polished and blasted samples of a Ti6Al4V biomaterial has been studied using an atomic force microscope. Surface RMS roughness and surface area have been measured at different scales, from 1 to 50 μm, while at distances below 10 μm the surface RMS roughness in both kinds of samples is not very different, this difference becomes significant at larger scanning sizes. This means that the surface roughness scale that could have a main role in cell adhesion varies depending on the size, shape and flexibility of participating cells. This consideration suggests that in cell–material interaction studies, surface roughness should not be considered as an absolute and independent property of the material, but should be measured at scales in the order of the cell sizes, at least if a microscopic interpretation of the influence of roughness on the adhesion is intended. The microscopic information is contrasted with that coming from a macroscopic approach obtained by contact angle measurements for polar and non-polar liquids whose surface tension is comprised in a broad range. Despite the very large differences of contact angles among liquids for each surface condition, a similar increase for the blasted surface with respect to the polished has been found. Interpretation of these results are in accordance with the microscopic analysis done through the use of a functional roughness parameter, namely the valley fluid retention index, evaluated from the AFM images, which has been shown not to correlate with the RMS roughness, one of the most commonly used roughness parameter.     

Polymer multilayer film formation studied by in situ ellipsometry and electrochemistry

Polyelectrolyte multilayer films adsorbed on gold surfaces were studied by combined ellipsometric and electrochemical methods. Multilayers were composed of “synthetic” (poly(4-styrenesulfonic acid) ammonium salt (PSS) and poly(allylamine hydrochloride) (PAH) (PSS/PAH)) and “semi-natural” (carboxymethyl cellulose (CMC) and chitosan (CHI) (CMC/CHI)) polyelectrolytes. It was found that only PSS/PAH Layer-by-Layer (LbL) assembled structures result in dense surface confined films that limit permeability of small molecules, such as ferri-/ferrocyanide. The PSS/PAH assemblies can be envisaged as films with pinholes, through which small molecules diffuse. During the LbL deposition process of these films a number of pinholes quickly decay. A representative pinhole diameter was found to be approximately 20 μm, which determines the diffusion of small molecules through LbL films, and yet remains constant when the film consists of a few LbL assembled polyelectrolyte bilayers. CMC/CHI LbL assemblies at gold electrode surfaces give very low density films, which do not limit the diffusion of ferri-/ferrocyanide between the surface of the electrode and the solution.     

Adsorption characteristics of metal-EDTA complexes onto hydrous oxides

The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (γ-Al₂O₃), were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AlOH₂⁺ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the “free” EDTA species were similar to those of the metal complexes. The results also showed that the adsorption of “free” EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (3.5) and exothermic at higher pH values (6.0). This implied that the surface preferred the NiHEDTA⁻¹ species rather than the NiEDTA⁻² species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the γ-Al₂O₃ particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ζ potential measurements. This model proved valid over a wide range of pH (3–10) and for both high (>50% coverage) and low (<10% coverage) surface loadings.     

Switching on stray electric fields in ferroelectric liquid crystal cells

In liquid crystal dot-matrix displays light may leak through the display area between the pixels. To obtain sufficient contrast this non-pixel area has to be made non-transmissive. For ferroelectric liquid crystal (FLC) displays this may be done by switching the material in the gaps between the picture elements to a non-transmissive state by the stray electric fields that occur during application of voltages to the pixel electrodes. This is experimentally studied for test cells with an electrically modified smectic layer structure. The gap region considered is an asymmetric environment of the FLC material, as the transparent conductive coating has been removed on one substrate, whereas on the other substrate a conductor covers the glass. The FLC molecules in the non-pixel area prefer to direct their dipoles towards the covered substrate. To switch the FLC material with the stray electric fields, it is a prerequisite to outweigh this preference. We made spatially resolved observations for various gap widths and various applied voltages on 2 μm thick FLC layers. With bipolar voltage pulses of 64 μs width each, amplitudes of about 25 V are needed to switch the FLC in 3·2 or 4·0 μm wide gaps. It was found to be more difficult to switch gaps that, are 7 μm wide than was anticipated on the basis of the results for 4 μm gaps. This is attributed to the surface polarization charge due to the FLC permanent dipoles built up at the FLC-glass interface. Experimental results supporting this explanation are presented.