Glass-liquid Interface Research Articles

Laser beam properties and microfluidic confinement control thermocavitation

Thermocavitation, the creation of a vapor bubble by heating a liquid with a continuous-wave laser, has been studied for a wide range of applications. Examples include the development of an actuator for needle-free jet injectors, as the pumping mechanism in microfluidic channels and nanoparticle synthesis. Optimal use in these applications requires control over the bubble dynamics through the laser power and beam radius. However, the influence of the laser beam radius on the bubble characteristics is not fully understood. Here, we present a way to control the beam radius from an optical fiber by changing the distance from the glass–liquid interface. We show that the increase in the beam size results in a longer nucleation time. Numerical simulations of the experiment show that the maximum temperature at nucleation is 237 ± 5 °C and independent of laser parameters. Delayed nucleation for larger beam sizes results in more absorbed energy by the liquid at the nucleation instant. Consequently, a larger beam size results in a faster growing bubble, producing the same effect as reducing the laser power. We conclude that the bubble energy only depends on the amount of absorbed optical energy and it is independent of the beam radius and laser power for any amount of absorbed energy. This effect contrasts with pulsed lasers, where an increase in the beam radius results in a reduction of bubble energy. Our results are of relevance for the use of continuous-wave laser-actuated cavitation in needle-free jet injectors as well as other applications of thermocavitation in microfluidic confinement.

Bessel beam with a micrometer-size central spot and interferometry for small volume bioliquid refractive index measurement.

This work demonstrates an interferometric technique to measure the refractive index (RI) of microliter volume of biosample solutions with the help of an optical fiber negative axicon and Bessel beam with the micrometer-size central spot. The RI measuring device consists of a broadband superluminescent diode (SLD) source along with an optical circulator and spectrometer. The axicon optical fiber probe packaged with a glass slide is connected to a broadband SLD source through an optical circulator, and a drop of microliter bioliquid sample is placed on the packaged probe's head. The reflected light from the glass-liquid interface couples back to the axicon probe and interferes with the reference beam generated at the air-glass interface of the axicon. The interference spectrum is further analyzed by applying the fast Fourier transform to calculate power from the respective interface and solve the Fresnel equation for the RI measurement. The RI of small sample volume ∼2µl of glucose solutions and bovine serum albumin protein solutions are reported as an application of the proposed method. This sensing platform shows promising applications in biomedicine for monitoring small volume various biosample concentrations. We also demonstrate RI measurement of flowing liquid sample using the developed setup in case the design can be considered for microfluidic channel research.

Nanoscale optical trap for fluorescent nanoparticles

Optical trapping has offered new potentialities in the manipulation of bacteria, living cells, organelles, or microparticles in micrometric volumes. Its extension to the nanometer scale may create exceptional opportunities in many areas of science. We report here the trapping of a single 100 nm radius particle in a nanometric volume. The trapping relies on stimulated emission of the fluorescent particle in the evanescent wave of a totally reflected Arago spot on a glass-liquid interface. The trapping volume is a 200 nm height and 125 nm radius cylinder. Such a nano-trap, aside from applications in nano-physics, may echo the needs of life scientists to manipulate ever-smaller biological samples.

Biological verification of the long-range effect for silicon light irradiation for planaria

The effect of hypersonic waves excited by a heterostructure based on a silicon wafer with natural oxide on the survival of planaria after their decapitation has been experimentally investigated. The aim of the work was to study the physical and biological factors affecting the regeneration of planarians. The main object of study was a model organism – planaria Dugesia Tigrina. The planaria were decapitated, and then they were monitored by their habitat for a week. Identification and counting of microorganisms, bacterial microflora inoculation, temperature, and pH control were carried out. To explain the mechanisms that occur under the influence of hypersound, a mathematical model of the passage of hypersonic waves through a thin layer of water near the glass-liquid interface was developed. In the process of regeneration after decapitation of the experimental group, it was found that in the experimental group exposed to hypersound, the survival of planaria was 60% higher than in the control. It was shown that in an aqueous medium along the glass-water interface, hypersonic propagation occurs with less attenuation than in the water column. This leads to a waveguide effect and improved transmission of hypersonic effects to the studied organisms.

Packaged Negative Axicon Optical Fiber Probe and Bessel Beam Interferometry for Refractive Index Measurement of Hazardous Liquid Samples

In this article, we demonstrate the packaging of an optical fiber negative axicon probe generating Bessel beam and its application in the measurement of refractive index (RI) of hazardous liquids. The negative axicon probe is placed inside cascaded capillary tube capped with a glass coverslip of RI ∼1.52 and thickness ∼0.15 mm. The packaged probe, connected to a broadband source through an optical circulator, is dipped in different hazardous liquid samples. The reflected light from the air-glass as well as the glass-liquid interface couples to the negative axicon probe and interferes with the reference beam generated at the air-negative axicon interface. The interference spectra recorded in a spectrometer through the circulator are analyzed applying Fast Fourier Transform (FFT). The FFT helps to separate and measure powers reflected from the different interfaces which are used to solve the Fresnel equation for estimating the RI of hazardous liquid samples, such as HCl, H2SO4 solutions. The resolution achieved is 1*10−4 ± 0.0005. The probe can be useful to measure RI of liquid samples remotely and in hazardous environmental condition.

Electrokinetic motion of a micro oil droplet under a glass slide.

Electrokinetic motion of a micro oil droplet beneath a glass slide was experimentally investigated in this paper. The micro oil droplets were released under the glass slide in an aqueous solution and the motion along the glass slide was measured by a microscope. The experimental results indicate that while the electrokinetic mobility increases with the applied electric field, it decreases with the oil droplet size and the ionic concentration of the aqueous solution, respectively. By changing the zeta potential of the glass-liquid interface using polybrene coating from negative to positive, the direction of the electrokinetic mobility is reversed and the absolute value of the electrokinetic mobility increases significantly. Finally, pH effects were also investigated, and it was found that the electrokinetic mobility of the droplets reaches a maximum at pH=6∼8.

Effect of pre-treatment of crystallized bioactive glass with cell culture media on structure, degradability, and biocompatibility

The silicate glass 45S5 Bioglass® (BG) is a potential scaffold material for bone regeneration because of its excellent bioactivity, biocompatibility and ability to form a strong bond with bone tissues, via the formation of an apatite layer on its surface. The evaluation of in vitro bioactivity in physiological body fluids, whilst challenging, can offer some insights for developing the bone-bonding ability of these glasses in vivo. In this study, we investigated the influence of three different cell culture and tissue fluid-like solutions on the dissolution and calcium-phosphate (CaP) based re-precipitation behaviour at the glass-liquid interface. We also examined pre-treatment of BG with these biological solutions, and how its influence on bone-forming MG-63 osteoblastic cell proliferation, viability and adhesion. The biological solutions used in this comparative study were: commercial cell culture medium (DMEM), a DMEM solution without organic components (DML) and a simulated body fluid (SBF), incorporating TRIS-buffer. Incubation of BG in these solutions over 28 days resulted in differences in weight loss, solution pH and ion release, and the development of CaP-based surface layers. XRD and FT-IR analyses showed clear differences in the characteristics of the CaP-based coating layers formed by the different solutions. The interfacial reactivity between the glass and the solutions depended on the composition and properties of the solutions. The formation of the CaP layer occurred more rapidly in SBF due to the presence of TRIS-buffer, which also significantly accelerated glass dissolution, further reducing the BG mass in SBF. MG-63 osteoblasts proliferated and spread more rapidly across the surfaces of all pre-conditioned BG, compared to fresh BG. The experimental results of this work help clarify differences between in vitro bioactivity of BG observed in cell culture solutions and in vivo BG bioactivity.

A Method To Measure Protein Unfolding at an Air-Liquid Interface.

Proteins are surface-active molecules that have a propensity to adsorb to hydrophobic interfaces, such as the air-liquid interface. Surface flow can increase aggregation of adsorbed proteins, which may be an undesirable consequence depending on the application. As changes in protein conformation upon adsorption are thought to induce aggregation, the ability to measure the folded state of proteins at interfaces is of particular interest. However, few techniques currently exist to measure protein conformation at interfaces. Here we describe a technique capable of measuring the hydrophobicity, and therefore the conformation and folded state, of proteins at air-liquid interfaces by exploiting the environmentally sensitive fluorophore Nile red. Two monoclonal antibodies (mAbs) with high (mAb1) and low (mAb2) surface activity were used to highlight the technique. Both mAbs showed low background fluorescence of Nile red in the liquid subphase and at a glass-liquid interface. In contrast, at the air-liquid interface Nile red fluorescence for mAb1 increased immediately after protein adsorption, whereas the Nile red fluorescence of the mAb2 film evolved more slowly in time even though the adsorbed quantity of protein remained constant. The results demonstrate that hydrophobicity upon mAb adsorption to the air-liquid interface evolves in a time-dependent manner. Interfacial hydrophobicity may be indicative of protein conformation or folded state, where rapid unfolding of mAb1 upon adsorption would be consistent with increased protein aggregation compared to mAb2. The ability to measure protein hydrophobicity at interfaces using Nile red, combined with small sample requirements and minimal sample preparation, fills a gap in existing interfacial techniques.

A new flow pattern map for flow boiling in microfin tubes

The flow patterns during flow boiling of R134a in a horizontal microfin tube have been measured optically in a contiguous glass tube. The microfin tube is made of copper with a total fin number of 60 and a helix angle of 18°. The fin height is 0.25mm and the inner tube diameter at fin root is 8.72mm. The copper tube is 0.5m long. It is heated electrically. The glass tube is made of borosilicate glass. The inner tube diameter is 8.8mm.The experiments have been conducted at the saturation temperature 10°C and the heat flux 10,000 Wm2. The mass flux has been varied between 15 and 300 kgsm2 in steps of 5 and 10kgsm2, respectively. The vapor quality in the glass tube has been varied between 0.1 and 0.9 in steps of 0.1.The optical measurements based on the different intensity of reflected laser light at the phase interfaces. No reflections have been detected at the glass–liquid interface while there have been reflections at the glass–vapor interface. The laser source has been rotated by 360° around the glass tube. Every 2.25° measurements of the intensity have been made.Because of the different wetting behavior flow pattern maps for smooth tubes are not suitable for determining the flow patterns in microfin tubes. Due to the helical structure of the fins there is a new flow pattern, the helix flow, where the liquid phase flows helically through the microfin tube. Because of the large wetted surface this leads to an increase of the heat transfer coefficient.The transition curves for a new flow pattern map for microfin tubes are being derived.

Random walk patterns of a soil bacterium in open and confined environments

We used microfluidic tools and high-speed time-lapse microscopy to record trajectories of the soil bacterium Pseudomonas putida in a confined environment with cells swimming in close proximity to a glass-liquid interface. While the general swimming pattern is preserved, when compared to swimming in the bulk fluid, our results show that cells in the presence of two solid boundaries display more frequent reversals in swimming direction and swim faster. Additionally, we observe that run segments are no longer straight and that cells swim on circular trajectories, which can be attributed to the hydrodynamic wall effect. Using the experimentally observed parameters together with a recently presented analytic model for a run-reverse random walker, we obtained additional insight on how the spreading behavior of a cell population is affected under confinement. While on short time scales, the mean square displacement of confined swimmers grows faster as compared to the bulk fluid case, our model predicts that for large times the situation reverses due to the strong increase in effective rotational diffusion.

Through-Thickness Velocity Profile Measurements in an Elastohydrodynamic Contact

This work presents for the first time through-thickness velocity profiles obtained in an EHL contact by photobleached imaging. The velocity profile was inferred by following the evolution of the shape of a photobleached plug formed through the thickness of the fluorescently doped lubricant, oligomer polybutene (PB), in the contact when shear was applied. The proposed methodology was validated by successfully obtaining the expected linear profile with PB experiencing Couette flow. The methodology was then applied to PB in an EHL contact. The variation of the profiles within the contact area was also investigated. The velocity profile of PB in an EHL contact severely deviates from the common linear assumption and exhibits inhomogeneous shear: three regions of varying shear rate have been observed. The phenomenon is shown to be neither due to thermal nor diffusion effects. PB also shows significant slip at the glass-liquid interface. The amount of slip varies with position in the contact. Possible causes, such as pressure-induced viscosity enhancement, as well as the significance of the findings and the benefits of the technique are discussed. The linear velocity profile in an EHL contact is usually assumed for both the film thickness and friction predictions. The profile has, however, never been measured experimentally until now. This work enables the validation of conventional assumptions and the study of flow heterogeneity of lubricants in a contact. This facilitates an improved understanding of the rheology of confined lubricant and hence more accurate predictions of tribological properties.

Improved glass–PDMS–glass device technology for accurate measurements of electro-osmotic mobilities

We demonstrate an improved glass–PDMS–glass device technology based on reactive ion etch of a thin PDMS layer for microfluidic channel forming and glass to glass bonding. Reactive ion etch process has been optimised to achieve a high etch rate and a minimum surface roughness. Typically, microfluidic channels of 2.5μm height and 50μm width are obtained on a wafer scale, providing a patternable glass–liquid interface larger than 95% of the total channel wall surface. To illustrate the capability of this technique, electrokinetic characterization of soda-lime glass (D-263) surfaces are presented.

Temperature and depth dependence of order in liquid crystal interfaces

We have studied the depth dependence and temperature behavior of the ordering of smectic-A films close to the smectic A-nematic transition, deposited on grated glass. X-ray grazing incidence geometry in reflection mode through the glass substrate was used to characterize the samples. Our results indicate the presence of a structure similar to the helical twist grain boundary phase. The structure has two maxima, one close to the glass-liquid crystal interface and another about 8μm above the surface. The structure at 8μm is the one that dominates at higher temperatures. In addition, we find that order is preserved to temperatures close to the nematic-isotropic transition temperature for the deeper gratings. We find also a dependence of the orientation of the structure with the depth of the grating and the elastic constant of the liquid crystal.

Optical fiber extrinsic refractometer to measure RI of samples in a high pressure and temperature systems: Application to wax and asphaltene precipitation measurements

An optical fiber extrinsic sensor for measurement of changes in the refractive index of liquids confined in chambers for high pressure and temperature experiments is described. One head sensor composed by two fibers is fixed in front of a high pressure and temperature cell filled with the sample. The operation principle is based in the reflectivity dependence in the refractive index of the glass–liquid interface. Excellent results and a sensitivity of 10 −5 RI were obtained for pure liquids. The applicability of the sensor is demonstrated following the changes in the refractive index for pure liquids at different pressure and temperatures and by measuring the asphaltenes and wax precipitation in crude oils under pressure. The extrinsic probe designed for refractive index measurement proves to be a reliable tool for measuring heavy organics deposition in crude oils under high pressures and temperatures where the sample to be measured is not very accessible.

Theory of the acoustic realignment of nematic liquid crystals.

When an ultrasonic wave is applied to a nematic liquid-crystal cell, the molecules change their orientation, leading to a change in the optical intensity transmitted through the cell. Modeling this acousto-optic effect involves three separate theoretical issues: (a) calculating the intensity of sound transmitted through the cell walls into the liquid crystal, (b) determining the consequent realignment of the liquid crystal, and (c) deriving the change in optical transmission through the cell. In this paper, we present a theory that addresses all three of these issues, and thereby reproduces the behavior seen in experiments. The theory shows how the performance depends not only on the liquid-crystal material properties, but also on the geometrical parameters of the system, such as the thickness of the glass walls, thickness of the liquid-crystal layer, angle of the ultrasonic wave, viewing angle, and boundary condition at the glass-liquid crystal interface. The theory predicts that the strong dependence on viewing angle still allows an optical image to be seen for realistic dimensions.

Temperature Dependence of the Buried Glass-Smectic Liquid Crystal Interface

We have studied the temperature behavior of the buried Glass-Liquid Crystal Interface for 8CB films of varying thickness deposited into a grated surface, using grazing incidence X-ray scattering. Our results indicate the presence of both a chevron structure and a structure similar to the helical twist-grain-boundary (TGB) phase the evolve as a function of temperature. We find that smectic order is preserved at temperatures well above the smectic A - nematic transition, especially in films of intermediate thickness. A depth analysis of the films indicates that the intensity of the scattering peak increases close near the center of the intermediate thickness films. This evolution is accompanied by a rotation of the smectic layers due to flow in the sample.

Optical method for two-dimensional ultrasonic detection

Two-dimensional detection of ultrasonic waves is based on pressure-induced changes of optical reflectance at a glass–liquid interface, imaged with a time-gated video camera. The method is used to record optoacoustic waves generated after irradiation of optically absorbing targets with 6 ns long laser pulses. Measurements of absolute pressure values with high temporal and spatial resolution (in the range of 10 ns and 10 μm, respectively) is demonstrated. The sensitivity is varied between 0.19% and 0.81% gray level modulation per bar. The detector plane is optically transparent, making it possible to irradiate the sample through the detector without disturbing the acoustic measurement. Two-dimensional recording of ultrasonic waves is ideally suited for the analysis of acoustic emission from small sources and for optoacoustic imaging of optical absorption differences in an opaque material.

Measurement of Dose Rate at the Interface of Cell Culture Medium and Glass Dishes by Means of ESR Dosimetry Using Thin Films of Alanine

Previous studies on human cervical cancer cells (NHIK 3025) have indicated that the cells, when X-irradiated in suspension, appeared to be more radiosensitive than when they were irradiated attached to glass dishes. However, this result depends on dosimetry, which is difficult in the situation where cells are attached to glass dishes due to backscattering electrons at the glass-liquid interface. Recently developed dosimetry that is based on detection of radiation-induced stable radicals in alanine and uses ESR spectroscopy offers a possibility for more relevant dosimetry at the glass-liquid interface than the previous estimates of doses based on Fricke dosimetry. Thin alanine films (>/=10 microm) were used to measure dose at the interface by irradiating the films while they were placed tightly against the bottom of dishes and covered with 1 mm of wax simulating the medium above cells. Fricke dosimetry was also performed, with different depths of Fricke solution in the dishes, to elucidate the contribution to the dose delivered by backscattering electrons at the glass-liquid interface. A dose rate of 1.9 Gy/min was measured with a thin layer (0.2-0.3 mm) of Fricke solution in petri dishes made of glass. However, this estimate appears to be too high, due to a contribution to dose by short-ranged electrons generated when the X rays passed through a steel lid 4.5 cm above the dishes. Dosimetry using alanine films resulted in dose rates of 1.15 and 0.87 Gy/min at the interfaces of glass-liquid and plastic- liquid, respectively. Hence there is a significant contribution to dose from backscattering electrons on dishes made of glass. The reason for our previous observation of a difference in radiosensitivity between cells irradiated in suspension and cells irradiated attached to glass appears to be a lack of accurate dosimetry at the glass-liquid interface.

Measurement of the complex refractive index of liquids in the infrared using spectroscopic attenuated total reflection ellipsometry: correction for depolarization by scattering

With spectroscopic ellipsometry one can measure the real and imaginary parts of the refractive index of a medium simultaneously. To determine this index in the infrared for a number of technical liquids, use was made of attenuated total internal reflection at the glass-liquid interface of a specially designed prism. This attenuated total reflection approach warrants minimal signal loss and is, for strongly absorbing liquids, the only way to measure the complex refractive index. A surprising phenomenon, observed when BK-7 prism glass was used, is scattering in the vicinity of the absorption wavelengths of the glass. A simple model that can be used to describe the relations among absorption, scattering, and depolarization was successfully used to correct the measurements. Refractive indices for demineralized water, Freon 113, heptane, benzene, gas oil, and crude oil in the wave number range from 5000 to 10,000 cm(-1) (1-2 µm) are presented.

Wetting layers caused by surface ionization in a near-critical binary liquid mixture.

Kayser [Phys. Rev. Lett. 56, 1831 (1986)] has proposed that surface ionization provides the long-range force responsible for the wetting layers seen in the binary liquid mixture ${\mathrm{CS}}_{2}$+${\mathrm{CH}}_{3}$${\mathrm{NO}}_{2}$ on borosilicate glass substrates. We use scaling theory to find the equation of state for the near-critical mixtures, and then derive a free-energy functional for the wetting layer that incorporates surface ionization forces. This model includes critical adsorption effects and is well suited for calculations of wetting layers in the single-phase region. Within the uncertainties of the material properties, reflectivities of the glass-liquid interface derived from the theoretical concentration profiles do agree with experimental reflectivity values for wetting layers in the one-phase region. We confirm that surface ionization produces the dominant long-range force in ${\mathrm{CS}}_{2}$+${\mathrm{CH}}_{3}$${\mathrm{NO}}_{2}$ on glass.