Accumulating microparticles and direct-writing micropatterns using a continuous-wave laser-induced vapor bubble

Through the enhanced photothermal effect, which was achieved using a silver film, a low power weakly focused continuous-wave laser (532 nm) was applied to create a vapor bubble. A convective flow was formed around the bubble. Microparticles dispersed in water were carried by the convective flow to the vapor bubble and accumulated on the silver film. By moving the laser spot, we easily manipulated the location of the bubble, allowing us to direct-write micropatterns on the silver film with accumulated particles. The reported simple controllable accumulation method can be applied to bimolecular detection, medical diagnosis, and other related biochip techniques.     

Condensation of analyte vapor species in graphite furnace atomic absorption spectrometry

A shadow spectral digital imaging technique (SSDI) with charge coupled device (CCD) camera detection was used to investigate the spatial and temporal distribution of condensation clouds of analyte species generated in a graphite furnace during the atomization of 5–40-μg masses of metals. Complex, non-uniform structures of aerosol species located away from both the graphite tube axis and walls are prevalent. Species giving rise to observed signals are likely clusters of metals in the case of Ag, Au and Pd, and oxide aerosols for elements such as Al, Ca and Mg. Source scatter often arises during the atomization of relatively large masses of analytes (or during the atomization of real samples containing high concentrations of concomitant matrix species) in the graphite furnace due to such aerosol formation. The SSDI technique is an extremely useful aid to the elucidation of this phenomenon and imaging at several wavelengths using both line and continuum sources in combination with (thermal, incandescent) emission from these structures permits a more complete picture to be developed.     

Disjoining pressure and capillarity in the constrained vapor bubble heat transfer system

Using the disjoining pressure concept in a seminal paper, Derjaguin, Nerpin and Churaev demonstrated that isothermal liquid flow in a very thin film on the walls of a capillary tube enhances the rate of evaporation of moisture by several times. The objective of this review is to present the evolution of the use of Churaev's seminal research in the development of the Constrained Vapor Bubble (CVB) heat transfer system. In this non-isothermal "wickless heat pipe", liquid and vapor flow results from gradients in the intermolecular force field, which depend on the disjoining pressure, capillarity and temperature. A Kelvin-Clapeyron model allowed the use of the disjoining pressure to be expanded to describe non-isothermal heat, mass and momentum transport processes. The intermolecular force field described by the convenient disjoining pressure model is the boundary condition for "suction" and stability at the leading edge of the evaporating curved flow field. As demonstrated by the non-isothermal results, applications that depend on the characteristics of the evaporating meniscus are legion.     

Condensation of supersaturated n-butanol vapor on charged/neutral nanoparticles of D-mannose and L-rhamnose

The effects of size, charge, and solubility on the condensation of supersaturated n-butanol vapor on monodisperse nanoparticles of D-mannose and L-rhamnose are investigated in a flow cloud chamber. The dependence of the critical supersaturation S(cr) on particle size in the range from 30 to 90 nm is determined experimentally. The results show that the experimental S(cr) decreases with increasing particle size and solubility, qualitatively in agreement with the prediction by the Volmer theory of nucleation on soluble particles and by the Kohler theory, but quantitatively smaller than both theoretical predictions. The condensation of supersaturated vapor on singly positive/negative charged particles with diameters of 30, 60, and 90 nm is examined, and no obvious charge effect and sign preference are observed.     

Condensation of supersaturated water vapor on charged/neutral nanoparticles of glucose and monosodium glutamate

The effects of size, charge, dissolution, and dissociation on the condensation of supersaturated water vapor on monodisperse nanoparticles of glucose and monosodium glutamate (MSG) were investigated in a flow cloud chamber (FCC). The dependence of the critical supersaturation, S(cr), on particle size in the range of 30 to 90 nm and on temperature in the range of 10 to 50 degrees C were determined experimentally. The results show that the experimental S(cr) decreases with increasing particle size at a rate in reasonable agreement with the predictions of the Kohler and Volmer theories of nucleation for soluble particles, but decreases with increasing temperature at a rate higher than the prediction of the Volmer theory. The dissociation of MSG into ions lowers the experimental S(cr) to a value smaller than that for the more soluble glucose, agreeing with predictions. The experimental S(cr) is smaller than the predictions of both theories, and the discrepancy cannot be fully explained by the reductions in surface tension due to the dissolution of particles and curvature dependence. The condensation of supersaturated vapor on singly positively charged particles with diameters of 30, 60, and 90 nm was also examined, and no obvious charge effect on S(cr) was observed.     

Single molecule studies of quantum dot conjugates in a submicrometer fluidic channel

A microfluidic and optical system was created for the detection and analysis of single molecules in solution. Fluidic channels with submicrometer dimensions were used to isolate, detect and identify individual quantum dots conjugated with organic fluorophores. The channels were fabricated in fused silica with a 500 nm square cross section. The resulting focal volume of approximately 500 aL reduced fluorescent background and increased the signal to noise ratio of single molecule detection. The channels also enabled the rapid detection of 99% of quantum dots and organic fluorophores traversing the focal volume. Conjugates were driven through the channels electrokinetically at 2.3 kV cm(-1), excited with a single 476 nm wavelength laser and detected with a confocal microscope. Fluorescence emission was collected simultaneously from green (500-590 nm) and red (610-680 nm) regions of the spectrum. Signal rejection was minimized by the narrow and symmetric emission spectra of the quantum dots. To demonstrate efficient multicolor detection and characterization of single molecule binding, Qdot 655 Streptavidin Conjugates were bound to Alexa Fluor 488 molecules and individually detected. Photon counting histogram analysis was used to quantify coincident detection and degree of binding. Fluorescence correlation spectroscopy was used to measure the mobility of bound and unbound species. The union of fluidic channels with submicrometer dimensions and quantum dots as fluorescent labels resulted in efficient and rapid multiplexed single molecule detection and analysis.     

Condensation reactions of a comenaldehyde derivative

Condensation reactions of comenaldehyde methyl ether (I) with malonic acid, ethyl cyanoacetate, and cyanoacetamide to give β-(5-methoxy-4H-pyran-4-on-2-yl)acrylic acid (II), ethyl 2-cyano-3-(5-methoxy-4H-pyran-4-on-2-yl)propenoate (III), and 2-cyano-3-(5-methoxy-4H-pyran-4-on-2-yl)propenamide (IV), respectively, are described. Ultraviolet absorption spectra for 2-hydroxymethyl-5-methoxy-4H-pyran-4-one, I and II are presented.     

The dynamics of a bubble ensemble in a cavitation field

A system of equations was obtained to describe the dynamics of bubbles in a cavitation cloud taking into account the interaction of pulsating bubbles involved in translational motion. The kinetics of cavitation bubble concentration changes, changes in the compressibility of the liquid, and phase transitions within a cavitation bubble and in the neighboring volume of the liquid were taken into account. The role played by bubble deformation in a cavitation cloud was considered. The Bernoulli pressure effect was shown to be negligible. The interaction of cavitation bubbles was a substantial factor that strongly influenced the dynamics of bubbles. It was suggested that there was at least one more mechanism that reduced sonoluminescence intensity from the multiple-bubble cavitation field, namely, a fairly high efficiency of sonoluminescence quenching could additionally be related to the arrival of a cumulative liquid stream at the central cavitation bubble region, where the concentration of active species was high. The dynamics of bubbles in the cavitation field is not only related to the expansion and compression of cavitation bubbles in the acoustic field, but also governed to a great extent by their interaction, translational motion, deformation, and the influence of cumulative streams penetrating the bubbles.     

Parametrical studies of electroosmotic transport characteristics in submicrometer channels

Spatially two-dimensional nonequilibrium mathematical model describing electroosmotic flow through a submicrometer channel with an electric charge fixed on the channel walls is presented. This system is governed by the hydrodynamic, electrostatic, and mass transport phenomena. The model is based on the coupled mass balances, Poisson, Navier-Stokes, and Nernst-Planck equations. Nonslip boundary conditions are employed. The effect of an imposed electric field on the system behavior is studied by means of a numerical analysis of the model equations. We have obtained the following findings. If the channel width is comparable to the thickness of the electric double layer, the system behaves as an ion-exchange membrane and the dependence of the electric current passing through the channel on the applied voltage is strongly nonlinear. In the case of negatively (positively) charged walls, a narrow region of very low conductivity (so-called ionic gate) is formed in the free electrolyte near the channel entry facing the anode (cathode) side. For a wide channel, the electric current is proportional to the applied voltage and the velocity of electrokinetic flow is linearly proportional to the electric field strength. Complex hydrodynamics (eddy formation and existence of ionic gates) is the most interesting characteristics of the studied system. Hence, current-voltage and velocity-voltage curves and the corresponding spatial distributions of the model variables at selected points are studied and described in detail.     

Control parameters of flame spreading in a fuel container

The processes involved in flame spreading over liquid fuels are subject of this work. A heat and momentum transfer analysis has been undertaken for fuel temperatures below the flash-point that confirms (within this range of temperatures studied in this work) that flame spreading is assisted by a convection pattern ahead of the flame. This assistance mechanism, which is not observed for solid fuels, is the origin (for lower temperatures) of a pulsating behaviour of the flame. A first experimental determination of the characteristic horizontal length of this assistance zone will be given. The analysis of our data lead us to conclude that flame spreading can be reduced by simultaneously preventing the formation of the convection zone and reducing the fuel surface temperature.     

Sonochemiluminescence from a single cavitation bubble in water

Single bubble feels the pressure: Sonochemical luminescence has been detected in a single-cavitation bubble within a narrow pressure domain below the sonoluminescence threshold. The parameter space of single-bubble sonochemistry is distinct from that of single-bubble atomic and molecular line emissions.     

Imaging of copper oxygenation reactions in a bubble flow

Reactions of gases with liquids play a crucial role in the production of many bulk chemicals. Often, the gas is bubbled into the chosen reactor. Most of the processes at the gas–liquid interface of the bubbles and in their tails are not fully understood and warrant further investigation. For this purpose, NMR imaging or Magnetic Resonance Imaging has been applied to visualize some of the processes in the bubble tail. To generate sufficient contrast, a magnetogenic gas–liquid reaction associated with a change of magnetic state, from diamagnetic to paramagnetic, was employed. In this work, a copper(I)‐based compound was oxidized to copper(II) to exploit relaxation contrast. To match the speed of the rising bubbles to the acquisition time of the spin‐echo imaging sequence, polyethylene glycol was added to increase the viscosity of the reacting solution. Images of the oxygen ingress into a static solution as well as of oxygen bubbles rising in the solution are presented. In both cases, changes in magnetism were observed, which reported the hydrodynamic processes.     

Condensation of a non-wetting fluid on a solid surface

Theoretical studies of a drop moving under condensation from the surrounding vapor, have been provided. Two cases are considered. In the first, the rate of condensation is large that the drop "moves" because condensation has changed its dimensions. The model provided here shows that the rate of spreading is a constant, proportional to the heat flux and inversely proportional to the macroscopic contact angle. This compares well with available experimental data. The other model where the rate of condensation is small, is taken from existing results and comes close to explaining one set of experimental data. It is based on the use of viscous forces as the primary rate mechanism. Its shortcomings have been discussed.     

Condensation phenomena in nanopores: a Monte Carlo study

The nonequilibrium dynamics of condensation phenomena in nanopores is studied via Monte Carlo simulations of a lattice-gas model. Hysteretic behavior of the particle density as a function of the density of a reservoir is obtained for various pore geometries in two and three dimensions. The shape of the hysteresis loops depend on the characteristics of the pore geometry. The evaporation of particles from a pore can be fitted to a stretched exponential decay of the particle density. Phase-separation dynamics inside the pore is effectively described by a random walk of the non-wetting phases. Domain evolution is significantly slowed down in the presence of a random wall-particle potential and gives rise to a temperature-dependent growth exponent. A geometric roughness of the pore wall only delays the onset of a pure domain growth.     

Water vapor transport in a series of polyarylates

Water vapor sorption and transport properties are presented for a series of polyarylates. The effect of making various structural changes in the polymer backbone has been studied, by comparing the transport properties to those of the base polymer bisphenol A isophthalate. All the polymers exhibit an upturn in the sorption isotherm at moderate to high activities, which in most cases is caused by plasticization of the polymer by water vapor. The response of water vapor to the structural changes is compared to that of the permanent gases reported previously for these materials. A good correlation is established between the diffusion and permeability coefficients and the fractional free volume. A similar behavior is seen in gases. However, the solubility coefficient, which decreases with increase in the fractional free volume, shows an opposite effect. Water vapor has the ability to hydrogen bond with itself and interact strongly with the polymer. In the case of solubility, the water vapor–polymer interaction plays an important role and overrides the effect of the increase in fractional free volume. The structural changes described here have similar effects on the permeation of water in a series of polysulfones reported earlier, as seen here for the polyarylates.     

Interference technique in grazing-emission electron probe microanalysis of submicrometer particles

The distribution of X-ray radiation from a small particle, situated on a flat support and excited by an electron beam, is analyzed theoretically. The interference pattern, appearing within the region of grazing take-off angles, is shown to contain valuable information about the particle structure and composition. It also provides the resources for extracting this information that are valid even in conditions of extremely low signals (e.g. for very small samples), when the effect of background suppression for angles of the total reflection region is insufficient. Characteristic elements of the interference pattern are found that are relatively independent on the noise caused by the support radiation. Algorithms for this noise correction and elimination are proposed. An approximate formalism for the particle structure retrieval, based on the Fourier-transformation properties, is described. Potential applications of the proposed technique are illustrated with numerical models of spherical and hemispherical particles, both homogeneous and of ‘core-and-shell’ structures, with total sizes between 30 and 200 nm. The additional possibilities of studying the shape, structure and elemental composition of small particles have been demonstrated. The general conditions of the experimental realization of proposed methods are discussed.     

Analysis of pressure-driven air bubble elimination in a microfluidic device

We report an analysis of pressure-driven bubble elimination for a gas-permeable microfluidic device. In this study, we described bubble elimination in a microfluidic device employing a gas permeation model and calculated the removal efficiency of bubbles. The correction factor for the simplified model was estimated with respect to the applied pressure. Based on the established model, the required time to remove a trapped bubble with a certain area was shown to be within an error of 11.58% by comparison with experimental results. Exploiting the model equation, we were able to completely remove the air bubbles appearing during the process of filling a microfluidic device with an aqueous solution.     

Effective pressure and bubble generation in a microfluidic T-junction

To improve the existing trial-and-error process in designing a microfluidic T-junction, a systematic study of the geometrical (mainly the channel length) effects on the generated bubbly/slug flow was conducted to figure out basic design guidelines based on experimental and theoretical analyses. A driving system with dual constant pressure sources, instead of the commonly used dual constant volume-rate sources (such as two syringe pumps), was chosen in this study. The newly proposed effective pressure ratio (P(e)*) has revealed its advantages in excluding the surface tension effect of fluids. All the data of generated bubbly/slug flow for a given geometry collapse excellently into the same relationship of void fraction and effective pressure ratio. This relationship is insensitive to the liquid viscosity and the operation range is strongly affected by the geometrical effect, i.e., the channel length ratio of downstream to total equivalent length of the main channel in a T-junction chip. As to the theoretical design and analysis of gas-liquid-flow characteristics in a microfluidic T-junction, which is still sporadic in the literature, the proposed semi-empirical model has successfully predicted the operation boundaries and the output flow rate of bubbly/slug flow of different investigated cases and demonstrated its usability.     

Solid particle in the boundary layer of a rising bubble

The hydrodynamic interaction of a solid particle and the boundary layer around a rising bubble is analyzed in the before-contact state (BCS) of a flotation act. The lagging of the particle behind the basic outer flow is accounted for. The forces acting on the particle are qualitatively examined. A new term is introduced in the force balance — the migration force. An expression for the collision efficiency is proposed that concerns a particle already entrained in the bubbles boundary layer.     

Internally self-assembled submicrometer emulsions stabilized with a charged polymer or with silica particles

Internally self-assembled submicrometer emulsions were stabilized by F127, by the charged diblock copolymer K151, by L300 particles, and by sodium dodecyl sulfate (SDS). The stabilization of all investigated internal phases and the impact of the stabilizer on them are discussed. The use of charged stabilizers results in a highly negative zeta potential of the emulsion droplets, which can be exploited as a means to control their adsorption onto charged surfaces. Small-angle X-ray scattering and dynamic light scattering were used to determine the internal structure and size of the emulsion droplets, respectively.