A strategy for characterizing the mixing state of immiscible aerosol components and the formation of multiphase aerosol particles through coagulation

We demonstrate that the coagulation of two aerosol droplets of different chemical composition can be studied directly through the unique combination of optical tweezers and Raman spectroscopy. Multiple optical traps can be established, allowing the manipulation of multiple aerosol droplets. Spontaneous Raman scattering allows the characterization of droplet composition and mixing state, permitting the phase segregation of immiscible components in multiphase aerosol to be investigated with spatial resolution. Stimulated Raman scattering allows the integrity of the droplet and uniformity of refractive index to be probed. The combination of these spectroscopic probes with optical tweezers is shown to yield unprecedented detail in studies of the coagulation of decane and water droplets.     

Manipulation and characterization of aqueous sodium dodecyl sulfate/sodium chloride aerosol particles

Aerosol optical tweezers coupled with Raman spectroscopy can allow the detailed investigation of aerosol dynamics. We describe here measurements of the evolving size, composition, and phase of single aqueous aerosol droplets containing the surfactant sodium dodecyl sulfate and the inorganic salt sodium chloride. Not only can the evolving wet particle size be probed with nanometer accuracy, but we show that the transition to a metastable microgel particle can be followed, demonstrating that optical tweezers can be used to manipulate both spherical and non-spherical aerosol particles. Further, through the simultaneous manipulation and characterization of two aerosol droplets of different composition in two parallel optical traps, the phase behavior of a surfactant-doped particle and a surfactant-free droplet can be compared directly in situ. We also illustrate that the manipulation of two microgel particles can allow studies of the coagulation and interaction of two solid particles. Finally, we demonstrate that such parallel measurements can permit highly accurate comparative measurements of the evolving wet particle size of a surfactant-doped droplet with a surfactant-free droplet.     

Spectroscopy of growing and evaporating water droplets: exploring the variation in equilibrium droplet size with relative humidity

We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2-6 microm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from the droplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstrate that it is possible to measure the equilibrium size and composition of an aqueous droplet with variation in relative humidity. For this benchmark study we investigate the variation in equilibrium size with relative humidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueous aerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be in excellent agreement with the predictions of K?hler theory. We suggest that this approach could provide an important new strategy for characterizing the thermodynamic properties and kinetics of transformation of aerosol particles.     

Characterizing the formation of organic layers on the surface of inorganic/aqueous aerosols by Raman spectroscopy

We demonstrate that nonlinear Raman spectroscopy coupled with aerosol optical tweezers can be used to probe the evolving phase partitioning in mixed organic/inorganic/aqueous aerosol droplets that adopt a core-shell structure in which the aqueous phase is coated in an organic layer. Specifically, we demonstrate that the characteristic fingerprint of wavelengths at which stimulated Raman scattering is observed can be used to assess the phase behavior of multiphase decane/aqueous sodium chloride droplets. Decane is observed to form a layer on the surface of the core aqueous droplet, and from the spectroscopic signature the aqueous core size can be determined with nanometer accuracy and the thickness of the decane layer with an accuracy of +/-8 nm. Further, the presence of the organic layer is observed to reduce the rate at which water evaporates from the core of the droplet with an increasing rate of evaporation observed with diminishing layer thickness.     

Observation of the binary coalescence and equilibration of micrometer-sized droplets of aqueous aerosol in a single-beam gradient-force optical trap

The binary coalescence of aqueous droplets has been observed in a single-beam gradient-force optical trap. By measuring the time-dependent intensity for elastic scattering of light from the trapping laser, the dynamics of binary coalescence have been examined and the time scale for equilibration of a composite droplet to ambient conditions has been determined. These data are required for modeling the agglomeration of aqueous droplets in dense sprays and atmospheric aerosol. Elastic-light scattering from optically trapped particles has not been used previously to study the time-resolved dynamics of mixing. It is shown to offer a unique opportunity to characterize the binary coalescence of aqueous droplets with radii from 1 to 6 μm. The study of this size regime, which cannot be achieved by conventional imaging methods, is critical for understanding the interactions of droplets in the environment of dense sprays.     

Optical manipulation and characterisation of aerosol particles using a single-beam gradient force optical trap

The application of optical tweezers (a single-beam gradient force optical trap) to the manipulation and characterisation of aerosol particles is discussed in this tutorial review. Optical tweezers allow not only the indefinite control over a single droplet, but control over arrays of particles. Typical particle sizes span the 1-10 microm diameter range. When coupled with spectroscopic techniques for probing evolving particle size (with nanometre accuracy), composition, phase and mixing state, detailed investigations of the thermodynamic properties of aerosol, the kinetics of particle transformation, and the nature of interparticle forces and coagulation can be undertaken.     

Evaporation of ethanol/water droplets: examining the temporal evolution of droplet size, composition and temperature

The evolving size, composition, and temperature of evaporating ethanol/water aerosol droplets 25-57 microm in radius are probed by cavity enhanced Raman scattering (CERS) and laser induced fluorescence. This represents the first study in which the evolving composition of volatile droplets has been probed with spatial selectivity on the millisecond time scale, providing a new strategy for exploring mass and heat transfer in aerosols. The Raman scattering intensity is shown to depend exponentially on species concentration due to the stimulated nature of the CERS technique, providing a sensitive measure of the concentration of the volatile ethanol component. The accuracy with which we can determine droplet size, composition, and temperature is discussed. We demonstrate that the CERS measurements of evolving size and composition of droplets falling in a train can be used to characterize, and thus avoid, droplet coagulation. By varying the surrounding gas pressure (7-77 kPa), we investigate the dependence of the rate of evaporation on the rate of gas diffusion, and behavior consistent with gas diffusion-limited evaporation is observed. We suggest that such measurements can allow the determination of the vapor pressures of components within the droplet and can allow the determination of activity coefficients of volatile species.     

Retrieval of the complex refractive index of aerosol droplets from optical tweezers measurements

The cavity enhanced Raman scattering spectrum recorded from an aerosol droplet provides a unique fingerprint of droplet radius and refractive index, assuming that the droplet is homogeneous in composition. Aerosol optical tweezers are used in this study to capture a single droplet and a Raman fingerprint is recorded using the trapping laser as the source for the Raman excitation. We report here the retrieval of the real part of the refractive index with an uncertainty of ± 0.0012 (better than ± 0.11%), simultaneously measuring the size of the micrometre sized liquid droplet with a precision of better than 1 nm (< ± 0.05% error). In addition, the equilibrium size of the droplet is shown to depend on the laser irradiance due to optical absorption, which elevates the droplet temperature above that of the ambient gas phase. Modulation of the illuminating laser power leads to a modulation in droplet size as the temperature elevation is altered. By measuring induced size changes of <1 nm, we show that the imaginary part of the refractive index can be retrieved even when less than 10 × 10(-9) with an accuracy of better than ± 0.5 × 10(-9). The combination of these measurements allows the complex refractive index of a droplet to be retrieved with high accuracy, with the possibility of making extremely sensitive optical absorption measurements on aerosol samples and the testing of frequently used mixing rules for treating aerosol optical properties. More generally, this method provides an extremely sensitive approach for measuring refractive indices, particularly under solute supersaturation conditions that cannot be accessed by simple bulk-phase measurements.     

Phase, morphology, and hygroscopicity of mixed oleic acid/sodium chloride/water aerosol particles before and after ozonolysis

Aerosol optical tweezers are used to probe the phase, morphology, and hygroscopicity of single aerosol particles consisting of an inorganic component, sodium chloride, and a water insoluble organic component, oleic acid. Coagulation of oleic acid aerosol with an optically trapped aqueous sodium chloride droplet leads to formation of a phase-separated particle with two partially engulfed liquid phases. The dependence of the phase and morphology of the trapped particle with variation in relative humidity (RH) is investigated by cavity enhanced Raman spectroscopy over the RH range <5% to >95%. The efflorescence and deliquescence behavior of the inorganic component is shown to be unaffected by the presence of the organic phase. Whereas efflorescence occurs promptly (<1 s), the deliquescence process requires both dissolution of the inorganic component and the adoption of an equilibrium morphology for the resulting two phase particle, occurring on a time-scale of <20 s. Comparative measurements of the hygroscopicity of mixed aqueous sodium chloride/oleic acid droplets with undoped aqueous sodium chloride droplets show that the oleic acid does not impact on the equilibration partitioning of water between the inorganic component and the gas phase or the time response of evaporation/condensation. The oxidative aging of the particles through reaction with ozone is shown to increase the hygroscopicity of the organic component.     

Characterizing multiphase organic/inorganic/aqueous aerosol droplets

The partitioning of an immiscible and volatile organic component between the gas and aqueous condensed phases of an aerosol is investigated using optical tweezers. Specifically, the phase segregation of immiscible decane and aqueous components within a single liquid aerosol droplet is characterized by brightfield microscopy and by spontaneous and stimulated Raman scattering. The internally mixed phases are observed to adopt equilibrium geometries that are consistent with predictions based on surface energies and interfacial tensions and the volume fractions of the two immiscible phases. In the limit of low organic volume fraction, the stimulated Raman scattering signature is consistent with the formation of a thin film or lens of the organic component on the surface of an aqueous droplet. By comparing the nonlinear spectroscopic signature with Mie scattering predictions for a core-shell structure, the thickness of the organic layer can be estimated with nanometer accuracy. Time-dependent measurements allow the evolving partitioning of the volatile organic component between the condensed and vapor phases to be investigated.     

Ultrasensitive absorption spectroscopy of optically-trapped aerosol droplets

High-sensitivity optical absorption measurements on individual sub-picoliter aqueous droplets are reported using aerosol optical tweezers to simultaneously manipulate and characterize a sample droplet and a control droplet for comparison. It is demonstrated that the detection sensitivity to trace analytes is set by the weak absorption by the solvent, water, and that absorbances less than 5 x 10(-7) can be measured over pathlengths of less than 10 microm. The potential applications of this approach to analyze aerosol particle composition and to perform trace analysis are discussed.     

光镊技术研究硝酸铵在超粘气溶胶中的挥发性

大气颗粒物中挥发性物质的气粒分配问题是大气科学研究的热点.选择典型的高粘度态模型体系、硝酸铵/蔗糖体系以及硝酸铵/硫酸镁体系,利用光镊-受激拉曼光谱技术原位获得液滴的自发拉曼和受激拉曼信号,同时观察回音壁(WGM)模式,利用米氏散射理论对一系列的WGMs峰位在给定范围内的粒子半径和折射率进行模拟计算,通过Maxwell方程精确计算了两个体系中硝酸铵在不同相对湿度(RH)下的有效饱和蒸汽压值,结果表明,在低湿度下的超粘态液滴中硝酸铵的有效饱和蒸汽压比纯硝酸铵的饱和蒸汽压低1～3个数量级.显然,低相对湿度下,液滴中硝酸铵的挥发受到了抑制.     

Comparative thermodynamic studies of aqueous glutaric acid, ammonium sulfate and sodium chloride aerosol at high humidity

Aerosol optical tweezers are used to simultaneously characterize and compare the hygroscopic properties of two aerosol droplets, one containing inorganic and organic solutes and the second, referred to as the control droplet, containing a single inorganic salt. The inorganic solute is either sodium chloride or ammonium sulfate and the organic component is glutaric acid. The time variation in the size of each droplet (3-7 microm in radius) is recorded with 1 s time resolution and with nanometre accuracy. The size of the control droplet is used to estimate the relative humidity with an accuracy of better than +/-0.09%. Thus, the Kohler curve of the multicomponent inorganic/organic droplet, which characterizes the variation in equilibrium droplet size with relative humidity, can be determined directly. The measurements presented here focus on high relative humidities, above 97%, in the limit of dilute solutes. The experimental data are compared with theoretical treatments that, while ignoring the interactions between the inorganic and organic components, are based upon accurate representations of the activity-concentration relationships of aqueous solutions of the individual salts. The organic component is treated by a parametrized fit to experimental data or by the UNIFAC model and the water activity of the equilibrium solution droplet is calculated using the approach suggested by Clegg, Seinfeld and Brimblecombe or the Zdanovskii-Stokes-Robinson approximation. It is shown that such an experimental strategy, comparing directly droplets of different composition, enables highly accurate measurements of the hygroscopic properties, allowing the theoretical treatments to be rigorously tested. Typical deviations of the experimental measurements from theoretical predictions are shown to be around 1% in equilibrium size, comparable to the variation between the theoretical frameworks considered.     

Electrochemical Detection of Droplets in Microfluidic Devices: Simultaneous Determination of Velocity,Size and Content

Microfluidic devices were designed to electrochemically detect in a two‐phase flow the velocity, size and content of aqueous droplets containing redox species. The principle of these determinations is based on the analysis of a unique chronoamperometric response recorded during the passage of a droplet over channel microelectrodes. Two configurations of electrochemical cell with different geometries were investigated both theoretically and experimentally. Velocity and size of droplets, as well as internal recirculating convection within droplets, were evaluated from chronoamperometric curves by specific transition times depending on the cell configuration. In addition, the droplet content was probed from the Faradaic current controlled by mass transport and by internal hydrodynamic regime. For droplet velocity and size, experimental data were systematically compared to optical measurements. All the results demonstrated the high performance of the electrochemical detection reached under these conditions. They successfully validate the concept of self‐consistent electrochemical detections of aqueous droplets within microchannels for the simultaneous determination of their velocity, size and content.     

An optical method for determining size distributions of water droplets

A method for determining aerosol size distributions by single laser-shot single droplet cavity enhanced Raman scattering (CERS) is presented. Droplets are illuminated with the tripled output from a Nd:YAG laser at 355 nm and the CERS fingerprint acquired with a spectrograph and CCD. Droplets with radii in the range 10–50 μm are probed. The extension of this to the determination of a distribution of droplet sizes is illustrated. We suggest that the CERS signature from water could be used to determine droplet size while the observation of Raman scattering from other constituents could be used to identify trace chemical constituents within water droplets.     

Controlled shrinkage and re-expansion of a single aqueous droplet inside an optical vortex trap

This paper describes the shrinkage and re-expansion of individual femtoliter-volume aqueous droplets that were suspended in an organic medium and held in an optical vortex trap. To elucidate the mechanism behind this phenomenon, we constructed a heat- and mass-transfer model and carried out experimental verifications of our model. From these studies, we conclude that an evaporation mechanism sufficiently describes the shrinkage of aqueous droplets held in a vortex trap, whereas a mechanism based on the supersaturation of the organic phase by water that surrounds the droplet adequately explains the re-expansion of the shrunk droplet. The proposed mechanisms correlated well with experimental observations using different organic media, when H2O was replaced with D2O and when an optical tweezer was used to induce droplet shrinkage rather than an optical vortex trap. For H2O droplets, the temperature rise within the droplet during shrinkage was on the order of 1 K or less, owing to the rapid thermal conduction of heat away from the droplet at the microscale and the sharp increase in solubility for water by the organic phase with slight elevations in temperature. Because most chemical species confined to droplets can be made impenetrable to the aqueous/organic interface, a change in the volume of aqueous droplets translates into a change in concentration of the dissolved species within the droplets. Therefore, this phenomenon should find use in the study of fundamental chemical processes that are sensitive to concentration, such as macromolecular crowding and protein nucleation and crystallization.     

Probing the evaporation of ternary ethanol-methanol-water droplets by cavity enhanced Raman scattering

Cavity enhanced Raman scattering is used to characterise the evolving composition of ternary aerosol droplets containing methanol, ethanol and water during evaporation into a dry nitrogen atmosphere. Measurements made using non-linear stimulated Raman scattering from these ternary alcohol-water droplets allow the in situ determination of the concentration of the two alcohol components with high accuracy. The overlapping spontaneous Raman bands of the two alcohol components, arising from C-H stretching vibrational modes, are spectrally-resolved in stimulated Raman scattering measurements. We also demonstrate that the evaporation measurements are consistent with a quasi-steady state evaporation model, which can be used to interpret the evaporation dynamics occurring at a range of pressures at a particular evaporation time.     

Determining the composition of aqueous microdroplets with broad-band cavity enhanced Raman scattering

We demonstrate that broad-band cavity enhanced Raman scattering (CERS) can be used to determine the composition of binary alcohol-water aerosol droplets over a wide compositional range from 10% v/v to 90% v/v. In contrast to conventional CERS using narrow-band laser excitation, the excitation is provided by a broad-band Nd:YAG pumped dye laser. A change in the spontaneous spectrum resulting from the change of the linewidth of the excitation laser permits tuning of the sensitivity range over which the droplet composition can be determined by CERS. We demonstrate that this change in sensitivity can be estimated using a simulation of the change in the sensitivity to the species in spontaneous bulk phase measurements. We further show that the compositional calibration is independent of droplet radius in the range 33-56 microm. The compositional range over which CERS is sensitive can be controlled and optimised for any particular application by exploiting the dependence of the stimulated Raman scattering on the laser linewidth and wavelength. Thus, quantitative measurements of droplet composition can be made in situ with high accuracy, providing a valuable new tool for analysing aerosol composition.     

Laser photophoretic migration with periodic expansion-contraction motion of photo-absorbing microemulsion droplets in water

When the water-in-oil (w/o) microemulsion droplets including the Co(III)-pyridylazo complex as the photo-absorber were irradiated with a continuous-wave Ar(+) ion laser (514.5 nm), we have observed unexpected phenomenon that photo-absorbing microemulsion droplets in water repeated the expansion and the sudden contraction during the laser photophoretic migration. The period of the expansion-contraction cycle was inversely proportional to both the concentration of the complex and the irradiated laser power and was independent of the initial size of the droplet. The mechanism of the periodic motion of the droplet was investigated by local temperature measurement and Raman microscope spectroscopy. It was suggested that the first step was the phase separation of the bicontinuous microemulsion droplet into the normal w/o microemulsion outer phase and the aqueous inner phase in the droplet, which was caused by the laser-induced temperature gradient inside the droplet. Subsequently, an expansion of the inner aqueous phase was induced by the percolation of the external water by thermo-osmosis, which was caused by the laser-induced temperature gradient between the inside and the outside of the microemulsion liquid membrane of the droplet. When the liquid membrane became thinner to a critical thickness, the inner aqueous phase was released and the droplet shrank into the original size. The proposed mechanism can give an account of the unique cyclical motion.     

Controlled generation of droplets using an electric field in a flow-focusing paper-based device

Droplet-based microfluidics is a modular platform in high-throughput single-cell and small sample analyses. However, this droplet microfluidic system was widely fabricated using soft lithography or glass capillaries, which is expensive and technically demanding for various applications, limiting use in resource-poor settings. Besides, the variation in droplet size is also restricted due to the limitations on the operating forces that the paper-based platform is able to withstand. Herein, we develop a fully integrated paper-based droplet microfluidic platform for conducting droplet generation and cell encapsulation in independent aqueous droplets dispersed in a carrier oil by incorporating electric fields. Through imposing an electric field, the droplet size would decrease with increasing the electric field and smaller droplets can be produced at high applied voltage. The droplet diameter can be adjusted by the ratio of inner and outer flow velocities as well as the applied electric field. We also demonstrated the proof of concept encapsulation application of our paper device by encapsulating yeast cells under an electric field. Using a simple wax printing method, carbon electrodes can be integrated on the paper. The integrated paper-based microfluidic platform can be fabricated easily and conducted outside of centralized laboratories. This microfluidic system shows great potential in drug and cell investigations by encapsulating cells in resource-limited environments.