Microscopic Droplets Research Articles

The Molecular Mechanism of Nanodroplet Stability.

Mixtures of nano- and microscopic oil droplets in water have recently been rediscovered as miniature reaction vessels in microfluidic environments and are important constituents of many environmental systems, food, personal care, and medical products. The oil nanodroplet/water interface stabilized by surfactants determines the physicochemical properties of the droplets. Surfactants are thought to stabilize nanodroplets by forming densely packed monolayers that shield the oil phase from the water. This idea has been inferred from droplet stability measurements in combination with molecular structural data obtained from extended planar interfaces. Here, we present a molecular level investigation of the surface structure and stability of nanodroplets and show that the surface structure of nanodroplets is significantly different from that of extended planar interfaces. Charged surfactants form monolayers that are more than 1 order of magnitude more dilute than geometrically packed ones, and there is no experimental correlation between stability and surfactant surface density. Moreover, dilute negatively charged surfactant monolayers produce more stable nanodroplets than dilute positively charged and dense geometrically packed neutral surfactant monolayers. Droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole, and hydrogen-bonding interactions. The difference between extended planar interfaces and nanoscale interfaces stems from a difference in the thermally averaged total charge-charge interactions in the two systems. Low dielectric oil droplets with a size smaller than the Debye length in oil permit repulsive interactions between like charges from opposing interfaces in small droplets. This behavior is generic and extends up to the micrometer length scale.

Investigation of factors affecting condensation on soiled PV modules

Condensation contributes to soiling of photovoltaic modules by trapping dust particles and, in certain conditions, leaving a material bridge between particles and the surface after evaporating. This study investigated the effects of four parameters on condensation on soiled surfaces: (i) relative humidity (RH), (ii) surface–dew point temperature difference, (iii) hygroscopic dust content, and (iv) surface wettability. Natural and synthetic dust mixtures of various compositions were studied via water adsorption isotherms, XRD, ion chromatography and optical microscopy, on hydrophilic and hydrophobic surfaces, in the lab and field. It was found that water uptake by surface dust was strongly dependent on its content of hygroscopic material, and such material allowed microscopic condensation droplets to exist on a soiled glass coupon even when it was significantly warmer than the dew point. A hydrophobic PTFE surface did not greatly retard the onset of condensation, compared to a glass surface, but did inhibit its growth. The implications for anti-soiling coatings are that it would be a difficult goal to eliminate condensation, and their performance will be influenced by dust composition and factors affecting condensation run-off such as RH and tilt angle. Variation in such parameters may partly explain inconsistent results from coating field trials reported so far.

Study on the relationship between emulsion stability and droplet dynamics of a spontaneous emulsion for chemical enhanced oil recovery

ABSTRACTSpontaneous emulsion (SE) has attracted increasing attention, especially in the development of low-permeability reservoirs (with an average throat radius of 0.1–2 µm) for enhanced oil recovery. In this work, based on multiple light scattering principles, the relationship between emulsion stability and the droplet dynamics of SEs was investigated. The results showed that the synergistic effect of surfactant and polymer was crucial for oil emulsification in brine, since the stability of the emulsion was greatly improved. The emulsion stability and droplet dynamics depend on the temperature, concentration, and type of emulsifier. The optimal combination system had the lowest Turbiscan stability index value, and the emulsion stability time was more than 2000s. The average droplet size was 1.50 µm, and the droplet migration rate was 7.21 mm/h. The stability of the emulsion was resulted from the microscopic droplet dynamics. By reducing the migration rate of the droplets, stability of the emulsion can be obtained. Finally, the stability and droplet dynamics mechanism of the system were explained by using a schematic representation of the various equilibriums in the spontaneous emulsification flooding system.

Transfer of labile organic matter and microbes from the ocean surface to the marine aerosol: an experimental approach

Surface ocean bubble-bursting generates aerosols composed of microscopic salt-water droplets, enriched in marine organic matter. The organic fraction profoundly influences aerosols’ properties, by scattering solar radiations and nucleating water particles. Still little is known on the biochemical and microbiological composition of these organic particles. In the present study, we experimentally simulated the bursting of bubbles at the seawater surface of the North-Eastern Atlantic Ocean, analysing the organic materials and the diversity of the bacteria in the source-seawaters and in the produced aerosols. We show that, compared with seawater, the sub-micron aerosol particles were highly enriched in organic matter (up to 140,000x for lipids, 120,000x for proteins and 100,000x for carbohydrates). Also DNA, viruses and prokaryotes were significantly enriched (up to 30,000, 250 and 45x, respectively). The relative importance of the organic components in the aerosol did not reflect those in the seawater, suggesting their selective transfer. Molecular analyses indicate the presence of selective transfers also for bacterial genotypes, highlighting higher contribution of less abundant seawater bacterial taxa to the marine aerosol. Overall, our results open new perspectives in the study of microbial dispersal through marine aerosol and provide new insights for a better understanding of climate-regulating processes of global relevance.

Measurement of contact angles of microscopic droplets by focal length method

We present a method to measure contact angles of microscopic droplets with a conventional microscope that possesses a precision focus adjustment stage. The droplets are modeled as spherical caps that act as lenses. Their focal length is determined by measuring the distance from the substrate surface to the level where a sharp image of the aperture stop is observed. The lens diameter is found by edge detection of a microscope image of the microdroplets. The spherical cap model relates the focal length and diameter of such lenses to the contact angle of the used liquid with known refractive index. The measurement procedure was applied to condensed water droplets on a silicon substrate covered by its native oxide layer. The results are found to be in good agreement with conventional, goniometric sessile drop measurements of the advancing contact angle.

On the transcritical mixing of fuels at diesel engine conditions

Whilst the physics of both classical evaporation and supercritical fluid mixing are reasonably well characterized and understood in isolation, little is known about the transition from one to the other in the context of liquid fuel systems. The lack of experimental data for microscopic droplets at realistic operating conditions impedes the development of phenomenological and numerical models. To address this issue we performed systematic measurements using high-speed long-distance microscopy, for three single-component fuels (n-heptane, n-dodecane, n-hexadecane), into gas at elevated temperatures (700–1200K) and pressures (2–11MPa). We describe these high-speed visualizations and the time evolution of the transition from liquid droplet to fuel vapour at the microscopic level. The measurements show that the classical atomization and vaporisation processes do shift to one where surface tension forces diminish with increasing pressure and temperature, but the transition to diffusive mixing does not occur instantaneously when the fuel enters the chamber. Rather, subcritical liquid structures exhibit surface tension in the near-nozzle region and then, after time surrounded by the hot ambient gas and fuel vapour, undergo a transition to a dense miscible fluid. Although there was clear evidence of surface tension and primary atomization for n-dodecane and n-hexadecane for a period of time at all the above conditions, n-heptane appeared to produce a supercritical fluid from the nozzle outlet when injected at the most elevated conditions (1200K, 10MPa). This demonstrates that the time taken by a droplet to transition to diffusive mixing depends on the pressure and temperature of the gas surrounding the droplet as well as the fuel properties. We summarise our observations into a phenomenological model which describes the morphological evolution and transition of microscopic droplets from classical evaporation through a transitional mixing regime and towards diffusive mixing, as a function of operating conditions. We provide criteria for these regime transitions as reduced pressure–temperature correlations, revealing the conditions where transcritical mixing is important to diesel fuel spray mixing.

The interfacial structure of water droplets in a hydrophobic liquid

Nanoscopic and microscopic water droplets and ice crystals embedded in liquid hydrophobic surroundings are key components of aerosols, rocks, oil fields and the human body. The chemical properties of such droplets critically depend on the interfacial structure of the water droplet. Here we report the surface structure of 200 nm-sized water droplets in mixtures of hydrophobic oils and surfactants as obtained from vibrational sum frequency scattering measurements. The interface of a water droplet shows significantly stronger hydrogen bonds than the air/water or hexane/water interface and previously reported planar liquid hydrophobic/water interfaces at room temperature. The observed spectral difference is similar to that of a planar air/water surface at a temperature that is ∼50 K lower. Supercooling the droplets to 263 K does not change the surface structure. Below the homogeneous ice nucleation temperature, a single vibrational mode is present with a similar mean hydrogen-bond strength as for a planar ice/air interface.

Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles.

Recent laboratory studies indicate that the hydrated form of crystalline NaCl is potentially important for atmospheric processes involving depositional ice nucleation on NaCl dihydrate particles under cirrus cloud conditions. However, recent experimental studies reported a strong discrepancy between the temperature intervals where the efflorescence of NaCl dihydrate has been observed. Here we report the measurements of the volume specific nucleation rate of crystalline NaCl in the aqueous solution droplets of pure NaCl suspended in an electrodynamic balance at constant temperature and humidity in the range from 250 K to 241 K. Based on these measurements, we derive the interfacial energy of crystalline NaCl dihydrate in a supersaturated NaCl solution and determined its temperature dependence. Taking into account both temperature and concentration dependence of nucleation rate coefficients, we explain the difference in the observed fractions of NaCl dihydrate reported in the previous studies. Applying the heterogeneous classical nucleation theory model, we have been able to reproduce the 5 K shift of the NaCl dihydrate efflorescence curve observed for the sea salt aerosol particles, assuming the presence of super-micron solid inclusions (hypothetically gypsum or hemihydrate of CaSO4). These results support the notion that the phase transitions in microscopic droplets of supersaturated solution should be interpreted by accounting for the stochastic nature of homogeneous and heterogeneous nucleation and cannot be understood on the ground of bulk phase diagrams alone.

Microscopic observations of riming on an ice surface using high speed video

Microscopic droplet riming events on an ice surface have been observed using high speed video. Observations included greater propensity for droplet spreading at temperatures higher than −15°C on flatter ice surfaces, and subsequently, the formation of growing rime spires into the flow, allowing glancing droplet collisions and more spherical freezing of smaller droplets. Insight into differences between laboratory observations of the Hallett-Mossop process is offered, relating to the nature of droplet spreading associated with the structure of the rimer surface prior to impact. Observations of a difference between air speed and resulting droplet impact speed on an ice surface may affect interpretations of riming laboratory studies, and may explain recent observations of a high secondary ice production rate in supercooled layer clouds.

Effects of the Molecular Structure of a Self-Assembled Monolayer on the Formation and Morphology of Surface Nanodroplets

The formation and morphology of microscopic droplets on a chemically modified surface are important for many droplet-related applications. In this study, we examined the formation and morphological characteristics of nanodroplets produced in the same process of solvent exchange on a gold surface coated with a methyl-terminated alkanethiol monolayer. From atomic force microscopy images, we obtained the contact angles of polymerized nanodroplets in 12 combinations of the length of a straight alkyl chain and the type of droplet liquid. Our results show a significant decrease in the number density of the droplets as the number of methyl groups extends from 8 to 12 or 14. The contact angle of the droplets on octanethiol is significantly larger than that on dodecanethiol or tetradecanethiol, possibly because of the screening effect from the monolayer. Our results demonstrate that under the same solution conditions the morphology of surface nanodroplets is sensitive to the detailed molecular structures of the monolayer on the substrate. This finding has important implications for understanding static wetting on the microscopic scale and the origin of three-phase contact line pinning.

Single Microgels in Core-Shell Equilibrium: A Novel Method for Limited Volume Studies.

The interactions of cationic surfactant dodecyltrimethylammonium bromide and cationic protein cytochrome c with anionic polyacrylate microgels have been investigated in microscopic liquid droplets by means of a micropipette technique at ionic strength 0.01 M and pH 8. Experiments on single microgels in solutions of limited amounts of the surfactant provide the first evidence of microgels in a stable biphasic core-shell state with the surfactant partitioned to the shell. Under the same conditions, the protein is found to distribute uniformly in the microgels. Quantitative data in the form of swelling and binding isotherms are presented and compared with literature data for macrogels and with predictions of a recent gel theory. Theory is found to be in semiquantitative agreement with the experiments. The importance of polyion-mediated attractions between the protein molecules is analyzed theoretically and proposed to explain the continuous but highly cooperative binding isotherms.

Role of Electrolyte Evolution on Damage Distributions and Rates during Atmospheric Corrosion Under Sodium Chloride Droplets

Corrosion under sodium chloride droplets on copper is being studied via electrochemical modeling and experimentally characterized under rigorously controlled laboratory conditions. The objective of this work is to understand the governance of thin alkaline films that emanate from saline droplets upon corrosion initiation on corrosion rates and damage distributions. The radial spreading of thin alkaline films has been reported for both macroscopic and microscopic saline droplets on a variety of alloys that exhibit active corrosion under humid conditions [1-6]. This process is termed secondary spreading. It is resultant from oxygen reduction at cathode sites near the drop edge which produce concentrated hydroxide solution (e.g., NaOH) that subsequently spreads as a film over the metal surface [1,2,7,8]. These films are characteristically comprised of fields of microdroplets on the order of 100 μm diameter or less and can spread several times the original droplet radius from which they emanated. A number of investigators have characterized secondary spreading, demonstrating the potential for these films to serve as sites for oxygen reduction [7]. Unknown is the extent to which these secondary spreading films provide cathodic current to the corroding system and, in turn, their influence on corrosion rates and damage distributions. In this work, we explore the degree to which secondary spreading films influence corrosion that occurs under NaCl droplets on copper for a range of droplet diameters (50 µm to 1 mm). Our approach is to (1) develop a continuum-based model of the system (with non-continuum informed model extensions) to explore the influence of the film under various conditions and (2) define the physicochemical properties of the droplet-film system and experimentally realize the influence of the film on corrosion behavior to inform the model. Specifically, a transient, multiphysics finite element model is being constructed to predict current and potential distributions during the initial stage of corrosion over a range of droplet-film geometries and electrochemical parameters. Initial model assumptions include Nernst-Planck transport kinetics in non-dilute solution and the absence of heterogeneous reactions in the bulk electrolyte; both assumptions will be improved upon in subsequent iterations. Additionally, Butler-Volmer electrochemical kinetics is assumed, with oxygen reduction as the only cathodic reaction and metal oxidation as the only anodic reaction. The behavior of the droplet-film system is being experimentally studied via electrochemical capillary techniques and the physicochemical evolution of the droplet and film characterized to inform and benchmark the model. Key to the experimental efforts is the ability to rigorously control electrolyte geometry. To do so, a novel testing platform has been constructed that will allow production of varying size microdroplets and allow electrochemical probing within them. Experiments are currently under way to characterize the effect of restricted size secondary spreading regions on corrosion kinetics and damage distributions using this system. Together, this integrated computational and experimental approach seeks to deliver new mechanistic insight into atmospheric aerosol-driven corrosion.

Large Scale Flow-Mediated Formation and Potential Applications of Surface Nanodroplets.

Microscopic droplets located on a solid substrate in contact with an immiscible liquid promise a broad range of applications in miniaturized analytical techniques, fabrication of antireflective coatings, high-resolution near-field imaging techniques, and many others. A simple method of producing oil nanodroplets with desirable morphology is a bottom-up approach called solvent exchange, where nanodroplets nucleate and grow, as a good solvent of oil is displaced by a poor solvent. In this work, we have achieved the production of surface nanodroplets over a large surface area on planar or curved surfaces, guided by the principles of the solvent exchange. The droplet size is uniform over the entire surface of a planar or curved substrate and tunable. The production rate is extremely high at 10(6) nanodroplets per second. This advance in the nanodroplet production provides a general platform for droplet-based applications. Here we demonstrate that the application of surface nanodroplets in microextraction of hydrophobic solute (dye) from its highly diluted aqueous solution and in situ detection of the dye in a simple process, and in fabrication of highly ordered array of microlens arrays and polymer-capped microstructures by simple processes.

Spatiotemporal Interpolation Methods for the Application of Estimating Population Exposure to Fine Particulate Matter in the Contiguous U.S. and a Real-Time Web Application.

Appropriate spatiotemporal interpolation is critical to the assessment of relationships between environmental exposures and health outcomes. A powerful assessment of human exposure to environmental agents would incorporate spatial and temporal dimensions simultaneously. This paper compares shape function (SF)-based and inverse distance weighting (IDW)-based spatiotemporal interpolation methods on a data set of PM2.5 data in the contiguous U.S. Particle pollution, also known as particulate matter (PM), is composed of microscopic solids or liquid droplets that are so small that they can get deep into the lungs and cause serious health problems. PM2.5 refers to particles with a mean aerodynamic diameter less than or equal to 2.5 micrometers. Based on the error statistics results of k-fold cross validation, the SF-based method performed better overall than the IDW-based method. The interpolation results generated by the SF-based method are combined with population data to estimate the population exposure to PM2.5 in the contiguous U.S. We investigated the seasonal variations, identified areas where annual and daily PM2.5 were above the standards, and calculated the population size in these areas. Finally, a web application is developed to interpolate and visualize in real time the spatiotemporal variation of ambient air pollution across the contiguous U.S. using air pollution data from the U.S. Environmental Protection Agency (EPA)’s AirNow program.

(Invited) The Influence of Flux Changes on Semiconductor Nucleation and Crystal Growth from a Liquid Metal Electrode

This presentation will discuss the latest findings in regards to the concept of mediating nucleation and growth of a solute crystal inside a liquid metal electrode through heterogeneous electrochemical reduction reactions at the electrolyte/electrode interface. The focus will be to compare/contrast experimental and modeling data that pertain to the flux conditions that favor and inhibit nucleation and crystal growth steps. Two systems will be discussed. First, conditions that favor large-grain crystal growth from a bulk liquid pool electrode will be discussed. Second, flux conditions that permit a single crystal to grow from a microscopic liquid metal droplet will be presented. The possibility of using AC amperometry in both systems to initiate and affect crystal growth will also be discussed.

Large Deviation Analysis of Rapid Onset of Rain Showers.

Rainfall from ice-free cumulus clouds requires collisions of large numbers of microscopic droplets to create every raindrop. The onset of rain showers can be surprisingly rapid, much faster than the mean time required for a single collision. Large-deviation theory is used to explain this observation.

Optics of water microdroplets with soot inclusions: Exact versus approximate results

We use the recently generalized version of the multi-sphere superposition T-matrix method (STMM) to compute the scattering and absorption properties of microscopic water droplets contaminated by black carbon. The soot material is assumed to be randomly distributed throughout the droplet interior in the form of numerous small spherical inclusions. Our numerically-exact STMM results are compared with approximate ones obtained using the Maxwell-Garnett effective-medium approximation (MGA) and the Monte Carlo ray-tracing approximation (MCRTA). We show that the popular MGA can be used to calculate the droplet optical cross sections, single-scattering albedo, and asymmetry parameter provided that the soot inclusions are quasi-uniformly distributed throughout the droplet interior, but can fail in computations of the elements of the scattering matrix depending on the volume fraction of soot inclusions. The integral radiative characteristics computed with the MCRTA can deviate more significantly from their exact STMM counterparts, while accurate MCRTA computations of the phase function require droplet size parameters substantially exceeding 60.

Mathematical modeling of planar and spherical vapor–liquid phase interfaces for multicomponent fluids

Development of methods for accurate modeling of phase interfaces is important for understanding various natural processes and for applications in technology such as power production and carbon dioxide separation and storage. In particular, prediction of the course of the non-equilibrium phase transition processes requires knowledge of the properties of the strongly curved phase interfaces of microscopic droplets. In our work, we focus on the spherical vapor–liquid phase interfaces for binary mixtures. We developed a robust computational method to determine the density and concentration profiles. The fundamentals of our approach lie in the Cahn-Hilliard gradient theory, allowing to transcribe the functional formulation into a system of ordinary Euler-Langrange equations. This system is then split and modified into a shape suitable for iterative computation. For this task, we combine the Newton-Raphson and the shooting methods providing a good convergence speed. For the thermodynamic roperties, the PC–SAFT equation of state is used. We determine the density and concentration profiles for spherical phase interfaces at various saturation factors for the binary mixture of CO2 and C9 H20 . The computed concentration profiles allow to the determine the work of formation and other characteristics of the microscopic droplets.

Spindle-shaped nanoscale yolk/shell magnetic stirring bars for heterogeneous catalysis in macro- and microscopic systems.

A new type of spindle-shaped nanoscale yolk/shell magnetic stirring bar containing noble metal nanoparticles was prepared. The as-synthesized Pd-Fe@meso-SiO2 not only showed impressive activity and stability as a heterogeneous catalyst in a macroscopic flask system, but also acted as an efficient nanoscale magnetic stir bar in a microscopic droplet system.

Self-transport and self-alignment of microchips using microscopic rain.

Alignment of microchips with receptors is an important process step in the construction of integrated micro- and nanosystems for emerging technologies, and facilitating alignment by spontaneous self-assembly processes is highly desired. Previously, capillary self-alignment of microchips driven by surface tension effects on patterned surfaces has been reported, where it was essential for microchips to have sufficient overlap with receptor sites. Here we demonstrate for the first time capillary self-transport and self-alignment of microchips, where microchips are initially placed outside the corresponding receptor sites and can be self-transported by capillary force to the receptor sites followed by self-alignment. The surface consists of hydrophilic silicon receptor sites surrounded by superhydrophobic black silicon. Rain-induced microscopic droplets are used to form the meniscus for the self-transport and self-alignment. The boundary conditions for the self-transport have been explored by modeling and confirmed experimentally. The maximum permitted gap between a microchip and a receptor site is determined by the volume of the liquid and by the wetting contrast between receptor site and substrate. Microscopic rain applied on hydrophilic-superhydrophobic patterned surfaces greatly improves the capability, reliability and error-tolerance of the process, avoiding the need for accurate initial placement of microchips, and thereby greatly simplifying the alignment process.