Single Microdroplets Research Articles

Microdroplet Dissolution into a Second-Phase Solvent Using a Micropipet Technique: Test of the Epstein−Plesset Model for an Aniline−Water System

The Epstein-Plesset model was originally derived for the dissolution of a single gas bubble in an infinite aqueous solution (Epstein, P. S.; Plesset, M. S. J. Chem. Phys. 1950, 18, 1505-1509). The micropipet manipulation technique was previously shown to test this theory on air microbubbles and air-filled lipid-coated microparticles accurately and appropriately (Duncan, P. B.; Needham, D. Langmuir 2004, 20, 2567-2578). This same theory is now tested to model liquid microdroplet dissolution in a well-defined solution environment. As presented previously for the gas-bubble system, holding a single microparticle at the end of a micropipet was not shown to affect the dissolution profile and allowed isotropic diffusion significantly, a necessary condition for the validation of the theory. Here, an aniline-water system with an initial droplet diameter of 50 microm was used as a model liquid-liquid system. A microdroplet of aniline in an aqueous solution presatureated with aniline at distinct levels was tested, as was the reverse system of a water droplet in an aniline solution. The dissolution lifetime was shown to increase with increasing medium saturation fraction according to the Epstein-Plesset time-dependent theory (including the time required to establish the stationary layer) neglecting interfacial tension. The droplet lifetime can be increased by an order of magnitude (from about 10 to 100 s) by increasing the saturation fraction from 0 to 0.9 and by another order of magnitude by increasing from 0.9 to 0.99. The technique proved to be an accurate and appropriate method to test the dissolution of single liquid microdroplets in a second liquid solution and establishes a systematic experimental and theoretical approach to the investigation of the formation of polymer and other microparticles.

Evaporation-Induced Particle Microseparations inside Droplets Floating on a Chip

We describe phenomena of colloidal particle transport and separation inside single microdroplets of water floating on the surface of dense fluorinated oil. The experiments were performed on microfluidic chips, where single droplets were manipulated with alternating electric fields applied to arrays of electrodes below the oil. The particles suspended in the droplets were collected in their top region during the evaporation process. Experimental results and numerical simulations show that this microsepration occurs as a result of a series of processes driven by mass and heat transfer. An interfacial tension gradient develops on the surface of the droplet as a result of the nonuniform temperature distribution during the evaporation. This gradient generates an internal convective Marangoni flow. The colloidal particles transported by the flow are collected in the top of the droplets by the hydrodynamic flux, compensating for evaporation through the exposed top surface. The internal flow pattern and temperature distribution within evaporating droplets were simulated using finite element calculations. The results of the simulation were consistent with experiments using tracer particles. Such microseparation processes can be used for on-chip synthesis of advanced particles and innovative microbioassays.

Rates of Homogeneous Ice Nucleation in Levitated H2O and D2O Droplets

Rates of homogeneous nucleation of H2O droplets in a temperature range from 236.37 to 237.91 K and of D2O droplets from 241.34 to 242.33 K were measured. The single microdroplets consisted of pure H2O or D2O and were levitated in an electrodynamic balance. In comparison to H2O, D2O shows a stronger tendency to nucleate. Over the investigated temperature interval, D2O droplets need to be supercooled less by 1.1 K compared to H2O droplets in order to arrive at the same nucleation rate. This is in good agreement with the higher degree of intermolecular association in liquid D2O, a fact which has been well established previously both from theory and experimental studies.

Kinetic Analysis of Ion Pair Extraction of an Alkyl Sulfate Anion across a Liquid/Liquid Interface by Fluorescence Microspectroscopy and Microelectrochemistry of Single Microdroplets

Mass transfer of an alkyl sulfate anion with a (ferrocenylmethyl)trimethylammonium cation from water into single oil microdroplets was kinetically studied by microcapillary injection, fluorescence microspectroscopy, and microelectrochemistry. The partitioning ratio and the extraction rate significantly depended on the alkyl chain length of the alkyl sulfate anion. The extraction rate for the n-dodecyl sulfate or n-tridecyl sulfate ion was proportional to both the alkyl sulfate ion and ferrocene derivative concentrations while that for the n-undecyl sulfate ion was proportional to the anion concentration alone. The results are discussed in terms of transfer of the individual ions across the microdroplet/water interface and adsorption/ion pair formation of the alkyl sulfate ion at the microdroplet/water interface.

Measurement of Activity Coefficients from Unsteady State Evaporation and Growth of Microdroplets

Techniques are presented for determination of activity coefficients of binary systems from unsteady state evaporation and growth of single microdroplets in controlled environments. A high-precision light scattering method based on resonances observed in light scattering by microdroplets was used to determine the size and composition of a microdroplet as functions of time. The techniques were validated through data on growth of glycerol microdroplets in slowly developing water vapor concentration fields and evaporation of microdroplets, containing volatile dimethylphthalate (DMP) and nonvolatile dioctylphthalate (DOP), in vapor-free atmospheres. When the water vapor concentration in the surrounding gas changes slowly a glycerol droplet maintains a dynamic equilibrium with the water vapor; thus the activity coefficient of water was determined from knowledge of the droplet composition and the water vapor saturation ratio in the gas phase. The activity coefficients of DMP were determined on the basis that the instantaneous evaporation rate of a DMP-DOP microdroplet in a vapor-free atmosphere is equal to the product of the activity of DMP and the evaporation rate of a pure DMP droplet. The activity coefficient values obtained from microdroplet experiments are highly reproducible and agree with data available in the literature.

Homogeneous nucleation of supersaturated KCl-solutions from single levitated microdroplets

Single levitated microdroplets of aqueous potassium chloride (KCl) solutions are investigated as a function of temperature and humidity of their surroundings. Supersaturated solution droplets are obtained from the evaporation of water. The phase transition into solid microparticles is investigated as a function of temperature and humidity, where the experimental conditions favor homogeneous nucleation. The heat of solution of supersaturated KCl-solution droplets containing variable salt concentrations is derived, which is compared to previous results on dilute and saturated KCl-solutions. It is found to be negative, contrary to saturated and dilute solutions. This gives evidence for the formation of KCl clusters in supersaturated solutions prior to nucleation. The rates of homogeneous nucleation from supersaturated KCl solutions are reported, where the temporal evolution of homogeneous nucleation is investigated. Processes occurring on different time scales are identified. These are discussed along with previous works on homogeneous nucleation of pure water and salt solutions.

The Freezing of n-Alkanes C15H32 and C17H36: An Unsual Mechanism of Homogeneous Nucleation

Abstract The nucleation kinetics of n-alkane droplets with carbon numbers of 15 and 17 was observed in an electrodynamic balance. Changes in the elastic light scattering pattern of the single levitated microdroplets indicate the phase transition liquid to solid. Measurements of the nucleation rates gave information on the dynamics of the nucleation process and allow to propose a new mechanism for the nucleation. Large induction times observed for the C15H32 droplets indicate that the nucleation starts after a solid layer has been built up at the surface.

Optical levitation of single microdroplets at temperatures down to 180 K.

A little light levitation: Optically transparent microdroplets can be levitated by means of the photon pressure exerted by a slightly focussed laser beam. The picture shows the forces operating on a particle during this process. The operation of this levitation techniques is reviewed on the basis of generalized Lorenz–Mie theory. The behaviour of levitated microparticles with respect to changes in composition and size can be investigated by Raman and Mie scattering. The feasibility of this technique for levitation studies down to 180 K is demonstrated.

Nucleation Behavior of n-Alkane Microdroplets in an Electrodynamic Balance

The nucleation behavior of n-alkane droplets with carbon numbers ranging from 14 to 17 was observed in an electrodynamic balance. Changes in the elastic light-scattering pattern of the single levitated microdroplets indicate the phase transition from liquid to solid. Cooling/heating experiments showed larger supercooling temperatures than expected for alkane droplets with an alkane/air interface. Measurements of the nucleation rates of C15H32 and C17H36 gave additional information about the dynamics of the nucleation process and allowed us to distinguish homogeneous from heterogeneous nucleation.

Competitive quenching fluorescence immunoassay for chlorophenols based on laser-induced fluorescence detection in microdroplets.

An improved biomonitoring system for the analysis of 2,4,6-trichlorophenol (TCP) in urine samples has been developed. The principle of the biosensor device is the detection of laser-induced fluorescence (LIF) in single microdroplets by a homogeneous quenching fluorescence immunoassay (QFIA). The competitive immunoassay occurs in microdroplets (d = 58,4 microm) produced by a piezoelectric generator system with 10-microm-diameter orifice. A continuous Ar ion laser (488 nm) excites the fluorescent tracer; its fluorescence is detected by a spectrometer attached to a 512 x 512 cooled, charge-coupled device camera. Fluorescence is quenched by specific binding of TCP polyclonal antibodies to the fluorescent tracer (hapten A-fluorescein); the quenching effect is diminished by the presence of the analyte. Thus, an increase in the signal is produced in a positive dose-dependent manner when TCP is present in the sample. In 10 mM PBS buffer, the IC50 of the LIF-microdroplet QFIA is 0.45 microg L(-1) reaching a LOD of 0.04 microg L(-1). The QFIA with the same reagents performed in microtiter plate format achieved a LOD of 0.36 microg L(-1) in buffer solution. Performance in human urine was similar to that observed in the buffer. A LOD of 1.6 ,g L(-1), with a dynamic range between 4 and 149.5 microg L(-1) in urine, was obtained without any sample treatment other than dilution with the assay buffer. The detectability achieved is sufficient for occupational exposure risk assessment.

Microinjection-microspectroscopy of single oil droplets in water: an application to liquid/liquid extraction under solution-flow conditions

A microinjection-microspectroscopy system combined with a fluid manifold was developed to manipulate and analyze ‘single’ micro-oil-droplets in water. Water was poured into a flow cell set on a microscope stage through a microcapillary tube. A single tributyl phosphate (TBP) microdroplet was then pushed out to the water phase in the cell through an another tiny capillary tube and held on the top end of the tube. These procedures made it possible to inject a single microdroplet exclusively into the cell. The technique was applied to study a time course of liquid/liquid extraction processes of an Al-DHAB chelate (DHAB=2,2′-dihydroxyazobenzene) from water to single TBP microdroplets. The extraction rate of Al-DHAB was determined for individual microdroplets and the results were discussed in terms of the solution flow rate and the droplet diameter.

Uptake Dynamics and Diffusion of HCl in Sulfuric Acid Solution Measured in Single Levitated Microdroplets

The uptake dynamics of HCl by single sulfuric acid microdroplets under stratospheric conditions is investigated. The droplets are typically 30−70 μm in diameter and weakly charged so that they can be stored in an electrodynamic trap. The gas uptake of the droplets is monitored either by measuring their size by angle resolved Mie- scattering patterns or by electrostatic balancing of the droplets in the trap. At low temperatures and high sulfuric acid concentration (T < 190 K for 48 wt % H2SO4 and T < 195 K for 56 wt % H2SO4, respectively), liquid-phase diffusion inside the droplet is the rate-limiting step in the overall uptake process. In this regime, the diffusion coefficients Dliq of HCl in supercooled sulfuric acid solutions are found to increase strongly with temperature and H2O concentration. The results are discussed with respect to diffusion models that have been proposed recently. In contrast, at higher temperatures and lower sulfuric acid concentrations (30−40 wt % H2SO4, 185−207 K) gas-phase diffusion with subsequent accommodation/dissolution at the liquid surface determines the observed uptake velocity. A new method to deduce accommodation coefficients α is proposed.

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.

Thermodynamic and statistical studies of supersaturated ternary solutions.

The influence of chromium ions Cr3+ on properties of supersaturated aqueous solution of ammonium sulfate (NH4)2SO4 has been studied employing containerless electrodynamic levitation of single microdroplets of this solution with known concentration of Cr3+ ions. The water activity versus solute concentration was measured in three different sets of experiments corresponding to different initial concentrations of Cr3+ ions. Theoretical treatment of the experimental results obtained is developed employing the theory of supersaturated electrolyte solutions [Phys. Rev. E 52, 1325 (1995)]. This treatment has allowed the following two conclusions. First, spinodal concentration is a decreasing function of Cr3+ concentration. Second, solute clusters formed in the presence of Cr3+ ions demonstrate complex behavior with respect to binding water molecules: (a) at low and moderate supersaturations, they bind more water molecules than clusters formed in the absence of an impurity; and (b) at high supersaturations, they bind less water molecules than clusters formed in the absence of an impurity.

Imaging and spectroscopic analysis of single microdroplets containing p-cresol using the near-infrared laser tweezers/Raman microprobe system

The near-infrared (NIR) laser tweezers/Raman microprobe system features two charge-coupled device (CCD) cameras with holographic notch filters (HNFs) for the imaging and spectroscopic analysis of molecules in a single microdroplet (MD). One CCD camera and a HNF are used to record an image of the laser microprobe in a trapped MD. The other CCD camera and two HNFs are used with a polychromator to obtain a Raman spectrum of molecules in the MD. A dielectric multilayer coated beam splitter divides the scattered NIR light into two optical paths for the cameras. The system provides sufficient sensitivity to obtain a Raman spectrum of p-cresol contained in a single picoliter toluene MD and sufficient spatial resolution to record an image of the laser microprobe in a trapped MD simultaneously. Furthermore, a difference in the solubility for the p-cresol in bulk solvent and in the MD solvent was clearly observed using this system.

Investigations of chemical reactions between single levitated magnesium chloride microdroplets with SO2 and NOx by means of Raman spectroscopy and elastic light scattering

The combination of Raman spectroscopy with an electrodynamic levitation technique enables insitu and non-invasive investigations of chemical reactions in single microparticles without any disturbing substrate or wall effects. Time-dependent Raman spectra during the condensation of a magnesium chloride microparticle (a model substance of sea-salt particles) with water vapour are reported. The condensation of a MgCl2·6H2O microparticle is also investigated by means of elastic light scattering data. Additionally, the solvation of SO2 and NOx in a MgCl2·6H2O microdroplet is studied by Raman spectroscopy. The uptake of these gases is also determined from elastic light scattering data.

Direct structural observation of liquid molecules in single picoliter microdroplets using near-infrared Raman microprobe spectroscopy combined with laser trapping and chemical-tomographic imaging techniques

Two near-infrared (NIR) Raman microprobe spectroscopic techniques were developed for the direct structural observation of liquid molecules in single picoliter microdroplets. The use of NIR laser light as the excitation light source instead of visible laser light provides Raman spectra with lower fluorescence interference, which makes it easier to determine the structure of liquid molecules. Combining an NIR Raman microprobe with a laser trapping technique makes it possible to classify molecular species in a single picoliter microdroplet during laser trapping in solution. An NIR Raman microprobe chemical-tomographic (RMCT) imaging technique provides the three-dimensional distribution of molecular species in a picoliter microdroplet with a spatial resolution of several femtoliters in volume. Both techniques show sufficient sensitivity to obtain Raman spectra of liquid molecules in single picoliter microdroplets.

Investigations of Radical Polymerization and Copolymerization Reactions in Optically Levitated Microdroplets by Simultaneous Raman Spectroscopy, Mie Scattering, and Radiation Pressure Measurements

The combination of the optical levitation technique with Raman spectroscopy, elastic (Mie) scattering, and radiation pressure measurements enables in situ and real-time investigation of polymerization and copolymerization processes in single microdroplets. The time-dependent evolution of the polymerization and copolymerization reactions of styrene/unsaturated polyester resin (UP-resin) is characterized by observing the educt and product signals in the Raman spectra. The size, the refractive index, and the wavelength dependence of the refractive index of the optically levitated microparticle are determined from the position of the morphology-dependent resonances observed in Raman spectra, in elastic light scattering, and in radiation pressure measurements. This additional information about droplet properties as a function of time allows a better understanding of the entire reaction process within the microdroplet.

Combined Near-Infrared Raman Microprobe and Laser Trapping System: Application to the Analysis of a Single Organic Microdroplet in Water

A combined Raman microprobe and laser trapping system using near-infrared (NIR) laser light was developed for the investigation of single organic microdroplets. The NIR laser light is noninvasive and reduces fluorescence interference in the Raman spectrum for organic molecules. The focused laser beam used for the laser trapping of a microdroplet serves simultaneously as the laser microprobe for Raman measurement. With this system, the focused laser spot is about 1 μm in diameter, which is small enough for the laser trapping of a single toluene microdroplet in water. The system also makes it possible to visualize a focused laser spot together with a laser-trapped microdroplet by using holographic notch filters. The Raman spectrum for a single laser-trapped toluene microdroplet can be obtained from below 100 cm−1 to above 3000 cm−1 with a charge-coupled device (CCD) detector. Fluorescence interference in the Raman spectrum is completely removed by using NIR laser light. The signal-to-noise ratio (SNR), defined as the ratio of the peak height to the standard deviation of the baseline noise in the spectrum, exceeded 250 for the 1003 cm−1 band of a toluene microdroplet at 1 s, which is sufficient to allow identification of the molecular species of a microdroplet.

The effects of atmospheric organics on aqueous droplet evaporation

Difunctional organic oxygenates are shown to have substantial effects on the evaporation rates of aqueous solution droplets. These compounds have been detected in both urban and rural environments, and their particulate concentrations assessed by investigators over the past twenty years [Grosjean (1977) Ozone and other Photochemical Oxidants. National Academy of Sciences, Washington, D.C.; Grosjean et al. (1978) Environm. Sci. Technol., 12, 313–317; Rogge et al. (1991) Atmos. Environ. 27A, 1309–1330]. To understand the effect of difunctional organic oxygenates on the transport of water at the air/water interface of single aqueous droplets, the evaporation rates of aqueous systems containing model organics were measured. The influence of organics of this type on the evaporation rate of water was also compared to aqueous solutions containing ammonium sulfate and sodium dodecyl sulfate. Light-scattering techniques were used to measure the droplet size as a function of time for electrodynamically levitated single microdroplets under conditions of controlled humidity and temperature. These techniques consisted of phase functions (angular scattering data) and morphological resonance spectra. The evaporation rates for all the aqueous systems are compared to that of pure water and are found to be reduced by up to an order of magnitude.