Micrometer-sized Water Droplets Research Articles

Electrostatic Stabilization of Colloids in Carbon Dioxide: Electrophoresis and Dielectrophoresis

Over the past decade, steric stabilization has been achieved for a variety of inorganic and organic colloids in supercritical fluid carbon dioxide (scCO2). Herein we demonstrate that colloids may also be stabilized in CO2 by electrostatic forces, despite the ultralow dielectric constant of 1.5. Zeta potentials of micrometer-sized water droplets, measured in a microelectrophoresis cell, reached -70 mV corresponding to a few elementary charges per square micrometer of droplet surface. This degree of charge was sufficient to stabilize water/CO2 emulsions for an hour, even with water volume fractions of 5%. Hydrogen ions partition preferentially, relative to bicarbonate ions, from the emulsion droplets to the cores of surfactant micelles in the diffuse double layer surrounding the droplets. The micelles, formed with a low molecular weight branched hydrocarbon surfactant, prevent ion pairing of the hydrogen counterions to the negatively charged emulsion droplets. Dielectrophoresis of the water droplets at a frequency of 60 Hz leads to chains containing a dozen droplets with lengths of 50 mum. The ability to form electrostatically stabilized colloids in carbon dioxide is particularly useful in practical applications, because steric stabilization in CO2 is often limited by the poor solvation of the stabilizers.

Hilbert phase microscopy for investigating fast dynamics in transparent systems

We introduce Hilbert phase microscopy (HPM) as a novel optical technique for measuring high transverse resolution quantitative phase images associated with optically transparent objects. Because of its single-shot nature, HPM is suitable for investigating rapid phenomena that take place in transparent structures such as biological cells. The potential of this technique for studying biological systems is demonstrated with measurements of red blood cells, and its ability to quantify dynamic processes on a millisecond scale is exemplified with measurements of evaporating micrometer-sized water droplets.

Uptake of Aromatic Hydrocarbon Vapors (Benzene and Phenanthrene) at the Air-Water Interface of Micron-Size Water Droplets

Uptake of aromatic hydrocarbon vapors (benzene and phenanthrene) by typical micrometer-sized fog-water droplets was studied using a falling droplet reactor at temperatures between 296 and 316 K. Uptake of phenan-threne vapor greater than that predicted by bulk (air-water)-phase equilibrium was observed for diameters less than 200 μm, and this was attributed to surface adsorption. The experimental values of the droplet-vapor partition constant were used to obtain the overall mass transfer coefficient and the mass accommodation coefficient for both benzene and phenanthrene. Mass transfer of phenanthrene was dependent only on gas-phase diffusion and mass accommodation at the interface. However, for benzene, the mass transfer was limited by liquid-phase diffusion and mass accommodation. A large value of the mass accommodation coefficient, α = (1.4 ± 0.4) × 10−2 was observed for the highly surface-active (hydrophobic) phenanthrene, whereas a small α = (9.7 ± 1.8) × 10−5 was observed for the less hydrophobic benzene. Critical cluster numbers ranging from 2 for benzene to 5.7 for phenanthrene were deduced using the critical cluster nucleation theory for mass accommodation. The enthalpy of mass accommodation was more negative for phenanthrene than it was for benzene. Consequently, the temperature effect was more pronounced for phenanthrene. A linear correlation was observed for the enthalpy of accommodation with the excess enthalpy of solution. A natural organic carbon surrogate (Suwannee Fulvic acid) in the water droplet increased the uptake for phenanthrene and benzene, the effect being more marked for phenanthrene. A characteristic time constant analysis showed that uptake and droplet scavenging would compete for the fog deposition of phenanthrene, whereas deposition would be unimpeded by the uptake rate for benzene vapor. For both compounds, the characteristic atmospheric reaction times were much larger and would not impact fog deposition.

Local order and dynamics in supercooled water: a study by IR spectroscopy and molecular dynamic simulations.

Micron-sized water droplets in a cryogenic flow tube were probed by IR spectroscopy. The analysis of the IR spectra suggests that there is a relative increase of about 30% in the fraction, f(L), of low density domains in water on cooling over the temperature range between 300 and 240 K. The results derived from the experiments agree qualitatively with those of molecular dynamics (MD) simulations in terms of the increase in the f(L) values. The MD simulations show that the intensities of the mode at about 100 cm(-1) for the molecules in the low density domains are reduced in comparison to the average, while the intensities and frequencies of the librational mode at 600 cm(-1) are increased. Furthermore, the reorientations (dielectric relaxation times) in these domains are found to be somewhat slower, pointing to the fact that these low density "cages" live longer than the average local molecular environments in supercooled water.

Time-resolved explosion dynamics of H2O droplets induced by femtosecond laser pulses.

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed.

Superhydrophobic Carbon Nanotube Forests

The present study demonstrates the creation of a stable, superhydrophobic surface using the nanoscale roughness inherent in a vertically aligned carbon nanotube forest together with a thin, conformal hydrophobic poly(tetrafluoroethylene) (PTFE) coating on the surface of the nanotubes. Superhydrophobicity is achieved down to the microscopic level where essentially spherical, micrometer-sized water droplets can be suspended on top of the nanotube forest.

The effect of naphtha to bitumen ratio on properties of water in diluted bitumen emulsions

Properties of water in naphtha-diluted bitumen emulsion depend on naphtha to bitumen ratio. There is a critical dilution ratio, at which the system properties change abruptly. The critical dilution ratio coincides with an onset of asphaltene precipitation. The critical diluted ratio for the bitumen and naphtha used in this study is 4. At N/B<4, when diluted bitumen was contacted with water, micron-sized water droplets were found in the oil phase close to the oil–water interface without any agitation. This phenomenon was especially noticeable at N/B<1.5. The precipitation of asphaltene was detected at N/B≥4. If the precipitation took place in the presence of water, some of the precipitated particles showed optical anisotropy when observed under cross polarizers. The stability of water in diluted bitumen emulsion depended on the order of emulsion preparation. If water was emulsified into already diluted bitumen, the stability decreased with dilution as expected. If, however, water and naphtha were added at the same time, the stability first decreased until about N/B=1.5, then increased again, leveling of at N/B of about 4. Possible explanations for this behaviour are that it is due to the state of asphaltenes in the oil phase when water is emulsified or to yet unknown phase transition in the organization of the stabilizing layer present at the water–oil interface.

IMPRINT OF HONEYCOMB PATTERN ON PDMS ELASTOMER

Recently a method that utilizes the condensation of micrometer-sized water droplets on evaporating solutions of polymers has been reported for the preparation of porous thin films with fine hexagonal periodicity, honeycomb films. Here we report a method for imprinting a honeycomb pattern on the PDMS (poly(dimethyl siloxane)) elastomer. For the preparation of the honeycomb film, a benzene solution of amphiphilic copolymer was cast at high atmospheric humidity. On the film the PDMS precursor was poured and cured. The cured PDMS with the honeycomb film was peeled from the slide glass. The PDMS was rinsed with the solvent to remove the honeycomb film. The exposed PDMS surface was observed using SEM and AFM. The pattern-imprinted PDMS surface can be used as a template for patterning of materials, which are difficult to produce a honeycomb pattern by the conventional method described above. Here, we show some examples of such honeycomb-originated structures composed of polymers or nanoparticles, which were fabricated using the pattern-imprinted PDMS. The present study shows an example of the secondary uses of the self-organized structures for the cost-saving micro/nanofabrication of various materials.

Micropipette: a new technique in emulsion research

Micron-scale studies on emulsions have, to date, been largely limited to the imaging of colloidal structures. In this communication, micropipette techniques are introduced as a progression beyond mere visualization: using small suction pipettes, mechanical experiments are performed on individual emulsion drops, from which interfacial properties can be deduced. To demonstrate this technique, the interfacial tension, emulsion stability and adsorption characteristics are directly assessed at the surfaces of micron-sized water droplets that are dispersed in crude oil.

The glassy water–cubic ice system: a comparative study by X-ray diffraction and differential scanning calorimetry

Mixtures of various ratios of cubic ice and glassy water were obtained by so-called hyperquenching of micrometer-sized water droplets at cooling rates of ≈106–107 K s−1 on a substrate held at selected temperatures between 130 and 190 K. These samples were characterized by differential scanning calorimetry (DSC) and X-ray diffraction. The minimum deposition temperature to obtain almost entirely vitrified samples is ≈140 K. Glassy water prepared at this temperature exhibits on heating an endothermic step assignable to a glass→liquid transition, without the requirement for previous annealing. Cubic ice samples obtained by deposition at 160 and 170 K undergo on heating two distinct exothermic processes of comparable intensity. One centered at ≈230 K is caused by the phase transition to hexagonal ice. The other is centered at ≈201 K in a sample deposited at 170 K, and it shifts to ≈193 K on deposition at 160 K. The latter process is attributed to the increase in particle size, relief of non-uniform strain and/or healing of different kinds of defects. Since the temperature of this second exotherm depends on the deposition temperature of the sample, it merges on sample deposition at 190 K with the exotherm from the cubic→hexagonal ice phase transition. Therefore, this can lead to an overestimation of the heat of the cubic→hexagonal phase transition. For samples deposited at ⩽150 K, the low temperature exotherm merges with the intense exotherm due to glassy water→cubic ice phase transition. X-ray diffractograms and DSC scans of cubic ice samples of different thermal history show, after annealing at the same temperature of 183 K for 5 min, essentially identical patterns. Likewise, X-ray diffractograms of cubic ice made on heating hyperquenched glassy water or vapor-deposited amorphous solid water up to 183 K are indistinguishable. Cubic ice deposited at 190 K, or annealed at 183 K, contains at most 20% amorphous component which persists up to the cubic to hexagonal ice phase transition. This is in contrast to recent claims of Jenniskens et al. (J. Chem. Phys. 1997, 107, 1232) that cubic ice obtained by heating thin films of vapor-deposited amorphous water contains more than 50% of amorphous, or even liquid, water.

Homogeneous nucleation rates of supercooled water measured in single levitated microdroplets

Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

Colloidal forces between emulsified water droplets in toluene-diluted bitumen

Interaction forces between two stable micron-sized water droplets in toluene-diluted bitumen have been studied by colloidal particle scattering, a method recently developed to determine surface forces between colloidal particles. Our results show that electrostatic forces contribute little or nothing to droplet stabilization. Instead, the stabilization mechanism is proposed to be steric repulsion between rough and non-homogeneous stabilizing layers on droplet surfaces. The range of possible thicknesses of the stabilizing layer is determined. The force–distance relationships reflecting this surface roughness are also plotted.

Formation of ordered micro-porous membranes

Regular micro-porous polymeric membranes have recently been discovered by rapidly evaporating a solution of CS2 containing poly(p-phenylene)-block-polystyrene [#!ref1!#]. 1,2-dichloroethane (a chlorated solvent in which polystyrene gel phase has never been observed) is also found to produce ordered structures, which definitively excludes eventual effect of the gelation process during the membrane formation. The observation of the solution surface during the solvent evaporation reveals the growing of micron-sized water droplets trapped at the surface and forming compact aggregates. The study of the solution/water interface shows that the water droplets profile is in agreement with the pore shape observed in the membranes. Moreover, the copolymer was found to precipitate at the interface, forming a layer encapsulating the droplets and preventing their coalescence. In that way, the final structure results from the droplets stacking under the action of large surface currents. Finally, we argue that the decisive element in the formation of ordered structures is the ability of the polymer to precipitate at the solution/water interface, which seems to be related the star-polymer microstructure.

Hydrogen bonding in glassy liquid water from Raman spectroscopic studies

In order to investigate subtle differences in the local structure and bonding of amorphous ices, glassy liquid water was produced by rapid cooling of micrometer sized water droplets. Samples were first studied with power x-ray diffraction (PXRD) and differential scanning calorimetry (DSC) to ensure that the production technique resulted in amorphous material with low crystalline content. Solutions containing 5% D2O in H2O and 5% H2O in D2O were vitrified, cooled to 12 K, and used to collect Raman spectra of the decoupled O–D (ΔνOD=2440 cm−1) and O–H (ΔνOH=3302 cm−1) vibrations, respectively. Samples were then annealed from 90 to 180 K in temperature intervals of 10 K. During each annealing step the samples were held isothermally for 15 min before rapidly cooling them back down to 12 K for data collection. The transition from amorphous to crystalline material was observed to occur between 150 and 160 K. Raman frequency shift data were then used to estimate the distribution in the hydrogen-bonded O–H––O distances using known correlations. The local bond strengths and O–H––O distances in vitrified liquid water were found to be very similar to those in vapor-deposited amorphous water.

Micrometer Size Effect on Dye Association in Single Laser-Trapped Water Droplets

Molecular association of malachite green (MG) in single micrometer-sized water droplets dispersed in di-n-butyl phthalate was studied by means of scanning laser manipulation and absorption spectroscopy to elucidate characteristic features of chemical processes at the droplet/oil interface. At a high MG concentration, the droplet showed a new absorption band around 580 nm in addition to the monomer band at 615 nm. Analyses of the concentration-dependent absorption spectrum of MG in the droplet indicated that the band at 580 nm could be assigned to that of the MG dimer. The dimer formation of MG in the water droplet was shown to be facilitated with decreasing the droplet diameter, demonstrating the micrometer size effect on dye association. A role of the water/oil interface for the MG dimer formation is discussed.

Random occurrence of stimulated Raman scattering emission from liquid water microdroplets

Stimulated Raman scattering (SRS) spectra from micrometer-sized water droplets have been obtained in the range 2100 < Δν < 5100 cm-(1). A number of Raman bands have been individually identified (to our knowledge, for the first time), corresponding to fundamental OH- and OD-stretching vibrations and to vibrations of hydrogen-bonded molecular complexes. All bands exhibit the intense morphologydependent resonance features that are characteristic of SRS emission from microdroplets. SRS emission is apparently random from all bands; however, the frequency of occurrence varies widely, from bands where emission is seen on practically every laser shot to bands where emission is seen only once in > 10(4) laser shots. Possible causes of these noteworthy emission features are discussed, including the difficulty of coupling weak spontaneous Raman emission to both the intense pump beam and the morphologydependent resonances within the droplet.

Observations of Descartes ring stimulated Raman scattering in micrometer-sized water droplets

Stimulated Raman scattering (SRS) from micrometer-sized droplets, which results from coupling of spontaneous Raman emission to droplet morphology-dependent resonances (MDR's), exhibits unique characteristics. Spatial patterns consist of bright SRS arcs on the droplet rim. A second source of SRS emission has recently been observed from a ringlike region encircling the droplet axis near the geometric focus (the Descartes ring). Investigation of the time and spectral characteristics of Descartes ring SRS and its suppression by the addition of absorptive dye to the droplet reveals it to be an additional manifestation of droplet MDR's. We conjecture that the Descartes ring results when the MDR light is scattered by refractive-index inhomogeneities produced by the intense pump field within the droplet.

Generation and suppression of stimulated Brillouin scattering in single liquid droplets

Stimulated Brillouin scattering (SBS) from micrometer-sized single water and methanol droplets was observed with a Brillouin shift that varied within a range determined by the k-vector spread and the optical feedback provided by the droplet interface. The inability to observe SBS in carbon disulfide droplets is attributed to phase-modulation broadening of the input and Brillouin radiation resulting from the large intensity-dependent index of refraction of carbon disulfide.

Optical breakdown thresholds in liquid water and micron-size water droplets exposed to single laser pulses

Results are presented of experiments to determine the breakdown threshold of water exposed to laser pulses at wavelengths of 1.06 and 0.69 μ. The threshold power densities are (1.0±0.3)×1011 and (3.8±1.0)×1010 W/cm2, respectively. In order to estimate the optical breakdown thresholds inside micron-size water droplets, calculations are made of the maxima of the internal optical field distribution for incident radiation of wavelengths in the range 0.232–1.06 μ and droplet radii between 0.5 and 20 μ. These data are used to estimate the optical breakdown thresholds in water aerosols of different sizes and the results are compared with published experimental data.

Lateral Motion of Individual Particles in Channel Flow—Effect of Diffusion and Interaction Forces

Consideration is given to dispersed systems in which the particle behavior is a function of systematic motion. The following phenomena governing the movement of the particles are analyzed from the channel entrance: convective effects, translation due to gravity, fluid drag, and circulation induced by the flow. External lateral forces and mutual particle interactions are not considered. Results are obtained for the magnitude and overall effects of the lateral force due to circulation around the particles. The analysis is closely related to the author’s measurements of the deposition of micron-size water droplets in adiabatic vertical flow.